Enhancement mechanism of metakaolin on the thermomechanical performance of alkali‑activated slag foam concrete: Roles of multi-scale pore structure and shrinkage behavior

Alkali‑activated slag foam concrete (AASFC) is a promising insulation material in energy-efficient buildings, yet it is highly prone to shrinkage cracking and faces a trade‑off between strength and thermal performance. This study proposed a strategy that synergistically enhanced foam stability via liquid-film interface reinforcement and metakaolin (MK)-induced rheological regulation, while optimizing the pore structure of bubble walls through MK to improve volume stability and thermomechanical properties. Pearson and Spearman correlation analyses identified key factors influencing thermal performance and strength. Experimental results demonstrated that moderate MK (< 50 wt%) optimized foam sphericity and dispersion, while refining foam pore size by increasing the paste viscosity. Furthermore, MK favored the generation of low-thermal-conductivity N‑A‑S‑H gels over C‑A‑S‑H, which coarsened capillary pores and increased thermal path tortuosity, thereby mitigating autogenous/drying shrinkage and reducing thermal conductivity. Under comparable foam porosity, thermal performance was primarily governed by 50–200 nm pores (positive) and autogenous shrinkage (negative). Foam sphericity, 50–250 μm micropores, and matrix mesopores exerted the significant positive correlations with compressive strength. The optimized mixture with 50 wt% MK achieved a thermal conductivity of 0.078 W/(m·K) and a 28-day compressive strength of 4.0 MPa, with 55.6% and 68.4% reductions in autogenous and drying shrinkage, respectively. These findings would provide a technical basis for designing AASFC with combined high strength and good thermal insulation performance.