Soil Stabilization and Colloid Science ERNST A. HAUSER, Professor of Colloid Science, Massachusetts Institute of Technology
Introductory Remarks by the Chairman: A great problem is to scientists like a medieval fortress to beleaguering soldiers. The walls are scouted for weak• nesses, and effort is concentrated on v/hat is considered to be the weakest spot. If a breach is made, most of the men will use it to enter the fortress. A scien• tific pioneer, as any other pioneer, does not like crowds. Even though he had originally recognized the weakness and was the first in the breaching of the wall, he will leave the crowd now seeping through In order to attack other prob• lems and break new walls. Hauser is such a true pioneer and aworthy representative of colloid science, a discipline that is always out infront in attacking complex problems and that leaves the conquered country to be organized by her socially more-accomplished sister, physical chemistry. Hauser is well known all over the world for his pioneering accomplishments in colloid science and its application to the chemistry and technology of rubber and other elastomers and of clays and other silicate min• erals. His many contributions to science and technology are too numerous to even list m this place; several of these were of prime military importance during the Second World War. It is gratifying that Hauser consented to enrich this symposium with an inter• pretation of the relations between soil stabilization and colloid science.
• COLLOID science is always needed in position of plants or animals. For this order to explain the interaction of matter reason, the surface activity of these clays present in the colloidal range of dimen• will differ pronouncedly from that of pure sions with other substances. Any sub• clay. stance, organic or inorganic, falls within This condition has been aptly described the colloidal range if it is present in at byWinterkorn (4) as follows: "Clay min• least one dimension in the range between erals in natural soils are not as clean as a one micron and one milimicron. scraped bone; rather the mineral surface In soil stabilization, clays must always is normally in as close a relationship with be considered as very important and often adsorbed and synactive organic matter as determinant soil components; therefore, a a bone in a living being is with cartilage soimd knowledge of their structure, com• and muscle tissue." Also, microorgan• position and morphology is of great im• isms are normally present in natural and portance. often in stabilized soils. Their bearing on A vast amount of information on clays soil stabilization has been recognized by has been accumulated since 1925, when it only a few workers in the field (5). An was possible, for the first time with the explanation of the variability in pro• aid of new research tools and the formula• perties of clays will be offered before the tion of new concepts, to embark on a truly mfluence exerted on soil stabilization by fundamental study of clay minerals. This organic matter is discussed. gigantic work has resulted in a good, though still not complete, understanding of Genesis of Clay Minerals the occurance, composition, and colloidal Of all the theories pertaining to the or• properties of all clay minerals (1). igin of clay minerals, geologists and min• This contribution will cover the most- eralogists have largely accepted the "re• important facts of the colloid science of sidual clay" and the "transported clay" clay minerals and the most-reCent con• theories. The former is based on the as• cepts pertaining to them. Despite the sumption that the formation of clay min• emphasis placed here on the clay minerals erals is the result of surface weathering of and their properties, it should not be over• fresh rocks or is due to the action of solu• looked that clay minerals in natural soils tions. From a colloid-scientific point of frequently have adsorbed on their surfaces view this theory deserves special atten• organic matter resulting from the decom- tion. However, it does not offer a truly 58 59
satisfactory jexplanation of the genesis of form complex molecules which, however, the two basic components of clays, silica are chemically well-defined; this is dem• gel, and alumina or magnisia gel. onstrated by the fact that all these silicates Although some of the latest contribu• are capable of separating into crystallo- tions to the chemistry of silicates admit graphically well-defined particles. Silicic that colloid science plays an important acid owes this property to its high acidity role, the majority of scientists interested at elevated temperatures. At low tem• in the colloid science of silicates are peratures the acidity of silicic acid is so seemingly still not familiar with the fact small, however, that it can form a salt that colloid science had already made basic only with the strongest bases, as for ex• contributions to the theory of the genesis ample NaaO andK20. It Will be precipitated of silicates over a quarter of a century from these by the weakest acid, however, ago (2). even by water alone. This implies that most of the silicates are present in a Even as early as 1779, research was metastable equilibrium, even at normal done in the field of what is known today as temperatures. When water decomposition