Unlocking the potential of bio-Based composites: durability and sustainability of hemp, flax, and miscanthus with low-carbon binders

• Six bio-based composites studied with hemp, flax, and miscanthus shives. • Combined effects of biomass type and low-carbon binders (NPC, CL90-S) analyzed. • Hemp–NPC composite showed highest strength (0.81 MPa) and low thermal conductivity (λ = 0.12 W/m.K). • Flax–lime/cement blend achieved best vapor permeability (7.44 × 10⁻¹⁴ kg/m.s.Pa). • Miscanthus–lime/cement composite retained 70% of its strength after freeze–thaw cycles. • Life cycle analysis showed 70–130 kg CO₂/m³ reduction in carbon footprint. Bio-based insulation materials offer a promising solution for reducing building carbon footprints, but their mechanical and durability limitations hinder large-scale adoption. This study evaluates six composite materials made from hemp, flax, and miscanthus shives combined with low-carbon binders: natural prompt cement (NPC) and a hybrid binder in which 50% of NPC is substituted by air lime (CL90-S). A multi-scale characterization was conducted, covering physical, thermal, mechanical, hygrothermal, freeze-thaw durability, and life cycle performance. Results show that hemp’s stiffness combined with NPC reactivity enhances mechanical strength, flax’s fiber morphology improves vapor permeability, and miscanthus’ low hygroscopicity boosts freeze-thaw durability. Among the composites made exclusively of NPC (C100), hemp exhibited the highest compressive strength (0.81 ± 0.04 MPa), followed by miscanthus (0.19 ± 0.03 MPa) and flax (0.10 ± 0.02 MPa). In contrast, with the hybrid binder, the order remained the same, but the values differed: hemp reached 0.37 ± 0.034 MPa, followed by miscanthus (0.32 ± 0.04 MPa) and flax (0.17 ± 0.037 MPa). Flax-based composites had the highest vapor permeability (7.44 × 10⁻¹⁴ kg/m·s·Pa), while miscanthus maintained the best freeze-thaw resistance, retaining 70% of its strength after cycles. The simplified life cycle assessment shows that the net carbon footprint of NPC-based formulations is between 95 and 130 kg CO₂/m³, while substituting 50% of the NPC with CL90-S reduces it to a range of between 70 and 105 kg CO₂/m³.These findings underscore the critical role of biomass-binder interactions in optimizing bio-based composites for low-carbon construction applications.