Investigation of the hydrophobicity and microstructure of fly ash-slag geopolymer modified by polydimethylsiloxane

Abstract

The replacement of geopolymer for Portland cement exhibits the potential for reducing the CO2 footprint and saving natural resources during the production of building materials. However, geopolymer is a class of inorganic cementitious material, easy to suffer water intrusion with aggressive ions, such as chloride and sulphate, resulting in the destruction of structural integrity. In this study, hydroxyl-terminated polydimethylsiloxane (PDMS) was used as an additive to improve the hydrophobicity of fly ash-slag geopolymer. The microstructure was characterized using mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The results showed that the contact angle (CA) of the geopolymers increased from 57.32° to 127.64°, and the water absorption decreased from 6.96% to 1.61% as a function of the dosages of PDMS, contributing to the hydrophobic nature of the geopolymers. On the other hand, the addition of PDMS led to the formation of a less compact pore structure of geopolymers, accompanied by a transformation from internal gel pores into capillary pores and macropores. The pore enlargement will have adverse effects on the compressive strength of the geopolymers, but it can still meet the requirements for daily work. According to the analysis of microscopic test results, PDMS and N-A-S-H gel were likely to undergo dehydration condensation reaction in a strong alkaline environment, they were connected in the form of covalent bonds, so that the geopolymers have hydrophobicity.