Properties and microstructure of self-waterproof metakaolin geopolymer with silane coupling agents

Geopolymer is a new green eco-friendly gel material. However, one of the distinguishing features of geopolymer is prone to cause high water absorption and poor waterproof performance, deteriorating properties such as durability. This paper investigated the effect on the waterproof performances of different silane coupling agents (SCAs) and different SCAs dosages (0 wt%–4 wt%) of metakaolin-based geopolymer. The performance was tested from aspects of capillary water absorption, capillary porosity, water resistance, filter loss and water retention volume (WRV), contact angle, wettability, and hydration degree. Meanwhile, the microstructure mechanism was characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transformation infrared Spectroscope (FT-IR), and BET (Brunauer-Emmett-Teller). The results showed that an appropriate dosage, whichever SCA was incorporated, would significantly reduce capillary water absorption, capillary porosity, with water resistance, and contact angle being enhanced. The hydration degree showed that incorporating a small dosage of SCAs could boost the early hydration reaction of geopolymer systems, resulting in performance improvement. Under the same SCA dosage, the waterproof performance of KH560 [γ-(2,3-epoxypropoxy)propytrimethoxysilane]-modified geopolymer was better than that of KH550 [γ-ammoniapropyltriethoxysilane]-modified geopolymer. Moreover, the FT-IR and XRD tests revealed that the incorporation of SCAs hardly destroyed the main crystal structure of geopolymer. Besides, the SEM test demonstrated that moderate dosages of SCAs modified geopolymer tightly connected internally than control geopolymer. In addition, the BET test visually identified the reason for the performance improvement. The experiments revealed the hydration mechanism of SCA-modified geopolymer. By enhancing waterproof on geopolymer, the modified geopolymer could effectively improve the deteriorating effects of harmful ions on materials through the process of water transmission.