The objectives of this investigation were to measure the effects of moderate heat treatments, below the dehydroxylation temperature, on physical and chemical properties of a calcium-montmorillonite clay. Previous workers have noted the reductions in cation exchange capacity and swelling property after heating in the range 200°–400°C, and have suggested several possible explanations, such as hysteresis effect, increased interlayer attractions owing to removal of interlayer water, or changes in the disposition of interlayer or layer-surface ions.

The liquid limits of Ca-montmorillonite steadily decreased with increased treatment temperature, until they reached a constant level at 450° to 500°C. The plastic limit decreased slightly up to 350°C. Above this temperature samples could no longer be rolled into threads. The gradual change is in contrast with sudden major changes noted for weight loss (maximum rates of change at 100°C and 500°C), glycol retention surface area (maximum change at 520°C), and changes of peak intensity and breadth character after glycolation of the first diffraction maximum (530°C). Other properties showing more gradual reductions with heat treatment were: cation exchange capacity by NH4Ac method, d001 intensity (21 Å spacing) after storing at 100 per cent r.h. one month and re-wetting with water, and amount of calcium exchangeable without water soaking. Previous water soaking allowed much greater release of fixed Ca++ up to 450°C. Similar results were obtained with cation exchange capacities when samples were treated with In CaCl2 solution.

The 21.0 Å peak intensity curve showed close similarity to the liquid limit and plastic index curves in the low temperature range, and an explanation is suggested.