TEOS-IOTES Dual-Coating Protection for Concrete: Densification-Hydrophobization Against Chloride Ingress and Salt Freeze-Thaw Damage

Concrete is widely employed in structural engineering; however, its porous nature renders it vulnerable to chloride ingress and salt freezing cycles, ultimately compromising its durability. To address this, a penetrating primer based on tetraethyl orthosilicate (TEOS) was prepared in an ethanol-water co-solvent system, and a hydrophobic topcoat of isooctyltriethoxysilane (IOTES) was obtained via emulsification. A layered application on concrete surfaces yielded a TEOS–IOTES dual-coating protection system designed to enhance water repellency and thereby improve resistance to chloride penetration and salt freeze–thaw damage. Test results show that the dual coating markedly increased hydrophobicity, giving a water contact angle of 130° and reducing water absorption rate to below 0.01 mm/min0.5. Compared with single-layer treatments, the dual coating significantly lowered the free chloride diffusion coefficient (reached 83.74%). In terms of salt freezing cycle resistance, the dual-coating protection delayed surface scaling and increased the critical number of freeze–thaw cycles required for damage by 40%. Microstructural analyses indicate that the TEOS primer generates nano-SiO2 and C-S-H gels, refining pores and densifying the matrix, while the IOTES topcoat forms a durable hydrophobic layer that suppresses moisture and deleterious ion transport. The synergistic “densification–hydrophobization” mechanism substantially enhances concrete durability, offering a cost-effective and efficient surface-protection strategy with promising application potential.