Structural properties of volcanic precursors-based geopolymers before and after natural weathering

The structural properties of geopolymers based on locally available volcanic feedstocks were here investigated in the optics of the development of sustainable non-structural building materials. Geopolymer binders and mortars based on two volcanic pyroclastic residues from Mt. Etna (ejected ash and a paleosoil, named “ghiara”) were studied to assess the effects of natural weathering. The strength and durability of the resultant products were examined before and after six months of outdoor exposure by comparing ultrasound pulse velocity (UPV), specific weight, Brazilian Disk test (BD) and Digital Image Correlation (DIC) together with Dynamic Vapour Sorption (DVS) results. DVS data were also compared to those of pure salts (e.g. sulphates and carbonates) commonly occurring in geopolymers as efflorescences. The results obtained on unexposed and exposed samples and the relationship between the moisture transportation, efflorescence development, and structure degradation have shown a better response of the volcanic ash-based geopolymers to weathering in the hot summer Mediterranean climate zone than ghiara-based products. The latter are more affected by exposure conditions as revealed by the decrease of UPV, the influence of efflorescences on moisture adsorption-desorption curves and the consequent worsening of mechanical performances. The correlation between UPV and DVS applied for the first time to the study of volcanic precursors-based geopolymers exposed to natural weathering has proven to be an effective and useful tool for assessing the durability of materials.