Waste valorization of seashells and expanded polystyrene in lightweight hybrid mortar for social housing

Abstract The construction sector faces increasing demands to reduce its ecological footprint, particularly in social housing, where material cost-efficiency must be balanced with technical reliability. This research investigates a sand-free alternative aggregate matrix for wall-coating mortars by fully substituting natural quartz sand with waste expanded polystyrene (EPS) and crushed sururu seashells ( Mytella falcata ) on an absolute volume basis. Three experimental formulations were examined: a reference control, a 40% partial replacement (20% shells and 20% EPS), and a 100% complete replacement (50% shells and 50% EPS). Physical properties were assessed using modified geometric cold-saturation methods. Density decreased from 1752.6 kg/m 3 to 1421.6 kg/m 3 and 904.6 kg/m 3 across the formulations. Relative water absorption values shifted from 12.90% to 10.88% and 21.67%, while the absolute absorbed water mass decreased from 44.4 g to 38.6 g at full replacement. This indicates that the percentage increase is a mathematical artifact resulting from a 48.4% reduction in skeletal mass. The 14-day compressive strength declined from 12.60 MPa to 6.40 MPa and 1.85 MPa, yet remained within standard coating specifications. This approach aligns with UN SDG 12 (Responsible Consumption and Production). Overall, the findings indicate the preliminary feasibility of volume-based waste aggregate replacement, although further evaluation of long-term durability is required before standard field implementation.