Fig .3. Effect of fiber content on the compressive strength of geopolymer
[12-14, 18, 21, 23, 28, 33-41].
The effect of different fiber contents on 28 d compressive strength of geopolymer is listed in Fig. 3.
Due to the high rigidity of steel fibers, its addition improves the compressive strength of geopolymer. The experimental results of Zada Farhan show that 6mm steel fibers improve the compressive strength more efficiently than 12mm steel fibers[34], and that the alumina coating can improve the interfacial bonding strength between steel fiber and geopolymer, which is 151% higher than that of uncoated steel fiber[3]. With the increase of polypropylene fiber content, the decrease in strength is attributed to the formation of interpolymer voids and weak interfacial bonding zone. Murthy’s study shows that 1% is the best content for polypropylene fiber to improve the compressive strength of geopolymer[35], while Bellum’s research indicates that 2% is the best content for polypropylene fiber to improve the compressive strength of geopolymer[13], and Wang’s study considers 0.8% polyvinyl alcohol fiber the best dosage to improve the compressive strength of geopolymer. However, Wang and Li’s study indicates that a higher polypropylene fiber content represents a higher compressive strength[18] [37]. Xu has found that the optimal dosage of basalt fiber to improve the compressive strength of geopolymer is 0.6%, and that the comprehensive performance of 6mm basalt fiber is better than that of 3mm basalt fiber [38]. Wang’s study shows that a higher content of basalt fiber indicates a better compressive strength[14], and Bai’s study recognizes 0.4% glass fiber as the best dosage to improve the compressive strength of geopolymer. Glass fiber can eliminate the microcrack of polymer and improve the mechanical properties of geopolymer[28]. Silva’s research shows that a higher sisal fiber content means a better compressive strength[23]. Wongsa’s study implies that sisal fibers greater than 0.5% reduces the compressive strength of geopolymers[21], that the jute fiber optimal dosage is 0.5%, and that the cotton fiber optimal dosage is 0.5%[41]. More than 0.5% of coconut fiber decreases the compressive strength of the geopolymer[21].
As shown in Fig.4, internal tensile stress is generated when the fiber reinforced geopolymer is squeezed by external force. The appropriate amount of fiber can play a bridging role and effectively inhibit the crack propagation. The strength growth percentage of cement-based materials caused by fiber inclusions is usually lower than that of geopolymer, which can be explained by the ceramic-like and polymer-like properties of geopolymers and better fiber matrix interface. Geopolymer has finer pore distribution than cement-based materials.