Influence of mixed fibers on fly ash based geopolymer resistance against freeze-thaw cycles

The freeze-thaw cycle is one of significant indexes to evaluate the durability of the geopolymer. In this study, experimental research was employed to characterize the freeze-thaw behavior of mixed fiber-reinforced geopolymer mortars. Modified multi-walled carbon nanotubes (MWCNTs) with mass fractions of 0.05%, 0.10% and 0.15% and PVA fibers with volume fractions of 1.50%, 2.00% and 2.5% were used as the experimental parameters. The geopolymer mortar without modified MWCNTs and PVA fibers was used as the control. Visual inspection, relative dynamic elasticity modulus and mass loss were adopted to evaluate the resistance against freeze-thaw cycles of mixed fiber reinforced geopolymer mortars. The freeze-thaw damage model was built on the basis of the freeze-thaw cycles and the relative dynamic elasticity modulus. In addition, the pore structure characteristics and the micro-morphology were characterized by means of mercury intrusion porosimetry (MIP) and field emission scanning electron microscopy (FESEM). The experimental results show that the control geopolymer mortar is destroyed in 50 freeze-thaw cycles, while the mixed fiber reinforced geopolymer mortar incorporated with 0.10% modified MWCNTs and 2.00% PVA fibers can withstand 175 freeze-thaw cycles. Meanwhile, the mechanism of freeze-thaw failure and improvement of geopolymer mortars were revealed from microscopic and mesoscopic aspects. The MIP analysis can well establish that the utilization of modified MWCNTs and PVA fibers is an effective way to decrease the porosity of geopolymer mortars. The nano-nucleation effect, nano-filling effect, crack arrest effect and mechanical interlocking effect of modified MWCNTs make geopolymer mortars denser, decreasing the percentage of the capillary pore. The mechanical interlocking effect of PVA fibers ounterbalances the internal stress, which is caused by freezing the water.