Activation of Metakaolin/Slag Blends Using Alkaline Solutions Based on Chemically Modified Silica Fume and Rice Husk Ash

This study describes the use of alkaline silicate solutions produced by mixing silica fume (SF) or rice husk ash (RHA) with aqueous NaOH, as alternative silica-based activators for metakaolin (MK)/slag (GBFS) blended binders. Pastes prepared with these activators show similar trends in mechanical strength development as a function of activation conditions compared with the pastes obtained using commercial silicate solutions as activator. All activating solutions promote higher compressive strength development with increased contents of GBFS in the binders, which promotes the coexistence of aluminosilicate reaction products along with calcium silicate hydrate gel. Higher-silica binding systems prefer a higher GBFS content for optimal strength development compared to those with a lower overall SiO2/Al2O3 ratio. SF-derived activators give reaction products which are very similar to those obtained using commercial silicate solutions, as a consequence of the high reactivity of this precursor, supplying high concentrations of Si to the systems since the early stages of reaction. RHA-derived activators appear to have slightly delayed Si availability due to the less-reactive character of this precursor, which influences the relative rates of formation of the two types of gel in blended systems. These results show that activation of GBFS/MK blends with by-product derived silicate-based activators can generate mechanical strengths and structures comparable to those obtained using commercial silicate solutions.