Study on the Effect of Jute CNFs Addition on the Water Absorption and Mechanical Properties of Geopolymer Concrete

Abstract

This study investigates the influence of thermoplastic polyurethane (TPU) reinforced with jute cellulose nanofibers (CNFs) on the water absorption and mechanical properties of geopolymer concrete. The integration of TPU/jute CNF nanocomposites into geopolymer concrete is explored as a strategy to enhance both its durability and mechanical performance. Geopolymer concrete, a sustainable alternative to traditional Portland cement concrete, is known for its low carbon footprint, but it suffers from high brittleness and water absorption. The water absorption behavior of the modified concrete was assessed, revealing a significant reduction in water uptake due to the hydrophobic nature of TPU and the reinforcing effect of jute CNFs. Additionally, the mechanical properties, including compressive and flexural strengths, were evaluated to understand the impact of the nanocomposites on the structural integrity of the concrete. The addition of TPU/jute CNFs notably enhanced the splitting tensile strength (63.5%), compressive strength (59%), and water absorption (0.59%) of the composite, indicating a promising route for developing high-performance construction materials. The integration of 6 wt% of TPU/jute CNF nanocomposites was found to be optimal, resulting in a uniform matrix, reduced micro-cracks, and improved compressive strength due to enhanced adhesion between the nanocomposites and the geopolymer matrix. Furthermore, a curing temperature of 100 °C was identified as ideal, minimizing unreacted fly ash and enhancing adhesion strength, while higher temperatures (140 °C) led to material deterioration due to rapid water loss. The findings demonstrate that the addition of TPU/jute CNF nanocomposites not only improves resistance to water penetration but also enhances overall mechanical performance. This supports the development of more sustainable and resilient construction materials, contributing to global efforts to reduce the environmental impact of the construction industry. Future research should focus on the long-term durability of these composites under various environmental conditions to validate their effectiveness in real-world applications.

1. Introduction

As the world’s population grows, so does the need for housing and infrastructure development. This increased demand has led to a global surge in the use of Portland cement, the primary binder for construction materials, especially concrete. Currently, approximately 2.8 billion tons of Portland cement are used annually worldwide. Unfortunately, the production of Portland cement is known to generate between 0.6 and 0.8 kg of CO₂ per kilogram of cement [1], accounting for 5–7% of global CO₂ emissions. Carbon emissions are a major contributor to global warming. From 2015 to 2020, the direct CO₂ intensity in cement manufacturing increased by 1.8% annually. As noted by Alola et al. [2] and Sharma et al. [3], to achieve the goal of net-zero emissions by 2050, an annual reduction of 3% is necessary by 2030. Cement production predominantly involves clinker, a key component directly linked to CO₂ emissions through the decomposition of limestone during clinker production and fuel combustion. Consequently, there is a pressing need for alternative, sustainable binding materials to mitigate the carbon footprint associated with Portland cement production, particularly in addressing process emissions.

This has led to the development of concrete from different perspectives, resulting in the emergence of geopolymer concrete. Geopolymer concrete is a sustainable and environmentally friendly alternative to Portland cement, offering a lower cost of production [4]. This has prompted researchers to explore geopolymer concrete as an eco-friendly option. Geopolymer concrete technology holds significant promise for the commercialization, standardization, and repurposing of agricultural and industrial byproducts [5]. This eco-friendly material can reduce CO₂ emissions by up to 45% by replacing Portland cement in concrete.

According to Verma et al. [6], the geopolymerization reaction plays a vital role in developing the strength of geopolymer concrete, and some factors affect the geopolymerization process, such as curing conditions, alkaline solution content, and binding material content in the design mix. Due to high silica and alumina contents, pozzolanic materials such as fly ash, metakaolin, and rice husk ash were used as binding materials in geopolymer concrete. According to Kucukgoncu and Ozbayrak [7], fly ash has been identified as a suitable source of material for geopolymer concrete due to its high content of SiO₂ and Al₂O₃. In the geopolymeric reaction, SiO₂ and Al₂O₃ contained in fly ash react under highly alkaline conditions typically provided by NaOH and NaSiO₃ solution. These reactions eventually produced an amorphous 3D network of silicon and aluminum atoms linked by oxygen atoms. Geopolymer originates from the reaction between the aluminosilicate powder and highly concentrated alkaline solutions. The strength-gaining process of the geopolymer concrete depends on the geopolymerization reactions. The dissolution of aluminosilicate powder such as fly ash and fumed silica in alkaline solutions lead to the formation of sialate monomer (Si-O-Al-O), as shown in the equations below:

$${\mathrm{S}\mathrm{i}\mathrm{O}}_{2.}{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}+\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{S}\mathrm{i}{\left(\mathrm{O}\mathrm{H}\right)}_4+\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}^{4-}$$

(1)

$$\mathrm{S}\mathrm{i}{\left(\mathrm{O}\mathrm{H}\right)}_4+\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}^{4-}\to (-\mathrm{S}\mathrm{i}-\mathrm{O}-\mathrm{A}-\mathrm{O}-{)}_{\mathrm{n}}+{4\mathrm{H}}_2\mathrm{O}$$

(2)

where: SiO₂: silicon oxide; Si: silicon; Al₂O₃: aluminum oxide; Al: aluminum; H₂O: dihydrogen oxide; O: oxygen; OH⁻: hydroxide ion.

