Towards common specifications for low- and high-expansion cement-based materials exposed to external sulphate attacks

Abstract

External sulphate attack involves the penetration of sulphate ions in solution within a cementitious material. The equilibrium conditions are changed, and the ions released by hydration products can react with the sulphate ions and form new compounds. Depending on exposure conditions and cement compositions, expansive minerals, such as ettringite, can precipitate and cause cracking and, eventually, the collapsing of the material. Laboratory tests have been designed in order to qualify cements as sulphate resistant. The response to sulphate attack depending greatly on the cement composition, different types of degradation can be observed, with different severity and kinetics. Thus, it is difficult for a single test and indicator to reflect the wide range of degradation mechanisms. In this study, four CEM I Portland cements (including one sulphate resistant cement), one CEM III sulphate resistant slag cement and two CEM V slag-fly ash cements have been tested and the monitoring data have been analyzed with a new method coupling several indicators (based on length and mass variations and considering leaching) in order to represent as faithfully as possible, the degradation mechanism of the mortar samples. Two main degradation mechanisms have been spotted: one driven by expansion and the other by surface degradation. Three Portland cement-based mortar samples (CEM I cement with aluminate content >5%) went through massive expansion, thus being dismissed as sulphate resistant. On the other hand, the last CEM I cement and the CEM III samples went through slight to no expansion but were continuously affected by leaching. The two CEM V mortar samples had a particular degradation mechanism since they did not crack and mostly lost material through leaching, they showed low but significant expansion.