John *; Harrison STATE OF ILLINOIS DWIGHT H. GREEN, Governor DEPARTMENT OF REGISTRATION AND EDUCATION FRANK G. THOMPSON, Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chief URBANA REPORT OF INVESTIGATIONS — No. 102 THE BONDING ACTION OF CLAYS PART I— CLAYS IN GREEN MOLDING SANDS BY Ralph E. Grim and F. Leicester Cuthbert A Cooperative Research Project Conducted by THE STATE GEOLOGICAL SURVEY AND THE ENGINEERING EXPERIMENT STATION, UNIVERSITY OF ILLINOIS in Cooperation with THE ILLINOIS CLAY PRODUCTS COMPANY, JOLIET, ILLINOIS PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 1945 This report is also being published by the Engineering Experiment Sta- tion of the University of Illinois as Bulletin 357 nun STATE OF ILLINOIS DWIGHT H. GREEN, Governor DEPARTMENT OF REGISTRATION AND EDUCATION FRANK G. THOMPSON, Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chief URBANA REPORT OF INVESTIGATIONS — No. 102 THE BONDING ACTION OF CLAYS PART I— CLAYS IN GREEN MOLDING SANDS RALPH E. GRIM Petrographer, State Geological Survey F. LEICESTER CUTHBERT Special Research Associate in Petrography and Mechanical Engineering A Cooperative Research Project Conducted by THE STATE GEOLOGICAL SURVEY AND THE ENGINEERING EXPERIMENT STATION, UNIVERSITY OF ILLINOIS in Cooperation with THE ILLINOIS CLAY PRODUCTS COMPANY, JOLIET, ILLINOIS PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 1945 ORGANIZATION STATE OF ILLINOIS HON. DWIGHT H. GREEN, Governor DEPARTMENT OF REGISTRATION AND EDUCATION HON. FRANK G. THOMPSON, Director BOARD OF NATURAL RESOURCES AND CONSERVATION HON. FRANK G. THOMPSON, Chairman NORMAN L. BOWEN, Ph.D., D.Sc, LL.D., Geology ROGER ADAMS, Ph.D., D.Sc, Chemistry LOUIS R. HOWSON, C.E., Engineering *WILLIAM TRELEASE, D.Sc, LL.D., Biology EZRA JACOB KRAUS, Ph.D., D.Sc, Forestry ARTHUR CUTTS WILLARD, D.Engr., LL.D. President of the University of Illinois GEOLOGICAL SURVEY DIVISION M. M. LEIGHTON, Chief *Deceased. SCIENTIFIC AND TECHNICAL STAFF OF THE STATE GEOLOGICAL SURVEY DIVISION 100 Natural Resources Building, Urbana M. M. LEIGHTON, Ph.D., Chief Enid Townley, M.S., Assistant to the Chief Velda A. Millard, Junior Asst. to the Chief Helen E. McMorris, Secretary to the Chief GEOLOGICAL RESOURCES Coal G. H L. C. Cady, Ph.D., Senior Geologist and Head McCabe, Ph.D., Geologist (on leave) R. J. Helfinstine, M.S., Mech. Engineer Charles C. Boley, M.S., Assoc. Mining Eng. Heinz A. Lowenstam, Ph.D., Assoc. Geologist Bryan Parks, M.S., Asst. Geologist Earle F. Taylor, M.S., Asst. Geologist (on leave) Ralph F. Strete, A.M., Asst. Geologist M. W. Pullen, Jr., M.S., Asst. Geologist Robert M. Kosanke, M.A., Asst. Geologist Robert W. Ellingwood, B.S., Asst. Geologist George M. Wilson, M.S., Asst. Geologist Arnold Eddings, B.A., Research Assistant (on leave) Henry L. Smith, A.B., Asst. Geologist Raymond Siever, B.S., Research Assistant (on leave) John A. Harrison, B.S., Research Assistant (on leave) Mary E. Barnes, B.S., Research Assistant Margaret Parker, B.S., Research Assistant Elizabeth Lohmann, B.F.A., Technical Assistant Industrial Minerals J. E. Lamar, B.S., Geologist and Head H. B. Willman, Ph.D., Geologist Robert M. Grogan, Ph.D., Assoc. Geologist Robert T. Anderson, M.A., Asst. Physicist Robert R. Reynolds, M.S., Asst. Geologist Margaret C. Godwin, A.B., Asst. Geologist Oil and Gas A. H. Bell, Ph.D., Geologist and Head Carl A. Bays, Ph.D., Geologist and Engineer Frederick Squires, B.S., Petroleum Engineer Stewart Folk, M.S., Assoc. Geologist (on leave) Ernest P. DuBois, Ph.D., Assoc. Geologist David H. Swann, Ph.D., Assoc. Geologist Virginia Kline, Ph.D., Assoc. Geologist Paul G. Luckhardt, M.S., Asst. Geologist (on leave) Wayne F. Meents, Asst. Geologist James S. Yolton, M.S., Asst. Geologist Robert N. M. Urash, B.S., Research Assistant Margaret Sands, B.S., Research Assistant Areal and Engineering Geology George E. Ekblaw, Ph.D., Geologist and Head Richard F. Fisher, M.S., Asst. Geologist Subsurface Geology L. E. Workman, M.S., Geologist and Head Carl A. Bays, Ph.D., Geologist and Engineer Robert R. Storm, A.B., Assoc. Geologist Arnold C. Mason, B.S., Assoc. Geologist (on leave) C. Leland Horberg, Ph.D., Assoc. Geologist Frank E. Tippie, B.S., Asst. Geologist Merlyn B. Buhle, M.S., Asst. Geologist Paul Herbert, Jr., B.S., Asst. Geologist Charles G. Johnson, A.B., Asst. Geologist (on leave) Margaret Castle, Asst. Geologic Draftsman Marvin P. Meyer, B.S., Asst. Geologist Robert N. M. Urash, B.S., Research Assistant Ruth E. Roth, B.S., Research Assistant Stratigraphy and Paleontology J. Marvin Weller, Ph.D., Geologist and Head Chalmer L. Cooper, M.S., Assoc. Geologist Petrography Ralph E. Grim, Ph.D., Petrographcr Richards A. Rowland, Ph.D., Asst. Petrographer (on leave) William A. White, B.S., Research Assistant Physics R. J. Piersol, Ph.D., Physicist B. J. Greenwood, B.S., Mech. Engineer GEOCHEMISTRY Frank H. Reed, Ph.D., Chief Chemist H. W. Jackman, M.S.E., Chemical Engineer P. W. Henline, M.S., Assoc. Chemical Engineer James C. McCullough, Research Associate James H. Hanes, B.S., Research Assistant (on leave) Leroy S. Miller, B.S., Research Assistant Elizabeth Ross Mills, M.S., Research Assistant Coal G. R. Yohe, Ph.D., Chemist Herman S. Levine, B.S., Research Assistant Industrial Minerals J. S. Machin, Ph.D., Chemist and Head Delbert L. Hanna, A.M., Asst. Chemist Fluorspar G. C. Finger, Ph.D., Chemist Oren F. Williams, B.Engr., Research Assistant X-ray and Spectrography W. F. Bradley, Ph.D., Chemist Analytical O. W. Rees, Ph.D., Chemist and Head L. D. McVicker, B.S., Chemist Howard S. Clark, A.B., Assoc. Chemist William F. Wagner, M.S., Asst. Chemist Cameron D. Lewis, B.A., Asst. Chemist Herbert N. Hazelkorn, B.S., Research Assistant William T. Abel, B.A., Research Assistant Melvin A. Rebenstorf, B.S., Research Assistant Marian C. Stoffel, B.S., Research Assistant Jean Lois Rosselot, A.B., Research Assistant MINERAL ECONOMICS W. H. Voskuil, Ph.D., Mineral Economist Douglas F. Stevens, M.E., Research Associate Ethel M. King, Research Assistant PUBLICATIONS AND RECORDS George E. Ekblaw, Ph.D., Geologic Editor Chalmer L. Cooper, M.S., Geologic Editor Dorothy E. Rose, B.S., Technical Editor Meredith M. Calkins, Geologic Draftsman Beulah Featherstone, B.F.A., Asst. Geologic Draftsman Willis L. Busch, Principal Technical Assistant Portia Allyn Smith, Technical Files Clerk Rosemary Metzger, Technical Assistant Leslie D. Yaughan, Asst. Photographer Consultant: Ceramics, Cullen W. Parmelee, M.S., D.Sc, and Ralph K. Hursh, B.S., University of Illinois Mechanical Engineering, Seichi Konzo, M.S., University of Illinois Topographic Mapping in Cooperation with the United States Geological Survey. This report is a contribution of the Petrography Division. January 1. 1945 Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/bondingactionofc102grim ABSTRACT The Bonding Action of Clays Part I — Clays in Green Molding Sands For more than nine years the Illinois State Geological Survey has been studying intensively the composition, molecular structure, and properties of the clay minerals which are the primary components of clays and shales. The present investigation sought to extend this work to the particular clays used in bonding molding sands with the thought that the more economical production of better castings depends largely on a better understanding of the properties of molding sands and bonding clays. The silica sand com- ponent of molding sands has been studied extensively, but hitherto there has been little investigation of the clay. Clays are essentially aggregates of extremely small crystalline, usually flake-shaped, particles of one or more kinds of a small group of minerals known as the clay minerals. Since the properties of any clay depend largely on its clay mineral composition, a fundamental basis for the classification of bonding clays is provided. Such a classification, founded on clay mineral composition, is presented together with detailed determinations of the green strength properties that are characteristic of each class. The bonding value of one of the classes of bonding clays, i.e., halloysite, was discovered during the investigation reported herein. A theory of the bonding action of clays in molding sands, based on the individual properties of the clay minerals and the rigid character of the initial adsorbed water is presented which ascribes the bonding action of clay to a "wedge-block" at the junction of the quartz grains rather than to a gluing or adhesive effect. The characteristics peculiar to the various classes of bonding clay are explainable by this theory. It explains, for ex- ample, the unusual air-set strength characteristics of sands bonded with kaolinite or halloysite clay. Such sands develop greatly increased strength without an accompanying water loss when they are allowed to stand in the air after ramming. The bulk density characteristics of sands bonded with each class of clay are presented. An explanation for the variation of bulk density with amount of tempering water is suggested, based on the physical character of the water adsorbed by the clay. CONTENTS PAGE Introduction 1 Acknowledgments 2 Concept of the structure of clays 2 Size and shape of particles 3 Composition and structure of particles 4 The clay mineral concept 4 Cause of properties 5 Base exchange 5 Classification of bonding clays 6 Class I. Montmorillonite bonding clays 6 Class IA 7 Class IB 7 Class II. Halloysite bonding clays 7 Class III. Illite bonding clays 8 Class IV. Kaolinite bonding clays 8 Clays investigated 9 Montmorillonite clay from near Belle Fourche, South~ Dakota . 9 Montmorillonite clay from northern Mississippi 10 Halloysite clay from Lawrence County, Missouri .... 10 Illite clay from Grundy County, Illinois 10 Kaolinite clay from Grundy County, Illinois 10 Experimental procedure 10 Green compression strength in relation to tempering water . . .11 Bulk density in relation to tempering water 14 Green compression strength in relation to the amount and kind of clay in molding mixtures 19 At maximum green compression strength 19 At minimum bulk density 22 VI CONTENTS (Continued) PAGE Character of the clay coating of sand grains and distribution of tempering water in green sands 24 Sands bonded with the montmorillonite clay IA 24 Sands bonded with the montmorillonite clay IB 28 Sands bonded with the halloysite clay 30 Sands bonded with the illite clay 32 Sands bonded with the kaolinite clay 34 Concept of the bonding action of clays in green sand .... 38 Relation to intensity of mulling 42 Durability 43 Air-set strength 44 Concept of bulk density variations in molding sands .... 46 Summary of factors affecting the green bonding properties of clays . 50 Amount of clay mineral 50 Kind of clay mineral 50 Exchangeable bases 51 Miscellaneous components of clay 51 Summary 51 Bibliography 55 VII ILLUSTRATIONS FIGURE PAGE 1. Green compression strength versus amount of tempering water in sands bonded with varying amounts of montmorillonite clay IA 12 2. Green compression strength versus amount of tempering water in sands bonded with varying amounts of montmorillonite clay IB 12 3. Green compression strength versus amount of tempering water in sands bonded with varying amounts of halloysite clay 13 4. Green compression strength versus amount of tempering water in sands bonded with varying amounts of illite clay 13 5. Green compression strength versus amount of tempering water in sands bonded with varying amounts of kaolinite clay 14 6. Bulk density versus amount of tempering water in sands bonded with varying amounts of montmorillonite clay IA 15 7. Bulk density versus amount of tempering water in sands bonded with varying amounts of montmorillonite clay IB 16 8. Bulk density versus amount of tempering water in sands bonded with varying amounts of halloysite clay 17 9. Bulk density versus amount of tempering water in sands bonded with varying amounts of illite clay 18 10. Bulk density versus amount of tempering water in sands bonded with varying amounts of kaolinite clay 19 11. Curves showing the maximum green compression strength developed by each type of clay in relation to the amount of clay in the sand-clay mixtures . . 20 12. Curves showing the green compression strength at the minimum bulk density point developed by each type of clay in relation to the amount of clay in the sand-clay mixtures 23 13. Photomicrographs of sand bonded with 4 percent montmorillonite clay IA showing the smooth regular coating of the clay around the quartz grains (150X) 25 14. Diagrammatic sketch illustrating the character of the coating of the quartz grains in sands bonded with montmorillonite clay IA 26 15. Photomicrograph of sand bonded with 8 percent kaolinite clay showing the irregular coating of the clay around the quartz grains (150X) 35 16. Photomicrograph of sand bonded with 12 percent kaolinite clay showing the very uneven coating of the clay around the quartz grains (150X) .... 36 17. Diagrammatic sketch illustrating the character of the coating of the quartz grains in sands bonded with kaolinite clay 37 18. Sketch of montmorillonite clay-bonded sand, based on microscopic examination, to illustrate the smooth even coating of the quartz grains with flakes of mont- morillonite, and the wedge-shaped blocks at the junction of the clay-coated quartz grains holding the grains in place 39 19. Sketch of kaolinite clay-bonded sand, based on microscopic examination, to illustrate the irregular coating of the quartz grains with small flakes and large lumps of flakes, and the wedge-shaped blocks at the junction of the clay-coated quartz grains holding the grains in place 40 20. Curve showing the typical relation of bulk density to tempering water in molding sands 46 Vlll THE BONDING ACTION OF CLAYS PART I — CLAYS IN GREEN MOLDING SANDS Introduction It is agreed generally by those associated with the metal founding industry that the more economical production of better castings is depend- ent largely on a better understanding of the properties of molding sands and rebonding clays. Extensive study has been given the silica sand components of molding sand, 4 ' 16,17 * but until recently the amount of fundamental data obtained on clays for rebonding has been meager. This is obviously an important matter. In 1940, an agreement for a cooperative research project on the con- stitution and bonding and related properties of clays for metallurgical use was made by Dr. M. M. Leighton, Chief of the Illinois State Geological Survey, Professor M. L. Enger, Director of the Engineering Experiment Station of the University of Illinois, and Otis L. Jones. President of the Illinois Clay Products Company, Joliet, Illinois. This project was con- tinued for nearly four years. It was an outgrowth of nine years of intensive research on clay mineralogy and the molecular structure of clays by Dr. Ralph E. Grim and Dr. William F. Bradley. In this present project the work was placed under the general direction and supervision of Dr. Grim, who selected the samples, made the mineral determinations, and, with Dr. Cuthbert, planned the tests, interpreted the results, and formulated the classification of bonding clays. The State Geological Survey made available its laboratory facilities in clay mineral technology, where the mineralogical work was done and the samples pre- pared for testing, and the Engineering Experiment Station its facilities for foundry tests. The latter were carried out in consultation with 0. A. Leut- wiler, Professor and Head of the Department of Mechanical Engineering, C. H. Casberg, Professor of Mechanical Engineering, and C. E. Schubert, Assistant Professor of Mechanical Engineering. The Illinois Clay Products Company provided the funds for the pur- chase of special apparatus, for the employment of Dr. F. L. Cuthbert and assistants, and such other expenses as were special for this investigation. These funds were held in trust by the University of Illinois. The Illinois Clay Products Company also provided industrial information and advice. Tor references, see bibliography at end of report. 