Sulfate Resistance and Microstructure of Metakaolin Geopolymer Reinforced by Cellulose Nanofiber and Wollastonite

As an alternative to the ordinary Portland cement (OPC), it is imperative to improve the performance of geopolymer under harsh conditions (such as sulfate/seawater attack). Sulfate attack is one of the reasons which cause deterioration and damages of geopolymer structures. Thus, the strategy of sulfate resistance attracts attention of researchers. As a reinforcement for geopolymer, the cellulose nanofiber (from waste bamboo powder) facilitates the utilization of waste bamboo powder. High-purity cellulose nanofiber (CNF) was obtained by using alkali, bleaching and acid hydrolysis treatments on waste bamboo powder. The purification effect and environmental impact of CNF were compared by different bleaching methods. The optimal conditions (containing 5 wt% H2O2, 0.04 wt% MgSO4 and 0.05 wt% TAED) for CNF purification were determined. This work aims to investigate the effect of CNF and wollastonite (WS) on the sulfate resistance of metakaolin (MK) geopolymer. The deterioration mechanism of geopolymer reinforced by natural fiber (CNF and WS) exposed to sulfate environment was revealed by employing XRD, SEM and MIP. The influence of extreme sulfate environment on mechanical properties and microstructure was investigated. The analysis results of microstructure, mass loss rate and mechanical properties were also combined to evaluate the sulfate resistance. The outcomes show that the bleaching method of H2O2 can effectively remove impurities and enhance the purification effect of CNF. The optimum mix proportion of MK geopolymer reinforced by natural fiber (formulated as 2% of CNF, 10% of WS and 88% of MK) is obtained. The compressive and flexural strengths are increased by up to 76.1% and 135.5% (compared to the control group), respectively. The deterioration of geopolymer is related to the concentration of Na2SO4 and the immersion time. The degradation mechanism is attributed to the incomplete geopolymerization and the destruction of the pore structure due to sulfate attack exposure. The deterioration processes of geopolymer can be slowed down by introducing WS and CNF. It improves the sulfate resistance of the matrix from the structural aspect.