Both reactive MgO and CaO are alternative activators for ground granulated blast furnace slag (GGBS). In this study natural dolomite, as a source of MgO and CaO, was calcined at 800 °C (D800) and 1000 °C (D1000) in air. The activation of GGBS with the calcined dolomites was investigated using compressive tests, pH measurement of pore solutions, powder X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The results indicated that both calcined dolomites can effectively activate GGBS. D800 showed a relatively slower acceleration to GGBS than D1000 and both were slower than CaO. The use of D800 produced similar compressive strengths as did D1000 after 7 days curing but lower strengths at later ages. By increasing the dosage of activators, significantly higher strengths were obtained using D800 while only small increases were observed using D1000. The detected hydration products by XRD and TGA were mainly C–S–H and hydrotalcite-like phases, similar to those from other alkali-activated slags. The comparison to other activators indicated that using calcined dolomite can induce faster hydration of slag than using reactive MgO in the early age while slower than Portland cement in this study.