Geopolymer concrete is known for its low carbon footprint and superior thermal and chemical resistances, making it a promising alternative to traditional Portland cement. However, its relatively high brittleness and tendency for water absorption present challenges for its widespread adoption in diverse applications. These factors limit its widespread use in construction applications, especially in environments exposed to moisture. The gap lies in the need for solutions that not only maintain the environmental benefits of geopolymer concrete but also significantly improve its mechanical strength and water resistance. In the quest for sustainable and high-performance construction materials, the integration of polymers and fibers into concrete systems has garnered significant attention. According to Osamah and Ammar [8], the combination of fiber and polymer with a brittle matrix is an effective method to enhance flexural strength and toughening mechanisms as it reduces crack development under various loading and environmental effects such as shrinkage.

Thermoplastic polyurethane (TPU) is known for its flexibility, toughness, and high elasticity. When incorporated into concrete, TPU typically acts as a toughening agent, improving the ductility and energy absorption capacity of the material. According to Huang et al. [9], incorporating TPU particles into cementitious materials enhances the performance of polymer concrete, especially bonding strength and mechanical (compression and splitting tensile) strengths as compared with traditional concrete. TPUs act as energy absorbers, enabling concrete to deform more before failure. In addition, the incorporation of TPU in concrete also results in a positive impact to the water resistance and durability of the concrete. TPU’s hydrophobic nature improves the water resistance of concrete, thus reducing water absorption. This is especially important in environments where water ingress leads to degradation. Li et al. [10] found that TPU particles help seal micro-cracks, thereby reducing the pathways for water to enter the concrete.

Jute cellulose fibers are a sustainable and biodegradable reinforcement material. Their high tensile strength and fibrous nature make them suitable for enhancing the mechanical properties of concrete. Khan et al. [11] studied the incorporation of jute fibers (JFs) into concrete and found significant improvements in compression strength and splitting tensile strength. Based on the findings of the conducted test, an optimal proportion of 0.10% JF has been determined to be conducive to enhancing the compression strength and splitting tensile strength by 6.77% and 6.91%, respectively. JFs act as crack arrestors, reducing the propagation of micro-cracks under load. This behavior is particularly beneficial in improving the fracture resistance of the concrete matrix. Other than that, JF is naturally hydrophilic and can increase the water absorption rate of concrete, which may reduce its durability in moisture-laden environments. However, studies by Zakaria et al. [12] suggest that surface modification of JFs such as through chemical treatments can help mitigate this issue by making fibers more hydrophobic, thus improving the water resistance of the resulting concrete.

Polypropylene (PP) fibers were first utilized as reinforcement in concrete to enhance its flexural strength. Zhu et al. [13] conducted a study on the use of polypropylene fibers (PPFs) as reinforcement in geopolymer concrete at 1.5 wt% and found that the compressive and tensile strength of the prepared geopolymer concrete improved by up to 31% and 106%, respectively. Muhd et al. [14] investigated the effect of nylon66 fiber addition on the mechanical properties of geopolymer concrete, finding that the compressive strength of the prepared concrete increased by 9% with the optimum nylon66 fiber addition (0.5 wt%). However, further addition of nylon66 fiber resulted in a decrease in compressive strength. On the other hand, Md et al. [15] examined the influence of recycled rubber addition on the mechanical properties of geopolymer concrete, concluding that the compressive strength and modulus of elasticity decreased with increasing rubber content. For instance, a 33% reduction in compressive strength was recorded when 25 wt% of natural fine aggregate was replaced with crumb rubber. However, these strength and elasticity reductions can be minimized by using pre-treated rubber particles.

Although previous research has explored various fibers and polymers such as TPU, jute fiber, polypropylene, nylon66, and recycled rubber as reinforcements, there are no data on the use of TPU/jute CNF nanocomposites as reinforcement in geopolymer concrete. This material is relatively new in this context, and its potential to improve compressive and splitting tensile strengths and reduce water absorption in geopolymer concrete has yet to be explored.

This research explores the dual role of TPU/jute CNF nanocomposites in geopolymer concrete, focusing on their impact on water absorption and mechanical properties. By systematically studying the interactions between these nanocomposites and the geopolymer matrix, this study aims to provide insights into optimizing geopolymer concrete for enhanced performance under various environmental conditions. This study contributes to the construction industry’s decarbonization efforts by offering a sustainable, low-carbon, and high-performance alternative to traditional concrete. By improving the mechanical properties and durability of geopolymer concrete, the research supports international climate goals and promotes the development of eco-friendly construction materials for the future. The findings of this research could pave the way for more durable, sustainable, and resilient construction materials, aligning with global efforts to reduce the environmental impact of the construction industry.

