Expansion of CEM I and slag-blended cement mortars exposed to combined chloride-sulphate environments

This study investigates the effects of specimen curing duration, temperature, and slag composition on expansion of CEM I and composite slag-cement mortars exposed to a combined NaCl and Na2SO4 solution for up to 664 days. Test prisms prepared at 0.5 w/b ratio, were wet-cured for either 7 or 28 days prior to submersion in a combined salt solution at temperatures of 20 or 38 °C, to simulate temperate or warm tropical climates respectively. Equivalent reference specimens were stored in saturated limewater at 20 °C and tested in parallel. Mortar samples were used to investigate expansion and sorptivity, while corresponding paste specimens were prepared, cured and exposed under similar conditions for chemical and microstructural investigation. Such characterisation was performed on specimens immediately prior to exposure to salt solution and after the onset of expansion. The results show significant resistance to sulphate-induced expansion for specimens cured and exposed at 38 °C. For slag blends, the influence of exposure temperature was found to be more pronounced than curing duration. Differences in slag composition and curing duration also played key roles on the expansion resistance of mortar specimens. Expansion was attributed to the formation of ettringite crystals due to the reaction of aluminate phases of the binders with sulphate ions, although Friedel's salt and Kuzel's salt were also formed. The presence of chloride mitigated sulphate expansion of CEM I. For slag blends, it was shown that sulphate expansion was significantly reduced with increasing slag contents.