Fig. 13. (a) Effect of polyvinyl alcohol fiber content on sulfate resistance of geopolymer
(b) Appearance of geopolymer after freezing-thawing cycles[18]
Reproduced from [18], with permission from [Publisher]
The pure fly ash base polymer is destroyed after 50 freezing-thawing cycles, but the addition of 0.1% carbon nanotubes and 2% polyvinyl alcohol fiber withstands 175 freezing-thawing cycles[78]. Polyamide fiber can also improve the freeze-thaw cycle durability of fly ash base geopolymer[79]. Besides, the compressive strength of polypropylene fiber reinforced geopolymer increases during 56 freeze-thaw cycles, but decreases during 300 freeze-thaw cycles[80]. The durability of polyvinyl alcohol fiber reinforced geopolymer composites decreases with the increase of fly ash and bentonite. When the ratio of fly ash to cement is 1.8, the durability of polyvinyl alcohol fiber reinforced geopolymer composite is the highest in water[81]. After 150 cycles of wetting and drying, the durability behavior of fiber reinforced cement matrix composites in different solutions turns to be: water>MgSO4>Na2SO4(aq) [82]. As shown in Fig.13, the 0.6%-0.8% polyvinyl alcohol fiber and 1.0%-2.0% nano-SiO2 can enhance the geopolymer durability to the greatest extent, and the residual compressive strength can reach 57.3 Mpa-58.8 MPa after 25 freeze-thaw cycles[18]. The strength of slag fly ash base geopolymer prepared with polypropylene, steel fiber and polyamide fiber does not change significantly after 250 cycles, indicating its excellent durability[12]. The strength loss of basalt fiber reinforced metakaolin base geopolymer is small after 90 freeze-thaw cycles[83], and after 180 freeze-thaw cycles, the compressive strength of the geopolymer with 0.8-1.2% basalt fiber increases while the flexural strength decreases[39].
The addition of fibers can improve the internal structure and macroscopic mechanical properties of geopolymer[81]. Firstly, the fiber is uniformly distributed in the matrix to balance the internal stress caused by water freezing. Secondly, fiber fills the internal pores, and increases the density of the matrix. Therefore, the incorporation of fibers can significantly improve the freeze-thaw cycle resistance of geopolymer[78, 84].

5.4 Other performances

Additionally, the fiber performs excellently in the electric conductivity and high temperature resistance. Carbon fiber and ceramsite can also be used as conductive admixtures in mortar to effectively reduce the resistivity of mortar, but exercise a negative impact on its workability and strength development[85]. Fiber and hollow glass beads are added to geopolymer to prepare heat-insulating composite materials. The improvement effect of fiber on shrinkage resistance of materials is polyvinyl alcohol fiber, glass fiber and lignin fiber in order[6]. The influence of steel fiber, polypropylene fiber and polyvinyl alcohol fiber on the low temperature erosion bending strength and compressive strength of ultra-high performance concrete (UHPC) is also studied, and the results show that the addition of steel fiber and polyvinyl alcohol fiber increases the bending strength and compressive strength of UHPC, and that the bending strength of UHPC increases by 70.06% with the increase of the length-diameter ratio of steel fiber. The compressive strength of UHPC decreases after cryogenic attack. Polyvinyl alcohol fiber and Polypropylene fiber absorb more water, which affects the hydration of cement in UHPC, thereby reducing the strength of UHPC[86]. In order to enhance the adhesion between fiber and matrix, the surface of fiber is modified with chemical reagents such as graphene and silane to enhance the hydrophilicity and roughness of the fiber surface[87].