2. Materials and Methods

2.1. Preparation of TPU Nanocomposite

TPU/Jute CNF nanocomposites with 4 wt% of jute CNFs were prepared using a melt blending technique. We chose a 4 wt% of jute CNF composites due to their excellent mechanical properties as discussed in previous research by Syazwani et al. [16] on the effect of jute CNFs addition on the mechanical properties of TPU nanocomposites. Initially, the TPU granules (Figure 1a) were preheated at 80 °C for 6 h in an oven to remove any moisture content. Subsequently, the TPU and jute CNFs (Figure 1b) were mixed and compounded in an internal mixer (ThermoHAAKE Polylab Rheomix, Thermo Fisher Scientific, Waltham, MA, USA) at 180 °C with a rotor speed of 150 rpm. The blended material, containing 4 wt% of jute CNFs, was then crushed into smaller pieces (Figure 1c) (0.5 cm × 0.5 cm × 0.5 cm) using a polymer crusher machine.

2.2. Preparation of Neat Geopolymer Concrete and TPU/Jute CNF Nanocomposites-Reinforced Geopolymer Concrete

In this research, fly ash-based geopolymer concrete was selected as the reference material due to its status as an environmentally friendly alternative to traditional commercial concrete (OPC) [17]. Fly ash particles (Figure 2) collected from the Jimah Power Plant were used in the preparation of the geopolymer concrete. The prepared TPU/jute CNF nanocomposites were incorporated into the geopolymer concrete at varying weight percentages (0, 3, 6, and 9 wt%). These varying compositions were chosen based on the previous studies conducted by other researchers in the utilization of polymer particles as reinforcement in the geopolymer matrix to achieve an optimal balance between enhancing the mechanical and physical properties of the composite and avoiding negative effects that could arise from excessive reinforcement loading [18,19,20]. The geopolymer concrete was prepared by first mixing the aggregates, fly ash, and TPU/jute CNF nanocomposites before adding the alkaline solution. This method is commonly used in the preparation of geopolymer concrete.

The geopolymer concrete reinforced with TPU/jute CNF nanocomposites was prepared by accurately weighing each raw material according to the required composition. To ensure precision, the weighing parameters were set to four decimal places. The preparation of the geopolymer concrete was based on 3%, 6%, and 9% by weight of TPU/jute CNF nanocomposites.

The production of the geopolymer concrete followed a standard mixing process, conducted in the laboratory at room temperature. Fly ash, aggregates, TPU/jute CNF nanocomposites, and the alkaline activator were mixed and cast in the quantities listed in

Table 1. Mix proportion of geopolymer concrete reinforced with TPU/jute CNF nanocomposites.

Weight Percent (wt%)	Fly Ash (kg/m3)	Fine Aggregates (kg/m3)	Coarse Aggregates (kg/m3)	TPU/Jute CNF Nanocomposites (kg/m3)	Alkaline Activator (kg/m3)
0	409.17	613.63	1350.14	0	224.98
3	409.17	535.69	1350.14	77.93	224.98
6	409.17	457.75	1350.14	155.88	224.98
9	409.17	379.82	1350.14	233.81	224.98

to produce a geopolymer concrete mix of grade M40. All raw materials were weighed using a balance according to a ratio of 1:1.5:3.3. The concentration of NaOH was fixed at 12M as suggested by Mustafa et al. [21]. In order to prepare a 12M NaOH solution, 480 g of NaOH flakes was dissolved in 1L of water and left for 24 h to settle down before being mixed with Na₂SiO₃ to produce an activator solution. This is due to the high temperature emitted during and after the dissolving process. The alkaline activator was prepared approximately one hour before the mixing process by combining NaOH and Na₂SiO₃ at a 2.5:1 ratio. An exothermic reaction occurred during the mixing and stirring of the alkaline activator, releasing a significant amount of heat. Consequently, the prepared alkaline activator was left to settle before being used.

According to Abdullah et al. [22], the concentration of alkaline activator solution plays an important role in geopolymer system due to its function; it acts as a medium to synthesize silicate (Si⁴⁺) and aluminate (Al³⁺) from the source materials used. When the high concentration (12M) of NaOH is used, it will accelerate the dissolution process of Al³⁺ and Si⁴⁺ until the optimum value is achieved. From the research conducted by Mustafa et al. [21], they indicated that the highest compressive strength was present for the geopolymer matrix with 12M NaOH, and it exhibited the best mechanical properties.

