Thermodynamic modeling of sulfate attack on hardened cement paste containing slag and investigation of the effect of various sulfate solutions

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. However, there is no direct relationship between Ettringite or solids formation during sulfate attack and amount of expansion. 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. Therefore, the significance of theoretical and software modeling along with experimental studies, reducing the time and cost, increases insight of the obtained results. Thermodynamic modeling, in this paper, is employed according to the method of minimizing Gibbs free energy in order to have better understanding of the external sulfate attack on the cement paste; It is done by the GEM software which is able to calculate the stable phase as a function of reactants, temperature and pressure.
Type and volume of phases formed during the sulfate attack and factors affecting that such as sulfate solution with different concentrations were studied using the thermodynamical modeling. Hardened cement paste at 20 ° C and water-cement ratio of 0.5 were assumed in the modeling. Potassium/Sodium/Magnesium sulfates, among other sulfate solutions, with concentrations of 4, 44, 50, and 60 grams per litre were used. Since the slag systems generally exhibit appropriate behaviour in the high level of substitution, slag substitution up to 80 percent is examined. A replacement rate of slag lower than 40 percent, due to the increase of deleterious phases such as monosulfate and ettringite, is not optimum, however, the function of the slag replacement rate above 60% is satisfactory. According to the type and volume of formed products in sulfate solutions, we can conclude that magnesium sulfate solution has higher deterioration effect. Generally, the results correspond to existing studies agree with the experimental results.