Water vapour atmospheres are known to raise the temperature of the endothermic reaction of clay minerals in differential thermal analyses. In isothermal kinetic studies of dehydroxylation, it is shown that a stationary state is attained depending on the operating temperature. The rate of dehydroxylation is a function of specimen size and shape, which suggests a controlling influence by entrapped water vapour. Even at constant reaction temperatures, the rate of dehydroxylation varies greatly between the interior and the exterior of a specimen. The bearing of these results on thermal analyses and on kinetic studies generally is discussed.

The mechanism whereby water is absorbed by soils and thereby causes expansion and “heave” is discussed. The dependence of expansiveness on occurrence of montmorillonitic and illitic clay types is shown. Rapid methods of analysis used, include a micro-desiccator method of deriving water vapour pressure isotherms and the technique of glycol retention.

Cation exchange capacity (c.e.c.) determined by methylene blue can be regarded as a useful soil mechanical test in its own right. When a large number of liquid limit (LL) tests have to be done to determine variation in clays of the same general type, as on an engineering site, economy of operation could be achieved by carrying out many c.e.c. tests and few LL tests. It is only when comparing clays of widely different types that correlation is poor.

Fifty per cent. of the soils studied came from the United States and represent samples from sites in which engineering difficulties were encountered; therefore, it is significant that the clay in 50% of all soils examined was an illite-smectite type, while illite-chlorite comprised 15% of the total. For illite-smectite clays, quantitative estimation of the illite and smectite percentages is obtained from the total potash and glycol retention data. The agreement between measured and calculated cation exchange capacities is taken as supporting evidence of the reliability of the methods used. The chlorite component in the illite-chlorite clays was a high-iron chlorite. Regular and random stratification of illite and chlorite components was observed. The most important influence of composition on behaviour is the very high water sensitivity of the expansive minerals. Most of the soils studied which had caused stability or volume change problems in the field contained expansive minerals. For soils with fines of a single mineral species or a simple mixture of minerals, predictions of behaviour can be made with fair accuracy from a knowledge of soil composition. Where the soil fines are interstratified minerals or cemented crystals, predictions can seldom be made with accuracy. Interstratification or cementation usually inhibits the water sensitivity of normally expansive minerals. This fact seriously limits the value of soil compositional data to the engineer.

The adsorption of potassium by clay minerals under varying conditions of concentration and pH has been studied. It seems from the experimental data that montmorillonite shows the greatest adsorption at all pH values. The adsorption is found to vary with the concentration of potassium in the solution, pH of the solution and the nature of the cations in the exchange complex. Data are presented for kaolinite, montmorillonite, halloysite and natrolite.

Certain of the criteria used for distinguishing vermiculites in clays from smectites are critically examined, and attention is drawn to the urgent need for further research in this field. Difficulties inherent in the differentiation are heightened by the common occurrence of interlayer contamination in the naturally occurring minerals.