Compressive and wetting properties of MTMS-modified metakaolin geopolymers reinforced with nano-SiO2 and PVA fibers

Three-dimensional superhydrophobic metakaolin-based geopolymer composites show promise for functional construction materials, yet balancing hydrophobicity and mechanical performance remains challenging. In this study, metakaolin-based geopolymers were modified with methyltrimethoxysilane (MTMS) and reinforced with nano-SiO 2 (NS) and polyvinyl alcohol (PVA) fibers to establish a multiscale reinforcement system. Increasing NS content from 0.5 wt.% to 1 wt.% increased compressive strength from 5.50 to 7.16 MPa, while further addition caused a slight reduction. PVA fibers provided stronger reinforcement, increasing compressive strength to 18.87 MPa at 1.5 wt.%. The MTMS-modified reference and NS-only specimens exhibited high water repellency, with the maximum WCA reaching 157.27° , and all NS-only specimens maintained superhydrophobicity with WCA values above 150°. Although PVA fibers gradually reduced the WCA, the NS/PVA hybrid system provided a favorable balance between mechanical strength and water repellency. The maximum compressive strength reached 24.04 MPa, whereas superhydrophobicity was retained only in the hybrid specimen with 0.5 wt.% PVA. Microstructural analysis showed that Si–CH 3 groups introduced by MTMS reduced surface energy and provided the chemical basis for hydrophobicity. Nano-SiO 2 reduced visibly large voids and improved the apparent continuity of the matrix, whereas PVA fibers provided the dominant mechanical reinforcement through microscale stress transfer and mechanical anchorage. The combined system therefore exhibited complementary and dosage-dependent effects rather than a universally demonstrated synergistic strengthening effect.These results demonstrate a multiscale strategy for improving mechanical performance while regulating, rather than universally maintaining, the bulk water repellency of metakaolin-based geopolymer composites.