The weighed fine aggregates, fly ash, and TPU nanocomposites were dry-mixed for 4 to 5 min to ensure uniform mixing. Then, an alkaline activator was added at a liquid-to-fly ash ratio of 0.5 to prepare a wet mixture, which was continuously mixed for another 4 to 5 min to achieve a homogeneous blend. To further ensure the homogeneity of the mixture and facilitate the mixing process, coarse aggregates were added at the final stage of mixing (Figure 3). This sequential mixing and stepwise addition of all raw materials was performed to ensure uniform dispersion of TPU/jute CNF nanocomposites in the geopolymer matrix. The wet mixture, now containing fine aggregates, fly ash, TPU nanocomposites, and coarse aggregates, was mixed continuously for an additional 4 to 5 min.

The freshly mixed geopolymer was then poured into 50 mm × 50 mm × 50 mm cube molds and dia. 50 mm × 100 mm cylindrical molds for compression and splitting tensile tests. The samples were left at room temperature for 24 h before being de-molded. The prepared geopolymer concrete samples underwent four different curing processes: ambient curing and oven curing at 60 °C, 100 °C, and 140 °C. After 24 h and de-molding, some samples were left in the laboratory at ambient temperature, while the remaining samples were subjected to oven curing for 24 h with a heating rate of 5 °C/min. After the oven curing, the samples were removed and stored in the laboratory at room temperature until the day of testing. After 28 days, the samples were weighed and tested for water absorption, compressive strength, and splitting tensile strength.

2.3. Preparation for Durability Geopolymer Concrete and TPU/Jute CNFs-Reinforced Geopolymer Concrete

The durability of the prepared neat geopolymer concrete and geopolymer/TPU nanocomposite concrete was assessed through a water absorption analysis. In this research, the rate of water absorption of the geopolymer concrete samples was determined after 28 days according to ASTM C642 [23]. This test measured the rate of water absorption by recording the increase in the mass of the geopolymer concrete samples over a period of up to 48 h. The samples were first dried in an oven at 100–110 °C for 24 h to ensure they were fully dry, and the dry weight was accurately recorded. After drying, the weight of each sample was recorded before immersion. The samples were then immersed in water, and the weight of surface dry samples was taken and recorded at intervals of 5 min, 10 min, 30 min, 1 h, 12 h, 24 h, and 48 h after immersion. The water absorption rate of the prepared geopolymer concrete was calculated according to ASTM C642 [23] as follows:

$$Absorptionafterimmersion,\%=\left[{\displaystyle \frac{B-A}{A}}\right]\times 100$$

(3)

where $A$ is the mass of the oven-dried sample in air in g, and $B$ is the mass of the surface-dried sample in air after immersion in g.

2.4. Mechanical Testing Analysis of Prepared Geopolymer Concrete and TPU/Jute CNFs-Reinforced Geopolymer Concrete

Mechanical properties such as compressive and tensile strengths are crucial parameters for assessing the quality of the prepared geopolymer and geopolymer/TPU nanocomposite concrete. The compression test determines the material’s ability to bear loads in compression, while the splitting tensile test provides insight into the material’s tensile behavior, which is essential for understanding how the material will perform under different stress conditions. A compression test was conducted on the prepared neat geopolymer and geopolymer/TPU nanocomposite concrete using an NL 4000/006-A002 compression machine from NL Scientific Instruments after 28 days of curing. This analysis is vital for understanding the influence of TPU nanocomposite addition on the compressive strength of the geopolymer concrete. The samples were ensured to have smooth and parallel surfaces before testing. The sample was placed between the upper and lower platens of the compression testing machine. We ensured that the specimen was centered to avoid uneven loading. The cross-head speed used was 2 mm/min, following ASTM C109 [24] for concrete samples. The cube samples were prepared in accordance with this standard method, with dimensions of 50 mm × 50 mm × 50 mm. In this research, six samples were prepared for each composition to ensure the accuracy of the strength results. The compressive strength (${\sigma }_c$) is calculated using the formula:

$${\sigma }_c={\displaystyle \frac{P}{A}}$$

(4)

where P is the maximum load at failure in N, and A is the cross-sectional area of the specimen in mm².

Additionally, the prepared neat geopolymer and geopolymer/TPU nanocomposite concrete underwent splitting tensile testing to determine the tensile strength of the concrete. The splitting tensile test was performed on the 28-day cured cylindrical samples at room temperature using an NL 4000/006-A002 loading machine at a constant loading rate of 0.2 mm/min, following the standard test method ASTM C496 [25]. The sample used should have a clean, smooth surface and be free from cracks or imperfections. The cylindrical sample was placed horizontally between the platens of the compression testing machine. Two soft packing strips (wood or steel) were placed along the top and bottom of the cylinder to apply the load over a narrow strip rather than a point, distributing the load evenly across the diameter. These tests were conducted to analyze the effect of different TPU nanocomposite contents on the tensile strength of the prepared geopolymer concrete.

Like the compression test, six cylindrical samples of 50 mm × 100 mm of each composition were prepared according to ASTM C192 [26] to ensure the accuracy of the tensile strength results. Based on these tests, the splitting tensile strength of the concrete was calculated as follows:

$$T={\displaystyle \frac{2P}{\pi DL}}$$

(5)

where T is the splitting tensile strength in MPa, P is the maximum load on the sample in N, D is the diameter of the sample in mm, and L is the length of the samples in mm. Six samples were tested for each composition, and the average measured values were analyzed.

