Effect of high MgO content on the performance of alkali-activated fine slag under water and air curing conditions

The present study investigates the effect of magnesium oxide (MgO) on the respective performance of water- and air-cured alkali-activated fine slag (AAS). Ground granulated blast furnace slag (GGBFS) of two different fineness levels were used to produce two AAS mixtures. These mixtures were alkali-activated with sodium hydroxide and sodium silicate and prepared into samples by adding, respectively, 2.5%, 5%, 7.5%, 10% and 15% MgO by total binder weight. A series of tests, including slump flow, setting time, compressive strength, X-ray diffraction (XRD), scanning electronic microscopy (SEM) and thermogravimetric analysis were conducted in accordance with the relevant standards. In terms of findings, the addition of MgO promoted the hydration of AAS significantly at early curing ages. Optimum MgO content was 7.5% for slag-4000 specimens and 5% for slag-6000 specimens. Cracks were observed in the water-cured AAS samples, with greater cracking at higher levels of MgO content. Moreover, the results show that the main contribution of MgO is in the hydrotalcite-like phase (Ht), where it adds volume, which improves strength the AAS paste. The effect of MgO was significantly increased in the samples that used finer-particle GGBFS. Adding MgO contributed to the more formation of hydration products, which was observed in the present study using X-ray diffraction and SEM and derivative thermogravimetric analysis.