Influence of internal curing and viscosity modifiers on resistance to sulfate attack

Sulfate attack is one of the common degradation mechanisms for concrete in severe environments. While various strategies for minimizing sulfate attack are well recognized, including using an ASTM C150 Type V cement, employing supplementary cementitious materials, and/or reducing water-to-cementitious materials ratio, this paper explores two new approaches for increasing a mortar’s resistance to sulfate attack. In internal curing, fine lightweight aggregates (LWAs) are pre-wetted to provide additional curing water to maximize cement hydration and enhance the microstructure of the interfacial transition zone. The concurrent reductions in connected porosity should contribute to a reduction in the transport rates of sulfate from the environment into the concrete, while the isolated pores present in the LWA may help to accommodate the formation of expansive degradation products, such as ettringite, without creating substantial stresses and subsequent cracking. In the second approach, previously verified for its efficacy to reduce chloride ingress, a viscosity modifier is added to the concrete mixture to increase the viscosity of the pore solution and thus slow down the ingress of sulfates from the environment. While each approach is observed to significantly reduce the measured expansion of mortar bars in standard ASTM C1012 testing, the best performance is observed when the two are combined together by pre-wetting the LWA with a 50:50 solution of the viscosity modifier in water. With the combined approach, the time for the mortar bars to reach a critical expansion level of 0.05 % was over 80 % longer than that measured for the control mortar specimens. The expansion measurements are supported by accompanying measurements of mortar bar mass and surface resistivity throughout their exposure to the sulfate solution, along with micro X-ray fluorescence imaging and X-ray microtomography analysis of specimens extracted from the mortar bars after 9 months of exposure to the sulfate solution.