Understanding the generation and evolution of hydrophobicity of silane modified fly ash/slag based geopolymers

Silanes are widely used to enhance the corrosion resistance of cement-based materials by endowing the substrate with hydrophobicity. However, their applications in fly ash/slag based geopolymer (FSBG) are still rare. This study comprehensively investigates the hydrophobicity properties and reaction products of FSBG modified by isooctyltriethoxysilane admixture to reveal the mechanisms involved in the generation and evolution of hydrophobization effect. In spite of the verified compromise in compressive strength of modified FSBG for longer ages, the early strength at 1 day is amazingly found to slightly increase, ascribed to the heat release along with the hydrolysis of silane. The reduction of porosity is caused by the siloxane products of silane after condensation reactions and thus does not show consistencies to the decreasing compressive strength as well as the formation of hydration products. The amount of N-A-S-H gel is found to reduce more intensively compared to the C-A-S-H gel when silane is used. The evolution of contact angle on the surface of modified FSBG is dependent on the mutual effect of superficial density of silane and porosity, with the former controlled by the amount of coupling sites. The formula of contact angle evolution is proposed and can be potentially used to regulate and govern the hydrophobicity properties of FSBG, with the feasibility to be applied in other geopolymers and cement-based materials.