The thermal transformation of montmorillonite clay compacts has been studied in water vapour (0·4 atm) and dry atmospheres by thermal analysis, XRD, and solid state MAS NMR. The water vapour atmosphere decreased the thermal transformation temperature by about 200°C and produced a threefold improvement in the mechanical properties of the fired clay compacts (crushing strength and softening coefficient). These effects are attributed to the increased strength at the contact points between the grains due to polycondensation of silanol groups and may be a general phenomenon in a range of clay mineral types.
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