Silane-impregnation is one of the most used surface waterproofing treatments for concrete structures to improve the durability, but long-term engineering observations find that silane-impregnation will deteriorate during the service life of concrete structures. The deterioration of silane treatment undermines its effectiveness in protecting the concrete structures and improving their durability. However, the deterioration mechanism of silane-impregnated treatment has rarely been studied. To fill this gap, This paper studied the deterioration mechanism of surface hydrophobicity of silane-impregnated cement-based materials through laboratory accelerated tests. The accelerated tests include drying-wetting cycles by pure water and ultraviolet (UV) irradiation. Then, the silane-impregnated materials are characterized by infrared spectroscopy, pyrolysis gas chromatography-mass spectrometry, thermal analysis system, and dynamic water vapor desorption. The aim of this study is to provide a research basis for quantifying the usage life of surface silane impregnation treatment used in engineering.

Laboratory accelerated deterioration tests were conducted to study the deterioration mechanism of surface hydrophobicity of silane-impregnated cement-based materials. The accelerated tests include drying-wetting cycles by pure water and ultraviolet (UV) irradiation. For drying-wetting cycle tests, an experimental design of 4 h wetting time and 8 h drying time was adopted. The irradiance of ultraviolet (UV) irradiation tests is irradiance, and the irradiation cycle is 30 d. After accelerated deterioration tests, the silane-impregnated cementitious materials are characterized by infrared spectroscopy, pyrolysis gas chromatography-mass spectrometry, thermal analysis system, and dynamic water vapor desorption.

Infrared spectroscopy was conducted to study the silane molecular structural changes during accelerated deterioration tests; pyrolysis gas chromatography-mass spectrometry and thermal analysis system was conducted to characterize the silane content in cementitious materials; and dynamic water vapor desorption was conducted to characterize the hydrophobic performance of impregnated cementitious materials.

Under the action of drying-wetting cycles: 1) infrared spectroscopy results of impregnated cementitious materials showed that the relative content of different alkyl groups had not changed, indicating that the molecular structure of silane was stable; 2) Both pyrolysis chromatography-mass spectrometry and thermal analysis system results showed that the silane content in impregnated cementitious materials was decreasing, combining with the analysis of infrared spectroscopy results, silane in impregnated cementitious materials was most likely underwent hydrolysis reaction, and the hydrolyzed silane continuously detached from impregnated particles under the action of drying-wetting cycles; 3) In addition, the water vapor adsorption capacity of cement paste powders, the water sorptivity and contact angle of mortar specimens had all increased.

Under the action of UVB-313 ultraviolet radiation: 1) the silane impregnated on the surface of cement-based material particles underwent chemical bond breakage. Based on the wavelength of the ultraviolet radiation, it is inferred that this likely caused the breaking and recombination of tertiary carbon bonds. Infrared spectroscopy, gas chromatography-mass spectrometry, thermal analysis system, and water vapor adsorption capacity results all indicate this aging process; 2) Due to the direct impact of ultraviolet irradiation on the surface of impregnated materials, the water sorptivity and contact angle of the impregnated mortar specimens had changed significantly. However, it should be noted that this significant aging effect depends on the UV wavelength and irradiation energy of UVB-313 lamps using in the laboratory. In the natural environment, the irradiance of sunlight is much lower than that of UVB-313. Therefore, the accelerated UV testing results are primarily suitable for mechanistic research and cannot not be directly applied to the aging behavior of silane impregnations in natural environment.

The stability of cement-based materials impregnated with high-purity silane (isobutyl triethoxysilane) was confirmed through drying-wetting cycles and ultraviolet irradiation. The stability is attributed to the condensation network between silane monomers and the condensation reaction with the surface of cement-based materials (Si—O—Si bonds); The wavelength distribution of natural light and the barrier effect of cement-based materials make the deterioration caused by ultraviolet irradiation less likely to occur in natural environment. The deterioration caused by hydrolysis of silane should be more related to the effect of environmental water. Based on the study of the drying-wetting cycles results and the mechanism of silane hydrolysis, it can be reasonably inferred that the hydrophobicity degradation of cement-based materials surface (layer) impregnated with silane in natural environments is related to the hydrolysis of silane and the migration of hydrolyzed silane.

For the four different cement pastes used in this study, OPC showed the greatest decrease in water vapor adsorption capacity after silane impregnation, indicating that the silane impregnation effect of OPC is better than that of OFA, OSG, and OFS. Combined with the chemical reaction mechanism of silane impregnation, alkaline conditions can catalyze the impregnation reaction of silane. However, pozzolanic reaction, occurred in the three cementitious materials mixed with fly ash or slag, consumes the alkali reserve of the pore solution. Therefore, the alkalinity of the pore solution of OPC is the highest, resulting in the best silane impregnation effect.