Reactivity of volcanic ash in alkaline medium, microstructural and strength characteristics of resulting geopolymers under different synthesis conditions

The present paper investigated the reactivity of volcanic ash in alkaline medium and reported the effect of synthesis conditions on microstructural and mechanical properties of volcanic ash-based geopolymers. The reactivity of volcanic ash was carried out by leaching it under different NaOH concentrations (8, 10, and 12 M) and temperatures (27, 60, and 80 °C), and chemical composition of the filtrate measured by ICP-OES. The influence of silica modulus (1.4, 1.5, and 1.66) and curing temperatures (27, 60, and 80 °C) on microstructural characteristics of the resulting geopolymers was assessed by Calorimetry test, XRD, FTIR, ²⁷Al and ²⁹Si MAS-NMR, TGA, and FESEM–EDX. The dissolution behavior of Si is influenced by NaOH concentration, while Al is more sensitive to temperature. The low amount of dissolved species is correlated with the low amount of heat released. XRD, FTIR, ²⁷Al and ²⁹Si MAS-NMR, and TGA have shown the evidence of changes occurred during geopolymerization of volcanic ash and the extent of reaction with varying silica modulus and curing temperature. FESEM and EDX have shown that geopolymers obtained are poly(ferro-sialate-siloxo), poly(ferro-sialate-disiloxo), and poly(ferro-sialate-multisiloxo) binder types with Ca²⁺, Mg²⁺, and Na⁺ as charge-balancing cations. These structures are irrespective of silica modulus and curing temperature. The curing temperature is the main factor affecting the early compressive strength.