the colloid science of siliceous matter. In sets in, the equilibrium is disrupted. In his paper, "De TerraSilicea," the Swedish the absence of acid, oxides or hydroxides scientist, Torbem Bergman, stated: of the silicate bases are formed and silicic "Finally I must still think of those in• acid is liberated. In the presence of car• complete phenomena which depend on a bonic acid, humic acid, or sulfuric acid, seeming solubility. This siliceous liquor primarily those bases. will be attacked (alkali silicate solution--E. A. H.) is pre• which will form soluble salts with these cipitated by all acids, because the alkali acids. The moleculer structure of the prefers to hang onto them rather than to silicate is disrupted and at certain loca• the gravel. This precipitated gravel has a tions of the crystal lattice the acid-insol• very expanded and loose texture and is uble bases and silicic acid remain. When filled with water, so that it is twelve times liberated they are present in a molecular as heavy when moist than when dry. If disperse condition. Since they are insol• more water is added before adding acid, uble, they are not capable of forming a however, the solution remains clear, even molecular solution with water; oversatura- if more acid is then added than would be tion results, causing polymerization. This needed to neutralize the alkali. This is a can either occur in space-lattice fashion, peculiar phenomenon and the reason for it resulting in the formation of crystals, or, is probably the following: Through the if the over saturation is too great in com• dilution with water the siliceous particles parison to solubility and the ability to are very much separated from each other, crystallize too small, colloidal gels are or made finer and better distributed formed. The formation of colloids due to throughout the liquid. Although the par• the decomposition of the silicate is a con• ticles should settle out, being heavier than densation reaction." liquid, they cannot in this case overcome the resistance due to friction, because a greater force will be needed to accelerate We still have no truly satisfactory sedimentation than that resulting from the answer to the question of how clay minerals difference in specific gravity. The silica were actually formed or how their genesis particles remain suspended in the liquid can be explained in a way which takes into but at the same time are invisible due to account all the knowledge now at our dis• their fineness and transparency." posal. Many years ago the alteration of pure silica and alumina to kaolinite or The more-modern work which has been montmorillonite was demonstrated. In all overlooked dates from 1927, when the probability we are dealing herewith a com• German geologist, Schomstein, made a bination of the chemical reactions attendant fundamental contribution to the colloid upon leaching and a devitrification of the science of siliceous matter. The follow• glass, the reaction then proceeding through ing is a passage from one of his basic a hydration and adsorption of ions. In all papers (2): probability the silica and alumina gels re• "During the decomposition of silicates acted first to form halloysite; then, by the following must be borne m mind: In condensation, kaolinite or montmorillonite, the magma^ silicic acid is in chemical talc, mica, and other minerals, depending equilibrium with the bases. It is able to on the ions present. In any event, clays 60 / z 9 / 0 • 9 9 3 OH f F JO//
or A^o 3 0 Cf'i'* 3 OH * * JO// 4 5 9 9 9 9 n^%9,K9 9 6 OH
>cr 6b x> 6 0 ^i^Vi'i 6 0// VWWV 6 OH 7 5
i OM-t 4 O *• tOHf 4 SI ItO 4 SI Z OHt* 40 *• lOM 4 AifO *SU0MAt tOHi* 4 0 *! OH 4 SI 'ii, tOHt40*tOMt a ygoKAi * 41^ 10 tf o 4 5/ 40+2 OH yS^^ V
13 6 0 5/ 4 Si Mg,AI,F»* 4 0 + 20H O - O 3 AH- I Mg o - OH 4 0+ 2 OH • - K • - HiO 4 SI o - 6 0 OHj, The schematic drawing in the first column of each section represents the composition of the unit cell of the respective building unit or complete crystal lattice. All atoms have been projected into one plane The second columns give the number and type of atoms or groups for every lattice plane. 1 = silicon tetrahedron, 2 = aluminum octa• hedron, 3= magnesium octahedron, 4 = hydrated silica, 5 = gibbsite; 6 = brucite, 7 = halloysite; 8 = attapulgite; 9 = kaolinite, 10 = talc, 11 = nontronite, 12= mica (illite); 13= montmoriUonite (substituted)
Figure 1. Structural data of the most-important clay minerals and their building units. 61 owe their genesis to the presence of silica electrons, whereas each oxygen atom and alumina or magnesia gels and the needs two electrons for saturation, a changes they undergo with time or tem• silicon-oxygen tetrahedron is unsaturated, perature (1^, 2, 3). lacking four electrons. If the oxygens are A fact \^ich until quite recently has not replaced by hydroxyl groups, however, the found the attention it deserves is that, in system is saturated, since the missing spite of the inertness of most siliceous electron has been supplied by the hydrogen matter, the chemical reactivity of the sil• of the hydroxyl group. ica surfaces surpasses that of most stable oxides. Systematic research on the sur• face chemistry of silicates has led to the discovery of imexpected chemical reactions which result from the dehydration or poly• merization of silicic acid {2).
