Production and characterization of heat retardant fiber-reinforced geopolymer plates

Abstract

Keywords: Fiber-Reinforced; Geopolymer; Thermal Instulation; Metakaolin

1. Introduction

Concrete is the most used building material because of its mechanical properties, ease of handling, easy shaping, and availability of its raw materials. Producing and using that tremendous amount of concrete requires its typical constituent, ordinary Portland cement (OPC). OPC needs large amounts of natural resources for its production. 1.5–2.8 tons of raw materials are needed to produce 1

ton of OPC [1–3]. In addition, OPC production requires high temperatures of around 1500 oC, which causes large energy consumption. Thus, 12–15% of the total energy is used worldwide for all OPC production processes [4]. Energy consumption reaches about 40%, considering the construction industry. From the point of view of CO2 emissions, 5–8% of all CO2 emissions come from OPC production [5–7], and 1/3 of greenhouse gases emission are contributed by the construction industry [8]. Besides

*Corresponding author. *E-mail address: ucengiz@comu.edu.tr Published by Yıldız Technical University Press, İstanbul, Türkiye This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

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the CO2 emission, more toxic gases such as Sulfur trioxide (SO3) and nitrogen oxide (NOx) are released into the atmosphere, which accelerates global warming and causes acid rain [9]. Recently, research has focused on developing alternative materials to OPC due to environmental and energy concerns [10–12]. One promising alternative to OPC is geopolymer material, which uses raw materials rich in aluminosilicate content and an alkaline solution to activate it [13–16]. Most of the geopolymer synthesis methods occur in this process. For example, aluminosilicate substances used in geopolymers may come from natural sources such as metakaolin (from kaolinite) or industrial wastes like fly ash and ground granulated blast furnace slag [2, 17–19]. These precursors are blended with an alkaline solution of silicate with the addition of a base, usually concentrated sodium or potassium hydroxide. The resultant material form looks similar to concrete paste but without Portland cement. Geopolymers have many environmental and mechanical benefits over OPC [20, 21]. For example, fly ash-based geopolymers cause 80% less CO2 in the atmosphere and consume 60% less energy during their production processes than OPC [22]. In addition, geopolymers can have better compressive strength and durability [23, 24], adjustable strength and workability [25–27], lower shrinkage and creep [28], better resistance against chlorides, acid attacks [29, 30], superior fire resistance [31], and improved thermal insulation properties [28]. Moreover, geopolymers can reach 90% of their top compressive strength in the first 72 hours [32]. This specialty makes it an ideal choice for early strength applications [33]. Using these alternative products has increased attention in construction due to the critical solution of decarbonization and energy-saving. Integration and formulation of thermal properties have been proven preferable by improving the energy efficiency and durability of the buildings [34]. One of the under research and promising applications of geopolymers are fireproof and fire retardant products [35–37]. In addition, some publications have shown their potential as thermal barriers [38, 39]. These studies proved that geopolymers could be used against fire and show high thermal stability. One of the properties of the geopolymers, brittle behavior, puts a limitation on the application areas of geopolymer materials [40, 41]. Nowadays, researchers are trying to solve this problem to improve this mechanical property [42, 43]. Fiber-reinforcing is a well-known solution to overcome this problem. For this reason, researchers' studies with different types of fibers focused on reinforcing geopolymer [37], [44]. Polymer fibers are the first preferable for reinforcing the geopolymers [43]. Improving the mechanical properties, especially bending strength, is the main reason for adding fibers into geopolymer. The addition of some chemical fibers into geopolymers can reveal some benefits. Fiber reinforcing

can cause better properties, such as higher resistance to fire or a decrease in thermal conductivity, depending on their types [44, 45]. The main objective of this study is to produce an alternative environment-friendly thermal insulation material for the market. Moreover, this study aimed to produce this building material with superior heat resistance properties and comparable strength to the concrete produced with OPC. For this purpose, commercially purchased metakaolin was used as precursor material. Alkaline solutions were prepared with NaOH, KOH, SiO2, and Na2O. The geopolymerization was obtained by mixing metakaolin and the alkaline solution at room temperature. Cellubor (CB) and polypropylene (PP) fibers were used as reinforced materials to improve the heat resistance properties of the geopolymer products. The study resulted in a thermal comparison of fiber-reinforced geopolymer plates and plates prepared with OPC. It was found that the heat resistance properties of metakaolin plates containing fibers gave 55% better results than metakaolin plates without fibers.

