Hydrophobic metakaolin-modified sulfoaluminate cement: Physicochemical properties and accelerated steel corrosion behaviour

Sulfoaluminate cement (SAC)-based grouting materials used for the filling and reinforcement of fractured rock masses in underground engineering are prone to moisture ingress and steel reinforcement corrosion under high-humidity and aggressive environments, thereby compromising structural durability. To improve the hydrophobicity of SAC pastes and mitigate steel corrosion under accelerated conditions, metakaolin (MK) was modified using methyltrimethoxysilane to produce hydrophobic metakaolin (HMK). The effects of varying dosages of MK and HMK on the hydration behaviour, mechanical properties, pore structure, hydrophobicity and corrosion resistance of SAC pastes were systematically investigated. The results indicate that an appropriate MK content (4%) promotes the formation of C–A–S–H (calcium aluminosilicate hydrate) gel and improves the 28-day compressive strength; however, its intrinsic hydrophilicity increases water absorption, which may be unfavourable for moisture resistance and steel corrosion protection. In contrast, HMK introduces hydrophobic functional groups while retaining partial pozzolanic activity, reducing water absorption by 28.5% and increasing the water contact angle to 145°. Under identical impressed-current conditions, the mass loss of steel bars embedded in HMK-modified specimens was reduced by 34.9–48.1% compared with that of the corresponding reference specimens. Correlation analysis further reveals a strong positive relationship between water absorption and steel mass loss (r = 0.97), while the proportion of pores within the 20–50 nm range is also positively correlated with steel mass loss. Through the synergistic effects of pore structure regulation and hydrophobic shielding, HMK reduced moisture transport and steel corrosion under accelerated conditions, demonstrating its potential to improve the durability of SAC pastes in humid and chloride-containing environments.