2 CLAYS IN GREEN MOLDING SANDS The objectives of the study were: 1. To gain an understanding of the bonding action of clays. 2. To evaluate the bonding properties of the various clay minerals and their durability. 3. To determine to what extent and how bonding clays can be improved by the use of beneficiating substances commonly used for that purpose. 4. To determine whether or not new and better bonding materials could be developed and what their specific properties and limitations are. 5. To improve mold making practices which would have the advantages of lowering the costs, reducing defective castings, increasing production, and producing better castings. Many varieties of clays were investigated, many chemical and physical treatment processes were explored, and the part played by water in the bonding of sands by clay and water was studied. This paper presents certain of those results of this cooperative project which have to do with green sand, namely: 1. A fundamental classification of bonding clays, based on the mineral components of clays which are largely responsible for their bonding action. 2. A description of the green compression strength and bulk density properties that are characteristic of each class of bonding clays. 3. A theory of the bonding action of clays in green sands. 4. An explanation of the variation of bulk density in green sands. Other results of the cooperative investigation will be given in later reports. ACKNOWLEDGMENTS The authors wish to acknowledge the great assistance rendered by the sponsors, particularly of Mr. Otis L. Jones, President, and Mr. Arthur S. Nichols, Vice-President, of the Illinois Clay Products Company. Their interest in research made the work possible. Their continued counsel and encouragement very materially aided in the prosecution of the work. Professors 0. A. Leutwiler and A. P. Kratz of the Department of Mechanical Engineering offered many helpful comments during the study and in the preparation of the report. Finally the helpful interest and coun- sel of Dr. M. M. Leighton, Chief, Illinois State Geological Survey, and Professor M. L. Enger, Dean, College of Engineering, University of Illinois, is gratefully acknowledged. Concept of the Structure of Clays In the last fifteen years the improvement of tools and methods of microscopy and the development of new research tools, such as X-ray diffraction analysis, and the electron microscope, have extended tre- STRUCTURE OF CLAYS 3 mendously the research work on clays and have made possible valuable discoveries of the fundamental structure of clays and the cause of their properties. 6 ' 7> 10 Many laboratories have contributed to this new knowledge. Before proceeding to the discussion of the findings of the present inves- tigation of the bonding action of clays in green sands, it is important to summarize the present concepts of clays, for the benefit of those readers who have not had the opportunity to follow closely the research work of many laboratories. SIZE AND SHAPE OF PARTICLES If one could magnify a piece of clay about 25,000 times with a very powerful microscope, one would see that it is composed of a tremendous number of particles, each shaped like a flake or piece of paper. Each flake would have a surface diameter of the order of one inch and a thickness of a small fraction of an inch. In some clays one would see that the flakes were mixed together without any regular pattern or arrangement, or, if one were looking at a shale, one would see that each flake was lying flat, one on top of another. These flake-shaped units are the fundamental and essential units that make up clays and shales — they exert primary control over the bonding, plastic, and other properties of clay materials. The flakes out of which clays are built are usually so small that they possess colloidal properties. The so-called colloidal content of almost all clays is simply the quantity of these flakes that are smaller than a given size — -usually about 725,000 °f an m(m (2 microns). If one looked at a variety of clays under the very powerful microscope, one would find that some of them were built of nothing but flakes whereas others contained additional materials, usually in small amounts, that would appear to be impurities. These additional materials would appear to be granular rather than flake-shaped, consisting frequently of small grains of silica or sand, particles of organic matter, and bits of iron pyrites. If the clay was very gritty, one would notice that the number of silica grains was great and perhaps that the individual grains were larger than the flakes. The study of a variety of clays would lead to the general conclusion that all clays are composed chiefly of flake-shaped particles. Some clays are nothing but aggregates of flakes — many others contain additional minute particles, usually granular in shape, of a variety of materials in- cluding free silica (quartz), pyrite, organic matter, etc. This conclusion would be correct for all but a very few types of clay for which the description given above does not fit precisely. These few clays are com- posed essentially of extremely minute particles that are elongate, lath- shaped, or fibrous, instead of flake-shaped. Some of these unusual clays also have strong bonding power. They are considered later. 4 CLAYS IN GREEN MOLDING SANDS COMPOSITION AND STRUCTURE OF PARTICLES Since the flake-shaped particles (lath- or fibre-shaped in a few rare cases) are the essential building blocks of clays, it is necessary to study them in detail in order to learn the causes of the properties of clays. A large amount of such work has been done, and one of its results has been to show that the flakes are definite chemical compounds, chiefly of alumina, silica, and water, and that they are crystalline. The flakes are not hetero- geneous mixtures of alumina, silica, and/or other materials. If one could magnify the individual flakes many more thousands of times one would find them composed of atoms of oxygen, silicon, aluminum and hydroxyls (some flakes also contain atoms of iron, potassium, and magnesium) , and that these atoms were arranged in definite fixed patterns. If the flakes were not crystalline, the atoms composing them would be mixed in a random fashion without any plan or pattern. The flake-shaped units are truly minerals because they are natural compounds with definite compositions and structures. If the flake-shaped units in a variety of clays were studied, one would find that the structure and composition of the flakes are not always the same. For example, some flakes would contain a larger number of silicon atoms than others, and the arrangement of the atoms would be different in some flakes than in others. The study of a large number of different clays would show that almost all of the flakes that compose them can be classed into a few groups. One would find that some clays were made up entirely of flakes belonging to one group whereas other clays were built up of flakes belonging to two or even three of the groups. In other words a few groups of minerals are the essential components of almost all clays. Because these minerals are found primarily in clays, they have been called "clay minerals." Very careful study has shown that there are slight variations in the composition of flakes belonging to a single group, that is, each group can be broken down into a series of subgroups or species based on minor variations in composition. THE CLAY MINERAL CONCEPT The foregoing statements are generally agreed upon by all students of clays, and form the basis of what has come to be known as the "clay mineral concept" of the composition and structure of clay materials. Briefly stated this concept is as follows: Clays and shales are essentially aggregates of extremely minute, crys- talline, usually flake-shaped particles that can be classified on the basis of their structure and composition into a few groups which are known as clay minerals. Some clays are composed of particles of a single clay mineral STRUCTURE OF CLAYS 5 whereas others are mixtures of clay minerals. Some clays are composed entirely of clay minerals whereas others contain admixtures of quartz, pyrite, organic matter, etc. CAUSE OF PROPERTIES The clay mineral concept provides a key for an understanding of the properties of clay. The properties of the clay mineral flakes depend largely on the composition and structural arrangement of the oxygen, silicon, and other atoms that compose them. Each group of clay minerals will, there- fore, have its own set of properties because the structure and composition is distinctive and different for each group. Therefore, the properties of a clay with no admixtures depend upon the clay minerals that make it up. If one examined a series of clays that are about the same in all properties (bonding, plasticity, refractoriness, etc.), one would probably find that they were built of the same kind of flakes. Since the clay minerals are the primary factor controlling the properties of a clay, one should be able to predict the properties of a clay when its clay mineral composition is known. This can actually be done. The amount and grain-size of admixtures of free silica (quartz), organic matter, pyrite, and the amount and kind of soluble salts will also influence the properties of a clay if they are present. BASE EXCHANGE In order to fully understand the properties of clays, it is also necessary to consider another of their characteristics, namely the adsorption of certain atoms and molecules. Thus, if one were to pass a common sodium salt solution through certain kinds of clay under certain conditions, it would be found that the clay had removed some of the sodium from the solution, that is, the clay had adsorptive capacity. If the experiment were carried further and a solution of a calcium salt were passed through the clay that had adsorbed the sodium, one would find that some of the calcium was removed from the solution by the clay and the clay had lost some sodium to the solution. In other words the clay had exchanged some sodium for some calcium. This is known as the base-exchange reaction. The agent of the base-exchange reaction is the flake-shaped units, i.e., the clay minerals, of which clays are composed. The different clay minerals have widely different base-exchange capacity, or in other words, can hold and exchange vastly different amounts of sodium, calcium, etc. The base-exchange property is important because the bonding and plastic properties of clays vary, depending on whether the clay contains sodium, calcium, or some other adsorbed ion. The amount of variation in properties is of practical significance only in those clays composed of clay minerals that carry relatively large quantities of exchangeable atoms. 6 CLAYS IN GREEN MOLDING SANDS Classification of Bonding Clays Determinations were made of the bonding properties of several hun- dred samples of various types of clay including a large number of com- mercial bonding clays. Petrographic and X-ray analyses of the composition of these samples showed that they were composed of a variety of clay minerals. A detailed study 9 of these data has provided a fundamental classification of bonding clays based on clay mineral composition. The experimental work presented herein shows that this arrangement separates bonding clays into groups with distinctive properties. No claim is made that the classification is complete — future studies may reveal the need for additional classes or subdivisions of suggested classes. CLASS I. MONTMORILLONITE BONDING CLAYS One of the groups of clay minerals has been named montmorillonite after the town of Montmorillon in France, from where the first described sample was obtained. The composition of all members of this group can be expressed by the general formula (OH^AUSigOao ' nH 2 0. In certain members of the group some of the aluminum is replaced by iron or by magnesium and iron, and these replacements form the basis for subdividing the group. Thus, when a considerable amount of aluminum is replaced by iron the mineral is called nontronite. Almost all bentonite-type clays are composed of extremely minute particles of the montmorillonite clay minerals. Because of the characteristic structure of the montmorillonite molecule, clays composed of this type of clay mineral have a high capacity for adsorbing water. When pieces of dry montmorillonite clays are placed in water, they tend to increase in volume, i.e., swell, without at first losing their shape. Such clays in the presence of water tend to break up readily into extremely minute units, considerably less than /^ 5,000 inch, and to form relatively permanent gels. Montmorillonite clays do not withstand high temperatures; pure clay of this type fuses at temperatures as low as 1800° F. Montmorillonite clays become very plastic when moistened with water, and exhibit high shrinkage when the moisture is lost on drying. Clays composed of montmorillonite have the property of adsorbing considerable amounts of hydrogen, sodium, calcium, etc. (i.e., they have base-exchange capacity). These bases are adsorbed in quantities large enough to materially affect the properties. The bonding properties of montmorillonite clays vary, depending on the adsorbed base and on the particular member of the montmorillonite group present. Thus, two montmorillonite clays, identical in every respect except that one has adsorbed sodium and the other calcium, will have different properties. Also two montmorillonite clays containing the same CLASSIFICATION OF BONDING CLAYS 7 adsorbed base will have different properties if one of them has a consider- able amount of aluminum replaced by iron. These two factors form a logical basis for subdividing the montmorillonite class of bonding clays. Class I A. — Montmorillonite clays belonging to class I A are composed of montmorillonite in which the aluminum is replaced by some magnesium but not by appreciable iron, and in which sodium is the chief exchangeable base. Bentonites from the Black Hills area of Wyoming and South Dakota belong to this class of clays. Such clays swell greatly in water, are alkaline in reaction, and disperse easily and rapidly in the presence of water to form relatively permanent gels. Class IB. — Montmorillonite clays belonging to class IB have a consid- erable amount of aluminum replaced by iron. They carry calcium and sometimes also hydrogen as the chief exchangeable ions. Bentonites from northern Mississippi belong to this class of clays. They swell very slightly in water, are about neutral or slightly acid in reaction, and disperse easily and rapidly into very small particles in water without forming relatively permanent suspensions. It must be emphasized that there may be other bentonites with valu- able bonding properties that future study will show do not fit in either class IA or IB. It should be emphasized also that any classification of clay is an arbitrary thing because there are all gradations between clays rather than isolated types. Montmorillonite clays may, therefore, be found that will be intermediate between I A and IB, fitting equally well into either class, depending on what emphasis is placed on their properties. CLASS II. HALLOYSITE BONDING CLAYS Another group of clay minerals is called halloysite after d'Halloy who first described them. There are two members of the halloysite group; 12 ' 14 one has the composition (OH) 8 Al 4 Si 4 O 10 , the other has the composition (0H) 16 Al 4 Si 4 6 . The latter type has the most water and changes slowly to the former type by the loss of water at a low temperature (140° F.). So far as known, it was in the present investigation that the bonding properties of halloysite clay were first discovered and determined. Individual units of halloysite are lath-shaped, consequently clays composed of this mineral are made up of lath-shaped units. Clays com- posed of halloysite are very refractory with moderate shrinkage and plastic properties. Halloysite clays with high bonding properties are readily dispersed in water into very small units. They have low aclsorptive capacities for basic ions, and consequently adsorbed sodium, calcium, etc., are present in quantities too limited to cause a variation in bonding properties. 