Structure of Clay Minerals
In 1923Hadding and, a year later, Rinne Figure 2. Left: A silicon-oxygen tetra• were able to prove by X-ray-diffraction hedron (schematic). Hie oxygen atom on top studies that clays are not composed of has been drawn as a transparent sphere to matter in the amorphous state, as had been show the location of the silicon atom in assumed, but that they are crystalline in the cavity formed by the four oxygen atoms. structure. Since then our knowledge of the Right: An aluminum- (magnesium) oxygen structure of clays has been greatly in• octahedron (schematic). The oxygen atoms creased. Colloid science has offered a constituting the upper layer have been drawn transparent to show the location of better understanding of some of the phe• the aluminum (or magnesium) atom in the nomena which are so characteristic of canty formed between the oxygen (hydroxyl) clays and which could not be explained by layers. physico-chemical reasoning alone. Bearing in mind the fact that all clays Starting with orthosilicic acid we find are composed of silica and alumina or that in forming a silica sheet every oxygen magnesia, and first taking the unit cells of atom located in one plane is shared by two these components into consideration, it silicon atoms, except the oxygens located will immediately be realized how these at the e^es of the sheet, which remain structures can increase in dimensions. It unsaturated; the oxygens in the vertex will then also be seen how they can com• position of the tetrahedra have a hydrogen bine to form either simple double layers or attached. This structure is saturated, triple layers of silica and alumina or mag• therefore, and is known as a hydrated sil• nesia (Figure 1). ica sheet (Figure 3). If we join nonhy- A silicon atom binding four oxygen drated silicon tetrahedra to form such a atoms forms a tetrahedron; the radii and sheet, we find that all oxygen atoms are distances between the silicon and oxygens saturated except those in the vertex, edge, permit the four oxygens to touch, leaving and corner positions. Consequently, any a cavity just large enough to include the unsaturation occuring due to edge or cor• silicon (Figure 2, left). ner positions of atoms will be negligible The second building unit is a combina• compared to the surface unsaturation of tion of aluminum with six oxygens, where such a structure. the latter completely enclose the aluminum The second building unit--the aluminum atom (Figure 2, right). The aluminum can or magnesium octahedron—is capable of be exchanged for magnesium, but since forming similar sheets. Assuming that all magnesium is a larger atom it does not fit the oxygens are saturated by hydrogen, then exactly into the cavity; therefore, a slight two layers of hydroxyl groups, each in strain in the structure is set up. These close hexagonal packing, can be cemented two combinations are known as aluminum- together by magnesium atoms, so that six oxygen and magnesium-oxygen octahedra. hydroxyl groups share three magnesiums. These building units are held together Except for edges and corners, such a sheet by the sharing of electrons. Since each is saturated and neutral. Several of these silicon atom can share four of its outer composite layers can be held together by 62
secondary forces, such as van der Waals' bonds, and this results in the formation of forces. thin hexagonal platey particles. If the It is interesting to note that by cement• fracture is parallel to the c axis of the ing two of the hydroxyl sheets together with crystal, however, the bonds between Si magnesium, the high symmetry of a hexa• and O, Al and OH, or O must be broken. gonal configuration is disturbed and reduces These broken bonds are the basis for the to a monoclinic one, since the hydroxyl preferential adsorption of hydroxyl ions. sheets are staggered. The mineral built The adsorbed hydroxyl ions are hydrated; up according to the above structure is that means that they carry with them water known as brucite (Figure 1). molecules which become part of the water hulls. The particle and the cations lo• cated close to it constitute the Gouy- Freundlich diffuse double layer. If a small trace of an electrolyte, e. g., sodium hydroxide, is present, profound changes in the forces connected with the micelle will occur as a result of the pre• ferential ion adsorption on the particle. This results in the setting up of a con• siderable repulsive force at the edge of the lyosphere. The charge density on the par• ticle and its potential are high, and from a colloidal point of view the particle is at maximum stability. In this state the kaolinite slip is deflocculated or dispersed. Figure 3. Structure of an unsyimnetrical Fraeturi silicon-oxygen sheet; silica