3. Results and Discussion

3.1. Durability Analysis of Prepared Geopolymer Concrete and TPU/Jute CNFs-Reinforced Geopolymer Concrete

The water absorption characteristics of geopolymer concrete play an important role in determining the durability of the structure [21,27]. Penetration of water into the geopolymer concrete will result in the structure becoming brittle, leading to spalling of the concrete and ultimately reducing the life span of the structure. According to Shaikh [28], the rate of capillary absorption of water by concrete is a function of the penetrability of the pore system, whereas, for unsaturated concrete, the rate of ingress of water or other liquids is largely controlled by absorption due to capillary rise. In this research, the rate of water absorption of geopolymer concrete containing TPU nanocomposites is studied according to ASTM C642 [23]. The test results of the water absorption test of the prepared geopolymer concrete and TPU nanocomposite-reinforced geopolymer concrete are summarized in Figure 4 and Figure 5.

Figure 4 shows the effect of curing temperature on the water absorption rate of prepared geopolymer concrete with different amounts of TPU nanocomposite loadings. It was observed that the absorption rate of neat geopolymer concrete slightly increased with the increase in curing temperature, with the lowest rate recorded at room temperature at 1.77%. The absorption rate increased to 2.13% (corresponding to a 20.3% increase) at 60 °C curing temperature, and the highest rate was recorded at 140 °C curing temperature with 3.12% (corresponding to 76.3% increase). This finding was in line with Turkey et al. [29] in their research on the effect of glass powder addition on the properties of geopolymer concrete. These increases are due to evaporation resulting from the high curing temperature and the emergence of cracks that cause water to leak and reach into the concrete, thus increasing the rate of water absorption.

However, upon the addition of TPU nanocomposites in the prepared geopolymer concrete, a significant difference could be observed, where the water absorption rate of the concrete decreased with increasing temperature, with the lowest rate recorded by 6 wt% of TPU nanocomposites and a curing temperature of 100 °C with 0.59%. This might be due to the adhesion bonding of TPU nanocomposites with other constituents in the geopolymer concrete, which improved with increasing curing temperature. The interfacial bonding among the nanocomposites and constituents in the geopolymer concrete was improved with the increases in curing temperature and thus reduced the gaps (capillary transport mechanisms) [30] and emergence of cracks, which resulted in a reduction of water reaching into the prepared concrete. The high absorption rates for samples cured at room temperature can be attributed to insufficient crosslinking of the polymer matrix, leading to greater water permeability. Meanwhile, at 60 °C curing temperature, the moderate absorption rate indicates partial crosslinking of the geopolymer matrix, leading to a reduction in water uptake but not complete resistance. The absorption behavior shows that curing at 100 °C significantly improves water resistance, likely due to the formation of a tightly crosslinked polymer network. This temperature likely causes the geopolymer matrix to become highly hydrophobic, forming a barrier against water penetration. On the other hand, the water absorption rate showed an increased trend when the curing temperature increased to 140 °C. This might be due to the excessive loss of water during curing at this high temperature, which leads to the formation of micropores. Due to the high curing temperature, the dense geopolymer matrix starts to deteriorate, causing a rapid loss of water that results in the formation of more micropore cracks. Thus, water is absorbed through these micropores and cracks, thus increasing the water absorption rate.

The curves show a time-dependent behavior, making a time series analysis a suitable method for modeling water absorption behavior over time. Initially, all samples display exponential-like growth in water absorption rates in the first 30–60 min. After 720 min (~12 h), the absorption rate reaches a steady state, indicating saturation. This plateau behavior is consistent across all samples but occurs at different absorption levels.

Meanwhile, Figure 5 shows the effect of TPU nanocomposite loadings (wt%) on the water absorption rate of prepared geopolymer concrete at different curing temperatures. It can be observed that the water absorption rate of prepared geopolymer concrete increased with the increase in TPU nanocomposite loadings at room temperature curing. The absorption rate of neat geopolymer concrete cured at room temperature at 1.77% had increased to 3.47% (corresponds to a 96% increase) with the addition of 3 wt% of TPU nanocomposites. The rate continued to increase with the increase in amount of TPU nanocomposites added, with the highest rate recorded by 9 wt% with 3.56%. A similar trend was observed for other curing temperatures. These increases are attributed to the hydrophilic nature of jute CNFs, which contained cellulose and a lignin-containing hydroxyl group. Thus, the increase in TPU nanocomposite loadings in the prepared geopolymer concrete leads to an increase in the water absorption rate of the concrete.

