The utilisation of reactive magnesia or quicklime as novel activators for slag offers a range of technical and environmental benefits over conventional caustic alkali activators and showed great potential in soil stabilisation. This paper investigates the mechanical and microstructural properties of two model soils, i.e., a clayey soil and a slightly silty clayey sand, stabilised by ground granulated blastfurnace slag (GGBS) using various techniques including unconfined compressive strength (UCS) test, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). A number of MgO and CaO mixtures with different MgO/CaO ratios were adopted for slag activation. The activator to GGBS ratio was 1:3 and the dosage of the binder (including MgO, CaO and GGBS) was 12% by weight of the dry soil. The result demonstrated that the increasing MgO/CaO ratio in the binder led to an increase in the UCS of the stabilised clayey soil up to 90 days, due to the increased homogeneity of C–S–H gel structure, the decreased Ca/Si ratio of C–S–H gel and the increased amount of voluminous hydrotalcite-like phases. On the other hand, slag activated with MgO–CaO mixtures showed poorer mechanical performance than slag activated with either MgO or CaO alone for sand stabilisation. In addition, strength enhancement was observed for the stabilised clayey soil upon different soaking conditions up to 7 days. After 28 days, although binders with higher MgO/CaO ratios showed slight strength degradation upon soaking, they still exhibited higher strength than those with lower MgO/CaO ratios.