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.