Nevertheless, the water absorption rate of prepared geopolymer/TPU nanocomposites concrete started to decrease with the increase in amount of TPU nanocomposites and curing temperature. The absorption rate of the prepared concrete decreased to 1.52% (corresponding to a 14.1% increase) upon the addition of 3 wt% of TPU nanocomposites and curing at 100 °C. Further decreases were recorded with the lowest rate of 0.59% with the addition of 6 wt% of TPU nanocomposites and curing at 100 °C. This finding confirmed that the geopolymerization reaction between the fly ash and alkaline activator is a phenomenon of thermal decomposition requiring a higher temperature for the formation of polymeric chains of aluminosilicates bond. The adhesion strength between the nanocomposites and geopolymer concrete also became stronger, showing that a high temperature is needed to improve the adhesion and gap among the constituents in the concrete, which can result in reduced water penetration and absorption rate. This finding is contrary to the result found by Alomayri et al. [18] in their research on the effect of cotton fiber addition on the water absorption analysis of geopolymer concrete, where the water absorption rate of the prepared geopolymer concrete increased drastically upon the addition of cotton fibers (corresponds to 149% increases) as compared with neat geopolymer concrete. Li et al. [10] discussed the effect of glass fiber addition on water absorption in an analysis of geopolymer concrete. Glass fibers effectively reduce water absorption due to their non-porous nature and ability to block micro-cracks in the concrete. Glass fibers contribute to minimizing capillary pores, thus impeding water ingress and resulting in 12.4% decreases (optimum) with the incorporation of 4 wt% of glass fibers. Glass fibers create a dense matrix by reducing the number of interconnected pores, which results in less water absorption and improved durability. However, overuse of glass fibers can lead to workability issues, which may need to be mitigated with superplasticizers. On the other hand, Aly et al. [31] conducted their study on the effect of recycled rubber addition on the water absorption analysis of geopolymer concrete. According to the study, rubber particles, particularly recycled rubber, were found to lower water absorption rates in concrete due to the hydrophobic properties. They observed a 13% decrease in water absorption with the inclusion of 10% rubber aggregate compared with the conventional matrix.

3.2. Compressive Strength Analysis of Prepared Geopolymer Concrete and TPU/Jute CNFs-Reinforced Geopolymer Concrete

The influences of the incorporation of prepared TPU nanocomposites and curing temperature on the compressive strength of fly ash-based geopolymer concrete are illustrated in Figure 6. This test has been carried out on 50 mm cube samples according to ASTM C109. It was found that the compressive strength of the prepared geopolymer concrete was notably improved with increasing amounts of TPU nanocomposites of up to 6 wt% before it decreased.

The compressive strength of prepared geopolymer concrete increased by 142%, 167%, and 54% with the incorporation of 3%, 6%, and 9% of prepared TPU/jute CNF nanocomposites at room temperature curing, respectively, compared with the control geopolymer concrete (22.9 MPa) without nanocomposites addition. As expected, neat geopolymer has the lowest compression strength across all temperatures. The increase in strength with temperature reflects improved crosslinking but is limited by the lack of reinforcement. The compressive strength of the neat geopolymer obtained agrees with fly ash-based geopolymer concrete obtained by other researchers, such as Sherwani et al. [19] with 22.3 MPa. There is a significant improvement in compression strength with the addition of 3 wt% and 6 wt% of TPU/jute CNF nanocomposites, indicating that a higher reinforcement content contributes positively to compression strength. The superior performance at 6 wt% suggests better fiber–matrix adhesion and a more uniform distribution of TPU nanocomposites within the geopolymer matrix, as shown in Figure 7. Furthermore, the strength of the geopolymer concrete increased by 56% (58.1 MPa), 72% (71.3 MPa), and 25% (63.6 MPa) at 60 °C curing temperature; 49% (64.1 MPa), 59% (76.1 MPa), and 26% (66.8 MPa) at 100 °C curing temperature; and 61% (46.7 MPa), 69% (60 MPa), and 6% (41.7MPa) at 140 °C curing temperature, respectively, as compared with the neat geopolymer concrete, which indicated that the incorporation of TPU nanocomposites benefits the geopolymer concrete.

This result was attributed to the strengthening mechanism possessed by the added nanocomposites, where the nanocomposites are able to block crack propagation and redirect the cracks to the interface between the nanocomposites added and other constituents [32] in the geopolymer concrete. The prepared TPU nanocomposites have very good strength, which is attributed to the very high strength of jute CNFs [33]. This might be the reason for the improvement in the strength of the prepared geopolymer concrete. However, the strength of the geopolymers had decreased by 40, 21, 20, and 30% upon the addition of TPU nanocomposites beyond 6 wt%. This reduction in the compressive strength was attributed to the overflowing availability of unreacted TPU nanocomposites in the matrix. Hence, the excess amount of TPU nanocomposites results in agglomeration among the other constituents and a poor dispersion of the nanocomposites inside the geopolymer concrete mixture that can act as stress concentrators, reducing the mechanical performance of the geopolymer concrete. This decrease in performance could also be linked to insufficient matrix flow around the TPU nanocomposites during curing, causing imperfect bonding. This poor dispersion of TPU nanocomposites in geopolymer concrete can occur due to variety of factors, including the high surface energy of nanocomposites and its van der Waals forces. Like many other nano-sized materials, TPU/jute CNF nanocomposites also have a high surface area-to-volume ratio. This leads to high surface energy, which causes them to naturally agglomerate or cluster together, making it difficult to achieve uniform dispersion. In terms of van der Waals forces, the attractive forces between particles at the nano scale contribute to their tendency to stick together, forming clusters that are difficult to break apart during mixing. A similar finding was discussed by Ahmed et al. [34] in their study on the effect of nano-silica addition on the properties of geopolymer concrete.

