1. Introduction
Geopolymer is considered an alternative cementitious material that
alleviates environmental problems, and has the advantages of low
CO2 emission of production and the use of large volume
of industrial
wastes
when compared with Portland cement. The hardened geopolymers contain
amorphous and quasi-crystalline three-dimensional network gels by
linking of tetrahedrons, such as silicon-oxygen tetrahedron,
aluminum-oxygen tetrahedron and phosphorus-oxygen tetrahedron.
Considering the special chemical activation process and the resulting
three-dimensional network structure, geopolymers usually present the
advantages of rapid high strength, excellent corrosion resistance, and
high temperature stability, and can be extensively used as suitable
binders in building, heavy metal adsorption, traffic repair projects,
nuclear waste treatment and other fields.
However, geopolymers with high
strength are also highly brittle, and methods overcoming the brittleness
of geopolymers can be broadly divided into three categories: (a)
adjustment of substrates by chemical or physical means; (b) internal
reinforcement; and (c) providing external constraints. Of the three
solutions, the latter two are generally more efficient and less costly.
One
of the ways to improve brittleness is to add fibers to the geopolymer
matrix to increase its density and toughness. Zhang et al.[1]
introduced the concept of fiber factor that describes the comprehensive
influence of fiber content and fiber length. The concept of fiber factor
was used to prove that the toughening effect of polyethylene fiber on
geopolymer was significantly better than that of steel fiber, polyvinyl
alcohol fiber and basalt fiber on cement-based materials. Mohammed et
al.[2] found that the toughening effect of geopolymer with 8mm
polyvinyl alcohol fiber is better than that of geopolymer with 13mm
polyvinyl alcohol fiber, indicating that smaller fibers can better
bridge the cracks in grounding polymers, thus providing better
performance. Riahi
et
al.[3] prepared the alumina-coated steel fiber reinforced
geopolymer, and found that the flexural strength and compressive
strength increases by 17 % and 14 %, respectively compared with the
uncoated steel fiber. Rachel et al.[4] found that
the
addition of bagasse fiber into the geological polymer can reduce the
density and increase the sound velocity, thereby improving the sound
insulation and thermal insulation properties. However, such an addition
will reduce the compressive strength of the matrix, and
the
permeability increases with the increase of fiber content. Ma et
al.[5] added short carbon fibers into geopolymers, which can
significantly improve the rheological properties of geopolymers, and
found that the flexural strength and compressive strength of the
composites are improved and reach the peak in the case of a fiber
content of 3 %. When the fiber content increases to 4 %, the fiber
agglomerates and reduces the mechanical properties. The geopolymer with
short carbon fiber is endowed with the advantages of light weight, high
strength and good toughness,
which
opens up a road for the practical application of fiber reinforced
materials. Su et al.[6]
prepared
geopolymer using fly ash and slag as raw materials of aluminate
silicate, and incorporated different fibers and hollow microspheres into
the slurry to improve the properties of the composites. The results show
that
the
improvement effect of fibers on reinforced materials is in turn of
polypropylene fiber, alkali-resistant glass fiber and lignin fiber, and
that fiber can prevent the separation of hollow microspheres and
geopolymer matrix, inhibit the generation and development of cracks, and
finally improve the strength whether it is mixed fiber or single fiber.
Liu et al.[7] overcame the brittleness of geopolymer matrix using
four kinds of steel fibers. When the fiber content and length increase,
the fluidity of the mixture decreases and is not affected by the fiber
shape. The increase of fiber content and the decrease of the fiber
diameter contribute to the improvement of compressive strength, and the
bending performance improves with the increase of fiber volume and
length.
At
present, many studies have been conducted on fiber reinforced
geopolymers, but there are still few systematic summaries on the
addition of inorganic fibers and organic fibers to geopolymers.
To this end, this review focuses on the current status of research on
fiber reinforced geopolymer, and attempts to identify the limitations
and advances of fiber-reinforced geopolymer. Finally, the knowledge gaps
and remaining challenges for future work are discussed.