Fig. 6. Effect of fiber content on the tensile strength of
geopolymer[13, 21, 22, 36, 40, 45, 46]
The tensile strength of the geopolymer increases with the addition of
steel fiber because of its greater rigidity. Bellum’s study shows that
the best content of polypropylene fiber reinforced geopolymer is 2%,
and that the best tensile strength is 7.23MPa. The research conducted by
Farooq indicates that the tensile strength of geopolymer increases with
the increase of polyvinyl alcohol fiber content. Wang’s study presents
the maximum splitting tensile strength of 4.84MPa of the 6 mm basalt
fiber reinforced geopolymer. The tensile strength of the geopolymer
increases with the increase of the glass fiber content. The study of
Wongsa shows that sisal fiber reinforced geopolymer possesses the
greatest tensile strength, while the tensile strength of coconut fiber
reinforced geopolymer increases with the increase of coconut fiber.
However, Zhang’s research shows that the tensile strength increases by
4.6 %, 21.3 %, 4.3 % and 74.3 %, respectively by adding 1 % single
fiber PP, PVA, RPP and steel fiber into the geopolymer. Steel fiber is
most effective in improving tensile strength because of its high tensile
strength and stiffness, but the tensile strength of geopolymer with PVA,
PP and RPP increases by 38.2 %, 36 % and 48.9 % by the incorporation
of mixed fibers. In this case, it is more effective to mix PVA, PP, RPP
and steel fiber to improve the tensile strength of geopolymer than to
use single fiber merely[47].
Considering its own characteristics such as high rigidity and high
tensile strength, fiber can obviously improve the splitting tensile
strength when added to geopolymer. However, the mixed fiber has a better
improvement effect, and the mixture of PP and steel fiber presents the
best effect in the case of improving the splitting tensile strength.
4.4 Fracture toughness
Fracture toughness represents the ability of material to prevent against
crack propagation and is a quantitative index to measure the toughness
of material. The test methods of fracture toughness include direct
tensile method, compact tensile method, wedge splitting method and
three-point bending beam method. The fracture toughness of the material
is larger in the case of a constant crack size, and fracture energy is
defined as the energy required for the unit area of fracture propagation
in brittle materials, which reflects the energy change during the crack
development[48].
Fiber factors can describe the combined effect of fiber content and
length: the fracture toughness increases with the increase of the fiber
factor in the case of a fiber factor less than 600, indicating that the
fiber matters considerably in restricting the cracks at the initial
stage of geopolymer cracking. The fracture toughness of geopolymer
decreases when the fiber factor exceeds 600.
RI=V*L/D
Where RI denotes the fiber factor; V, the fiber volume content; and L/D,
the fiber length to diameter ratio.
Zhang et al.[1] studied the influence of the ratio of polyethylene
fiber to water-binder ratio on the fracture toughness of geopolymers,
and found that polyethylene fiber has a good toughening effect on
geopolymers. The fracture toughness of the matrix decreases when the
fiber content exceeds 1.05%. Due to the high fiber content, the matrix
has more pores and interfaces, resulting in more defects. Behzad et
al.[49] found that when 2% polyvinyl alcohol fiber is added to fly
ash base geopolymer, the fracture toughness can reach 0.436Km. It has
also been found that adding 0.5wt% cotton fiber can increase the
fracture toughness of geopolymer by 1.12MPa·m1/2.
Considering the poor dispersion of cotton fiber in slurry, the fracture
toughness decreases with the increase of fiber content. At the same
time, the dispersion of cotton fiber in geopolymer has a great influence
on the fluidity, adding 0.7 and 1.0wt% cotton fiber will greatly affect
the fluidity of the matrix, which must be compensated by increasing the
water content of the mixture, and increasing the porosity and
microcracks of the matrix. At 0.5wt% cotton fiber, the mechanical and
fracture properties of geopolymer composites are optimized. These cotton
fiber-reinforced geopolymers can be applied to panels or shingles of
siding, roofing, piping and cooling towers[50]. Ghasemzadeh et al.
studied the effect of a certain volume of steel and polypropylene fibers
on the fracture characteristics of ultra-high performance geopolymer
concrete based on abrasive blast furnace slag and silica fume. With the
increase of fiber content, the fracture energy increases compared with
the control group, and the substitution of polypropylene fibers for
steel fibers slightly reduces both types of fracture energy[51].
When the properties of fibers such as the hardness cannot be changed,
many scholars improve the fracture toughness of composites by filling
admixtures to improve the density of the matrix[52]. The fiber plays
a bridging role in the matrix and transmits internal stress in the
matrix, thereby improving the ability of resisting crack generation and
propagation.
4.5 Shear strength
Shear strength refers to the ultimate strength of the material when it
is cut, and reflects the ability of the material to resist against shear
sliding.
The influence of different types and volume fractions of fibers on the
shear strength of geopolymers is shown in Fig. 7. The optimal dosage of
steel fiber (SF) to increase the shear strength of geopolymer is 0.5%
and the increment is 56%. However, the shear strength of geopolymer
decreases when the dosage is increased to 1%. Given that the geopolymer
concrete with high blast furnace slag content has poor workability and
fiber dispersion may be affected by the high viscosity of sodium
silicate solution, the optimal content of steel fiber in geopolymer
decreases compared with that of steel fiber in cement of 2.5%. Besides,
the mixed fiber is better to improve the shear performance of polymer.
In order to obtain good shear strength with low fiber content, it is
recommended to add steel fiber and polypropylene fiber as well as steel
fiber and polyvinyl alcohol fiber to the geopolymer.