Influence of initial material properties on the degradation of mortars with low expansion cements subjected to external sulfate attack

Abstract

External sulphate attack consists in reactions between sulphate ions and hydration products causing crystallisation pressures then macroscopic expansion and cracking. The penetration of sulphates within the material is the rate-limiting step. Thus, laboratory tests can last a few years before giving usable results. Moreover some cement-based materials show relatively long latency period and low expansion, hence the need for accelerated testing procedure and/or alternative indicator to shorten the response time. It is possible to shorten the response time by stimulating the penetration of sulphates. This can be achieved by drying the material at 60 °C then to saturate it with a sulphate solution. The impact of this thermal pre-conditioning on the sulphate attack phenomenology still remains unclear. In this study, two sulphate-resisting cements, one Portland cement with low-C3A content (SR3) and a CEM III/B cement, and two blended slag-fly ash cements CEM V were subjected to external sulphate attack. The behaviour of mortars with and without thermal pre-conditioning was analysed. The microstructure of tested specimens was also studied using TGA, Hg-porosimetry, X-ray tomography, and XRD. Pre-conditioning did not influence the chemical properties of formed sulphate-rich phases but modified the microstructure of mortars with different trends depending on cement type. CEM V mortars with pre-conditioning showed finer porosity and lower volumes of precipitation. These modifications of mortar microstructure were consistent with the macroscopic behaviour of mortars with and without thermal pre-conditioning.