Thermodynamic Modeling of the Effects of Wollastonite-Silica Fume Combination in the Cement Hydration and Sulfate Attack

Sulfate attack is a series of physico-chemical reactions between hardened cement paste and sulfate ions. Sulfate ion penetration into the hydrated cement results in the formation of voluminous and deleterious phases such as gypsum and ettringite which are believed to cause deterioration and expansion of concrete. Concrete deterioration due to sulfate attack depends on many parameters, however, in experimental studies, the implementation of the parameters and obtaining the results in a short time are too difficult. In this paper the effect of wollastonite, with and without silica fume, on the performance of cement based materials during hydration and magnesium sulfate attack was studied by thermodynamic modeling. Thermodynamic modelling was carried out using the Gibbs free energy minimization program GEMS. By this method, in addition to investigating the type and volume of the produced material, the optimal substitution percentage of wollastonite and silica fume were studied as well. In sulfate attack, especially at higher percentages of substitution, wollasonite is not very effective in itself. Wollasonite replacement has a reverse effect on monosulfate and ettringite phases. Volume of these phases increases with addition of the substitution percentage. Substituting a portion of the cement with wollastonite and silica fume would improve sulfate resistance. Substitution of 5% of wollasonite and 10% of silica fume has shown the best performance, highest increase in C-S-H gel volume and reduction in harmful phases such as gypsum, ettringite and brucite.