Hydrothermal, autoclave-type experiments were conducted on water saturated, soft montmorillonite gels. The temperature was 105-200°C and the water pressure 15-30 MPa for 0.5-1 year, the confined samples being cooled and left to rest before XRD tests, compression experiments, and electron microscopy were made. All the samples showed a significant, temperature-dependent strength increase which was attributed primarily to a permanent change in microstructure. It had the form of heat-induced regrouping of expanded montmorillonite stacks at temperatures in the range of 100-150°C as concluded from electron microscopy, which also revealed the formation of amorphous silica and cristobalite at 200°C. This suggests that silica precipitation contributes to the strength increase at high temperatures. The actual mechanism of silica release and migration may be explained by considering Forslind's and Jacobsson's montmorillonite lattice forms for 'low' and 'high' temperature. If there actually is a transition between such forms, it is concluded to take place at a significantly higher temperature than 100°C and it may involve permanent elimination of the expandability. Also, it may result in the lattice charge change that is a prerequisite for alteration of smectite to hydrous mica.
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