Development of alkali-activated binders with rapid hardening and high early-age strength for bio-based construction materials

Bio-based construction materials incorporating plant aggregates exhibit slow hardening and low early-age mechanical performance, limiting their demolding, handling, and structural applications. These limitations are mainly attributed to chemical incompatibilities between lignocellulosic aggregates and cementitious binders. To address these challenges, a rapid-hardening alkali-activated binder was developed as a high-performance matrix for flax shive-based bio-based construction materials. A hybrid binder based on metakaolin (MK) and ground granulated blast furnace slag (GBFS) was optimised to enhance early-age reactivity and mechanical strength. The optimum formulation (30 MK–70 GBFS), cured at 40 °C, achieved a compressive strength of 74.7 MPa after 28 days. Microstructural analyses revealed the synergistic formation of N-A-S-H and C-(A)-S-H gels, producing a dense and homogeneous matrix. The interaction between the binder and flax shives was assessed, using aqueous shive extracts as mixing water to isolate the effect of water-soluble organic compounds. The extracts delayed the polycondensation peak but slightly reduced its amplitude, giving a low inhibition index (I < 10), and thermogravimetric analysis showed that hydration products still formed in their presence. These results establish the kinetic compatibility of the binder with the extractable compounds of flax shives. These findings support using MK–GBFS alkali-activated binders as rapid-hardening, high-early-strength matrices for bio-based construction materials.