8 CLAYS IN GREEN MOLDING SANDS Investigation has shown that the bonding properties of halloysite clays are dependent on the form of the halloysite that is present. In general the best bonding properties seem to be present when the clay is composed of mixtures of the two hydration forms or is in a transition stage between them. "White clay" from the Eureka district in Utah, recently placed on the market as a bonding clay as a result of this investigation, is composed of halloysite, and therefore belongs to this class of bonding clays. CLASS III. ILLITE BONDING CLAYS Another group of clay minerals is called illite after the abbreviation for the State of Illinois where it was first named by Grim, Bray, and Bradley. 8 The illite clay minerals have a very complex composition as can be seen by the following general formula for them (0H) 4 K y (Al 4 • Fe 4 • Mg 4 ■ Mg«) (Si 8 _ y ■ Al y ) 20 . There is considerable variation in the composition of the illites, par- ticularly in the amount of potash and water and in the ratio of silica to alumina. As a consequence there is considerable variation in the properties of clays composed of illite. Only some of the clays composed of illite have properties of value for commercial bonding use. In general these illite clays have relatively higher water and lower potash contents than those without commercial bonding values. Illite clays are not refractory (2500° F.±) and have moderate to low shrinkage. The illite clays with high bonding power disperse fairly easily in water to extremely minute particles with a slight tendency to form permanent gels. Their moderate adsorptive power toward basic ions permits too small a content of adsorbed calcium, sodium, etc., to cause appreciable variation in bonding properties. "Grundite" produced in Grundy County, Illinois, is a trade example of a bonding clay composed chiefly of illite (it contains also a small amount of kaolinite). CLASS IV. KAOLINITE BONDING CLAYS Another group of clay minerals is the kaolinite group. Members of this group have the composition (OH) 8 Al 4 Si 4 Oi . This is the same composition as that of the lower hydrated member of the halloysite group, but the atoms are arranged somewhat differently in the two groups of minerals. Many clays composed of kaolinite have low bonding properties whereas others, perhaps because of certain structural characteristics such as slight distortion of their atomic arrangement, have high bonding properties. The fireclays that are produced extensively in Illinois and Ohio for the bonding trade are made up of kaolinite with small amounts of illite. CLAYS INVESTIGATED 9 Kaolinite clays are refractory and have low shrinkage. They have very low adsorptive power for basic ions and consequently the adsorbed cal- cium, sodium, etc., are too scant to influence bonding properties. Clays Investigated Out of the large number of samples investigated an example of each class and subclass, according to the classification herein suggested, was selected for detailed study. Samples were chosen on the basis of being- representative of a class and having a small content of nonclay minerals. In order to obtain such samples of certain classes it was necessary to specially hand pick crude material or remove the nonclay mineral material from the crude clay. The investigation of such pure clay mineral samples permitted (1) the determination of the bonding properties that are peculiar to each class of bonding clays, and therefore provided data that are applicable to all clays belonging to the various classes, (2) the evalu- ation of the role played by tempering water in the bonding process, and (3) the development of a fundamental concept of the bonding action of clay and water and of bulk density variations in green sands. Many clays used in foundries contain components other than clay minerals, i.e., quartz, organic material, etc., but one can consider that the bonding properties of such clays are the result mainly of the clay minerals composing them, being modified somewhat by the nonclay minerals. An investigation of such clays encounters difficulties because when the heter- ogeneous mixture is tested, the effect of the several components cannot be evaluated. Because of the complex composition of such clays, experi- mental data obtained from them, without separation of the clay minerals, are applicable only to the sample tested and rarely lead to fundamental conclusions that enlighten foundry practice. Since the samples investigated were specially selected, the test data given for them are probably not precisely the same as those for any clay at present used in foundries. The data, however, can be applied generally to about all clays used in foundries and foundrymen can apply the con- clusions to their particular clays as soon as the class to which they belong is known. Montmorillonite clay from near Belle Fourche, South Dakota. — This clay is a bentonite and is an example of class IA. Analytical data* show that it is composed of about 90 percent montmorillonite and 10 percent quartz, feldspar, mica, and miscellaneous rare minerals. The clay is alka- line (pH = 9.04) and sodium is the principal adsorbed base as indicated •Analytical data, including microscopic, X-ray, differential thermal, and chemical, are the usual type and are not given herein. 10 CLAYS IN GREEN MOLDING SANDS by the following determinations by Weidman 19 made on a clay from the same general deposit: Na + 93.0; K + 6.1; Ca ++ 18.5; Mg ++ 0.7; exchange capacity corrected for sulphates 92.7 (all values are in milliequivalents per 100 grams). Montmorillonite clay from northern Mississippi. — This clay is a ben- tonite, and is an example of class IB. Analytical data show that it is com- posed of about 85 percent montmorillonite-type clay mineral of which an appreciable part is the iron-rich member (i.e. nontronite). About 15 per- cent of the sample is quartz, limonite, and miscellaneous rare minerals. The clay is slightly acid (pH = 6.5) and calcium is the principal adsorbed base as shown by the following determinations: Ca + + 92; Mg + + 8.7; K + 0.1; Na + trace; exchange capacity corrected for soluble salts 93 (all values are in milliequivalents per 100 grams) . Halloysite clay jrom Lawrence County, Missouri. — This clay is com- posed almost wholly of halloysite-type clay mineral, and is an example of class II. Analytical data indicate that it is a very intimate mixture of the lower and higher hydration forms of halloysite with the former predominating. It can be considered as representing a transition state between the two forms. Illite clay from Grundy County, Illinois. — This clay was especially separated from a sample of underclay of Pennsylvanian age in order to obtain a sample of relatively pure illite clay that is representative of those illite clays with high bonding strength. It is an example of class III. Analytical data indicate that after separation it is composed of about 70 percent illite, 15 percent kaolinite, and 15 percent grains of nonclay minerals, chiefly quartz. Kaolinite clay from Grundy County, Illinois. — This clay was especially prepared from a sample of a different underclay of Pennsylvanian age from the above sample in order to obtain a relatively pure kaolinite-type clay representative of those kaolinite clays with good bonding strength as an example of class IV. It was obtained from a natural fireclay by separating all the material finer than %5,ooo of an mcn (2 microns) by a wet classi- fication procedure. Analytical data show that after separation it is com- posed of about 60 percent kaolinite, 30 percent illite, and 10 percent quartz and miscellaneous rare minerals. Experimental Procedure Green compression strengths were determined, according to the stand- ard procedure of the American Foundrymen's Association 1 (A.F.A.), for mixtures of all samples using various amounts of clay up to 15 percent GREEN COMPRESSION STRENGTH vs. TEMPERING WATER 11 with varying amounts of tempering water. The weight of the standard sized specimen was recorded for all the determinations. Standard A.F.A. sand was used in all the experiments. On the basis of the size of the grains in this sand as shown by microscopic and sieve analyses, computations were made of the total surface area of the grains in a given volume of sand. Further computations were made of the thick- ness of the film of clay and water coating the sand grains at maximum green compression strength and minimum bulk density point for all samples at various concentrations of clay and sand. The mulled and rammed samples were studied microscopically to de- termine the distribution of the clay with respect to the quartz grains. Green Compression Strength in Relation to Tempering Water It has become well known after the early work of Ries and Nevin 18 that the maximum green compression strength (m.g.c.) of molding sands is developed with a definite amount of tempering water. The addition of more or less tempering water than the optimum amount is accompanied by a reduction of strength. Curves presenting the relation between green compression strength and the amount of tempering water for varying amounts of montmorillonite clay I A, montmorillonite clay IB, halloysite clay, illite clay, and kaolinite clay are given in figures 1, 2, 3, 4, and 5 respectively. Briggs and Morey 3 have published a similar set of curves for sands bonded with a bentonite that probably belongs to class IA. Their curves are like those in figure 1 except that they are somewhat steeper which may be the result of a slight difference in the bentonite or in the testing sand. For sand-clay mixtures with 4 percent or less of any of the clays, the curves showing the relation of tempering water to green compression strength are relatively flat. That is to say the strength of the mixture decreases gradually when more or less water is added than that necessary to develop maximum strength. The curves for all types of clay are steeper for mixtures with amounts of clay from 4 to 15 percent than with lesser amounts of clay. In the range from 4 to 15 percent of clay, the maximum green compression strength is attained within narrow moisture limits. The narrowness of the limit is not the same for all the types of bonding clays. In sands bonded with more than 4 percent clay, the curves for sands bonded with a given type of clay, except montmorillonite clay IB, have about the same shape regardless of the amount of clay. The general shape of the curves for sands bonded with illite clay (fig. 4) and kaolinite clay (fig. 5) is about the same and they have gentle sloping sides. The curves o W 24 y \ 15 > • j \ \ V I2 / / \ \ \ PERCENT CLAY s / \ / r 8 / s 7 \ \ 6 5 A 3 y f^ 2 2.5 3.0 3.5 PERCENT TEMPERING WATER Fig. 1. Green Compression Strength Versus Amount of Tempering Water in Sands Bonded With Varying Amounts of Montmorillonite Clay IA o z a '6 ^ /\ / /^ \ _ \ V / t \ \ \ \ / £ \ \ \ .15 / \ / ^ 12 ^10 PERCENT CLAT \ s / 7\ \ e / \ / 6 4 / / 2 3 3 5 4 4 5 PERCENT TEMPERING WATER Fig. 2. Green Compression Strength Versus Amount of Tempering Water in Sands Bonded With Varying Amounts of Montmorillonite Clay IB ?24 d / s s/ A / / \ // o \ V K ^ / \ \ / \ \ 15 / V \ / \ \ \,2 / f\ \ \ \ \ \ / \ \ \ \ MO PERC 1 ENT CLAY / \ \ \ \ \ \ / / \ \ r \ \ \ \ 8 / \ \ ^6 \ \ \ --2^ 3 \< 2.0 2.5 3.0 3.5 4.0 PERCENT TEMPERING WATER Fig. 3. Green Compression Strength Versus Amount of Tempering Water in Sands Bonded With Varying Amounts of Halloysite Clay i 1 \ V s \ I 15 / / 12 / V 10 P ERCE NT C LAY 8 6 — 4 2.5 3.0 3.5 40 PERCENT TEMPERING WATER Fig. 4. Green Compression Strength Versus Amount of Tempering Water in Sands Bonded With Varying Amounts of Illite Clay 14 CLAYS IN GREEN MOLDING SANDS S' / , /' S^5 / / ' SlO PERCENT CLA1 r / r N ^8 ^ X ^7 5 ^6 S 4 2.5 PERCENT 3.0 3.5 TEMPERING WATER Fig. 5. Green Compression Strength Versus Amount of Tempering Water in Sands Bonded With Varying Amounts of Kaolinite Clay for the sands bonded with montmorillonite clay I A (fig. 1) are somewhat steeper, and those for sands bonded with halloysite clay (fig. 3) are even more steep indicating that maximum green compression strength is devel- oped in a very narrow moisture range. The curves for sands bonded with 4 to 8 percent montmorillonite clay IB (fig. 2) are quite steep, whereas those for sands with higher clay con- tents have more gently sloping sides. In foundry practice, molding sands are used at "temper" which is a sand condition that develops for many sands when they are prepared with slightly more water than that required for maximum green compression strength (m.g.c). The actual amount of water in excess of that necessary for m.g.c. is not the same for all types of clay nor for sands with different amounts of the same clay. The true worth of a clay for bonding, therefore, is not determinable from the green compression strength obtained at a single moisture content. An accurate comparison of clays requires curves giving strength values throughout a range of tempering water contents. Bulk Density in Relation to Tempering Water As the amount of tempering water added to a dry sand-clay mixture is increased, the weight per unit volume of the mixture decreases to a given moisture content and then increases. The temper point of molding sands has been defined 2 as the moisture content at which the sand has minimum BULK DENSITY vs. TEMPERING WATER 15 *I52 \ \ \ \ \ \ \ \ V \ \ \ \ \ V \ 10 PERCENT CLAY ^ 15 \ \ \ \ \ ^ 8/ \ \ \ \ ^■6 \ \ \ \ \ 2 . — ^ l! i i 3.0 PERCENT 3.5 4.0 4.5 TEMPERING WATER Fig. 6. Bulk Density Versus Amount of Tempering Water in Sands Bonded With Varying Amounts of Montmorillonite Clay IA weight per unit volume. In figures 6, 7, 8, 9, and 10 curves are presented showing the weight per unit volume of mixtures containing different amounts of clay with varying amounts of tempering water. For each mix- ture the curve passes through a minimum point or the temper point of the sample according to this definition. Many foundrymen use the term "temper" to describe a green sand with a moisture content giving the best working properties. In the clays investigated, the best working conditions prevailed when the sands were slightly on the wet side of the minimum bulk density point (m.b.d.p.). It seems, therefore, that the two usages of the term "temper" do not coincide. The curves for the montmorillonite clay IA-bonded sands (fig. 6) are steep on the dry side of m.b.d.p. and moderately steep on the wet side, indicating that the bulk density changes rapidly with moisture content below the m.b.d.p. and gradually above it. The slope of the curve on the dry side becomes more gentle as the amount of clay increases, and conse- quently the m.b.d.p. is less sharp in higher clay content mixtures. The upward shift of successive curves in figure 6 indicates an increase in weight per unit volume at the m.b.d.p. as the amount of clay increases in the mixtures. The amount of increase in weight becomes relatively less as the clay content increases; thus a mixture with 15 percent clay is only slightly heavier than a mixture with 10 percent clay at the m.b.d.p., 16 CLAYS IN GREEN MOLDING SANDS ! 1 1 1 iO ! 1 i < _J Z o or i t^j . 