Development of Silicate-Based Surface Impregnation Solution with High Permeability for Restoration of Degraded Concrete

Abstract

In this study, for developing a silicate-based surface impregnation solution with high permeability to repair degraded concrete from the surface to the interior, experiments were conducted to investigate the effect of blending sodium hydroxide (NaOH) on the permeability of the lithium silicate solution (LS) that is one of generally used silicate surface impregnation materials. The high permeability of the modified LS solution, composed of LS, NaO, and water, was confirmed by solution immersion test using several grades of concrete and heated concrete with different strengths, and by ICP-AES analysis. Appropriate NaOH blend significantly increased the penetration of LS into concrete. The appropriate NaOH blends and the mechanisms of increasing permeability were investigated by the setting time test of fresh pastes of modified LS solution and unheated or 650°C-heated Portland cement paste powders, and the SEM-EDS, XRD, TG-DTA analyses of hardened pastes. As a result, when NaOH is blended in an amount such that the Na/Si molar ratio of NaOH-modified LS solutions is in the range of 0.5-1.25, they have high permeability. The addition of NaOH reduces the solubility of Ca(OH)₂, being a hydrate of Portland cement (PC), and therefore delays the calcium silicate formation between Ca(OH)₂ and LS. This delay prevents the penetration path of LS from being quickly blocked by calcium silicate, thus LS permeability is improved. However, the addition of excessive NaOH destroys other hydrates of PC to dissolve Ca ions, instead increasing the calcium silicate formation. In addition, the blend of NaOH would increase the alkalinity of neutralized concretes.