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<!<'^ 4 0*2 OH 6 Mg 4 0i- 2 0H 4 si 6 O 12 / K 6 0 60 4 5f 3Sii- lAl 401-ZOH 40-I-20H 4F,*** 4 Al 40 + 2 OH 4 01-20H SI 3 5// lAl 4 6 O 6 O I K 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.