Sulphate attack in slag-blended cementitious materials hydrated with sodium sulphate

Abstract

Deterioration of cementitious materials co-hydrating with highly concentrated sodium sulphate is a major durability problem for the use of the materials in low- or intermediate-level nuclear waste repositories. The sulphate attack is a physicochemical interaction of sulphate ions and cement matrix. Understanding the underlaying mechanisms and influencing factors are important to evaluate the long-term performance of the materials in the nuclear waste repositories. In this study, Portland cement and slag-blended cementitious materials in which the cement is replaced by 42% of slag were hydrated 13% of Na2SO4 by weight. The hydrated samples having diameter between 2.38 mm and 4.75 mm were immersed in water, sodium and magnesium sulphate solution to evaluate its degradation. Solid phases in the samples before and after the exposure were quantified by XRD/Rietveld analysis and the results were compared with thermodynamic equilibrium model. The model prediction agrees with experimental data except U-phase and ettringite. The experimental results show that the U-phase is not stable in exposure solution and it is changed to ettringite, but the model could not predict it. The effect of solution composition, the type of associated cation, and the presence of slag and its replacement ratio on the experimental and simulation results were discussed.