Abstract
The concept of geopolymers has been widely studied since it was proposed. However, the wide range of applications of geopolymers is affected by brittleness and poor crack resistance. In this study, the mechanical properties of geopolymers with single-doped PVA fibres, single-doped carbon nanotubes, and mixed PVA fibers and carbon nanotubes were studied respectively first. It was found that PVA fibres and carbon nanotubes had a positive effect on improving the mechanical properties of geopolymers, especially bending strength and flexural strength. Moreover, the incorporation of PVA fibre could improve the damage morphology of geopolymers. Additionally, the phase analysis, structural group analysis, and strengthening mechanism were studied via scanning electron microscopy, mercury intrusion porosimetry analysis, X-ray diffraction pattern characterisation, Fourier transform infrared spectroscopy analysis, and magic angle spinning nuclear magnetic resonance analysis. It was found that the strengthening effect of PVA fiber to the geopolymer was primarily a physical strengthening effect, whereas the strengthening effect of carbon nanotubes to the geopolymers was both chemical and physical. Finally, based on the previous study, a multi-scale dual-fibre strengthening mechanism was proposed. Micro-nano fibre composites were used to improve microstructure via physical and chemical effects. This is helpful to improve the performance and application of geopolymers. Furthermore, it lays a preliminary theoretical foundation for engineering applications and technical improvement of geopolymers in the future.
Highlights
Research on geopolymers began in the 1930s and 1940s
The results show that both singled-doped and mixed-doped PVA fibres and carbon nanotubes (CNTs) can improve the flexural strength of the geopolymer
Compared with the B0 group, the flexural strength of the P2 group with 1.2 kg/m3 PVA fibre increased by 11.6%, T4 group with 1.5‰ CNTs increased by 14.0%, and PT5 group with PVA fibres and CNTs increased by 20.9%
Summary
Research on geopolymers began in the 1930s and 1940s. Purdon (1940) used the reaction of slag and hydroxide to prepare a fast-setting high-strength cementing material and proposed the alkali catalysis mechanism. Saafi et al (2013) found that adding MWNCTs could improve the flexural strength, Young’s modulus, and flexural toughness of fly ash-based geopolymers effectively, while enhancing the fracture energy of geopolymers. Khater and Gawaad (2016) studied the effects of MWNCTs on the mechanical properties and microstructure of slag-based geopolymers and found that 0.1wt% and 0.4wt% of multi-walled CNTs can increase the TABLE 1 | Chemical composition of metakaolin and water glass. Yuan et al (2017) found that the flexural strength, elastic modulus, and fracture toughness of multi-walled CNT-reinforced geopolymers improved significantly after high-temperature treatment at 950–1200°C. There are few studies on the effect of CNT-strengthened geopolymers; there is insufficient research on their physical and mechanical properties. Based on the multi-scale microscopic analysis of the strengthening effect of geopolymers by PVA fibres and CNTs, a multi-scale dual-fibre strengthening mechanism was proposed
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