Utilization of photovoltaic waste glass as fine aggregate in cement mortar: Effects of glass type, nitric acid treatment, and fly ash on alkali-silica reaction expansion and strength

This study investigates the feasibility of using photovoltaic (PV) waste glass as a sustainable fine aggregate replacement for natural sand in mortar by evaluating its alkali-silica reaction (ASR) reactivity and mechanical performance. Two types of waste glass, cover glass and substrate glass, were recovered from copper indium gallium selenide-type PV modules and used to partially replace natural sand at different proportions. The effects of acid treatment, which is commonly applied during PV panel recycling, and fly ash incorporation were also examined. Accelerated ASR expansion tests, compressive strength measurements at 20°C and 40°C, and multi-scale analyses were conducted to assess the dissolution behavior and interfacial reactions of the glass in cementitious systems. Mortars incorporating cover glass exhibited greater ASR expansion than those with substrate glass, with 60% replacement giving the highest value, although all expansions remained below the JIS A 1146 threshold. Both acid treatment and fly ash effectively reduced expansion, especially in cover glass mortars. X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) confirmed higher solubility of cover glass, while acid treatment suppressed alkali and silica dissolution. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX) revealed interfacial products with high Ca/Si and low (Na+K)/Si ratios, indicating low swelling potential. Compressive strength was enhanced by pozzolanic densification at the glass-cement interface, particularly for untreated cover glass under elevated curing. However, fly ash addition to reactive glass mixtures reduced strength due to binder dilution and alkali consumption. These results provide insights into the mechanisms governing ASR mitigation and strength development in PV glass-incorporated mortars.