The Effect of Adding Fibers on Dry Shrinkage of Geopolymer Concrete

Despite their drastically different chemical ingredients and interactions, geopolymer concrete exhibits many of the same features as ordinary concrete. Among these properties is drying shrinkage. As in normal concrete, dry shrinkage in geopolymer concrete may cause cracking if the geopolymer concrete is bound, which affects the integrity of the structure in the future. It's important to measure drying shrinkage as soon as possible because it's the cause of early age cracking, which happens when the concrete isn't very strong. The purpose of this study is to determine how to reduce the dry shrinkage value of geopolymer concrete by using different types of fibers. Three types of fibers were used to determine their effect on the dry shrinkage of geopolymer concrete when compared with a reference mixture without the fibers. Metakaolin was used as a binder for the concrete geopolymer. As for the fibers, steel, carbon and polypropylene fibers were used in proportions of (0, 0.5, and 1%). The results showed an improvement in dryness shrinkage when adding fibers in general, with a difference in values between the different types of fibers. Steel fibers had the lowest amount of dry shrinkage. The temperature had a direct influence on the decrease in the extent of the shrinking, since the samples handled at higher temperatures had less dryness to begin with.


Introduction
Raw material characteristics, mixing ratios, processing methods, and other factors all influence shrinkage. Internal tension and even cracks in buildings are caused by excessive and uneven shrinkage, which affects the quality and durability of concrete [1][2][3]. Geopolymer concrete exhibits many of the properties of traditional concrete, although its chemical components and reactions are very different. However, only a few attempts have been made to assess the drying shrinkage of geopolymer concrete. Wallah & Rangan (2010) studied the drying of ocean-treated samples in the order of 1,500 microstrain, two to three times higher than that required for OPC-based parabolic cement [4,5]. A thermo-thermo-treated fly ash-based geopolymer concrete has dried after 3 months. Water is not directly integrated into the geopolymer gel product, unlike Portland cement. In the geopolymer gel, only a small amount of the mixing water remains as interstitial water [6,7]. Because MK-geopolymer pastes take a lot of water to mix, there is a lot of unbound or free water in the hardened paste that can evaporate at room temperature under low relative humidity circumstances. Despite the lack of chemically bonded water, structural stability is still required.
Under relatively typical climatic circumstances, excessive water loss might cause severe shrinkage cracking of specimens. One explanation for geopolymer drying shrinkage and cracking might be the formation of high capillary pressures between wet and dry regions of the micropore network, which has been shown to begin crack propagation in the microstructure [8]. Also, one of the reasons for the increase in dry shrinkage is the amount of the binder and the ratio of the alkaline liquid substance to the amount of the binder [9]. There have been attempts by Bell & Kriven (2009) [10] to avoid dry shrinkage and crack proliferation by modifying the pore structure to reduce capillary porosity and control water loss during processing. It was observed that the dry shrinkage of heat treated geopolymer concrete was generally lower compared to that of heat-treated concrete at ambient conditions [11,12]. This attribute was most likely due to the water that is released during ambient-curing, which then evaporates over time, especially within the first two weeks. Therefore, it effectively and influentially contributes to the properties of geopolymer concrete and makes it more durable [13] as well as making it a desirable option in the future when manufacturing various structural components [14].
On the other hand, Khan et al. explained the effect of curing temperature on dry shrinkage and creep of geopolymer compared to ordinary concrete consisting of Portland cement (OPC), where the increase in temperature led to less shrinkage and creep [15]. While Al-Hedad et al. (2020) found that adding reinforcement to the geopolymer concrete reduces the coefficient of thermal expansion and dry contraction compared to that without reinforcement [16]. Also, Gailitis et al. (2019) compared the shrinkage deformation between foam concrete and geopolymer concrete and found that the geopolymer shrinkage is less than that of foam concrete [17]. Many studies have addressed the effect of adding fiber to concrete. The chemical and microstructure differences between geopolymer concrete and ordinary concrete can lead to different binding properties between the matrix and the added fiber. As a result, the fibers in a geopolymer matrix may not necessarily perform the same as the fibers in a regular concrete matrix. Integrating fibers into the cementitious matrix is a well-known approach to improving the flexural properties and post peak characteristics of corresponding composites because fibers regulate crack propagation and widening under various forms of mechanical loading or shrinkage [18]. Deboning, sliding, and dragging fibers out are local processes that promote bridging action during micro and macro matrix cracking. This mechanism raises the energy requirement for the fracture to propagate [19].
Fibers of varied materials and geometric qualities, which may be classified into two categories: high modulus and low modulus, are utilized in construction [20]. Each group improves the matrix's individual properties. Metallic fibers, on the other hand, increase flexural strength due to their higher stiffness, while nonmetallic fibers, with a higher aspect ratio and surface contact area, regulate matrices' plastic shrinkage [21]. The effectiveness of fibers in geopolymer matrices is influenced by their intrinsic characteristics, fiber content, geopolymer precursors, curing conditions, and composite age [22]. The fiber/matrix interface, on the other hand, plays a critical role in the overall mechanical properties of composite structures; a strong contact interface has the ability to pass load from the matrix to fibers with high load bearing power, while fibers with inert surfaces result in weak interfacial contact, which leads to interface deboning and composite failure [23]. Because geopolymer is made mostly of water, the wettability of the reinforcement may be utilized to assess if the fibers and the matrix have a strong connection. Good wettability, which is defined by a low contact angle, aids the creation of a strong connection and the degree of efficacy of reinforcement in a geopolymer composite [24]. Noushini et al. (2016) investigated the effect of monofilament polypropylene fibers and monofilament structural polyolefin fibers on the long-term creep and shrinkage of fly ash-based geopolymer concrete and discovered that the addition of PP and PO fibers with a volume fraction of 0.5% resulted in reduced drying shrinkage and increased compression creep for fly ash-based geopolymer concrete [25].
In this study, further investigations are being conducted in order to evaluate the effect of fibers on concrete geopolymers upon initiation of the shrinkage test. The shrinkage tests started 24 hours, 3, 7, 14, 28, 56 days after casting. This drying shrinkage is a cause of premature concrete cracking, so evaluate it as soon as possible. According to the standard ASTM, or ANSI C157 [26], the curing temperatures were (35 and 75°C). This work will contribute to increasing the amount of experimental data available in the literature on the time-dependent behavior of concretions of geopolymer based on metakaolin content. This paper provides preliminary experiments on the effect of the addition of different types of fibers on the drying shrinkage of geopolymer concrete. Especially since Iraqi metakaolin is used in the production of this type of geopolymer.  According to ASTM C618, the binding material in the creation of geopolymer concrete is Iraqi metakaolin. The chemical composition of metakaolin as determined by the analysis is shown in Table 1 and Figure 2, with silica oxide (SiO2) accounting for 55.99%, aluminum oxide (Al2O3) for 38.32%, and iron oxide (Fe2O3) for 1.735%, and calcium oxide (CaO) accounting for less than 0.7%. The metakaolin utilized can be classified according to ASTM C618 based on these findings. It is primarily pozzolanic, with silicon dioxide reacting with calcium hydroxide from the cement hydration process to form the calcium silicate hydrate (CSH) gel, which produces cementitious compounds appropriate for geopolymer application. The presence of calcium ions resulted in a rapid reaction time; As a result, the geopolymer will harden quickly and cure faster [27].

