Double-life sustainable construction materials from alkali activation of volcanic ash/discarded glass mixture

Volcanic ash, according to the large amount of silica and alumina, may be considered as feedstock for geopolymers. However, the relatively low reactivity, mostly due to the relatively low amount of amorphous phase, implies the introduction of ash as minor component in complex mixtures and the activation with highly concentrated alkaline solutions. This paper aims at improving the sustainability of ash conversion into inorganic polymers with adequate strength-to-density ratio, by minimizing the addition of valuable compounds and including discarded material. Fine powders of volcanic ash from Mt Etna (Italy), in fact, were activated with NaOH solutions at low molarity (3 M), with a variable water/solid ratio (0.35–0.42), after mixing with waste glass powders, from cullet purification. The adopted ash/glass proportion (50 wt%-50 wt%) was intended to favour the reuse of inorganic polymers, by firing at 950 °C, in turn causing the transformation into porous glass-ceramics with a remarkable strength-to-density ratio. A significant foaming was effectively observed, due to decomposition of hydrated alkali alumino-silicates developed upon hardening. Foams with excellent strength-to-density ratio were also obtained by thermal transformation of highly porous cold consolidated materials.