1 D - rvi \ s o \ .00 ' ) / /\ / \ ^ / L / ^ 3.5 4.0 TEMPERING WATER Fig. 8. Bulk Density Versus Amount of Tempering Water in Sands Bonded With Varying Amounts of Halloysite Clay whereas the mixture with 10 percent clay is considerably heavier than one with 5 percent clay at the m.b.d.p. The character of the curves for the montmorillonite clay IB (fig. 7) is about the same regardless of the clay content. They have relatively gentle slopes on both the dry and wet side of m.b.d.p. Unlike the mont- morillonite clay IA, in mixtures up to 8 percent clay, there is only a very slight upward shift of successive curves indicating a very small increase in weight per unit volume at m.b.d.p. as the amount of clay increases. In mixtures with more than 8 percent clay there is a decrease in weight at m.b.d.p. as the amount of clay increases. The curves for the halloysite clay-bonded sands (fig. 8) are U-shaped, indicating very great changes in bulk density with slight variations in the amount of tempering water on both the wet and dry side of the m.b.d.p. The variation of bulk density with amount of tempering water is greater for sands bonded with the halloysite clay than with any other type of clay. The weight per unit volume at m.b.d.p. increases with increasing amounts of the halloysite in mixtures with up to 8 percent clay. With amounts of clay less than about 6 percent, mixtures with the halloysite clay weigh about the same per unit volume as mixtures with the montmorillonite clay IA. In mixtures with more than 6 percent clay, sands bonded with this montmorillonite clay are heavier. The character of the curves for the sands bonded with the illite clay and kaolinite clay (figs. 9 and 10) are the same within the limits of experi- mental error. The curves have gentle slopes on both the dry and wet side of the m.b.d.p. The curves for the illite clay- and kaolinite clay-bonded sands are similar to those for sands bonded with the montmorillonite clay IA except that the dry side is steeper for sands with less than about 8 percent of this montmorillonite clay. 18 CLAYS IN GREEN MOLDING SANDS 2 170 < Q a. JT C _AY \ S \ 4 > v \ \ s \ \ N ^ 3.0 PERCENT 3.5 4.0 TEMPERING WATER Fig. 9. Bulk Density Versus Amount of Tempering Water in Sands Bonded With Varying Amounts of Illite Clay The weight per unit volume at m.b.d.p. for a given amount of clay is about the same in sands bonded with either the kaolinite clay or the illite clay. In both cases the weight per unit volume is greater than that of the montmorillonite clay- or the halloysite clay-bonded sands containing equal amounts of tempering water. In sands bonded with either the illite clay or the kaolinite clay, the weight per unit volume at m.b.d.p. increases sharply as the amount of clay increases. The curves for the illite clay-bonded sands are distinctive in that increasing amounts of clay cause only very slight shifts to the right. This means that sands bonded with large amounts of the illite clay develop m.b.d.p. with smaller amounts of water than do sands bonded with any of the other types of clay. GREEN COMPRESSION STRENGTH vs. CLAY 19 15 k 10 PE RCEN T CL AY \ s \ \ 6 \ v. \ ■? 6 \ N \ / ' \ \ \ / 5.5 6.0 Fig. 10. Bulk Density Versus Amount of Tempering Water in Sands Bonded With Varying Amounts of Kaolinite Clay The dry side of the curves for kaolinite clay mixtures with more than 4 percent clay show a reversal at the dry end. Curves for some of the sands bonded with the other types of clay also suggest that the gradual increase in weight on the dry side of the m.b.d.p. is reversed at very low moisture contents. Green Compression Strength in Relation to the Amount and Kind of Clay in Molding Mixtures AT MAXIMUM GREEN COMPRESSION STRENGTH A curve is presented in figure 11 for each type of clay investigated showing the maximum green compression strengths of mixtures with clay contents up to 15 percent. The montmorillonite clay IB gives the highest strength values at all of the clay contents investigated. The halloysite clay and the montmorillonite clay IA have about the same strength in mixtures up to 6 percent clay — with larger amounts of clay the montmorillonite clay is the stronger. Kaolinite clay is slightly stronger than the illite clay 20 CLAYS IN GREEN MOLDING SANDS in mixtures up to about 12 percent clay, whereas the illite clay is slightly stronger in mixtures with more than 12 percent clay. In sands bonded with the montmorillonite clay IB, the strength in- creases approximately in proportion to the amount of clay up to about 8 percent clay. Additional clay up to 10 percent causes only a very slight increase in strength, and the addition of clay in excess of 10 percent causes no increase in strength. ■id. .„„.,.. 28 MON TMOf *ILLO IB NITE CLAY -y MONTMORILLONITE CLAY 1 A 24 ^ LOYS ITE CLAY "" ! / / * 20 / ^A~ / / s-~* KAOL INITE CLAY 16 // j / s / / / i / i / / * ! / / 1 12 /* ,y 8 / / / I 4 ' A '/ / s V 1 i 6 8 10 PERCENT CLAY 12 Fig. 11. Curves Showing the Maximum Green Compression Strength Developed by Each Type of Clay in Relation to the Amount of Clay in the Sand-Clay Mixtures In sands bonded with the montmorillonite clay IA, halloysite clay, or kaolinite clay, the strength increases in proportion to the amount of clay up to about 9 percent clay. As the clay content is raised from 9 to 12 per- cent there is a small increase in strength, and from 12 to 15 percent, there is only a very slight increase in strength. GREEN COMPRESSION STRENGTH vs. CLAY 21 Table 1. — Green Compression Strength per Unit of Clay in Lb. per Sq. In. Percent clay Montmorillonite clay IA Montmorillonite clay IB Halloysite clay Illite clay Kaolinite clay At maximum green compression strength 2 3 .060 .079 .117 .156 .047 .070 4 5 .101 .114 .178 .191 .098 .121 .041 .058 .044 .060 6 7 .137 ,148 .197 .196 .137 .132 .067 .071 .072 .082 8 9 .150 .150 .191 .176 .133 .126 .079 .082 .092 .094 10 12 15 .146 .127 .103 .160 .132 .106 .119 .104 .085 .084 .084 .076 .092 .084 .072 At minimum bulk density 2 3 .070 .080 .082 .108 .050 .069 4 5 .087 .091 .118 .125 .087 .102 .040 .050 .034 .032 6 7 .090 .091 .120 .119 .114 .124 .057 .061 .033 .037 8 9 .101 .101 .114 .109 .130 .122 .064 .068 .042 .045 10 12 15 .098 .089 .075 .104 .095 .084 .112 .095 .078 .069 .073 .075 .043 .036 .029 The sands bonded with the illite clay show a proportionate increase in strength with increase in clay content up to about 12 percent. Unlike other clays, in mixtures with more than 12 percent of illite clay there is only a slight reduction in bonding strength per unit of clay. The relative bonding strengths of the various types of clay in mixtures with different amounts of clay are presented another way in table 1 which gives the bonding strength per unit of each clay in mixtures up to 15 per- cent clay. The data show clearly that the maximum strength for all of the clays is developed in sands when they contain rather narrowly limited amounts of bond. With higher or lower amounts of bond than the optimum, the strength developed per unit of clay remains about the same or de- creases. Further, the optimum clay content for the development of maxi- mum green compression strength is not the same for all classes of bonding clays. The curves in figure 11 and the data in table 1 illustrate a further fact, namely, the relative difference in the maximum strength developed by the 22 CLAYS IN GREEN MOLDING SANDS different types of clay is greatest in low clay-content sands. In high clay- content sands the difference in bonding power of the various clays de- creases considerably. For example, montmorillonite clay IB-bonded sands are stronger than montmorillonite clay IA-bonded sands in mixtures with up to about 12 percent clay. With more than 12 percent clay, there is little difference in strength. As another example, with 5 percent clay, montmorillonite clay IA- bonded sands are twice as strong as those bonded with the same amount of kaolinite clay; with 10 percent clay they are only 60 percent stronger, and with 15 percent clay they are only 45 percent stronger. The foregoing data may be stated still another way: If for some reason a high clay-content mixture is required, the type of clay makes little differ- ence in the strength the mixture will develop. In foundry practice at the present time, there is a trend toward the use of sands with high contents of clay — as much as 25 percent clay. Sands with high clay contents have about the same strength regardless of whether they are bonded with a kaolinite, a montmorillonite, or one of the other classes of clays. The foundryman can, therefore, select the type of bond for his sands entirely on the basis of refractoriness, durability, surface characteristics, etc. In other words, for such sands, the kaolinite- and halloysite-type clays would have peculiar advantages because of these properties not possessed by the other types of clay. AT MINIMUM BULK DENSITY A curve is presented in figure 12 for each clay showing the green com- pression strength at the minimum bulk density point in mixtures with clay contents up to 15 percent. In mixtures with less than about 6.5 percent clay, the montmorillonite clay IB-bonded sands are strongest; in sands with from 6.5 to about 12 percent clay the halloysite clay-bonded sands are strongest, and from 12 to 15 percent clay the montmorillonite clay IB-bonded sands are strongest. Sands bonded with the montmorillonite clay IA have less strength than those bonded with the montmorillonite clay IB at all clay contents up to at least 15 percent. Sands bonded with the illite clay are weaker than those bonded with either montmorillonite clay or the halloysite clay in all mixtures except those with high clay contents in which the strengths are about equal. The sands bonded with kaolinite clay are lower in strength than those bonded with the other clays. A reason for this characteristic is that mini- mum bulk density (m.b.d.) is developed at very high moisture contents in sands bonded with large amounts of this clay — consequently the strength is low. The kaolinite clay curve flattens above 10 percent indicating that GREEN COMPRESSION STRENGTH vs. CLAY 23 there is no increase in strength at m.b.d.p. as clay in excess of 10 percent is added to mixtures. Unlike the kaolinite clay, illite clay-bonded sands with high clay con- tent develop m.b.d.p. at relatively low moisture contents. Consequently the illite clay-bonded sands with high clay contents are strong at m.b.d.p. — as strong in fact as those bonded with either montmorillonite clay or the halloysite clay. 28 2 24 ^ 20 co z o If) If) UJ * v / -ILL TE C :lay / // / / / r V / / / • s K N AO LINIT E CL AY / / / s S V /^ *'' 10 12 PERCENT CLAY Fig. 12. Curves Showing the Green Compression Strength at the Minimum Bulk Density Point Developed by Each Type of Clay in Relation to the Amount of Clay in the Sand-Clay Mixtures In the halloysite clay-bonded sands the increase in strength at m.b.d.p. is proportionate to the amount of clay up to about 8 percent clay. With more than 8 percent clay, the strength increases only slightly as the clay content increases. In both montmorillonite clays the strength increases in proportion to the amount of clay up to about 9 percent. In mixtures with more than 9 percent clay, the relative increase in strength is slightly reduced for mont- morillonite clay IB, and considerably reduced for montmorillonite clay IA. 24 CLAYS IN GREEN MOLDING SANDS The relative bonding strength of the various types of clay is shown also in table 1 which gives the bonding strength per unit for each clay in mix- tures up to 15 percent clay. The data in the tables indicate the same con- clusions as those derived from a consideration of the curves in figure 14. A comparison of the maximum green compression strength and the strength at m.b.d.p. brings out the following conclusions: The strength of the halloysite clay-bonded sands at m.b.d.p. is only slightly less than the maximum strength, whereas the strength of sands bonded with either class of montmorillonite clay is considerably less. The explanation is that m.b.d.p. is developed in the halloysite clay-bonded sands with only slightly more water than that required for maximum green strength. In sands bonded with either montmorillonite clay considerably more water is required for m.b.d.p. than for maximum green strength — consequently the strength at m.b.d.p. is considerably reduced. Because of this factor, the halloysite clay-bonded sands are stronger at m.b.d.p. for some clay contents than are sands bonded with the montmorillonite clays. Similarly the illite clay-bonded sands develop m.b.d.p. at moisture contents only slightly above that required for maximum green strength, hence the strength at m.b.d.p. is only slightly less than the maximum. This is particularly true for high clay-content sands and explains why the illite clay-bonded sands have about the same strength at m.b.d.p. as sands bonded with either montmorillonite clay or the halloysite clay when the clay content is high. In actual foundry practice the tendency is to use sands slightly on the wet side of m.b.d.p. Consequently the differences in the strength of the various types of clay experienced in foundry use would be those at m.b.d.p. rather than the maximum strengths. This may not hold for very high clay- content sands because in such cases foundry "temper" might have a some- what different relation to m.b.d.p. Character of the Clay Coating of Sand Grains and Distribution of Tempering Water in Green Sands sands bonded with the montmorillonite clay ia As illustrated in figures 13 and 14, the quartz grains in sands bonded with this clay have a smooth regular coating. The clay is made up of flake-shaped units of montmorillonite, and when it is mulled with sand and water, the montmorillonite is broken down to its individual unit flakes and these units are plastered regularly on top of each other to make up the coating of the quartz grains. Microscopic study of the coated quartz grains shows that a distinctive feature of the coating is the absence of DISTRIBUTION OF TEMPERING WATER 25 Fig. 13. Photomicrographs of Sand Bonded With 4 Percent Montmorillonite Clay IA, Showing the Smooth Regular Coating of the Clay Around the Quartz Grains. 150X 26 CLAYS IN GREEN MOLDING SANDS aggregates or larger particles of clay which have not been broken down during mulling. When water is added to the sand-clay mixture, it penetrates into the montmorillonite and forms a film that coats the surfaces of each individual flake of the montmorillonite. The thickness of the water film, of course, increases as the amount of tempering water increases. When the film is thin, the water molecules composing the film occupy definite fixed positions with respect to the surface of the montmorillonite flake units. Such water in which the molecules are oriented and fixed in position is not fluid, but solid and rigid. 13 It follows that the first tempering water added to a sand-clay mixture assumes a rigid condition on the surface of the mont- CLAV COATING AT MAXIMUM GREEN COMPRESSION STRENGT CLAY WATER C3 MOLECULAR LAYERS) OXYGEN AND HYDROXYL ATOMS ALUMINUM " '.*.' 'j OXYGEN AND HYDROXAL ATOMS SILICON " OXYGEN " WATER (4 MOLECULAR LAYERS) Fig. 14. Diagrammatic Sketch Illustrating the Character of the Coating of the Quartz Grains in Sands Bonded With Montmorillonite Clay IA morillonite flakes. As additional water is added the film becomes thicker in stepwise fashion, that is, the first water forms a layer one molecule thick, additional water forms a second layer another molecule thick and so on. Water can be held in a rigid condition only to given thickness. Beyond this thickness the rigidity gradually tapers off so that there is no abrupt boundary between rigid and fluid water held on the montmorillonite flakes. The cause of the orientation of the water molecules and consequent rigidity resides in the arrangement of the atoms that make up the mont- morillonite itself. The effect of the atom arrangement in the montmoril- lonite extends to a limited distance from the surface of the flakes, hence the rigid character of the water also extends to a limited distance from the surface. DISTRIBUTION OF TEMPERING WATER 27 Ice is an example of rigid solid water. As the temperature falls the water molecules lose their mobility and when freezing takes place become fixed into a rigid definite pattern, thereby making up the ice. In the clay- water film the rigid condition is not caused by a temperature drop but by the structure of the atoms in the clay and the character of the water molecules. The thickness of the water and montmorillonite films surrounding the quartz grains is given in table 2 for sand-clay mixtures of various clay con- tents when there is just enough water to develop maximum green compres- sion strength. The values given in table 2 are based on the assumption, which is valid for montmorillonite, that water penetrates between each unit flake of montmorillonite. The data indicate that in all mixtures containing more than enough clay to have pronounced bonding value (5 percent clay) the thickness of the water film at maximum green compression strength is approximately 8A. A water film thickness of 8A is about equal to the thick- ness of three water molecules.