Sustainable stabilization of waste foundry sands in alkali activated glass-based matrices

Abstract

Since cements and concrete are highly energy-demanding, sustainable construction materials represent a crucial aspect in mitigating environmental pollution. The study proposes a promising solution for transforming waste into new and highly sustainable construction materials according to alkali activation process. The unemployed fractions of boro-alumino-silicate pharmaceutical glass (BASG) and waste foundry sands (WFS) were employed in the production of synthetic conglomerates. Glass powders were firstly suspended into ‘mild’ NaOH/KOH aqueous solutions (2.5–5 M) for 180 minutes at 500 rpm, at room temperature or at 40°C. Conglomerates were later produced by embedding of foundry sands in slurries of alkali activated glass, for BASG:WFS ratios of 1:1, 1:2 or 1:3. The mass of sand embedded is a function of the grain size distribution. A final drying phase, lasting 7 days at 40°C, promoted the consolidation of the newly-formed materials due to the formation of strong siloxane bonds. The obtained conglomerates were mechanically tested under compression. Interestingly, the composites exhibited a compressive strength-to-density ratio comparable with commercial insulating lightweight concrete and Plaster of Paris. Moreover, the leachates from conglomerates were subjected to optical emission spectroscopy in inductively coupled plasma (ICP-OES) to verify the stabilization of hazardous elements. The methodology selected for the stabilization of pollutants is compliant with UNI EN 12457–2:2002. The results show that the release of critical pollutants (Ba, Cr, Cd, Co, Mo, Ni) was maintained below the European limits. The research highlights the possibility to manufacture conglomerates starting from waste raw materials. Furthermore, stabilizing activity towards hazardous elements is achieved by all composite materials.