Fig .5. Effect of fiber content on the flexure strength of geopolymer
[12-14, 34, 36, 38-40, 43, 44][14, 21, 23, 28, 40]
The influence of different fiber dosages on the flexural strength of geopolymer is shown in Fig. 5. Steel fiber significantly improves the flexural strength, which is greatly related to the stiffness of steel fiber itself, and the flexural strength increases with the increase of the volume fraction of steel fiber in geopolymer. In the case of a PP fiber dosage of 2%, the flexural strength of the matrix can be greatly improved. Ramamohana[13] found that after adding polypropylene fiber and steel fiber, the bending strength in Day 28 increases by 26.36 % and 57.79 %, respectively, compared with that of pure slurry geopolymer. Chuan Wang[14] found that polypropylene, glass and basalt fiber can increase the flexural strength of pure paste by 16.07 %, 30.35 % and 35.36 %, respectively.
Besides, it has also been found that plant fibers such as sisal fiber, coconut fiber and jute fiber are better than inorganic fiber and organic fiber in improving the flexural strength of geopolymer. For instance, 2% sisal fiber can improve the flexural strength of the matrix by 300%.
Adding fiber in geopolymer can improve the flexural strength of the matrix and greatly improve the bending toughness. The reason why the fiber can enhance the flexural strength of the matrix is as follows: (1) the geopolymer has multiple cracks in the process of bending deformation; (2) fiber bridging plays an important role in crack stability and multi-crack induction; and (3) the existence of fiber changes the stress distribution during the specimen deformation process, and distributes the stress evenly in the specimen, thereby improving the flexural strength.

4.3 Tensile strength

The tensile properties of fiber reinforced composites are greatly affected by the matrix type, fiber type, fiber volume, loading rate, interface bonding strength and even the type of the used impact machine.
The tensile failure process of composites is a progressive damage process: considering the existence of defects, some fibers will fracture first in the initial loading process, and local thermoplastic deformation will occur in the matrix and interface near the fiber fracture. Besides, the redistribution of microscopic stress deformation will occur, accompanied by more fiber failure and local plasticity, considerable fiber instability failure and the final failure of composite materials. It can be found that the tensile strength failure of composites depends on various loss evolutions including fiber fracture and inelastic deformation of the matrix and the interface.
The effects of different fiber dosage or length on the 28-day tensile strength of geopolymers are listed in Fig. 6. Given the brittleness of the geopolymer, the addition of fiber can greatly improve the tensile properties of the matrix, which is not only attributed to the high tensile strength and elastic modulus of the fibers, but also the fact that stress in the sample can be transferred to the fibers through the interface with the geopolymer matrix.