glass (sche• matic) (after W. H. Zachariason). If aluminum replaces the magnesium in the octahedral sheets, changes have to be made in order to retain a neutral sheet. Aluminum can share three electrons, magnesium only two. Consequently, one third of the places formerly occupied by magnesium must remain vacant. There• fore, the aluminum octahedral sheets will not be so densely packed as the magnesium octahedral sheets. This mineral is known as gibbsite. Just as silicon-oxygen tetrahedral sheets are built up from single silicon-oxygen tetrahedra, gibbsite from the aluminum- Figure 4. The formation of E units and D oxygen octahedra, and brucite from units at the fracture surface of crystal• magnesium-oxygen octahedral sheets, the line SiC^ (after W. A. Weyl). minerals of the kaolinite, montmorillonite, If we now look at the structure of a clay and illite groups are built up by joining mineral of the montmorillonite group, we gibbsite or brucite sheets to silicon-oxygen find that these substitutions result in tetrahedral sheets (1). establishing net residual negative charges on certain sections of the lattice, which in Some Colloid-Scientific Considerations turn cause attraction. Sodium is the most- common cation found in naturally occurring According to its crystallographic classi• montmorillonites. The crystal units of fication, kaolinite is a monoclinic system montmorillonite are held together quite which displays good cleavage. In the loosely by weak O-O bonds. The sodium formation of fragments the fracture occurs ions are primarily adsorbed on the sur• along the basal cleavage plane and also faces of the silica sheets and to a negligi• normal to it. Fracture along the cleavage ble extent on the fracture along the c-axis. plane does not rupture primary valence When hydration occurs these ions pry the 63 particles apart and adoublelayerof appre• characteristic of colloids cannot be har• ciable thickness is formed. Montmorillon- nessed into rigid mathematical concepts. itic clays thus exhibit a high degree of swelling (6). Ion Exchange Silica in its various modifications represents a three-dimensional network of Of all the phenomena exhibited by min• SiO« tetrahedra in which the O*" ion is erals which form colloidal micelles when shared by two Si*"*^ ions. in contact with water, the most important In the interior of silica the building unit is the ion-exchange reaction. This reac• is the Si*"^ /0*"» group. In contrast there- tion was originally recognized by Thompson y—if-) in his investigation of the properties of to, two types of groups must be expected soils (2). The ultimate clay crystal carries a net in the surface, namelyg.4+ (_0°.)' 3 *'»<*si*+ negative charge. This is the result either (—g—) O*". The first, known as a defic• of anion adsorption onto its surface or of iency unit, or D unit, shows an oyxgen an unbalanced crystal lattice. Whatever deficiency resulting in an excess positive the basic cause, the individual ultimate charge; the second, known as an excess clay particle may be pictured as a very- unit, or E unit, has oxygen in excess over complex anion. To balance the charge the the stoichiometric ratio and carries a particle will tend to adsorb the necessary negative charge (Figure 4) (2). number of cations available in the environ• This may lead to an electron transfer ment.' These cations then act as the so- from the E unit to the D unit so that the called counter ions. When dispersed in potential field is reduced and with it the water they will hydrate to a degree depend• energy content of the nascent surface. For ing on their valency and hydratability, silica such an electron transfer does not dissociate to a certain extent from the sur• seem very probable, since this would lead face of the particle, and thereby form a to the formation of Si''*' ions having eight- diffuse electric double layer, thus giving plus-one outer electrons. It is the insta• rise to the formation of a colloidal micelle. bility of this electron configuration which Such a clay particle may therefore be com• most probably excludes such an electron pared with a dissociated electrolyte, the transfer from contributmg to the stability size of one of its ions falling within the of the nascent surface of silica. colloidal range of dimensions. This information indicates the complex• For example, just as a soap-like so• ity of the problems pertaining to the pro• dium oleate in hard water will exchange its perties exhibited by clayey substances and sodium ions for the less-hydrated calcium demonstrates one phase of the importance ions, the counter ions of the clay particle of colloid science for soil stabilization. will be exchanged with ions from the dis• In dealing with colloidal clays it