Carbonated manufactured sand as a sustainable substitute for natural fine aggregates in mortar

The depletion of natural sand due to over-extraction and strict environmental regulations has accelerated the search for sustainable alternatives. This study explores the feasibility of producing CO₂-sequestered sand from industrial by-products such as coal fly ash and carbide sludge. Artificial lightweight coarse aggregates (ALCA) were synthesized by mixing 70% fly ash and 30% carbide sludge, followed by autoclaving at 5 bar pressure. The hardened ALCA were then crushed and ground to obtain fine aggregates (< 4.75 mm), which were subsequently subjected to accelerated carbonation under different CO₂ pressures (3, 5, and 7 bar) and carbonation durations (8, 16, 24, 48, and 72 h). Fine aggregate obtained after carbonation of ALCA at 7 bar CO₂ pressure for 8 h yielded carbonated sand (C-sand) with compressive strengths of 19 MPa and 25 MPa at 7 and 28 days, respectively, comparable to natural sand. To evaluate performance in mortar, natural sand was partially replaced with C-sand at 0%, 20%, 40%, 60%, 80%, and 100% levels. A 20% replacement achieved a 28-day compressive strength of 38.52 MPa, closely matching the control mix. The optimized blend of 20% C-sand and 80% natural sand, developed as CO₂-sequestered manufactured sand (CM-sand), satisfied IS 383-2016 requirements and corresponded to Zone II gradation. It exhibited a fineness modulus of 1.89, water absorption of ~ 6%, specific gravity of ~ 2.51, loose bulk density of ~ 0.76 g/cm³, rodded bulk density of ~ 0.92 g/cm³, and a void ratio of ~ 0.63. Importantly, the process sequesters ~ 22.08 kg of CO₂ per cubic meter of mortar, offering an eco-friendly, resource-efficient alternative to natural sand in construction applications.