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Concrete due to its special feature, is the most widely consumed material in the world, after water. But the production process of ordinary Portland cement as a main component of conventional concrete, has major disadvantages such as high amount of CO₂ emission and high energy consumption. Therefore, it seems necessary to find an alternative to ordinary Portland cement. In recent years, geopolymer has been introduced as a novel green cementing agent and environment-friendly alternative to the Portland cement which can eliminate the extensive negative of ordinary Portland cement production process. According to the needed engineering characteristics perspective in civil engineering, the geopolymer concretes have better chemical and mechanical properties than the ordinary ones such as high compressive, flexural and tensile strength, rapid hardening, resistance against high heat and firing, low penetration, resistance against salts and acids attacks, and low creep. On the other hand, in terms of technical characteristics, concrete has some disadvantages, most notably low tensile strength and consequently low ductility. Therefore, the use of different fibers in the concrete mixture and the manufacture of fiber reinforced concrete is considered as an appropriate solution to eliminate these defects. Also, fiber reinforced geopolymer concrete is known as a novel type of concretes with higher ductility than ordinary concretes. In this experimental study, two types of polymer fibers, including simple polypropylene fibers and 4-element polyolefin hybrid fibers, were used to manufacture fiber reinforced geopolymer concrete specimens. In this regard, fiber reinforced and non-fiber geopolymer concrete specimens were made and cured in 80 °C for 24 hours. After curing, specimens were placed in the ambient condition and associated tests including: density, 3-days water absorption, 7-and 28-days compressive, Brazilian indirect tensile and three point flexural strengths, were performed to study effect of fibers on metakaolin-based geopolymer concrete mechanical properties. Also, to study effect of fibers on high-temperature resistance of metakaolin-based geopolymer concrete, specimens weight and compressive strength loss percentage after exposure to high temperatures up to 800 °C, were measured. The obtained results indicated that using fibers in geopolymer concrete mixture, result in increasing compressive, indirect tensile and flexural strengths and also decreasing in density and 3-days water absorption. Further, the use of hybrid fibers due to their ability to inhibit the cracking process from both micro and macro levels, significantly improved compressive, indirect tensile and flexural strengths compared to simple fibers. The optimum amount of 4-element polyolefin fibers for compressive, tensile and flexural strength improvement was measured 0.2%, 0.2% and 0.15% (by volume), respectively. Also, the optimum amount of polypropylene fibers for compressive, tensile and flexural strengths improvement was measured 0.2% (by volume). In term of high-temperature resistance, although the polymer fibers reduced the risk of the explosive sapling of geopolymer concrete specimens due to generation micro channels which were randomly distributed in concrete because of melting of fibers, resulting in less weight loss than non-fiber specimen, but on the other hand, the compressive strength loss of polymer fiber reinforced specimens were higher than non-fiber one. Overall, it can be concluded that these fibers did not have a significant effect on the high-temperature resistance of geopolymer concrete.
 

Keywords: fiber reinforced geopolymer concrete, hybrid fibers, polyolefin fibers, compressive strength, tensile strength, flexural strength, high-temperature resistance.

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