Transformation of Clay Minerals by Calcium Hydroxide Attack

Abstract
Calcium hydroxide was allowed to react with various clays, other silicates, and quartz at slightly elevated temperatures for several months. The reaction products were examined by X-ray diffraction, DTA, and electron microscopy, and were shown to be poorly-crystallized calcium silicate hydrates of the tobermorite family, and calcium aluminate hydrates. Quaternary phases were not detected, but some isomorphous substitution probably occurred. The extent of reaction was shown to be such that under appropriate conditions almost all of the clay mineral was decomposed. Electron micrographs of the reacted materials indicated that attack occurred from the edges of the particles, and in general the remaining unattacked portion of the clay did not suffer appreciable loss of crystallinity. It was postulated that the reaction involved progressive dissolution of the mineral at the edges of the particles in the strongly basic environment maintained by calcium hydroxide solution, followed by separate precipation of the reaction products.In these experiments the calcium silicate hydrate generated by the reaction between lime and quartz was uniformly calcium silicate hydrate gel (CSH (gel)); reaction with kaolinite and montmorillonite produced either CSH (gel) or calcium silicate hydrate (I) (CSH (I)) depending on the conditions of the reaction. At 60°C the alumina-bearing phase was tricalcium aluminate hexahydrate; at lower temperature the phase produced was a hexagonal material closely resembling 4CaO.Al2O3.13H2O (C4AH13) but retaining a constant 7.6Å basal spacing regardless of its state of wetness or dryness.It was found that under appropriate conditions the formation of CSH(I) resulted in as effective a cementation as did the formation of CSH (gel).

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