is im• persion medium if the resulting micelle possible to e:q)lain their properties by would have less tendency to hydrate and rigid mathematical laws, as so success• carry a lower charge, or both. This re• fully applied in physico-chemistry. This action is, of course, the more pronounced should not imply that the use of any mathe• the more ions of a high degree of hydration matics in this field is of no value at all; are present in the clay under investigation. however, there is little to be gained by The magnitude of this ability to adsorb ca• drawing conclusions pa.sed on mathematics tions depends primarily on the structural alone, disregarding the peculiarities of the configuration of the nucleus of the colloidal colloidal state of matter. The state in• clay micelle. It is usually expressed in volved in colloid science is characterized miUiequivalents of cations per hundred by a high surface-over-volume ratio. In grams of clay. accordance with the electronic theory of By the same reasoning, however, clays valency, an ion located in the surface of a may also adsorb anions where net positive particle must be far-more reactive than charges are setup in the crystal lattice or one in the interior. Therefore, matter where the hydrogen of a hydroxyl group is present in the colloidal state must be exchanged for a stronger ion, like PO*—. much-more reactive than its chemical Generally speaking, therefore, it may be composition alone would indicate. This stated that ion exchange will follow the also e}q)lains why most of the properties Hofmeister or lyotropic series, at least 64
for cations, i.e., the higher the atomic essential to obtain the correct information. weight of an ion, the more firmly it will be held by the exchanger. Therefore, the ex• Soil Stabilization change reaction for monovalent ions will follow the series Li
1 1 1 1 1 1 aeo 0.70 0.00 0.90 /.O0 I.IO EXCHANGE CAPACITY IN MCQ /S. Figure 5. Ihe radii of hydrated ions ver• sus exchange capacity at equilibrium.
Sr For this reason, the search for better sta• is accomplished the permeability of the bilizers for fine-grained soils is being soil decreases, because a continuous soil- pursued quite actively. polymer-water structure is formed. The A primarymilitary problem is to obtain soil water is also held more closely to the an almost immediate Improvement of the soil structure by the formation of the poly• structural properties of naturally occur• mer structure and its dipolar alignment. ring soil. The treatment must be effec• On June 4, 1946, United States Patent tive with various naturally occurring soils 2,401,348 was issued to E. A. Hauserand even if they differ pronouncedly in their E.M. Dannenberg for molding composi• water content, so that they will show an tions using base-exchange solids like ben- appreciable strength both in shear and in tonites in finely divided form to which an tension without being affected by water ionizable salt of a polymerizable carbox- after they have been reacted. It must be ylic acid was added. This compound con• possible to incorporate the stabilizer into tains the polymerizable olefinic group of the naturally occurring soil without actually the salt. In Claim 1 of the patent the use removing the soil from the site; also, the of acrylic acid salts was claimed specif• stabilizer should be effective in very small ically (2). proportions. Although this patent did not refer to the One approach toward the solution of this use of a polymerization catalyst which problem is the incorporation into the soil would permit the reaction to take place at of a monomer which can be poljnnerized normal temperature, it must be con• thereafter at normal temperature, thereby sidered, nevertheless, as the original idea forming effective bonds or links with the for working out the acrylate and similar soil as a basic part of the new structure. methods for soil stabilization. The monomer to be employed must be Whenever a base-exchange solid like able to enter into ion exchange with a base- sodium bentonite is used, the reaction has exchangeable soil or to provide an effective to be carried out in such a way that the bond with soils exhibiting no base-exchange sodium ions which act as coimter ions in properties. In the latter case the poly• the clay micelle may be exchanged, so that merized monomer will form long-chain the clay becomes of the nonswelling type. polymers into which soil particles are As work continued it was found that salts interwoven. of multivalent cations, e.g., lead acry- The monomer should be ionizable and lates, generally give satisfactory results. represent a water-soluble salt. If the As a result of this work a patent on organic part of the monomer is negatively "Stabilization of