Development of a durability indicator to forecast the efficiency of preventive measures against external sulphate attack

Currently, the C3A content of binders is considered the most important factor contributing to external sulphate attack (ESA) deterioration. However, portlandite is also deemed to play a major role in ESA development. Yet, there are very few researches on this topic. This paper evaluates physical (i.e., induced expansion and mass variation, ultrasonic pulse velocity, dynamic modulus of elasticity, modulus of rupture and compressive strength) and chemical (i.e. X-ray diffraction and thermogravimetry) properties of seven mortar mixtures presenting distinct binders (i.e. cement types, inert fillers and supplementary cementing materials) and exposed to different sulphate solutions (i.e. sodium and magnesium). Correlations are conducted between data obtained in the laboratory, and a theoretical approach to describe cementitious mixtures' susceptibility against ESA is then proposed. Results show that the proposed durability indicator (i.e., predicted portlandite amount and potential of ettringite formation) are well correlated with ESA-induced expansion and damage. Moreover, the influence of portlandite on ESA seems to depend on the type of sulphate attack (i.e., Na2SO4 or MgSO4). Finally, highly reactive SCMs and consequent higher portlandite consumptions seem to increase the overall deterioration due to MgSO4 exposure.