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.