Effect of the combined treatment with inorganic and organic agents on the surface hardening and adhesion properties of cement-based materials

Highlights: • The MgSiF₆ reacts to immobilize soluble K⁺ and Ca²⁺ to harden and roughen the repaired surface. • The leaching rate of Ca²⁺ is reduced by 62.53% while adhesion strengths after the repair are increased by up to 40%. • The MgSiF₆ on the repair surface promotes silane coupling agent activation and cross-linking. • The immobilizing reaction of free SiF₆^2- and K⁺ is finished within seconds in liquid phase -- Abstract: In this paper, a stable but highly reactive potassium silanol coupling agent (PSCA) solution for cementitious materials repair was prepared via the cationic complexation method under −20 °C. Furthermore, here we report an innovative magnesium fluorosilicate (MF)- and PSCA-combined treatment for repairing the surface of old cement-based materials. The effect and mechanism of MF and PSCA on the chemical reaction, adhesion properties, durability, and surface morphology of cement-based materials were systematically studied. The results revealed that MF acts to immobilize soluble Ca²⁺ (with the dissolution rate of Ca²⁺ decreased by 62.53%), to harden the surface (with an increase from 0.975 GPa to 2.420 GPa), as well as to roughen the surface (with the roughness average increased by 80.61%). Moreover, after spraying the surface with PSCA, MF promotes cross-linking by forming a potassium salt with K⁺ to form organic-inorganic chemical bonds. Indeed, the adsorption orientation effect formed by MF and PSCA leads to a good coupling effect; therefore, the leaching rate of Ca²⁺ is reduced by 62.53%. By adopting such a surface-treatment method, the slant-shear strength and the pull-off adhesion strength after the repair are increased by ~12% to 40%, while the water aging resistance is remarkably improved.