Alkaline Solution
The alkaline solution is made from a mixture of a 12 molarity sodium hydroxide solution and a sodium silicate solution. To form the NaOH solution, NaOH (which is presented in flakes and pellets) granules were dissolved at 98% purity in water. Table 2 shows the properties of NaOH. In the case of sodium silicate solution, the ratio of Na2O to SiO2 and H2O affects the concentration of the solution, according to Table 3.

Fibers
The fibers used in this study are carbon, steel with hooked ends steel, and polypropylene. Table 4 and Figure 3 shows the characteristics of each.

High Range Water Reducing (HRWR)
For the production of geopolymer concrete, modified sulfated formaldehyde naphthalene condensation primer was used. KUT PLAST SP 400 HRWRA compliant with BS 5075 and ASTM C494 Type F in this study. The use of this type of HRWR results in a very high level of water reduction, and, as a result, a noticeable increase in strength can be gained along with maintaining good operability. Table 5 shows the main properties of KUT PLAST SP 400 HRWRA.

Fine Aggregate
Natural fine aggregates supplied by Samarra were used. All geopolymer concrete mixes contained fine aggregates with a maximum size of 4.75 mm and were graded into zone 2 limitations. Tables 6 and 7 were also used to classify fine aggregates that met the criteria of Iraqi Standard No. 45/1984 Zone 2 [28].

Coarse Aggregate
In all of the combinations, crushed gravel from Samarra was utilized as coarse aggregate. Gradient, specific gravity, sulfate concentration, absorption, and fine materials were all tested, according to Iraqi standards IQS 45/1984 [28]. Tables 8 and 9 show the grades and physical properties of the coarse aggregate which are respectively shown.

