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].