Modelling approach to the prediction of Equilibrium Phase Distribution in Slag-Cement Blends and their Solubility Properties.

Abstract

AbstractBlended cements containing mixtures of granulated blast-furnace slag (BFS) and Portland cement give low permeability matrices with initially favourable leach characteristics. Their retention arises from a combination of physical and chemical effects which include high pH and sorption. It is not practical though, in either laboratory or site-based experimentation, to determine changes in matrix chemistry over long timescales and the development of realistic models for long-term property predictions therefore becomes increasingly important. This paper pursues the development of such a model enabling changes in chemical and mineralogical balances during ageing to be predicted.Phase development is assessed in the system CaO-Al2O3-SiO2-MgO-H2O. In the relevant composition range, the phases occurring include crystalline hydrates: portlandite, (Ca(OH)2); gehlenite hydrate, (2CaO.Al2O3.SiO2.8H2O); a hydrotalcite-structured phase (nominally 6MgO.Al2O3.(OH)x.yH2O). an AFm type phase, (nominally 4CaO.Al2O3 SO3.12H2O); and a poorly crysiallised c~lcium silicate hydrogel, C-S-H. All five phases are observed to occur together in slag-cements which are still hydrating. Given, as input, the chemical analyses of both the slag and the cement, and the initial blending proportions, the model predicts the equilibrium distribution between the five components and additionally, the Ca/Si ratio of the C-S-H. The aqueous chemistry in the system is predicted from the calculated phase distribution and appropriate solubility products.