Integral superhydrophobicity in cement matrix via in-situ hierarchical micro-nano roughness

Abstract

Creating hierarchical matrix roughness from micron to nano scales remains a tough challenge for developing integral superhydrophobic cement-based materials. This study presents a novel approach employing calcium sulfoaluminate (CSA) cement, selected for its intrinsic capacity to generate hybrid crystalline–amorphous hydration products and multiscale porosity, to construct a three-level bulk roughness structure: micron (5–50 μm), submicron (100 nm–5 μm), and nano (5–100 nm). The origins of the hierarchical roughness and its underlying mechanism on enhancing hydrophobicity were investigated in comparison with an ordinary Portland cement system. The results indicate that the CSA matrix comprised 19.8–22.1 vol% un-hydrated clinkers, 75.2–76.8 vol% hydration products, and pores. This specific phase distribution with broad microhardness ranges endowed the micron-scale roughness. Gel clusters, prismatic ettringite (AFt, 28.6–30.0 wt%), fuzzy aluminum hydroxide (AH<inf>3</inf>) and C–S–H gels (38.5–40.8 wt%), and meso/nano pores (24.3–28.4 vol%), formed irregular, pervasive 3D textures contributing to the submicron and nano-scale roughness. Additionally, multilayered flower-like phases, considered as silane–Ca2+–CSA hybrids, were extensively formed in the superhydrophobic matrix, providing low–surface–energy components and additional submicron-scale roughness. The synergy between this intrinsic hierarchical texture and 1 % silane modification achieved a water contact angle of 159.1° and an 88.8 % reduction in water sorptivity, offering a distinctive design strategy for superhydrophobic, durable CSA-based materials applicable to coatings, repair materials, and 3D-printed components.