Fabrication and properties of porous geopolymers modified with n-octyltriethoxysilane

Porous geopolymer is a new green inorganic thermal insulation material, which is consistent with the sustainable development of ecological environment. However, the durability and mechanical properties of porous geopolymer materials are easy affected by efflorescence phenomenon. In this work, the effect of n-octyltriethoxysilane on pore structure and properties of metakaolin/slag-based porous geopolymer was investigated. Moreover, the hydrophobic mechanism was discussed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM) and scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS). The results showed that porosity of porous geopolymer increased with the increase of n-octyltriethoxysilane, and the pore size distribution gradually narrowed. Compressive strength of materials decreased with the addition of n-octyltriethoxysilane. However, water absorption showed a reduction of 50%-63.7% in compared with that of blank group, and contact angle was as high as 152.6°. Furthermore, the impact of n-octyltriethoxysilane on thermal conductivity was not significant. In addition, the efflorescence precipitated on specimens almost could be ignored when 1.5% n-octyltriethoxysilane was introduced. From a microscopic perspective, the addition of n-octyltriethoxysilane to porous geopolymer could reduce cracks on materials, increase the multistage roughness and grafted hydrophobic functional groups to the surface of geopolymer, which resulted in hydrophobic properties. ● Porous geopolymer materials were modified to be hydrophobic using n-octyltriethoxysilane with an integral method. ● The incorporation of n-octyltriethoxysilane endowed the porous geopolymer with remarkable hydrophobicity, which refined the pore structure and led to an increased porosity. ● The addition of 1.5% n-octyltriethoxysilane significantly mitigated the detrimental efflorescence phenomenon. ● The hydrophobicity stemmed from the combined effect of reduced microcracks, induced multi-scale surface roughness and the grafting of hydrophobic functional groups.