Synergistic use of sintered fly-ash aggregate and wood waste in ultra-lightweight green concrete

Green concrete refers to concrete incorporating waste materials to reduce environmental impact and consumption of natural resources. This study investigated a novel ultra-lightweight green concrete in which 100% of the aggregate skeleton was composed of sintered fly ash aggregate (Certyd) and wood waste (sawdust and wood chips), partially replacing natural fine and coarse aggregates. The research aimed to evaluate how varying proportions of these waste materials affect key properties and to identify mixtures that balance low density, strength and water absorption. An experimental programme with a fixed CEM I 42.5R Portland cement content (400 kg/m³) and a constant total water content was designed using a two-factor central composite design (fine fraction balance X; coarse fraction balance Y) and second-order response surface regression. Fine aggregate was partially or fully replaced by fine Certyd (0–2 mm) and sawdust (0–2 mm), while coarse aggregate was replaced by coarse Certyd (4–16 mm) and wood chips (2–20 mm). Wood aggregates were pre-treated (mineralised) with aluminium sulfate and hydrated lime to improve compatibility with cement. After ambient air-dry curing, 28-day compressive strength, oven-dry bulk density and 24 h water absorption were measured and analysed. The composites achieved densities of 651–1031 kg/m³, compressive strengths of 1.8–7.3 MPa and water absorption of 25.6–39.8%. The best-performing mixture reached 7.26 MPa at a density of 1031 kg/m³ and 25.6% absorption, corresponding to a high share of mineralised sawdust and a low share of coarse wood chips. Quadratic response surface models for strength, density and absorption showed good statistical adequacy (R² ≈ 0.63–0.64; p < 0.001), confirming the dominant influence of the coarse-phase factor (Y) and the mainly curvature-type effect of the fine-phase factor (X). These model-based findings define a quantitative performance window for dual-waste, ultra-lightweight concretes and provide a tool for preliminary mix optimisation. The results indicate that, although strength remains below that of purely mineral lightweight concretes, the proposed material is competitive within the family of wood-modified lightweight composites and is suitable for non-structural or semi-structural insulating elements that prioritise low density and high waste utilisation.