This paper reports results of strength and reaction products of blast-furnace slag cements activated using a mixture of MgO-NaOH with a ratio 50/50 incorporated at 4, 6 and 8% relative to the slag mass. Pastes and mortars were prepared and cured at 20 °C and 60 °C-24 h-20 °C up to 90 days. The results indicated that the combination of MgO-NaOH acts as an effective alkaline activator of the slag, as samples cured at 20 °C developed 54 MPa at 90 days, while those at 60 °C-24 h-20 °C had a rapid strength gain at early ages but in the long run strengths of ~42 MPa at 90 days were observed. The microstructures consisted of (C-(A)-S-H), third aluminate hydrates and hydrotalcite. The formation of the latter increased at higher MgO:NaOH concentration lowering the Al uptake by C-S-H. MgO and Mg(OH)2 were not detected, indicating an effective incorporation of Mg2+ ions into the atomic structure of the reaction products.