The influence of MgO addition on the performance of alkali-activated materials with slag-ice husk ash blending

This paper investigates the effects of adding magnesium oxide (MgO) on the engineering properties and microstructure of alkali-activated slag (AAS)-rice husk ash (RHA) paste that has been produced using an aqueous mixture of sodium hydroxide and sodium silicate as the crucial activator. Three percentages (20%, 30% and 40%) of RHA as partial replacement of slag and four percentages (2.5%, 5%, 7.5% and 10%) of MgO in the binary mixture of slag and RHA were used to manufacture the MgO-modified, alkali-activated slag-RHA mixture (AASR). These mixtures were then compared with the reference AAS, which was manufactured without RHA and MgO. Experimental results showed that the higher RHA and MgO levels caused a negative effect on the workability of AASR fresh samples. The 20% RHA percentage samples exhibited the best engineering properties of all of the samples and superior engineering properties to the reference AAS. Moreover, adding 7.5% MgO significantly improved the mechanical properties and thermal conductivity of the AASR paste samples. Finally, microstructural examination using X-ray diffraction (XRD), scanning electronic microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) identified hydrotalcite-like phase (Ht), C-S-H as the primary hydration products of the MgO-modified AASR, included with cristobalite, and un-hydrated MgO.