## 1 Introduction  
Geopolymer (GP)-based concrete, first brought into light in the 1970s by a French scientist [PERSON], is an environmentally-benign material with a relatively lower carbon footprint compared to conventional concrete made from ordinary Portland cement (OPC) [1; 2; 3; 4]. Referred to as a next generation concrete, and also chemically classified as a polysililate, GP can be considered as a versatile, inorganic paste similar to zeolite materials with an amorphous microstructure [5; 6; 7]. Geopolymers are generally formed through a reaction taking place between aluminum-silicate minerals and alkaline solution at ambient temperatures. During this 'geopolymerization' reaction, -Si-O-Al-O- bonds, similar to amorphous feldspar, are formed when the source material reacts with the alkaline activator [8; 9; 10; 11]. The alkaline activator, which is another important factor that determines the performance of GP-based materials, can be sodium hydroxide (NaOH) or potassium hydroxide (KOH), which is taken in conjunction with other compounds such as sodium silicate (Na\({}_{2}\)SiO\({}_{3}\)) or potassium silicate (K\({}_{2}\)SiO\({}_{3}\)), sodium sulfate (Na\({}_{2}\)SO\({}_{4}\)), etc. [12; 13]; however, due to the availability and cost effectiveness, NaOH and Na\({}_{2}\)SiO\({}_{3}\) solutions are generally employed [14; 15; 16; 17; 7; 18]. There are a number of reports in the literature [19; 20; 21; 4; 22] highlighting that the binder material should be one that is rich in aluminum and silicon. Other related studies [23; 24; 25; 26; 18] have indicated that components such as FA, slag, and metakaolin are often used as the source material(s) as they are rich in oxides of silica and aluminum. It has been also reported that metakaolin is the most commonly used source material due to its consistent composition; however, it is relatively expensive [27]. Finely powdered FA, on the other hand, has since become a more popular source material. In addition to the fact that it is a waste residue from coal-based power-plants (thus making it more sustainable), FA-based GP concretes exhibit increased residual strengths after exposure to elevated temperature conditions (even up to 800 \({}^{\circ}\)C) [28; 29; 30]. It was also reported that the metakaolin-based pastes underwent a significant decrease (34%), while FA-based GP pastes displayed a 6% increase in the residual strength after exposure to 800 \({}^{\circ}\)C [31]. The gel microstructure of FA-based GP concretes generally control the internal moisture content, and thus plays a major role in increasing the resistance to spalling when exposed to elevated temperatures [32; 33]. A study to evaluate the strength of FA-based geopolymer pastes in comparison to OPC pastes, after being exposed to elevated temperature levels, revealed that the former was more resistant to degradation than the latter [34]. In addition, it was also established that the residual compressive strength remained somewhat constant at temperatures 800-1000 \({}^{\circ}\)C for GP-based concretes, while the compressive strength fell to zero at 800 \({}^{\circ}\)C for the OPC-based concrete [35]. In another report, the compressive strength of FA-based GP pastes, reinforced with carbon and basalt fibers at 0.5, 1, and 1.5% by weight, after being exposed to temperatures of 200, 400, 600, and 800 \({}^{\circ}\)C was tested [26]. The results revealed better compressive strengths, lower volumetric shrinkage, and lower mass losses from samples comprising of 1% fibers. Furthermore, specimens reinforced with carbon fibers performed better compared to specimens incorporating basalt fibers over the temperature range that was employed. [PERSON] et al. [36] studied the influence of melamine and steel fibers on the compressive and bending strengths of FA-based GP mortar specimens.