Fig. 9. SEM images of geopolymer with different 3mm basalt fiber contents (a-0%; b-2%; c-4%; d-6%; e-8%; f-10%)[38]. Reproduced from [38], with permission from [Publisher]
The drying shrinkage value of fly ash base geopolymer added with steel fiber is in the range of 264-297 microstrain, that of fly ash base geopolymer added with polypropylene fiber ranges 394-424 microstrain[12], and that of the matrix can be minimized by adding 0.6% polypropylene fiber to the geopolymer[57]. Given that basalt fibers promote the development of C-S-H, the addition of this fiber to the geopolymers can reduce the drying shrinkage of the matrix,. The combination of CASH and NASH leads to the refinement of the pores, and the fiber acts as micropolymer in the matrix, which can well disperse the sample stress [58]. As is shown in Fig. 8, a longer basalt fiber indicates a better shrinkage rate in the case of constant basalt fiber content, and the shrinkage rate of the sample with 6mm basalt fiber is lower than that with 3mm basalt fiber. As is shown in Fig. 9, when the fiber content is constant, a longer fiber indicates fewer roots per unit volume. In this case, the binding energy between fiber and slurry increases with the increase of the fiber length[38]. Adding 0.4% polypropylene fiber and 0.5% expansion agent MgO to the geopolymer can effectively reduce the shrinkage of the matrix[59]. The dry shrinkage rate of fly ash and slag base polymer with 0.3 and 0.6 volume fraction of PVA fiber is 20.53% and 45.69% at Day 7, and 26.92% and 41.18% at Day 28, respectively[60].
The drying shrinkage of the matrix can be reduced when fiber is added to the geopolymer. Since the fiber restrains the extension of micro-cracks in the specimen, it bears part of the stress caused by the shrinkage of the matrix, thus reducing the shrinkage strain of the material[61].

5.2 Chemical resistance

There are many types of chemical erosion, mainly including sulfate erosion and chloride ion erosion. Corrosion is caused by the reaction of sulfate ions and chloride ions from the external environment with hydration products, which results in expansion and cracking, and thereby reduces the matrix properties[62]. It is of great significance to study the effect of fiber-reinforced geopolymers on chemical erosion, since chloride ion and sulfate erosion and freeze-thaw cycle are easy to occur in marine environment. Fig.10(a) describes the breakdown mechanism of chemical erosion (SO42- and Cl- attack) and physical processes (freeze-thaw cycles), and Figures 10(b) and (c) show the SEM before and after Cl- and SO42-attack. The matrix eroded by Cl- presents cubic crystals (NaCl after drying), indicating that Cl-invades the internal voids of the geopolymer cementification. However, the matrix changes from a uniform and dense one to a porous one after sulfate erosion[18].