From aggregate to composite: Design of flax-based concretes through thermal treatment and hemp synergy for controlled construction practices

Bio-based composites are promising materials for low-carbon, moisture-regulating building envelopes, yet flax–lime composites are limited by the high-water sensitivity of flax shives, which disrupts binder hydration and reduces mechanical robustness. This study addresses the challenge of controlling flax–binder water competition while preserving the open porosity required for hygrothermal performance. Flax–lime and flax–hemp composites were investigated using three water-management strategies: baseline mixing, flax pre-wetting, and mild thermal treatment of flax shives (80–160 °C) combined with water-to-binder adjustment. A multi-scale experimental approach linked shive morphology and chemistry to composite properties through microscopy and thermal analysis, water absorption kinetics, thermal conductivity, compressive strength, and vapor permeability. Thermal treatment at 120 °C for 2 h reduced early water uptake by 32% and decreased intrinsic shive thermal conductivity by 20%. At the composite scale, treated flax at 14% shive content lowered thermal conductivity from 97.7 to 74.0 mW·m−1 K−1 while maintaining mechanical reliability, further improved by water-to-binder optimization. Pre-wetting enhanced strength but significantly reduced vapor permeability. A vapor-transport percolation threshold was identified near 14% shive content. Overall, mild thermal treatment combined with controlled water balance offers a robust hygrothermal–mechanical compromise for breathable, low-impact flax-based infill materials.