Other than that, the effect of curing temperature on the compressive strength of prepared geopolymer concrete has also been studied. It was noticed that the compressive strength of prepared geopolymer concrete increased with the increase in curing temperature up to 100 °C before slightly decreasing at 140 °C as compared with room temperature curing. These increases are due to the dissolution of solid particles being accelerated at higher curing temperatures, thus improving the polymerization reaction of the precursors in the first geopolymerization of the system. Moreover, the increases in the strength of the prepared geopolymer concrete up to 100 °C can be attributed to the reaction that occurred in the TPU nanocomposites added, which resulted in the enhancement of the interfacial transition zone with the presence of the right amount of TPU nanocomposites at optimum curing temperature; that can reduce the amount of porosity and increase the compactness of the prepared geopolymer concrete, as shown in Figure 7. However, it was noted from Figure 6 that the compressive strengths of prepared geopolymer/TPU nanocomposites slightly dropped by 17, 10, 12, and 30% at a curing temperature of 140 °C. This was probably due to the excessive loss of water during curing at this relatively high temperature, causing the prepared geopolymer to dry out. This dried-out phenomenon in geopolymer concrete is likely to affect the strength negatively as there is less moisture in the system. It can be concluded in this study that heat acted as a catalyst to the geopolymerization reaction and the optimum curing temperature of geopolymer concrete reached 100 °C. The highest compressive strength was recorded at 6% of nanocomposite addition as compared with other compositions at all curing temperatures.

The obtained compressive strength was in contrast with the findings by other researchers on the effect of fiber addition on the strength of geopolymer concrete. The compressive strength of geopolymer concrete decreased significantly (corresponding to a 49% decrease) with the addition of nylon fibers [14]. A similar finding was obtained by Alomayri et al. [18] on the effect of cotton fibers on the compressive strength of geopolymer concrete [16]. This might be due to the weak interfacial connection between the matrix and fibers, which is attributed to the hydrophobic nature of the fibers, thus preventing the fibers from inhibiting cracks in geopolymer. It is different from the TPU/jute CNF nanocomposites used in this research where the interfacial connection between the geopolymer matrix and nanocomposites improved with the elevated curing temperatures.

Referring to the obtained compressive strength, it is proven that the prepared geopolymer/TPU nanocomposite concrete has the potential to be used in the construction industry as well as in seismic situations. According to ACI 369.1M-17 [35], the required compressive strength of plain concrete for seismic buildings is in the range of 21 to 80 MPa.

3.3. Splitting Tensile Strength Analysis of Prepared Geopolymer Concrete and TPU/Jute CNFs-Reinforced Geopolymer Concrete

A splitting tensile test was performed on the prepared neat geopolymer and geopolymer/TPU nanocomposite concrete after 28 days of curing. The influence of TPU nanocomposite addition and curing temperature on the splitting tensile strength of the prepared geopolymer concrete is illustrated in Figure 8. This test was carried out on dia. 50 mm x 100 mm geopolymer concrete cylinders according to ASTM C496 [25]. It was found that the splitting tensile strength of the prepared geopolymer concrete was notably improved, parallel with the compression strength with an increasing amount of TPU nanocomposites up to 6 wt% before decreasing.

