This thesis describes the results of an investigation into thermal clay stabilisation, ultimately involving the use of iron oxide. It has long been thought that the process of laterisation involves the formation of hydrous iron oxides and their subsequent dehydration over geological time. It is of interest to the engineer to accelerate this process, but little success has been reported hitherto. The present study has shown that in the presence of activated silica and fine iron oxide, clay minerals as diverse as montmorillonite and kaolinite can be induced to yield very strong compacts, almost comparable with brick, in a matter of days at ambient temperatures. After extensive investigation of the optimal conditions for thermal stabilisation and activation of clays and soils, the optimal process finally developed involved heating a fine-grained soil to a temperature high enough to destroy its water sensitivity, adding finely divided iron oxide, and then introducing a sodium silicate and/or sodium hydroxide solution. After compaction the mixture sets into a hard material. The products responsible for this stabilisation have been shown to be strong, durable, and water resistant. They have been identified by X-ray and electron probe as a series of complex iron silicates (having various substitutions); five of the major compounds responsible for the stabilisation appear to be hitherto unreported in the literature. Although the reaction mechanism has not yet been fully elucidated, it seems possible that it will provide important insights into the practicability of achieving accelerated laterisation in clay soils, and permit the development of a range of new durable and strong products based on ordinary soil rather than pure clay minerals; since the method of preparation appears to be insensitive to many natural impurities, and the quality of product may even be improved thereby.