* It can be concluded, therefore, that maxi- mum green compression strength is developed when there is just enough tempering water added so that the water between each unit of montmoril- lonite is three molecules thick. *This is based on the assumption that the water molecules are loosely packed. 13 Table 2. — Sands Bonded With Montmorillonite Clay IA Thickness of Thickness of water film per Percent clay Percent Water lost at 110° C. clay film in unit cells of Thickness of water film unit cell of montmoril- Ratio of water film to clay m cc. a montmoril- in microns lonite in film thickness lonite Angstrom units Values at maximum green compression strength 2 1.0 20.0 631 0.946 15.0 1.63 3 1.25 25.0 955 1.19 12.5 1.36 4 1.35 27.0 1288 1.30 10.1 1.10 5 1.45 29.0 1626 1.41 8.6 0.93 6 1.65 33.0 1973 1.63 8.3 0.90 7 1.85 37.0 2326 1.84 7.9 0.86 8 2.10 42.0 2681 2.12 7.9 0.86 9 2.40 48.0 3057 2.45 8.0 0.87 10 2.60 52.0 3426 2.68 7.8 0.85 12 3.30 66.0 4214 3.47 8.1 0.88 15 4.15 83.0 5454 4.52 8.3 0.90 Values at minimum bulk density 2 1.1 22.0 631 1.041 16.5 1.8 4 1.7 34.0 1288 1.642 12.8 1.4 6 2.6 52.0 1973 2.564 12.9 1.4 8 3.15 63.0 2681 3.174 11.8 1.28 10 3.8 76.0 3426 3.915 11.4 1.24 15 5.15 103.0 5454 5.617 10.3 1.12 B Values based on 2000-gram mixtures. 28 CLAYS IN GREEN MOLDING SANDS It seems certain that the effect of montmorillonite in causing water to become rigid would extend at least through three water molecules, and it can be concluded, therefore, that the water coating the montmorillonite flakes (and this is essentially all the tempering water) is in a rigid condition when maximum green compression strength is developed. Since the bonding agents, clay and water, are both in a rigid condition when maximum green compression strength is developed, it follows that a concept of green bonding in molding sands (see page 38) must be based on a rigid condi- tion of the material causing bonding. The thickness of the water film (8A) is slightly less than the thickness of the individual units of montmorillonite (9.2A). The coating of each quartz grain may, therefore, be visualized (fig. 14) as an alternation of layers of montmorillonite and rigid solid crystalline water with the water layers slightly thinner than the montmorillonite layers. The thickness of the water and montmorillonite films surrounding the quartz grains in sand-clay mixtures with various amounts of the clay when there is just enough water to develop minimum bulk density is given also in table 2. The data show that mixtures with enough clay to have pronounced bonding value develop minimum bulk density when there is enough water to coat each flake surface of montmorillonite with a film of water about 12A thick. This water thickness is equal to a layer four water molecules thick. It is significant that minimum bulk density is developed when the water per sheet of montmorillonite is one molecule thicker than that re- quired for maximum green compression strength. It will be indicated later (see page 47) that the m.b.d.p. for sands bonded with this type of clay is developed when the water film is thick enough to begin to lose its rigidity. The coating of the quartz grains at m.b.d.p. can be visualized as an alternation of layers of montmorillonite and water with the water layers somewhat thicker than the montmoril- lonite layers (fig. 14). The individual water layers are not uniformly rigid, the central portion of each layer being less rigid than that part directly in contact with the montmorillonite. SANDS BONDED WITH THE MONTMORILLONITE CLAY IB Microscopic study of sands bonded with this clay shows that the quartz grains have a smooth even coating, and that there are no large aggregates or particles of clay present in the coating. The coating of the quartz grains appears to be made up of individual flake-shaped units of montmorillonite plastered regularly on top of each other. In table 3, computed values are given for the thickness of the water film coating each unit flake of montmorillonite. The value for the water DISTRIBUTION OF TEMPERING WATER 29 film thickness per flake of montmorillonite is 12. 7A in 4 per cent bentonite mixtures and 9.4A in 15 percent bentonite mixtures. This decrease in thickness of the water film per unit of montmorillonite is probably more apparent than real because it is likely 7 that water does not penetrate all of the unit surfaces in this type of montmorillonite and because relatively more such surfaces would be unavailable to water in the mixtures with higher contents of the clay. Table 3. — Sands Bonded With Montmorillonite Clay IB Thickness of Thickness of water film per Percent clay Percent water Water lost clay film in Thickness of unit cell of Ratio of water at 110° C. unit cells of water film montmoril- film to clay in cc. a montmoril- in microns lonite in film thickness lonite Angstrom units Values at maximum green compression strength 2 1.15 23.0 631 1.09 17.3 1.88 4 1.7 34.0 1288 1.64 12.7 1.39 6 2.25 45.0 1973 2.22 11.2 1.22 8 2.7 54.0 2681 2.72 10.1 1.10 10 3.2 64.0 3426 3.29 9.6 1.04 12 3.85 77.0 4214 4.06 9.6 1.04 15 4.7 94.0 5454 5.13 9.4 1.02 Values at minimum bulk density 2 1.35 27.0 631 1.27 20.1 2.18 4 2.1 42.0 1288 2.03 15.8 1.72 6 2.85 57.0 1973 2.81 14.2 1.54 8 3.55 71.0 2681 3.58 13.4 1.46 10 4.3 86.0 3426 4.43 12.9 1.40 12 5.3 106.0 4214 5.58 13.2 1.44 15 5.95 119.0 5454 6.55 12.0 1.30 a Values based on 2000-gram mixtures. The computed thickness of the water film per unit of montmorillonite in mixtures with more than 6 percent bentonite is in excess of the thickness of a sheet of water three molecules thick and less than a sheet four mole- cules thick. Since, as noted above, there are probably some surfaces un- available to water, a logical conclusion seems to be that the available surfaces of the units of montmorillonite are coated by a film of water four molecules thick. The montmorillonite clay IB differs, therefore, from the montmorillonite clay IA in holding four instead of three molecules of water per unit of montmorillonite at maximum green compression strength. If, as seems likely (see page 38) , maximum green compression strength develops when the clay mineral units are coated with an amount of water equal to that which can be held rigidly, montmorillonite clay IB can retain four mole- cules of water in a rigid condition whereas montmorillonite clay IA can 30 CLAYS IN GREEN MOLDING SANDS retain only three molecules in a rigid condition — the fourth molecular layer showing a distinct reduction of rigidity. Later it will be shown that this extra water retaining power of the montmorillonite clay IB provides an explanation for the higher green compression strength of this clay. The cause of the difference in water retaining power of the two mont- morillonite clays probably resides in the difference in the exchangeable bases which they carry. As noted earlier in the report the chief exchange- able base in the montmorillonite clay IA is sodium, in the montmorillonite clay IB is calcium. The coating of the quartz grains at maximum green compression strength can be visualized as an alternation of layers of montmorillonite and rigid solid crystalline water with the water layer slightly thicker than the montmorillonite layers. The alternation is not so perfect rhythmically as in the case of montmorillonite clay IA because of some relatively thicker clay layers indicated by the suggestion that not all basal surfaces are available to water. The data given in table 3 indicate that the computed thickness of the water film per unit of montmorillonite at the minimum bulk density point is about 13A in mixtures with more than 6 percent clay. This computed thickness is slightly less than the thickness of a sheet of water five mole- cules thick. By reasoning similar to that suggested for the water film thick- ness at maximum green compression strength, it appears likely that the actual water film thickness on available units of montmorillonite at m.b.d.p. is five molecules, or one molecule thicker than that required for maximum green strength. Like montmorillonite clay IA, montmorillonite clay IB reaches m.b.d.p. with just enough tempering water in addition to that required for maxi- mum green strength to build an extra layer on each montmorillonite unit, one molecule thick. The coating of the quartz grains at m.b.d.p. can be visualized as an alternation of layers of montmorillonite and water layers as described above except that the water layers are one molecule thicker. Furthermore, the water layers are not uniformly rigid — the central portion being less rigid than that directly in contact with the montmorillonite. SANDS BONDED W T ITH THE HALLOYSITE CLAY The quartz grains of sands bonded with the halloysite clay have a smooth regular coating.* Halloysite is made up of elongate, thin, lath- shaped units with an ultimate thickness of individual laths of approxi- mately 7A. The coating appears to be composed of such units plastered regularly on top of each other. *There are two forms of halloysite (see page 7) and the bonding properties are not the same for both forms or for all mixtures of the two forms. The discussion herein presented applies only to the halloysite clays that have high bonding strength. DISTRIBUTION OF TEMPERING WATER 31 In table 4 the water required to yield maximum green compression strength is given for sands containing varying amounts of halloysite clay. The data in the table show that the amount of water per unit of clay neces- Table 4. — Sands Bonded With Halloysite Clay Percent clay Percent water Water lost at 110° C. in cc. a Thickness of clay film in unit cells of halloysite Thickness of water film in microns Thickness of water film per unit cell of halloysite in Angstrom units Ratio of water to clay film thickness Values at maximum green compression strength 2 1.15 23.0 1009 1.088 10.8 1.54 3 1.35 27.0 1531 1.291 8.4 1.20 4 1.70 34.0 2055 1.642 8.0 1.14 o 2.05 41.0 2606 2.000 7.7 1.10 6 2.35 47.0 3161 2.318 7.3 1.04 7 2.60 52.0 3727 2.590 6.9 0.97 8 2.95 59.0 4306 2.973 6.9 0.96 9 3.25 65.0 4898 3.312 6.8 0.94 10 3.60 72.0 5503 3.709 6.7 0.94 12 4.30 86.0 6754 4.480 6.6 0.92 15 4.95 99.0 8740 5.400 6.2 0.86 Values at minimum bulk density 2 1.2 24.0 1009 1.131 11.2 1.60 4 1.9 38.0 2055 1.835 8.9 1.27 6 2.55 51.0 3161 2.515 7.9 1.13 8 3.05 61.0 4306 3.074 7.0 0.99 10 3.65 73.0 5503 3.760 6.8 0.95 15 5.15 103.0 8740 5.617 6.4 0.89 'Values based on 2000-gram mixtures. sary to obtain maximum green compression strength is fairly constant in all mixtures with more clay (6 percent) than that required to develop the greatest bonding power per unit of clay. The mixture with 15 percent clay contains slightly less water and appears to be an exception to the fore- going statement. Data are insufficient to determine if this indicates a definite relationship for all mixtures with very large amounts of halloysite clay. Computations of the thickness of the water film thickness reveal that maximum green compression strength is developed in sands with more than 6 percent halloysite when the tempering water is equal to a film of water 6-7A thick for the surface of each lath of halloysite (column 6, table 4). This value is greater than the thickness of two water molecules and some- what less than that of a water layer three molecules thick. It is likely, be- cause of certain structural characteristics of halloysite that not all of the individual lath surfaces of halloysite are available to water, and that actual thickness of the water sheets on the surfaces available is greater than 32 CLAYS IN GREEN MOLDING SANDS the computed values. A favored interpretation is that the thickness of the water films on the available surfaces is at least three water molecules. Halloysite clay-bonded sands possess air-set strength (see page 44) which suggests that such sands immediately after ramming are composed of quartz grains coated with an alternation of clay layers and rigid water layers plus some liquid water. In the course of time as air-set strength develops the latter water molecules assume a fixed position and become completely rigid. The water required to develop minimum bulk density is given also in table 4 for sands bonded with various amounts of the halloysite clay. The values suggest that the thickness of the water film per unit of clay de- creases as the content of halloysite in the sand increases. In mixtures with more than 8 percent halloysite, the decrease in thickness is very small and is not greater than the experimental error in determining the values. A comparison of the water values at maximum green compression strength and minimum bulk density shows that the amount of water re- quired for m.b.d.p. is only slightly greater than that for maximum green strength. Minimum bulk density and maximum green compression strength both appear to be developed when the amount of water present is equal to a film of water at least three molecules thick, on the available surfaces of the individual laths. The halloysite clay-bonded and illite clay- bonded sands are unique in this respect since sands bonded with the other types of clay require more water to develop m.b.d. than maximum green compression strength. SAXDS BONDED WITH THE ILLITE CLAY A microscopic examination of sands bonded with this type of clay shows that the individual sand grains have a somewhat irregular uneven coating. The coating is composed of minute flake-shaped units of illite that are regularly plastered around the quartz grains, and also of larger crystals and aggregates of illite that are stuck irregularly in the fine particles to give the coating an irregular appearance. Unlike montmorillonite, which breaks down readily and completely on mulling, illite is broken down only in part to very fine flake-shaped particles when it is mulled with sand and water. The remainder is in the form of larger crystals and aggregates. The values given in table 5 indicate that the ratio of clay to water re- quired to develop maximum green compression strength is about constant for all amounts of clay in excess of about 8 percent clay. In column 6 of table 5 a computation is made of the thickness of water per unit sur- face of illite for sands with various clay contents based on the assumption that all the water is held on the surface of the illite particles and all the DISTRIBUTION OF TEMPERING WATER 33 illite unit surfaces are available to water. These assumptions are approxi- mately correct for the sands bonded with the montmorillonite clays, but not for sands bonded with illite clay because of the large number of ag- gregates with surfaces not penetrated by the water. The values given are about equal to a layer two water molecules thick, whereas the actual thickness on available surfaces is more likely at least three molecules. Table 5. — Sands Bonded With Illite Clay Percent clay Percent water Water lost at 110° C. in cc. a Thickness of clay film in unit cells of illite Thickness of water film in microns Thickness of water film per unit cell of illite in Angstrom units Ratio of water film to clay film thickness Values at maximum green compression stren gth 4 1.65 33.0 I486 1 . 593 10.7 1.07 6 1.70 34.0 2276 1.677 7.37 0.74 8 1.93 38.6 3101 1.945 6.27 0.63 10 2.22 44.0 3962 2.2^7 18 12 2.57 51.4 4S63 2.708 5. Si 0.56 15 3.22 64.4 6293 3.513 5 . 58 0.56 Va ues at minimum bulk density 4 1.77 35 . 