Alkali-fusion as an effective method for activating low-reactivity silicate wastes: A comparative study

Abstract

Silicate wastes are used as supplementary cementitious materials (SCM) in Portland cement production, and as precursors to produce alkali-activated materials such as geopolymers to lower the environmental impact of construction. The high crystallinity of some of these wastes lowers their reactivity and hence the production of cementing hydrates. This paper includes a comparative analysis of the efficiency of alkali-fusion to activate three distinct types of silicate waste sourced from diverse sectors. The paper underscores the robustness of this method and its applicability in a wide composition range. It provides a solid foundation for the application of alkali-fusion across a wide spectrum of industrial waste types. Fly ash (FA), crop bottom ash (CBA) and aluminium processing sludge (APS); were fused with NaOH at 600 °C. XRD and FTIR analyses revealed that alkali-fusion disrupts highly cross-linked Si-O-Si structures transforming them into more reactive phases (sodium aluminosilicate and sodium silicates). Alkali-fusion is efficient even at low NaOH dosage(15 %). 15 % NaOH-fusion increased the mechanical activity index of FA, CBA and APS by 108.19 %, 494.61 % and 27.96 % respectively. The mineral composition of the wastes changed at 50 % alkali content: quartz was largely digested, and sodium silicate emerged as the dominant phase. After alkali-fusion, dense and compact matrices with rich hydrates were found evidencing the enhancement of the pozzolanic reaction. Alkali-fusion at relatively low temperature (600 °C) is successful in activating low-reactivity silicate waste, transforming the waste into valuable SCMs.