Reaction kinetics of dolomite and portlandite

The reactions of dolomite with portlandite have been studied at well-spaced temperatures between 20 degrees C and 85 degrees C using XRD Rietveld, TG and SEM methods. The results show quantitatively the faster rates of reaction at higher temperature. Isothermal reaction has, at most, a short induction period after which the degree of reaction is linear with time. An exponential equation is presented to predict the initial rate of reaction as a function of temperature. The microstructure shows that large equant calcite crystals, up to 30 mu m, develop on the rhomboid cleavage surfaces of dolomite. Pressed pellets of dolomite develop calcite at both exterior surfaces and in the interior, where calcite develops as a secondary phase but is not morphologically well-formed crystals. Pellets remain coherent in the course of reaction. The porous pellets have much unfilled space inside and often exhibit portlandite agglomerates with development of Hadley grain structures at the sites of former dolomite crystals, especially when reacted at lower temperatures. The reaction is not topotactic but requires dissolution, transport, nucleation and growth. Transport occurs often over considerable distance relative to the sizes of crystals. (C) 2014 Elsevier Ltd. All rights reserved.