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].