4 1486 1.710 11.5 1.15 6 2.2 44.0 2276 2.170 9 . 53 0.95 8 2.55 51.0 3101 2 . 570 8.29 0.83 10 2.8 56.0 3962 2.885 7.2S 0.73 12 3.06 60.12 4863 3.167 6.51 0.65 15 3.30 66.0 6293 3.60 5.72 57 a Values based on 2000-gram mixtures. It is probable that the water held on the surface of the illite units is rigid crystalline water. The coating of the sand grains can be visualized, as composed largely of minute flakes of illite mixed with some larger par- ticles with rigid solid water on the surface of the illite flakes. The water required to develop minimum bulk density is given also in table 5 for sands bonded with various amounts of the illite clay. The values indicate that the amount of water per unit of clay decreases as the clay content increases. The explanation for this decrease in water require- ments is not known. The values given in table 5 indicate that m.b.d.p. is attained with very little more water than that required for maximum green compression strength. The difference between the moisture required for m.b.d.p. and maximum green compression strength is largest at intermediate clay con- tents and smaller in high or low clay-content mixtures. 34 CLAYS IN GREEN MOLDING SANDS SANDS BONDED WITH THE KAOLINITE CLAY The coating of the individual grains in sands bonded with this clay is irregular and uneven as illustrated in figures 15, 16, and 17. A microscopic examination of the coating with very high magnification shows it to be composed of large crystals and aggregates that are chiefly kaolinite dis- seminated through a matrix of extremely small clay mineral flakes. The coating differs from that found in sands bonded with the illite clay by being more irregular and having a greater proportion of large crystals and aggregates. Kaolinite, like illite and unlike montmorillonite, is not readily reduced to extremely small units when it is mulled with sand and water. Consequently, some large particles of kaolinite persist through the mulling operation and are found in the coating. In kaolinite water does not penetrate between all the individual unit flakes. It is, therefore, impossible to make reliable computations of the surface of the kaolinite available to water and, consequently, the thickness of the water on the surfaces of the flakes at maximum green compression strength cannot be calculated. The values given in table 6, showing the tempering water required to develop maximum green compression strength, indicate that the amount of water per unit of clay necessary for minimum strength is about constant regardless of the amount of clay, in all mixtures* with clay in excess of that required (6 percent) to develop appreciable bonding action. *Data are not at hand to determine if this statement holds for sands with very high contents of clay. Table 6. — Sands Bonded With Kaolinite Clay Percent clay Percent water Water lost at 110 J C. in cc. a Thickness of clay film in microns Thickness of water film in microns Ratio of water film to clay film thickness Values at maximum green compression strength 4 1.15 23.0 1.486 1.11 0.75 5 1.35 27.0 1.877 1.32 0.70 6 1.40 28.0 2.276 1.40 0.62 7 1.55 31.0 2.684 1.54 0.57 8 1.70 34.0 3.101 1.73 0.56 10 2.10 42.0 3.962 2.16 0.55 15 3.40 68.0 6.293 3.71 0.59 Values at minimum bulk density 4 1.9 38.0 1.486 1.835 1.2 6 2.55 51.0 2.276 2.515 1.1 8 3.3 66.0 3.101 3.326 1.1 10 3.8 76.0 3.962 3.915 1.0 15 6.05 121.0 6.293 6.60 1.0 a Values based on 2000-gram mixtures. DISTRIBUTION OF TEMPERING WATER 35 Fig. 15. Photomicrograph of Sand Bonded With 8 Percent Kaolinite Clay Show- ing the Irregular Coating of the Clay Around the Quartz Grains. 150X 36 CLAYS IN GREEN MOLDING SANDS Fig. 16. Photomicrograph of Sand Bonded With 12 Percent Kaolin ite Clay Showing the Very Uneven Coating of the Clay Around the Quartz Grains. 150X DISTRIBUTION OF TEMPERING WATER 37 The coating of the quartz grains at maximum green compression strength can be visualized as a heterogeneous mixture of relatively large and small flakes and aggregates of kaolinite. In mixtures with more than 6 percent clay, water makes up about 60 percent of the volume of the coating. Kaolinite clay-bonded sands develop air-set strength so that im- mediately after ramming some of the water is not completely rigid. This water becomes completely rigid when air-set strength develops so that shortly after ramming the coating of the quartz grains is a mixture of clay mineral units and aggregates with rigid water. The relatively smaller pro- portion of water to clay required to develop maximum green strength in COATING SAND GRAIN Fig. 17. Diagrammatic Sketch Illustrating the Character of the Coating of the Quartz Grains in Sands Bonded With Kaolinite Clay kaolinite clay-bonded sands than in those bonded with montmorillonite clays is explained by the smaller amount of surface available in coatings made up of kaolinite-type clay. The data in table 6 also show that sands bonded with all amounts of kaolinite clay require considerably more tempering water to develop minimum bulk density than maximum green compression strength, and that the amount of water per unit of clay at m.b.d.p. decreases very slightly with increasing amounts of clay in the mixture. The coating of the quartz grains at m.b.d.p. has the same character- istics as at maximum green compression strength except that the propor- tion of water to clay is greater, and there is more incompletely rigid water immediately after ramming. At m.b.d.p. the volume of water and clay is about equal in sands with more than about 6 percent clay. 38 CLAYS IN GREEN MOLDING SANDS Concept of the Bonding Action of Clays in Green Sand The concept of the bonding action of clay and water in molding sands that agrees best with experimental data is that of a wedge and block at the contact of the sand grains holding them in place. As illustrated on fig- ures 18 and 19, where the quartz grains are in contact with each other, the clay-water coating forms a wedge-shaped mass. It is this wedge-shaped block, which has maximum solidarity and rigidity at maximum green compression strength, that locks the sand grains in position and gives the mixture its strength. Each grain is, of course, in contact with several other grains so that each grain is fixed in place by a number of clay wedges. The bonding action of clay and water in molding sands is not that of glue or adhesive causing the grains to stick together. The evidence sug- gesting the wedge-block concept and at the same time opposing any theory of glueing action is summarized and discussed below. (a) Green sands have very low tensile strength. This is opposed to any theory of glueing action. (b) The maximum green compression strength is developed when all the water molecules are in a solid rigid condition or when, as in the case of clays with air-set strength, the maximum degree of rigidity is reached that develops immediately after ramming. The strength is reduced greatly by the presence of very small amounts of water not in a highly rigid state. It is unlikely that very slight changes in the character and amount of water would cause such great changes in strength if bonding action were a glueing phenomenon. (c) The wedge-block concept satisfactorily takes into account the green strength properties that are distinctive for the various classes of bonding clays. (d) The wedge-block concept provides a satisfactory basis for an understanding of how variations in the intensity of mulling produces dif- ferent effects on the green strength of the various classes of bonding clays. (e) The variation in the durability of the different classes of bonding clay can be explained satisfactorily on the basis of the wedge-block concept. (f) The increased strength developed in sands bonded by certain types of clay when they are allowed to stand in the air after ramming, i.e., air-set strength, can be accounted for by the wedge-block concept. (g) The wedge-block concept is entirely consistent with the concepts and experimental data on dry strength. This subject will be considered in detail in another report. The strongest block of a given size offering the greatest resistance to the movement of the sand grains would be homogeneous, completely rigid, CONCEPT OF BONDING ACTION 39 and free from planes of weakness. The clay-water coating of the sand grains of the montmorillonite clay-bonded sands is essentially homo- geneous (fig. 18) and at maximum green compression strength all the tempering water is rigid. It follows that the blocks holding the grains in place are strong and the sand has high strength. On the other hand the m Fig. 18. Sketch of Montmorillonite Clay-Bonded Sand, Based on Microscopic Examination, to Illustrate the Smooth Even Coating of the Quartz Grains With Flakes of Montmorillonite, and the Wedge- Shaped Blocks at the Junction of the Clay-Coated Quartz Grains Holding the Grains in Place 40 CLAYS IN GREEN MOLDING SANDS sand grains in the kaolinite clay-bonded sand have an irregular coating containing large aggregates which provide planes of weakness (fig. 19). It follows, therefore, that the wedge-block holding the quartz grains in place is weaker for kaolinite clay-bonded sands than for montmorillonite clay- bonded sands. As the test data have shown, this is actually the case. Fig. 19. Sketch of Kaolinite Clay-Bonded Sand, Based on Microscopic Examination, to Illustrate the Irregular Coating of the Quartz Grains With Small Flakes and Large Lumps of Flakes, and the Wedge-Shaped Blocks at the Junction of the Clay-Coated Quartz Grains Holding the Grains in Place CONCEPT OF BONDING ACTION 41 It has been shown that sands bonded with montmorillonite clay IA have coatings containing sheets of water three water molecules thick per unit of montmorillonite, and that in the case of montmorillonite clay IB the coating is four water molecules thick per unit of clay at maximum green compression strength. Therefore in sands of the same clay content, the coating would be thicker when the latter clay is used. The wedge- block between the grains therefore would be larger, and since the water is rigid and the coating is homogeneous in both cases, the larger block would be expected to be the stronger. An explanation is provided, therefore, for the higher green strength of the montmorillonite clay IB. It is characteristic of sands bonded with all types of clay, when there is more than 4 to 6 percent clay in the mixture, that the maximum green compression strength is developed in a very narrow moisture range. This character also follows from the wedge-block concept, if it is assumed that there is a definite maximum thickness of water that is fixed rigidly on each clay mineral basal surface, since a slight amount of incompletely rigid water would greatly weaken the wedge or block. This assumption seems valid because it agrees with structural data regarding clay and water, 13 and because it permits an understanding of bulk density variations in rammed sand-clay mixtures (see page 46). The great variation in strength with slight variations in the amount of tempering water cannot be explained on any "glueing" concept of bonding action, since a slight change in amount of water should change only slightly the bonding power of a glue. The curves showing tempering water in relation to green compression strength are relatively flat for mixtures with less than 4 to 6 percent clay regardless of the class of bonding clay. A probable explanation is that the amount of clay is so small that other factors, such as the adhesion of quartz and water, also play a part in the bonding action. When there is enough clay to exert appreciable bonding strength, the curves develop a characteristic shape. A comparison of the tempering-water — green-compression-strength curves for the various classes of clay shows that the curves for the halloy- site clay-bonded sands are the steepest. The explanation for the steepness of the curves for the halloysite clay-bonded sands is probably somehow related to the property of halloysite (see discussion of m.b.d.p.) of holding absorbed water to a given amount in a rigid condition with any additional water being completely non-rigid. It will be indicated later that additional water beyond that held entirely rigid is held with gradually reduced rigidity in the other clay minerals with the possible exception of the illites. Although the explanation has been obscure, it is a well-known fact that the mixing of a small amount of montmorillonite type clay with a clay belonging to one of the other classes causes an increase in bonding strength 42 CLAYS IN GREEN MOLDING SANDS that is out of proportion to the amount of montmorillonite clay added. The explanation based on the suggested concept is that the montmorillonite breaks down into extremely small particles which occupy the open spaces in the coatings caused by the aggregates and larger particles of the other clay minerals. The montmorillonite acts to fill and seal the spaces, thereby eliminating planes of weakness and as a result the strength is increased. It should be emphasized that there must be comparatively few planes of weakness in a coating of kaolinite clay or illite clay as otherwise there would be no bonding strength at all. By the same token only a very little montmorillonite clay is required to heal the few planes of weakness, and the resulting effect is large. As shown in figure 11 and table 1, in mixtures with more than about 10 percent clay, the maximum bonding strength per unit of clay increases more for the illite clay and the kaolinite clay than for the montmorillonite and halloysite clays. That is, for the types of clay that provide homoge- neous coatings and wedges, once a wedge-block of certain size is attained, a larger wedge-block gives no greater strength. On the other hand for the types of clays providing nonhomogeneous wedge-blocks, the larger the wedge-block the greater the strength provided. This seems logical, because the larger a nonhomogeneous block the less chance there would be for a plane of weakness to penetrate completely through it. The initial strength of a clay depends very largely on the degree to which it is broken down in the mulling operation because this determines the uniformity of the coating of the quartz grains and therefore the strength of the wedge-block. Initial strength may not show the various classes of clay in their true relationships in actual foundry practice. Several other factors including durability, air-set strength, and relation to intensity of mulling must be considered. The following discussion of these factors is not meant to be exhaustive, but to analyze them in relation to the suggested concept of bonding action. RELATION TO INTENSITY OF MULLING It follows from the foregoing discussion of the bonding action of clays that as the mulling action applied to a sand bonded with an illite or kaolinite clay is intensified or lengthened, the number of larger clay par- ticles in the coating of the quartz grains will be reduced, thereby increasing the uniformity of the coating and accordingly increasing the strength of the clay. As shown in table 7, the statement does not apply to montmoril- lonite clay IA-bonded sands because this class of clay breaks down com- pletely with very little mulling and, therefore, little if any strength is gained by long or intensive mulling. Long or intensive mulling tends to narrow the gap between the strength of the different classes of clays. CONCEPT OF BONDING ACTION 43 Table 7. — Effect of Time of Mulling 1 on Green Strength Green compression strength in lb. per sq. in. Mulling time in minutes Mixture with 12 percent kaolinite clay at 2.8 percent tempering water Mixture with 4 percent montmorillonite clay IA at 2.2 percent tempering water 1 2 3 5 7 6.6 7.6 8.4 9.6 10.3 4.0 4.4 4.6 4.9 5.0 "Mulling was done in an intensive high-speed muller. DURABILITY The strength of a sand after repeated use without clay additions is one measure of its durability. In sands bonded with kaolinite or illite clays, some of the larger aggregates which cause