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.