Chemical reactivity of zeolitized volcanic waste materials from Sardinian quarries in the production of eco-friendly pozzolanic mortars for their sustainable recovery

The aim of this study is to verify the chemical reactivity between calcium hydroxide Ca(OH)₂ and 26 volcanic rocks from different quarry wastes and outcrops of Sardinia to assess their potential use as pozzolans in mortar production and their sustainable recovery in construction materials field. The rocks were analysed from a chemical-mineralogical-petrographic point of view using transmitted light optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), multiple thermal (TGA–DTA) analyses. In addition, petrophysical (porosity, real and bulk densities, water absorption, etc.) and mechanical tests (compressive, flexural and Point Load strengths) were determined on the experimental mortars made up with the more reactive selected pozzolans. The volcanic rocks are characterised by a highly variable content of glass (from 5 % to 99 vol%) that strongly influences the chemical reactivity with lime, and by the presence of zeolites that may affect the lime-reaction. The results show that the pozzolans here studied exhibit highly variable reactivity with hydrated lime, as function on their compositional and microstructural features. The reactivity of pozzolans is primarily influenced by (i) weight fraction (variable between 5 % and 99 vol%), microstructure and hydration degree of the amorphous matrix, (ii) microporosity and specific surface area; and (iii) chemical-mineralogical alteration of the rock. The most reactive samples have a glassy microstructure with characteristic concentrically trending micro-cracks, or a diffuse microporosity made up by interconnected pores (mainly <10 microns in size). Samples with intermediate or poor reactivity contain moderate to low fractions of glass or exhibit significant mineralogical variability. Rock alteration negatively affects reactivity by forming secondary phases, which reduce the glass-to-mineral ratio. Furthermore, the presence of zeolites, clay-minerals, dolomite and calcite in both pyroclastites and perlitic facies, negatively affects the reactivity. Among the perlites samples, those with a devitrified matrix and secondary minerals (e.g., Ca-erionite) show particularly low reactivity. This study confirms that several volcanic lithologies in Sardinia can be exploited as raw materials for the production of pozzolanic mortars, thanks to their low production cost. In the cement industry, highly reactive pozzolans can (i) bond with portlandite produced during Portland cement hydration (ii) reduce clinker production, thereby lowering gas emissions, and (iii) decrease binder production costs by partially using low-cost raw materials from quarries or industrial waste. • High glass/crystallinity ratio and microcraks favors pozzolan-lime reactivity. • Higher surface area of pyroclastic pozzolan pumice increases reactivity with lime. • Medium-low glass and/or mineralogical variability negatively affect the reactivity. • Glass hydration produces micro-cracking and microporosity (and thus the reactivity). • Glass hydration triggers alteration and new secondary phases hindering reactivity.