Fig. 10. (a) Chemical erosion and freeze-thaw cycle damage
mechanisms;[63]
SEM before and after Cl- attack; (c) SEM before and
after SO42- attack[18]
Reproduced from [18,63], with permission from [Publisher]
The resistance to chloride ion erosion can be characterized by
resistivity. A sample with a lower total charge passing through is
provided with a higher chloride penetration resistance[64]. The
local geopolymer possesses high permeability and low resistivity, so the
permeability of chloride ion is also high. Adding fiber can reduce
chloride ion penetration, and polypropylene fiber is more effective than
steel fiber. Considering the conductivity of steel fiber itself and the
formation of rust around it, the resistance to chloride ion erosion is
not as good as that to polypropylene fiber[65]. Zhu et al.[66]
studied the influence of liquid-solid ratio and slag substitution rate
on the chloride ion resistance of geopolymer, and found that chloride
ion permeability largely depends on the porosity and tortuosity of
composites. The porosity of composites decreases with the decrease of
the liquid-solid ratio, and the decrease of porosity and increase of
tortuosity are beneficial to the decrease of chloride ion permeability.
Banana fiber and coconut shell fiber improve the toughness of the
matrix, enhance the durability of the composite against acid erosion,
and coconut shell fiber presents a better effect[67]. The mass loss
of bamboo fiber composite is about 18% when exposed to an acidic
environment, and it possesses good durability[68]. In 10% and 32%
hydrochloric acid solutions, the compressive strength of carbon fiber
reinforced geopolymer decreases by 66% and 61.3%, and the mass loss
decreases by 5.3% and 3.7%, respectively, which can be used for
improving the durability of reinforced concrete bridges[69].
The degradation of geopolymers by sulfate erosion mainly focuses on the
effect of pores and cracks on the matrix. The porosity and pore size of
geopolymers increase with the increase of the sulfate concentration,
which will thus reduce the compressive strength. However, a mixture of
organic fibers (polypropylene and polyethylene) and inorganic minerals
(wollastonite) can improve the sulfate resistance of the
composite[26]. After 15 sulfate cycles, the compressive strength of
the polymer with 0.2% polypropylene fiber, 0.3% basalt fiber and 0.4%
steel fiber reaches 67.9MPa[69]. Besides, it is also found that
steel fiber and propylene fiber are acid resistant, while synthetic
fiber performs weakly in this aspect[70]. The durability of the
geopolymer with a fiber content of 1.2 kg/m3 remains
good after soaking 60 times in 5% sodium sulfate solution[71].