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].