Mixture Design and Specimen's Preparation
Seven sets of mixtures were made and tested to see how different types and amounts of fibers affected the drying shrinkage of metakaolin-based geopolymer concrete. The aggregate content, both coarse and fine, was maintained at a consistent level across totally mixes. The concentricity of NaOH was 12 Molar, sodium hydroxide to sodium silicate weight ratios equaled (1:1) also surface area of metakaolin to be (2300) m 2 /kg, heat curing process was employed for these mixtures, with a temperature of 35 and 75 °C for 3 days.
Following that, the specimens are kept in the laboratory at the appropriate temperature until the time of test. Test was measured at different ages (1, 3, 7, 14, 28, and 56) days. Detailed information and mix proportions for metakaolin-based geopolymer concrete mixes are shown in Table 10.

Results and Discussion
The results of the length change experiment were used to determine drying shrinkage, and the (FGPC) specimens were tested for a total of 56 days after a three-day period of heat curing (at temperatures of 35 and 75 °C). After that, the samples were placed in the laboratory at ambient temperature until the time of the tests. Figures 4 and 5 depict the relationship between all of the mixes' changing length results at various testing times at 35°C and 75°C.
The length change of all mixes was lower than that of the reference mix (GPF0), as shown in Figure 4. This might be due to the heat treatment improving the micro-structural growth and pore amendment of the geopolymer paste [29]. The shrinkage test is performed according to the ASTM C157 test method. The measurement of dry shrinkage begins (24) hours after the casting is completed. The heat cured mixes at 75°C showed much lower dry shrinkage than the heat cured samples at 35°C, as shown in Figure 5. The fact that the majority of the geopolymerisation process occurs during the heat curing phase may have contributed to the heat cured samples' reduced dry shrinkage at 75°C. Tables 11 and 12 show that all fiber reinforced GPCs had smaller shrinkage strain than the reference mix (GPF0), which did not contain any fiber. The reduction in shrinkage is likely due to the greater tensile strength of the matrix material and/or the friction bonding of fibers in the matrix [30]. Drying shrinkage in geopolymer concrete is expected to be linked to an increase in pressure applied and tensions in the structural capillary network, resulting in concrete contraction only if the concrete is regulated [31]. When compared to the reference mix, the matrix in (GPS2) containing steel fibers had a greater tensile strength than (GPF0), as assessed by an indirect tensile test. This indicates that the matrix is more durable (as shown in Table 12), resulting in less dry shrinkage and better control of micro-cracking in the steel fiber-containing matrix (GPS2). The increased length of the steel fibers, as well as the end-hooked design, allow for improved matrix-fiber interface mechanical interlocking when using steel fibers. This results in increased friction, which helps to neutralize a portion of the shrinkage energy, resulting in less shrinkage of the fiber reinforced concrete mix.  (Day)  GPF0  GPS1  GPS2  GPC1  GPC2  GPP1  GPP2   1  700  550  452  582  501  615  517   3  766  616  495  653  554  678  569   7  864  689  528  734  599  769  612   14  933  697  543  810  628  830  641   28  986  706  551  881  642  895  649   56  997  712  559  894  661 899 654

Conclusion
All mixes show that the dry shrinkage of metakaolin-based and heat-cured geopolymer concrete is significantly lower than that of room-temperature-cured concrete. The addition of fibers helped in developing the mechanical properties of metakaolin-based geopolymer concrete compared to non-fibrous ones. The higher the fiber content, the lower the dry shrinkage of the geopolymer concrete. Based on the tests performed and the results obtained in this study, it can be concluded that various fibers can be used as reinforcement for geopolymer concrete, as they do not cause negative effects on the mechanical properties of concrete but enhance the behavior of geopolymer concrete during dry shrinkage. The results of the tests also showed the effect of heat curing on the value and rapidity of dry shrinkage. When heat curing was increased, the dry shrinkage value was lower, because most of the geopolymerisation process takes place during the heat treatment stage, which contributed to reducing the dry shrinkage of the samples treated at high temperature. Also, the results showed that the addition of steel fibers with the end hook led to the lowest value of dry shrinkage compared to the rest of the other types of fibers. The length of the fiber and the back hook shape of the steel fiber help absorb part of the shrinkage energy, which affects the dry shrinkage value.

Data Availability Statement
The data presented in this study are available in article.