3. Results And Discussions

Figure 2. Flame test. and gas regulator. The thermocouple thermometer used in this study is UDL 100 brand. It measures the front and back side temperature of the plates. The flame test was applied directly on the panel from a 25 cm distance clamped between the aluminum panel, as seen in Figure 2. The plates' front and back surfaces temperature are recorded at a speed of one-tenth of a second with J-type temperature meters connected to the computer. At this time, the flameprone plate surface was also measured using an infrared

Fiber-containing geopolymer plates, fiber-free geopolymer, and Kalekim plates were fabricated. The study consists of 2 stages. The first step was to perform flame tests of fiber-free metakaolin and Kalekim plates and to determine the time it takes for the back side temperature to reach 50 o C. The second stage of the study was the flame tests of the fiber plates and the determination of flame retardancy. A procedure has been developed to compare the study flame test results. Accordingly, the plate's maximum temperature reached by the front side was TFrontMax. When the TFront-Max temperature was determined, the back temperature of the plate was recorded as backplate temperature "TBack" (24±0.5 oC). In addition, the time elapsed from the start of the flame test to the occurrence of these temperatures (TFront-Max and TBack) was recorded as the "tBack" time. TBack (oC) temperature and

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Table 3. Flame test results of metakaolin plates containing different ratios of CB fiber

Table 4. Flame test results of metakaolin plates containing PP fiber in different ratios

the tBack (s) time were defined as the initial temperature and time of the back plate. The first time the back plate reaches 50 oC, it was reported as "tBack (s) @ 50 oC" in the test records. This value measures how long after the start of the experiment the backplate reaches 50 °C. The difference between the time of the front and back of the plate to reach the same temperature was expressed as "Δt(s)." This period shows how long the plate transmits Back the heat from the front side to the backside. Firstly, the flame test of the fiber-free Kalekim plate was performed. The heat delay time of the Kalekim plate was found to be 12 seconds. In the same experiment, when it is made with a metakaolin plate that does not contain any fiber, it is seen that the back side of the plate reaches 50 oC in the 21st second. Therefore, the metakaolin plate provided heat retardancy 9 seconds longer than the Kalekim plate (Table 2). The back side of the fiber-free metakaolin plates was observed to reach 50 oC 9 seconds later than the Kalekim plate. This heat transfer delay was due to the metakaolin structure [48]. The back plate temperature of metakaolin reached 27.9 oC after 39 s of the start of the experiment at the 909.8 oC of the front plate. Regarding the Kalekim plate, the TBack reached 32.2 oC at 16 s after the experiment at the 880 oC of the front plate. Therefore, when the duration of reaching 50 oC is compared to both plates, the Metakaolin plate was reached after 32 s than the Kalelim plates, which was essential delaying of heat transfer. Regarding fiber-reinforced plates, CB, PP, and CB+PP mixed fibers were used to obtain fiber-containing metakaolin plates. Flame test analysis results of CB series metakaolin plates containing cellubor fiber in different ratios are shown in Table 3.

