Durability and hygrothermal performance of metakaolin-based geopolymer composites reinforced with hemp shiv and miscanthus fibers

This study presents sustainable geopolymer composites incorporating high volumes of hemp shiv (HS) and miscanthus fibers (MF) as bio-based, low-carbon alternatives to conventional insulation materials. Metakaolin-based matrices were reinforced with up to 350 wt% plant aggregates, and tested for physical, mechanical, hygrothermal, fire, and environmental performance. The composites achieved ultra-low thermal conductivities (0.088 W/m.K for 350HS and 0.096 W/m.K for 350MF) and compressive strengths above 0.75 MPa, suitable for non-load-bearing applications. Despite higher water absorption, immersion had minimal effect on thermal performance. Mercury intrusion porosimetry highlighted the role of triple porosity in optimizing insulation and strength. Water vapor resistance factors as low as 2.4 demonstrated outstanding breathability, while fire testing confirmed Euroclass A1–A2 behaviour without the use of synthetic foaming agents. Environmental and economic assessments, normalized to a thermal resistance of R = 2.5 m 2 .K/W, revealed CO 2 emissions near zero and production costs as low as 13.1 €/m 2 substantially outperforming commercial insulators. These results demonstrate a scalable pathway toward cost-effective, carbon-negative insulation materials for sustainable construction. • All samples met fire resistance Euroclass A1 or A2 without additives. • Near-zero CO 2 emissions and lower cost than conventional insulation materials. • Composites retained ≥ 0.75 MPa compressive strength, meeting EN 998–1 for non-structural use.