the irregular coating and low strength are broken each time the sand is mulled. There is, therefore, a tendency in the successive use of such a sand to increase the uniformity of the clay coating and hence its strength. As sands are used, the high temperature of the metal burns out some of the clay bond and thereby reduces its strength. The strength of a sand during use is determined by the relation between the destruction of the bond by the heat and the de- velopment of new bond by the mulling. The clay aggregates are in effect a reserve bond that is released as the sand is used. The strength of sands bonded with kaolinite clay may actually increase for several heats after the first use. In contrast, sands bonded with montmorillonite clay may break down completely during the first mulling so that there is no reserve of bond to be released as the sand is reused. Among the other factors important to durability is the resistance of the clay to heat. This is not merely the fusion temperature of the clay, but the temperature at which vitrification begins. In considering the effect of heat on clays in molding sands, the vitrification temperature is important rather than the fusion temperature. Illite and montmorillonite clays have both low vitrification and fusion points. Halloysite has very high vitrifica- tion and fusion points. Kaolinite has a very high fusion point, but most clays composed of kaolinite begin to vitrify at a temperature considerably below this point. Another factor of importance is that small particles of a clay will fuse more quickly at a given temperature than large particles of the same clay. Other things being equal, therefore, clays yielding extremely 44 CLAYS IN GREEN MOLDING SANDS minute units in the mulled mixture would be more likely to undergo changes as a result of the heat of the casting process. AIR-SET STRENGTH Sands bonded with kaolinite or halloysite clays develop greatly in- creased compression strength when they are allowed to stand in the air after ramming (table 8). This so-called air-set strength develops with Table 8. — Air-Set Strength in Lb. per Sq. In. Green compres- sion strength Compression strength determined the following periods of time after ramming determined im- mediately after ramming 15 min. 30 min. 1 hour 3 hours Strength Moisture Strength Moisture Strength Moisture Strength Moisture Strength Moisture Halloysite clay (12 percent clay mixture) 7.0 3.0 4.75 5.95 14.9 4.0 4.45 5.7 20.8 4.3 31.5 5.2 5.4 9.5 3.82 5.1 51.0 52.5 2.45 3.95 Kaolinite clay (10 percent clay mixture) 7.6 3.8 13.2 3.35 17.2 2.95 21.8 2.55 30.0 1.50 4.0 5.2 5.5 4.75 8.0 4.40 13.5 3.7 40.0 1.95 Illite clay (12 percent clay mixture) 8.5 5.05 4.2 5.4 12.2 8.5 3.65 4.85 15.4 3.37 11.6 4.47 20.4 17.1 2.9 3.95 32.3 33.3 2.05 2.70 Montmorillonite clay I A (6 percent clay mixture) 7.75 5.87 3.2 4.6 9.90 2.85 7.0 4.3 11.0 2.66 8.6 4.0 15.8 2.3 11.3 3.70 27.6 22.8 1.70 2.9 small loss of tempering water and is considerably greater than the green compression strength that develops in a similar sand with an amount of tempering water equal to that in the air-set piece when air-set strength is determined. Sands bonded with other types of clay do not develop strength on drying without an accompanying loss of water, that is they do not have air-set strength. The explanation suggested for air-set strength, because it develops only with some classes of clays and because it develops without appreciable loss of tempering water, is that a certain amount of time (measured in minutes) CONCEPT OF BONDING ACTION 45 is required for the tempering water in certain classes of bonding clays to penetrate all the clay mineral surfaces and develop the completely rigid condition necessary for strength. Air-set strength develops particularly in sands that are prepared with more water than is required for maximum green strength. Such sands contain water which gradually penetrates the clay mineral particles and becomes completely rigid. In the process of allowing sand-clay mixtures to temper before testing there is time for the water to penetrate to the clay mineral surfaces. How- ever, when the mixtures are rammed, the relation between various lumps of clay and the individual clay mineral flakes is changed so that new inter- faces develop. The actual ramming operation probably disrupts some of the lumps of clay so that additional surfaces are available to the water. Further, the equilibrium between the flake surfaces and the water attained during tempering is probably disturbed by the ramming. After ramming, therefore, water must penetrate to new surfaces and become rigid to develop a new equilibrium. Montmorillonite IA clay is distinctive because the surfaces of the clay mineral composing it are penetrated thoroughly and very rapidly by water. In montmorillonite IB clay some few surfaces are not available to water but the available surfaces are immediately available and the unavailable surfaces do not become available. Therefore, the tempering water gets to all possible surfaces and becomes rigid almost at once. As a consequence, there is no tendency for strength to increase when a sand bonded with this type of clay is allowed to stand in the air. In sands bonded with halloysite or kaolinite clays some of the temper- ing water becomes fixed rigidly at once and some strength is developed — this is green strength. The remainder of the water is at first less rigid or perhaps liquid and gradually penetrates more inaccessible clay mineral surfaces and is gradually fixed. The quartz grains of sands bonded with kaolinite clay and halloysite clay are coated with some very fine particles of these clay minerals and also with larger aggregates of them. The water is fixed rigidly by the fine particles, very quickly yielding green strength. The penetration of water into the inaccessible surfaces associated with the aggregates requires a certain amount of time. Illite clay-bonded sands do not show air-set strength because the surfaces in aggregates that are unavailable tend to remain unavailable, that is, they are not gradually penetrated by water which becomes fixed to develop increased strength (air-set strength). Air-set strength should be an advantage in clays for many types of foundry work. Sands frequently possess their best working properties with more water than is required for green strength. This means that some of the inherent strength of the sand is lost if the sand is bonded with a clay 46 CLAYS IN GREEN MOLDING SANDS that does not develop air-set strength. It also means that no inherent strength is lost in sands bonded with clays developing air-set strength if the mold is allowed to stand a short interval before pouring. The existence of the air-set property indicates that the comparison of green strength is not always a fair way of comparing bonding clays. A better way would be to determine the strength of test specimens after they have been allowed to stand the same length of time that they would stand in the foundry after the mold was made and before it was poured. Concept of Bulk Density Variations in Molding Sands As shown in figures 6 to 10, a curve of the general type shown in figure 20 results when the weight per unit volume of rammed green sand is plotted against the amount of tempering water. The curve indicates that I! o w UJ < > M a < UJ v^ a. I O TEMPERING WATER (AMOUNT INCREASING — *■ ) Fig. 20. Curve Showing the Typical Relation of Bulk Density to Tempering Water in Molding Sands with increasing amounts of tempering water the bulk density first increases slightly, then decreases to a minimum value which in turn is followed by an increase in weight per unit volume. Based on the concept of the bonding action of clays in molding sands presented herein, the following explanation of the relation of tempering water to bulk density seems warranted. Water added up to the amount A (fig. 20) penetrates the clay, but is not enough to permit the clay mineral particles to be broken apart and plastered to the quartz grains. With less water than the amount indicated by A, there is little clay actually coating the quartz grains, and conse- BULK DEXSITY VARIATIONS 47 quently the clay and water are present in pore spaces between the grains. The tempering water is, therefore, a net addition to the weight per unit volume of the mixture. As point A is passed, the quantity of water becomes adequate to per- mit the clay mineral particles to break up and to develop the clay-water coating around the quartz grains. As the coating becomes thicker and more perfect with increasing amounts of tempering water, the packing of the grains becomes more difficult under the action of the rammer. The point of minimum weight per unit volume (point C) is the point where the quartz grains have the greatest resistance to packing and hence the greatest pore space after ramming. Point B indicates the point at which maximum green compression strength is developed in sands bonded with montmorillonite and kaolinite type clays which is at a lower moisture content than that required for minimum weight per unit volume. In sands bonded with illite and halloy- site type clays, points B and C almost coincide. With moisture contents higher than that indicated by C, the coating on the quartz grains gradually becomes soft permitting some of the coating to be squeezed into interstitial space, thereby increasing bulk density. As shown on page 29, sands bonded with montmorillonite clay IA develop minimum bulk density with tempering water just adequate to form a layer four molecules thick on each basal surface of montmorillonite, whereas maximum green compression strength is developed when there is water to coat each basal surface with a film only three molecules thick. Since maximum green compression strength corresponds to maximum rigidity in the clay- water film, it follows that a layer three molecules thick is the amount of water that can be held with maximum rigidity. The addi- tion of the fourth molecular layer of water causes the green strength to drop, and it can be concluded that this indicates that the rigidity of the water has been reduced. It follows that m.b.d.p. develops when the clay- water coating is slightly on the wet side of its maximum rigidity, or when some rigidity has been lost. It seems logical that a condition in the coating- would prevail just beyond maximum rigidity, and before there was enough water to greatly soften the coating, when it would offer the maxi- mum resistance to packing. In contrast to sands bonded with montmorillonite clay I A, sands bonded with montmorillonite clay IB develop maximum green compression strength when there is water to coat each basal surface of montmorillonite with a layer four molecules thick. That is, the latter type of montmorillonite can hold an additional molecular layer of water in a completely rigid condition — the explanation offered is based on the differences in the exchangeable 48 CLAYS IN GREEN MOLDING SANDS bases carried by the two montmorillonites. The addition of a fifth molec- ular layer of water to montmorillonite clay IB causes a drop in rigidity of water, and consequently the strength is reduced and minimum bulk density develops. The data are less clear for the other classes of bonding clays because of the development of air-set strength and because the water per unit of clay can be computed only approximately or not at all. However, the fact that the curves for m.b.d. versus tempering water are about the same for all classes of clay suggests that the explanation developed for the mont- morillonite type clays holds in a general way for all types of bonding clays. Thus, in the case of kaolinite clay, maximum green compression strength corresponds to the water content when the maximum degree of rigidity is reached that develops immediately after ramming. A small amount of additional water is adsorbed in a less rigid condition and the m.b.d.p. develops. With considerably more water than that required for m.b.d.p. some of it will have low rigidity and may be liquid water in pore spaces in the irregular coating of the quartz grains. If such a wet sand is allowed to stand after ramming, some of this water will penetrate into the clay and become rigid and fixed. Therefore as less of the water is liquid, the coating of the quartz grains becomes more uniform and more completely rigid with a consequent increase in strength, that is, air-set strength develops. After air-set strength develops, the relations of amount of tempering water to strength and to m.b.d.p. are different from those determined on sands immediately after ramming. Sands bonded with the halloysite clay develop maximum green strength when the tempering water is equivalent to a coating on available surfaces at least three molecules thick. Apparently this is the amount of water when the maximum degree of rigidity is reached immediately after ramming. Unlike sands bonded with montmorillonite or kaolinite clays, sands bonded with halloysite clay develop m.b.d.p. at about the same moisture content as maximum green compression strength. This suggests that for halloysite, water is held in a rigid condition to a given thickness and that additional water is completely nonrigid. If the additional water is completely non- rigid, it cannot be expected that it will increase the resistance to packing over that prevailing when all the water is completely rigid. The state- ments made for kaolinite clay in relation to air-set strength apply to sands bonded with halloysite clay, except that all the water in excess of that required for m.b.d.p. is probably completely nonrigid, or liquid, before air-set strength develops. Sands bonded with the illite clay probably develop maximum green strength when there is water to coat each available surface of illite with a BULK DENSITY VARIATIONS 49 film at least three molecules thick. Like halloysite, illite clay-bonded sands develop minimum bulk density and maximum green compression strength with about the same amount of tempering water. This suggests that addi- tional water to that which can be held rigidly by the illite is liquid. The general shape of the curves (see figs. 6 to 10) show that the relation of m.b.d. to tempering water is the same, within the limits of experimental error, for all types of clay except halloysite. The halloysite clay curves are steeper, indicating very rapid changes in bulk density with amount of tempering water. There is no satisfactory explanation for this character- istic of halloysite clay. Perhaps it would be expected on the wet side of m.b.d.p. since the water in addition to that required for m.b.d. p. would be essentially liquid and would therefore occur in pores and would increase the weight. On the other hand, illite clay has the same property of adsorb- ing rigidly a fixed amount of water with no tapering off of the rigidity of additional water, and yet the wet side of the curve is not steep. Dietert 4 defines the "temper point" of a molding sand as the point of minimum weight per unit volume. However, practical foundrymen place "temper" just on the wet side of the m.b.d.p. This is logical because a sand would undoubtedly work better in actual practice — have better flowability, etc. — when the coating of the grains is not at the point of maximum resistance to adjustment of grains, but on the wet side where adjustment of the grains to each other (flowability) would be less difficult. On the wet side of m.b.d.p. there is little or no liquid water in sands bonded with montmorillonite clays. At the same point sands bonded with halloysite clay may have liquid water before air-set develops, and they, therefore, feel distinctly wet immediately after mulling. It would seem necessary to control more closely the moisture content of halloysite clay-bonded sands, and perhaps handle them differently in actual foundry practice. The weight per unit volume at m.b.d.p. increases sharply as the amount of the kaolinite clay or the illite clay in the sand-clay mixture increases. There is a slight increase in weight per unit volume for sands bonded with montmorillonite clay IA, and for the halloysite clay or montmorillonite clay IB there is a very slight increase in weight per unit volume in sands with up to about 8 percent clay. In sands with larger amounts of these latter types of clay, the weight per unit volume actually decreases. Sands bonded with small amounts of kaolinite clay or the illite clay are slightly heavier per unit volume than those bonded with the other types of clay, whereas sands bonded with 8 percent or more of the former types of clay are considerably heavier than those bonded with equal amounts of either the halloysite clay or montmorillonite clays. The explanation for the variation in weight per unit volume for the different types of clays is not clear. 