Phase Relations and Solubility Modelling in the Cao- SiO2- Al2O3- MgO- SO3-H2O System: For Application To Blended Cementss.

Abstract

ABSTRACTCement blends are likely to form a large fraction of the materials in the near field of radioactive repositories. Being chemically reactive and slightly soluble, their effect on near field chemistry will be very marked. Predictions of nuclide migration in this zone will therefore depend in part on accurate modelling of cement chemistry.The models being developed require the establishment of correct phase relations in the Cao- SiO2- Al2O3- MgO- SO3-H2O system (the major cement components). More than 40 compatibility experiments were carried out to determine these relationships. Purified cement hydrates were shaken continuously in CO2- free water, for 10 weeks. The results from these experiments together with data from the literature, enabled us to construct a better validated phase relations scheme than has hitherto been possible.Interpretational problems were encountered. Thus, mixtures containing Ca(OH)2, gave rise to the precipitation of metastable phases. This is due to special dissolution/ precipitation equilibria obtaining at early age, allowing ‘AFm type’ (4CaO.Al2O3.SO3.12H2O) phases to form, where C3AH6 (3CaO.Al2O3.6H2O) and AFt (6CaO.Al2O3.3SO3.32H2O) are thermodynamically favoured.