Engineering and environmental performances of rice husk-based lightweight bio-concrete for applications in dry and hot Sudano-Sahelian context

The development of low-carbon and context-adapted construction materials is critical for hot dry climates such as the Sudano-Sahelian regions. This study investigates the physico-mechanical and hygrothermal performance of lightweight concrete produced by volumetric substitution of river sand (RS) with rice husk (RH). Five substitution levels (0 to 100% RH) were evaluated under three cement dosages (250, 300, and 350 kg/m³). Increasing RH content from 0 to 100% significantly reduced bulk density from 1580 to 444 kg/m³, accompanied by an increase in open water-accessible porosity from 31 to 65%. Compressive strength (Rc) at 28 days decreased from 8.3 to 0.3 MPa. However, mixtures containing 25% RH maintained compressive strengths up to 4.1 MPa, suitable for non-structural applications. The flexural strength at 7 days also decreased with increasing RH content, from approximately 1.4–1.7 MPa for the reference concretes to 0.3–0.6 MPa at 100% RH, depending on the cement dosage, corresponding to reductions of between 65 and 79%. Thermal conductivity (λ) decreased significantly (by more than 90%) with increasing RH content, from 1.3 to 0.09 W/m·K. Thermal diffusivity and effusivity followed similar trends, indicating reduced heat transfer. The incorporation of RH increased open porosity and pore connectivity, which raised vapor permeability. This higher permeability, in turn, promoted moisture absorption at high relative humidity, with absorption values ranging from 26 to 65% of dry mass at 97% relative humidity, confirming a better hygric regulation potential. Global warming potential (GWP), from a cradle-to-gate life cycle assessment, decreased from 322 to 219 kg CO₂ eq/m³. The lowest GWP normalized by Rc (38.8 kg CO₂ eq/MPa) was obtained for the reference concrete containing 350 kg/m³ of cement, whereas the lowest GWP normalized by thermal resistance, 1/λ (20 kg·CO₂·eq·W/(m·K)) was observed for the 100% RH concrete containing 250 kg/m³. These low normalized values reflect better eco-efficiency, i.e., a reduced carbon impact. This suggests that rice husk-based concrete is more suitable for non-load-bearing insulating applications than for structural applications. The substitution rates of 25% and 50% offer good compromises between structural and thermal efficiency and are recommended for non-load-bearing infill elements, while the content of 75% and 100% are recommended for insulating materials. These results highlight the technical feasibility of rice husk-based concrete as a locally sourced material, adapted to Sahelian climatic conditions.