Effect of cellulose nanocrystals on performance of PVA fiber-reinforced geopolymer composites: Reaction kinetics, bending behavior, and toughening mechanisms

Geopolymer composites are an attractive alternative to those based on ordinary Portland cement, owing to their lower carbon footprint and higher mechanical performance. However, brittleness is one of the major barriers limiting their use in construction applications. In this work, cellulose nanocrystal (CNC), a cost-effective and eco-friendly nanoparticle, was exploited in polyvinyl alcohol (PVA)-reinforced metakaolin-based geopolymer composite to further increase its mechanical performance. To this end, PVA-reinforced geopolymer composites were prepared by varying the CNC concentration from 0 wt% to 0.15 wt%. Three-point bending and fracture testing, combined with digital image correlation (DIC) and scanning electron microscopy (SEM), were performed to evaluate the bending and fracture behavior of samples containing CNC and reveal various toughening mechanisms. Composites with 0.15 wt% CNC recorded an increase of 190% in flexural strength and a 325 time of increase in fracture energy, compared to the control geopolymer (i.e. without PVA fibers and CNC). Isothermal calorimetry, Fourier transform infrared (FTIR) spectroscopy, and x-ray diffraction (XRD) were also utilized to characterize the heat flow, chemical bonds, and crystalline phases of the samples. These analyses revealed that CNC incorporation increased the amount of formed geopolymer gel with no significant changes in the crystalline structure and chemical bonding. In addition, stronger interfacial bonding between the PVA-geopolymer matrix was achieved, resulting in substantial improvements in flexural strength and fracture energy.