Alkaline activation of low-reactivity ceramics: Peculiarities induced by the precursors' dual character

Abstract

In the alkaline activation process, precursors with high reactivity are mostly used. If low-reactivity materials are utilized, it is necessary to take into account their dual character, i.e., that only their amorphous portion can react and the remaining crystalline part acts as microfiller. The composite character of obtained alkali-activated materials can then induce some peculiarities which are not observed for geopolymers based on high-reactivity precursors. In this paper, alkaline activation of low-reactivity ceramic grinding dust is investigated. Microstructure and phase composition of the prepared aluminosilicates are analyzed at first, and empirical geopolymer formulas in the form nM2O∙pCaO∙Al2O3∙xSiO2∙yH2O are derived in dependence on activator alkalinity and curing temperature. The dependence of flexural strength upon Al/(Na+½Ca) molar ratio calculated for the amorphous portion of the analyzed aluminosilicates is identified as the most distinct feature characterizing the links between microstructure, composition, and properties. The improvement of mechanical properties is related to the enhancement of matrix-aggregate composite effect and refinement of pore structure due to the incorporation of crystalline substances into the aluminosilicate matrix.