Mechanical and sustainable properties of autoclaved aerated concrete with high-volume fly ash and iron tailings sand

Coastal and island roadways require lightweight concretes that balance mechanical performance, transport resistance, and sustainability. This study investigates autoclaved aerated concrete (AAC) incorporating different fly ash–to–tailings sand ratios (FA/Sand = 0.08–1.82). A comprehensive evaluation was conducted, including compressive strength, drying shrinkage, thermal conductivity, sulfate spray–drying resistance, and microstructural analyses, together with life-cycle and cost assessments. Results show that increasing FA content progressively decreases bulk density from ~634 to ~573 kg·m⁻³ and thermal conductivity with ~12% reduction, but also reduces compressive strength to ~3.05 MPa at the 45% FA level due to a coarser and more connected pore structure. Under sulfate exposure, high FA mixes exhibited greater strength loss associated with accelerated ion ingress and the formation of expansive sulfate products. In contrast, mixtures with FA/Sand ratios of 0.27–0.55 developed refined pore structures and well-crystallized tobermorite, achieving an improved balance of strength retention and sulfate resistance. Sustainability analysis further demonstrated that partial sand replacement by FA reduced embodied CO₂ emissions and material cost per unit volume, although the marginal benefit diminished at higher FA levels. Overall, a moderate FA/Sand range was identified as optimal for achieving the combined objectives of structural performance, durability, and environmental efficiency in AAC production, offering a viable pathway for sustainable application in marine and island infrastructure.