Preliminary study on valorization of scraps from the extraction of volcanic minerals

Powders < 3mm of pumice and lapillus (quarry scraps of national volcanic minerals) were employed in the tailoring and characterization (from the physical, chemical and mechanical point of view) of geo-polymers and lightweight aggregates. Geo-polymers were obtained at room temperature by substituting 70 - 80 wt% of metakaolin by volcanic scraps and employing an alkaline solution with a Na2SiO3/NaOH ratio from 0.8 to 1 for pumice and lapillus series, respectively. Within 24 h and in water, bulk specimens resulted with good integrity, with pH values around 9-11 and conductivity increasing over time, but less for the lapillus-containing sample richest in metakaolin indicating more compactness. The porosity, ranging around 32-33% for all the samples, increased up to 45% for the formulation based on 80% of lapillus. The best mechanical performance was achieved by lapillus samples: compressive strength in the range 35-38 MPa against 6-8 MPa of pumice ones. Lightweight aggregates were created by powder sintering at 1000 °C for 1 hour of 85 wt% of volcanic scraps and 15 wt% of spent coffee grounds used as pouring agent. Additional formulations were realized adding 50 wt% of nourishing mixture P and K-containing in the form of animal bone meal and vegetable biomass ashes. The specimens resulted porous and light (porosity around 60%), with good capacity of water retains, and, except in two cases, with neutral pH and conductivity values below 2 mS/cm, indicating a possible use for substrate (growing layer) in roof gardens, green roofs, house gardens, etc..