This paper investigates the potential of combining Na2CO3 and reactive MgO as a sustainable activator for ground granulated blastfurnace slag. Two very different reactivity MgOs were added at 5–10 % and the Na2CO3 content varied from 4% to 8% by the weight of slag. The strength and drying shrinkage of the activated slag pastes were tested up to 90 d. It was found that the optimum reactive MgO addition was 5% regardless of the MgO type and Na2CO3 content. MgO with the higher reactivity significantly increased the early strength of the paste but had almost no effect on the strength at 90 d. On the other hand, the effect of the lower reactivity MgO on the strength was more profound at later ages and low Na2CO3 dosage. In terms of drying shrinkage, increasing the Na2CO3 content from 4% to 6% caused a remarkable decrease of drying shrinkage while increasing it from 6% to 8% had negligible effect. X-ray diffraction and thermogravimetric analysis revealed that the major hydration products were calcium silicate hydrate gel and hydrotalcite-like phases, similar to those in conventional alkali-activated slag cements. There was also a large quantity of calcite formed especially in the 8% Na2CO3 pastes due to causticisation. It was concluded that the combination of reactive MgO and Na2CO3 could serve as a potential sustainable activator for slags.