Figure 3. Metakaolin plates flame retardant time variation due to increased CB fiber content. The tback @50 oC of CB fiber-reinforced metakaolin increased depending on increasing CB fiber content after 3% wt in the metakaolin mortar. The variation of flame retardancy times according to the fiber ratio of metakaolin plates containing different ratios of CB fiber is given in Figure 3. When the CB-fiber content was up to 3% wt, no relation was observed between CB and the tback @50 oC, and heat transfer delaying remained constant. However, a linear trend was observed after 3% wt of fiber content due to the CB fiber heat resistance property. In addition, even with 1.96% wt of CB, the heat transfer delay was observed comparing the fiber-free metakaolin and Kalekim plates (Fig. 3). Regarding the PP fiber-reinforced metakaolin plates, no relation was observed between the heat transfer delay and fiber content, as shown in Table 4.

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Figure 4. Metakaolin plate flame retardant time change due to increased PP fiber content.

Figure 5. Change of flame-retardant time of metakaolin plate due to increasing CB+PP fiber mixture.

Table 5. Flame test results of metakaolin plates containing different ratios of CB+PP fiber mixture

The variation in flame retardancy times according to PP fiber content is given in Figure 4. This figure indicated that constant flame retardancy times were observed despite increasing the fiber ratio. It is due to the lower PP fiber melting properties. The PP fiber melts after 120 oC and fills the porous structure of the metakaolin. Thus, the heat-delaying properties are reduced due to the lowering space of the metakaolin. However, PP fiber-reinforced metakaolin plate has better heat retardancy than the fiber-free metakaolin and Kalekim plates. The CB and PP mixed fibers were also investigated for the thermal performance of metakaolin plates. The flame test results of CBPP series metakaolin plates containing CB+PP fiber mixture in different ratios are given in Table 5. The variation of flame retardancy times according to the fiber ratio of metakaolin plates containing CBPP fiber in different ratios is seen in Figure 5. This figure indicated that constant flame retardancy times were observed despite increasing the fiber ratio similarly to PP-containing metakaolin plates (Fig. 4). This similarity is due to the melting properties of PP fibers. However, the introduction of CB fiber in PP fiber-reinforced metakaolin plates showed more heat insulation (Fig. 5). In addition, CBPP fibers brought approximately three times stronger fire retardancy to the Kalekim plate and 1.5 times to the metakaolin fiber-free plate (Fig. 5).

4. Conclusions

This paper produced CB, PP, and CBPP fiber-reinforced geopolymer plates. The flame tests of the plates were carried out. First, according to the flame test results, all the mortars reinforced with fibers showed higher heat retardancy than the fiber-free metakaolin and Kalekim mortars. Depending on the increased CB ratio, it was observed that the temperatures of the back parts of the plates reached 50 oC in between 20 and 43 seconds. Furthermore, it was found that the metakaolin plate containing 5.66% by weight of CB showed approximately 55% more heat retardancy than the fiber-free metakaolin plate and 72% more than the fiber-free Kalekim plate. On the other hand, the performance of PP-containing metakaolin plates did not change seriously depending on the variation of PP content, and the time to reach 50 oC for the back plate was around 31±2 seconds. This result indicated that a small amount of PP in the building is enough to supply heat resistance. However, heat resistance performance remained stable despite the increasing PP content. It is due to the filling of the gaps with melting PP. Although the flame test results of CBPP series metakaolin plates containing a certain amount of PP and CB blended fibers are relatively good compared to plates containing only PP fibers but not good as CB fibers. The melting PP reduced the gaps between

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the CB fibers, so the heat retardancy performance decreased compared with the CB fiber-reinforced metakaolin plates. As a result of the experiments, it was found that the metakaolin boards containing approximately 6% by weight of CB in the structure showed 72% better heat resistance results than Kalekim boards and 55% better results than non-fiber metakaolin boards. This study is a pioneering study in producing fiber-containing plates, and it is expected that the produced plates will be used as building materials in the construction industry in future studies, as they have both heat resistance and fire retardant properties.

Ethics

There are no ethical issues with the publication of this manuscript.

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Financial Disclosure

This work was supported by Çanakkale Onsekiz Mart University, The Scientific Research Coordination Unit, Project number: FYL-2019-3084.

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