Ceramics-Silikáty 59, (1) 37 - 47 (2015)

The effect of different proportions and different reactivities of MgO on the drying shrinkage and compressive strength of alkali activated slag pastes (AAS) has been investigated. The slag was activated by 6 wt.% sodium hydroxide and liquid sodium silicate at ratio of 3:3 (wt.). The different reactivities of MgOs were produced from the calcination of hydromagnesite at different temperatures (550, 1000, 1250^oC). The results showed that the reactivity of magnesium oxide decreases with increasing the calcination temperature. Also, the drying shrinkage of AAS was reduced by the replacement of slag with MgOs. The highly reactive MgO accelerated the hydration of AAS at early ages. The replacement of slag with 5% MgO₅₅₀ increased one day compressive strength by ~26 % while MgO₁₂₅₀ had little effect. A significant increase in strength was observed after 7 days in case of replacement of slag with 5 % MgO₁₂₅₀. The MgO reacts with slag to form hydrotalcite likephases (Ht) as detected by XRD, FTIR spectroscopy, TGA/DTG analysis and SEM.