Effect of Reactive Magnesia on the Properties of Alkali Activated Slag Cement Pastes

The effect of different proportions of different reactive magnesium oxides (MgOs) on the compressive strength and drying shrinkage of alkali activated slag pastes (AASPs) has been investigated. The slag was activated by 6 wt. , % of sodium hydroxide (SH) and liquid sodium silicate (LSS) at a ratio of 33 wt. , %. The MgOs with different reactivities were produced from calcination of hydromagnesite at different temperatures (550, 1000, and 1250C). The results show that, the crystallinity of MgO increases with the calcination temperature, therefore its reactivity decreases with temperature. The highly reactive MgO550 accelerates the hydration and consequently increases the early compressive strength of AASPs, while the low reactive MgO1250 had little effect. The acceleration of AAS-MgO hydration at early ages is mainly due to the exothermic hydration reaction of MgO. The replacement of GBFS by 5 wt. , % of low reactive MgO1250 significantly improves the compressive strength at later curing ages. The replacement of slag by 5, 10 and 15 % of MgO550 reduces the drying shrinkage of AASPs by ~40, 57 and 77 %, respectively after 90 days. Also, the drying shrinkage of AAS is reduced by replacement of slag with different reactive MgOs. The MgO reacts with slag to form hydrotalcite like- phases (Ht) as detected by XRD, FTIR and TG/DTG techniques.