50 CLAYS IN GREEN MOLDING SANDS Summary of Factors Affecting the Green Bonding Properties of Clays On the basis of the discussion of the concept of the composition of clays and the bonding properties of various types of clay, it is possible to classify as follows the factors that determine the green bonding qualities of clays. AMOUNT OF CLAY MINERAL Many clays contain various amounts of granular particles of such minerals as quartz and feldspar in addition to the clay minerals. These granular particles are frequently large and they are not broken up on mulling. They disrupt the homogeneity of the coating of the quartz grains thereby weakening the wedge-block holding the quartz grains in place. Consequently such components have no bonding power and are dilutents of the bonding strength to the extent that they are present in a clay. In general, therefore, the larger the content of clay minerals (that is, the smaller the amount of these dilutents) , the greater the bonding strength. KIND OF CLAY MINERAL Data presented in this report have shown that the kind of clay mineral making up a clay determines to a large measure its bonding characteristics. Clays composed of clay minerals that disaggregate easily in water, for example bentonites composed of montmorillonite, quickly form homogene- ous regular coatings of the quartz grains into which tempering water finds easy access. Such clays have high strength. The strength of such clays is not improved by long intensive mulling or by fine grinding because the clay mineral breaks up and coats the grains uniformly without this extra application of work. Also such clays show no air-set strength because no appreciable time is required for the water to penetrate the flake masses. Other clay minerals that do not break down easily and completely on initial mulling have lower initial strength than the easily disaggregated clays because a lot of bonding power is locked up in the large aggregates of the coating and because the aggregates form planes of weakness. The bonding strength of such clays is improved by intensive mulling or finer grinding because this tends to break up some of the aggregates. Also the aggregates tend to give durability to the clay because some of them are broken up on successive mullings thereby releasing bonding power. In the case of sands bonded with kaolinite or halloysite clays, water tends to slowly penetrate the aggregates and in effect "knit" the aggregate into the coating, and, as a consequence, air-set strength develops. If, as in the case of illite-bonded sands, there is little penetration of the aggregates by water, there is very little air-set strength. SUMMARY 51 EXCHANGEABLE BASES This factor is important only to clay composed of minerals with high base-exchange capacity, such as bentonites composed of montmorillonite. The character of the exchangeable base affects bonding properties pri- marily by determining the thickness of the water film that can be held rigidly on the clay mineral surfaces. Thus, if in one clay, water three mole- cules thick is held rigidly on each unit surface and in another clay it is four molecules thick, the latter clay will develop a thicker rigid coating with the same amount of clay. Because of the thicker coating of the quartz grains the former clay will have greater green strength. This explana- tion is offered for the difference in strength of the two types of mont- morillonite clays. MISCELLANEOUS COMPONENTS OF CLAY Some clays contain small amounts of organic material, water-soluble salts, and hydroxides of iron. Certain of these components influence bond- ing properties favorably. Thus, it is well known that certain kinds of organic material increase the bonding strength of some clays. The mecha- nism whereby these components affect the bonding properties is not yet- clearly understood. Summary 1. Clays and shales are essentially aggregates of extremely minute crystalline, usually flake-shaped particles that are known as clay minerals. Some clays are composed of particles of a single clay mineral whereas others are mixtures of clay minerals. Some clays are composed entirely of clay minerals whereas others contain in addition grains of quartz, pyrite, organic matter, etc. 2. The properties of any clay or shale are determined chiefly by the kind of clay minerals that it contains. The properties of the clay minerals in turn are determined by the composition and structural arrangement of the oxygen, silicon, aluminum, and other atoms that compose them. A fundamental basis for the classification of bonding clays is provided by the clay mineral components because they largely determine the properties. Such a classification is presented, and it is shown by a detailed study of the green strength of samples representing each class that this plan of classification allows bonding clays to be separated into groups with dis- tinctive properties. Class I, Montmorillonite bonding clays: A. Aluminum in the montmorillonite replaced by some magnesium but not by appreciable iron, and with sodium as the chief exchangeable base. Example, bentonite from Black Hills area of Wyoming and South Dakota. 52 CLAYS IN GREEN MOLDING SANDS B. A considerable amount of the aluminum of the montmorillonite re- placed by iron, and with calcium and sometimes also hydrogen as the chief exchangeable base. Example, bentonite from northern Mississippi. Class II, Halloysite bonding clays: Example, the bonding clay recently placed on the market as "White Clay" from the Eureka district in Utah. The value of halloysite clay for bonding purposes was discovered in the course of work reported herein. Class III, Illite bonding clays: Example, "Grundite" produced in Grundy County, Illinois, contains about 60 percent illite. Class IV, Kaolinite bonding clays: Example, fireclays produced in Illinois and Ohio. 3. The general shape of the tempering-water versus green-compression- strength curves is much the same for sands bonded with all classes of clay except the halloysite. The curves for the halloysite clay-bonded mixtures are very steep, indicating a large variation of green strength with variation of moisture content. Until there is enough of any class of clay (about 4 percent) to exert appreciable bonding strength, maximum green compression strength is de- veloped throughout a wide moisture range because factors other than the clay, such as the adhesion of quartz and water, are effective. 4. Curves are presented for each type of clay showing the relation be- tween weight per unit volume of the sand-clay-water mixture, the amount of tempering water, and the amount of clay. The curves show that as the tempering water added to a dry sand-clay mixture is increased, the weight per unit volume of the mixture decreases to a given moisture content and then increases. In the sand-clay mixtures studied, the best working condi- tions are developed slightly on the wet side of the minimum bulk density point (m.b.d.p.). The relation of the bulk density of sand-clay mixtures to tempering water and to clay content varies considerably with the type of bonding clay in the mixture. 5. The maximum green compression strength of sands bonded with montmorillonite clay IB is greater than that of sands bonded with any other class of clay. At minimum bulk density point, the sands bonded with halloysite clay have the highest green compression strength in mixtures containing from 7 to 12 percent clay. With clay contents other than 7 to 12 percent, the sands bonded with montmorillonite clay IB have the highest strength at m.b.d.p. Data for the bonding strength per unit of clay show that additions of each clay, except the illite clay, in excess of a certain definite amount SUMMARY 53 (8-12 percent ± ) add little to the maximum green strength or the strength at m.b.d.p.; the actual amount of strength added varies with the type of clay. In illite clay-bonded sands, the strength increases at least up to 15 percent clay. 6. In sands bonded with the montmorillonite or certain halloysite clays, the sand grains are shown by microscopic examination to have a smooth coating of clay composed of extremely small units of montmorillonite or halloysite. In contrast the sands bonded with kaolinite or illite clays are composed of grains with an irregular coating containing many aggregate masses of the original clay that have not been reduced to extremely small particles in the mulling operation. Continued or successive mulling of sands bonded with the latter type of clays tends to break up the aggregate masses of the original clay, thereby releasing additional bonding strength. The aggregate masses of clay in the coating is actually a reservoir of bonding strength that is released as the mixture is used. The aggregates, therefore, provide durability. 7. In all mixtures with more than 5 percent montmorillonite clay IA, the tempering water required for maximum green compression strength is equal to that required to coat the basal surface of each unit cell of mont- morillonite with a water film three water molecules thick. In similar mix- tures bonded with montmorillonite clay IB, the water film is four water molecules thick. In mixtures bonded with both halloysite and illite clays a water film thickness of at least three water molecules is indicated. Compu- tations of the water film thickness per unit of clay cannot be made for kaolinite clay-bonded sands because the clay does not break down com- pletely and an unknown amount of surface is coated by water. 8. The initial water held on the clay mineral unit surfaces is loosely packed, with each water molecule occupying a definite position with respect to the surface. Such water in which the molecules are oriented and fixed in position is not fluid but is solid and rigid. It is indicated that at maxi- mum compression strength all of the water held on the clay mineral surfaces is in a rigid condition. Maximum green compression strength occurs at a moisture content corresponding to the maximum amount of rigid water retained by the clay. 9. The "wedge-block" theory of the bonding action of clays in molding sands is suggested, based on the concept of the rigid character of the water held by the clay when maximum green compression strength is attained. The bonding action is not that of a glue or adhesive causing the grains to stick together. On the basis of the suggested theory the bonding character- istics peculiar to the various classes of clays can be explained. For example, it explains why a small amount of montmorillonite clay added to 54 CLAYS IN GREEN MOLDING SANDS another type of clay has an effect on bonding properties out of all propor- tion to the amount added. 10. In mixtures with more than 6 percent montmorillonite clay IA the tempering water required for m.b.d.p. is equal to that required to coat the basal surface of each unit cell of montmorillonite with a water sheet four molecules thick, that is, a layer one molecule thicker than that required for maximum green strength. In similar mixtures with montmorillonite clay IB, tempering water at m.b.d.p. is equal to a layer five molecules thick per unit of montmorillonite, likewise one water molecule thicker than that required for maximum green strength. In mixtures bonded with illite or halloysite clay the thickness of the water film per unit of clay mineral is the same at m.b.d.p. and at maximum green strength, namely at least three molecules. Computations of water film thickness cannot be made for kaolinite clay mixtures. 11. An explanation of the variation in bulk density of molding sands is offered based on the concept that a certain limited amount of water is held on the basal surfaces of the clay mineral units in the rigid state and that this water by controlling the character of the coating of the quartz grains influences the ease of mutual adjustment or packing of the sand grains and consequently the reduction of pore space when the mix- ture is rammed. 12. Sands bonded with kaolinite and halloysite clays develop greatly increased strength without an accompanying water loss when they are allowed to stand in the air after ramming. This so-called air-set strength develops because a certain amount of time is required for the tempering water to penetrate the clay mineral surfaces in these types of bonding clays and develop a completely rigid condition. 13. The influence of variations in the composition of clays, other than in the clay mineral content, on bonding properties is discussed on the basis of the suggested theory of bonding action. BIBLIOGRAPHY 55 BIBLIOGRAPHY 1. Anon3 r mous, Testing and grading foundry sands and cla}'s: Am. Foundry men's Assoc, Chicago, 111., 4th Edition, 1938. 2. Anonymous, Foundry sand testing: H. W. Dietert Co., Detroit, Michigan, 1937. 3. Briggs, C. W., and Morey, R. E., Synthetic bonded steel molding sands: Trans. Am. Foundrymen's Assoc, vol. 47, pp. 653-724, 1939. 4. Dietert, H. W., and Valtier, F., Grain structure control insures mold permeability control: Trans. Am. Foundrymen's Assoc, vol. 41, pp. 175-192, 1931. 5. Dunbeck, N. J., American synthetic sand practice: Trans. Am. Foundrymen's Assoc, vol. 50, pp. 141-164, 1942. 6. Grim, R. E., The relation of the composition to the properties of clays: Jour. Am. Cer. Soc, vol. 22, pp. 141-151, 1939; Illinois Geol. Survey Cir. 45, 1939. 7. , Modern concepts of clay materials: Jour. Geol., vol. 50, pp. 225-275, 1942; Illinois Geol. Survey Rpt. Inv. 80, 1942. 8. Grim, R. E., Bray, R. H., and Bradley, W. F., The mica in argillaceous sedi- ments, Am. Min., vol. 22, pp. 813-829, 1937; Illinois Geol. Survey Rpt. Inv. 44, 1937. 9. Grim, R. E., and Cuthbert, F. L., The bonding properties of various types of clay; Manuscript in preparation. 10. Grim, R. E., and Rowland, R. A., The relation between the physical and the mineralogical character of bonding clays: Trans. Am. Foundrymen's Assoc, vol. 48, pp. 211-224, 1940; Illinois Geol. Survey Rpt. Inv. 69, 1940. 11. , Differential thermal analyses of clay minerals and other hydrous ma- terials: Am. Mineralogist, vol. 27, pp. 746-760, 801-818, 1942; Illinois Geol. Survey Rpt. Inv. 85, 1942. 12. Hendricks, S. B., On the structure of the clay minerals, dickite, halloysite, and hydrated halloysite: Am. Mineralogist, vol. 23, pp. 275-301, 1938. 13. Hendricks, S. B., and Jefferson, M. E., Structure of kaolin and talc-pyrophyllite hydrated and their bearing on water sorption of the clays: Am. Mineralogist, vol. 23, pp. 863-875, 1938. 14. Mehmel, M., On the structure of halloysite and meta-halloysite : Zeit. f. Krist., B. 90, pp. 35-43, 1935. 15. Morey, R. E., and Taylor, H. F., Some properties of synthetically bonded steel molding sands: Trans. Am. Foundrymen's Assoc, vol. 49, pp. 385-426, 1941. 16. Ries, H., and Conant, G. D., The character of sand grains in molding sands: Trans. Am. Foundrymen's Assoc, vol. 39, pp. 353-392, 1931. 17. Ries, H., and Lee, H. V., Relation between shape of grain and strength of sand: Trans. Am. Foundrymen's Assoc, vol. 39, pp. 857-860, 1931. 18. Ries, H., and Nevin, C. M., The cohesiveness test of foundry sands: Trans. Am. Foundrymen's Assoc, vol. 31, pp. 640-648, 1924. 19. Weidman, V. W., A study of the physical and chemical properties of bentonitic clays: Univ. of Ala., M.S. thesis, 1940.