Application of pretreated dealuminated kaolin as a modifier agent for alkali-activated slag cement

In Egypt, the alum industry is increasing rapidly, which leads to an increase in waste resulting from this industry, which was named dealuminated kaolin (DK). Recycling this waste into building materials presents the best solution for its disposal. In this article, for the first time, a study was conducted to investigate the possibility of recycling DK before and after suitable activation (FDK) into alkali-activated slag (AAS) cement. A new method was used to activate DK, which was exposed to 1000 °C for 2 h in the presence of 20 wt% sodium hydroxide (NaOH) to produce an amorphous material (FDK). The slag was partially replaced by different ratios (10-30 wt%) of this waste before (DK) and after activation (FDK). The setting time, compressive strength, and total porosity were monitored. Different methods were used to detect the effect of DK and FDK on the hydration products and microstructures. The results showed that the incorporation of DK into AAS cement prolonged setting time, decreased compressive strength, and increased total porosity due to its crystalline phase, low reactivity, and the increase in silica at the expense of calcium. Contrarily, the inclusion of 10% FDK shortened setting time, increased compressive strength, and decreased total porosity due to its amorphous phase, higher calcium silicate hydrate/calcium aluminate hydrate (CSH/CASH) gel formed, and higher dissolution of slag particles due to the formation of Na2O. The other ratios of FDK showed an adverse effect due to the excessive Na2O which delayed the condensation process and hindered slag dissolution, as well as the excessive silica amount at the expense of calcium.