Soils" was granted in charged, the cation of the monomer must 1953 (2). According to this patent, the be polyvalent, so that a positively charged dissociation of calcium acrylate is accom• complex monomer ion becomes available panied by a base-exchange reaction in for base exchange with the negatively which the ionized calcium acrylate ions charged soil particles. Furthermore, a replace sodium ions associated with the polyvalent cation is needed whenever the soil particles. cation is to serve as a link in cross-poly- After the soil has been intermixed with ' merization. calcium acrylate and a base-exchange re• It must also be borne in mind that a I certain quantity of free soil water is action has taken place, the soil-monomer needed when chemicals of this type are mixture can be polymerized at normal added to the soil, so that they become ion• outdoor temperature through the use of ized and immediately enter into the ex• an appropriate catalyst, e.g., one com• change reaction. Generally it can be stated prising aa oxidizing agent and a reducing that most soils have enough water pre• agent. sent to meet the requirements for poly• Calcium acrylate will not ionize to an merization. Any monomer which is not appreciable extent; therefore, a complex dissolved and base exchanged with the.soil colloidal electrolyte forms. Calcium ions particles remains available for 'poly• have a diameter greater than that of the merization in the final phase of the treat• "inscribed circle" (see Figure 6), which ment. prevents their penetration of the silica Most important is the formation of an layer of the clay particle; therefore, the Insoluble polymer in which the soil par• calcium ion will be attached to the gibbsite ticles become part of the polymer. If this layer of most clay particles. If a very- 66 moist soil is involved in the reaction, factory answer to the problems Involved in however, it becomes questionable whether soil stabilization. Soils should first be this can be accomplished. In most cases, subjected to systematic chemical and therefore, only a skeleton of calcium acry- bacteriological investigation. late surrounding the soil particles will Without paying more attention to these actually be formed and not a truly chemi- factors, it will never be possible to sta• ically combined structure. bilize soil in situ satisfactorily. What is still being widely overlooked today is the fact that, as previously men• Conclusions tioned, soils can never be considered as Colloid science is of extreme impor• being absolutely pure; the presence of de• tance for any work pertaining to the prob• composed plants (which act as nutrients lem of soil stabilization. For this reason for bacteria) and of other microorganisms the most-important facts pertaining to the must be taken into consideration (4,5). structure of clay minerals have been dis• Therefore, any work carried out with pure cussed. On the basis of the clay struc• clays or sands is only of theoretical im• tures, the colloidal properties of clays portance (9). Far more attention must be have been referred to with specific atten• paid to the reactivity of th6 different soils tion to the phenomenon of ion-exchange re• with their surrounding media to obtain actions. Based on this information, the truly successful results. Only to make application of acrylic resin exchangers is tensile and shear tests on specially molded discussed and the problems involved in the samples can never result in a truly satis• use of calcium acrylate are pointed out. References 1. Ernst A. Hauser, "Colloid Chem• the Surface Behavior of Bentonites and istry of Clays," Chemical Reviews, Vol. Clays," Soil Science, Vol. 41 pp. 25-32 37, pp. 287-321 (^SiS). (1936). 2. Ernst A. Hauser, "Silicic Science," 7. J. E. Gleseking, "The Exchange of D. Van Nostrand Co., Inc., New York Large Organic Cations by Clays," Paper (1955). No. 6 of the Symposium on the Science of 3. C. Edmimd Marshall, "The Colloid Soil Stabilization, Highway Research Chemistry of the Silicate Minerals," Board, (1955). Academic Press, Inc., New York (1949). 8. Hans F. Winterkom, "Principles 4. Hans F. Winterkom, "Physico- and Practice of Soil Stabilization." Colloid chemical Properties of Soils," Proc. 2nd Chemistry (J. Alexander, Editor) Vol. VI, Int. Conf. Soil Mech. & F.E. Vol. I, pp. pp. 459-492, Reinhold Publishing Corp. 23-29 Rotterdam (1948). New York (1946). 5. Hans F. Winterkom, "Fundamental 9. Hans F. Winterkom, "Job Expe• Approach to the Stabilization of Cohesive rience with Exchange Ions Involving Inor• Soils," Proc. Highway Res. Bd. Vol. 28, ganic and Organic Ions," Symposium on pp. 415-422, (1948). Exchange Phenomena in Soils, ASTM Spec• 6. Hans F. Winterkom, "Studies on ial Publication No. 142 (1952).