A KINETIC AND MECHANISTIC STUDY OF THE DEHYDROXYLATION OF CALCIUM HYDROXIDE

A series of complementary kinetic studies of Ca(OH)2 dehydration showe d that the reaction is deceleratory throughout and that isothermal yie ld-time data were satisfactorily described by the first order equation . Arrhenius parameters and relative reaction rates are sensitive to th e reactant mass and its compaction/dispersal within the reaction zone. Rates of water evolution are significantly influenced by the pressure of water vapour prevailing, the local concentrations of water vapour within the reactant mass and the rates of intracrystalline and intercr ystalline escape of water vapour from the reactant assemblage. The the oretical significance of the present observations are discussed in the context of electron microscope observations and the extensive literat ure already available concerned with this reaction. We formulate a rea ction model that takes due account of previous proposals, for homogene ous or for heterogeneous-type mechanisms. We conclude that reaction ad vances inward from initial crystal boundaries but that water eliminati on is by diffusive loss from within an extended zone of maintained rea ction crystal structure. Dehydration does not occur at a sharp reactan t-product interface, unlike many solid state processes, and the kineti c characteristics are more deceleratory than the requirements of the c ontracting volume rate expression. Water loss is not closely followed by recrystallization; these steps are separated in space and time.