Reactive magnesia (MgO) and carbide slag (CS) were used to activate ground granulated blastfurnace slag (GGBS) to stabilise a marine soft clay, and the stabilised clays were subjected to accelerated sodium sulfate attack. The results indicated MgO-GGBS stabilised clay was nearly inert to this sodium sulfate solution. The resistance of CS-GGBS stabilised clay to sodium sulfate attack was lower than that of MgO-GGBS stabilised clay, but higher than that of Portland cement stabilised clay. Unlike CS-GGBS stabilised clay, there was no calcium aluminate hydrate or alumino-ferrite monosulfate produced in MgO-GGBS stabilised clay, and hence no ettringite, which would cause deterioration, formed when exposed to sodium sulfate.