Neat geopolymer concrete (0 wt%) shows the lowest splitting tensile strength across all temperatures, demonstrating the impact of reinforcement on enhancing mechanical properties. The increase in the splitting tensile strength with temperature up to 100 °C suggests that TPU/jute CNF nanocomposites benefit from some degree of thermal processing, but they lack reinforcement to maximize strength. Figure 8 shows that the addition of TPU nanocomposites brings about notable enhancements in the tensile strength of geopolymer by 59.3, 63.5, and 24% with the additions of 3, 6, and 9 wt% of TPU nanocomposites at room temperature curing as compared with the neat geopolymer concrete. At 3 wt%, there is a noticeable improvement in the tensile strength compared with 0 wt%, indicating that even a small addition of TPU/jute CNF nanocomposite significantly enhances the load bearing capacity. On the other hand, the splitting tensile strength of the prepared geopolymer increased by 24.7, 31.0, and 11.8% at 60 °C curing temperature; 22.4, 26.3, and 12.2% at 100 °C curing temperature, and 26.9, 30.1, and 3% at 140 °C curing temperature, respectively, as compared with the neat geopolymer concrete. These increases are desirable as they indicate that the incorporated TPU nanocomposites are beneficial to the geopolymer concrete as the existence of TPU nanocomposites in the mixture can help reduce the porosity of geopolymer concrete. This argument is in line with the finding by Amirreza et al. [20] in their study on the effect of waste polymeric materials in slag-based geopolymer concrete. Other than that, it is shown by the figure that geopolymer concrete with 6 wt% of TPU nanocomposites exhibited the highest splitting tensile strength with 4.35, 4.45, 4.85, and 4.54 MPa at room temperature, 60, 100, and 140 °C curing temperatures. This suggests that 6 wt% is close to an ideal reinforcement level where the distribution and dispersion of TPU/jute CNF nanocomposites within the geopolymer matrix are optimal. At this level, the nanocomposites provide a significant improvement in tensile strength due to better stress transfer and reduced agglomeration. Other than that, the increases in the splitting tensile strength of geopolymer/TPU nanocomposites concrete were also attributed to the good bond of the geopolymer binder with the TPU nanocomposites (Figure 7), which provided high crack control in the geopolymer. In addition, the cellulose nanofiber in jute has hydroxyl groups that may chemically interact with the geopolymer’s aluminosilicate network, leading to an improvement in their chemical adhesion. Meanwhile, TPU is known for its flexibility, which acted as a toughening agent, absorbing and dissipating energy during loading. Its elastomeric nature helps to prevent brittle failure in geopolymer concrete, which is typically more prone to cracking under tensile stress. Thus, these TPU/jute CNF nanocomposites may enhance the matrix’s resistance to crack growth and allow the material to deform before failure through an effective stress transfer mechanism from matrix to reinforcement. Furthermore, the observation of similar behaviors of prepared samples during compression and splitting tensile tests is highly attributed to the uniform distribution of TPU nanocomposites in the samples. On the other hand, at 9 wt%, while there is still an improvement over the neat geopolymer concrete, the splitting tensile strength is not substantially higher than that at 6 wt%. This may indicate that beyond a certain threshold (around 6 wt%), the reinforcement does not contribute proportionately to the mechanical properties. Possible reasons for this plateau effect include the agglomeration of nanofibers at higher concentrations, which can create weak points or stress concentration sites within the matrix or difficulties in achieving uniform dispersion of the nanocomposites. In the case of the effect of curing temperature on the splitting tensile strength of prepared geopolymer concrete, a very similar trend to that of compressive strength was also observed where the strength increased with increasing curing temperature up to 100 °C before slightly decreasing at 140 °C as compared with room temperature curing. At lower temperatures (RT and 60 °C), the splitting tensile strengths show gradual increases for all wt% sample. These increases can be attributed to initial curing, which helps in the crosslinking of the polymer matrix. The TPU/jute CNF nanocomposites begin to interact more effectively with the geopolymer matrix as the curing temperature rises. These interactions are due to enhanced molecular mobility and better wetting and adhesion of the matrix around the nanocomposites. It has been demonstrated that higher curing temperature brings about improvement in the splitting tensile of geopolymer concrete. This relates to the completion of the geopolymerization reaction at higher temperatures. The maximum splitting tensile strength is observed at 100 °C for all samples, indicating that this temperature is the optimal point for curing. At this stage, the geopolymer matrix reaches an ideal level of crosslinking and alignment, resulting in better load transfer between the geopolymer matrix and TPU/jute CNF nanocomposites. Under this condition, most of the geopolymerization is completed and the thermal yields a fast improvement in the splitting tensile strength. Higher temperatures enhance the diffusion of polymer chains and improve interfacial bonding between the TPU/jute CNF nanocomposites and the geopolymer matrix. The increased adhesion and bonding likely lead to a stronger geopolymer concrete with improved splitting tensile strength.

On the other hand, the splitting tensile strength of prepared geopolymer concrete showed a decreasing trend at a curing temperature of 140 °C. This decline suggests that further temperature increase may lead to thermal degradation or overcuring of the geopolymer concrete. Overcuring can cause embrittlement, a reduction in elasticity, and a potential breakdown of chemical bonds within the matrix or at the matrix–reinforcement interface. This degradation reduces the load-bearing capacity and mechanical performance of the composite material. The behavior indicates that the matrix starts to degrade or oxidize at higher temperatures, leading to weakened structural integrity. This might be due to the excessive loss of water during curing at this high temperature, resulting in the ‘dry out’ of the prepared geopolymers. This ‘dry out’ has caused a rapid loss of water in the geopolymer concrete, which results in the formation of micropores and a significant amount of crack that deteriorates the strength of the geopolymer concrete. Thus, as stated in the previous section, heat acted as a catalyst to the geopolymerization reaction before reaching an optimum curing temperature of 100 °C in this study.

4. Conclusions

The TPU/jute CNF nanocomposites were successfully incorporated into geopolymer concrete to enhance its mechanical properties and water absorption behavior. This study demonstrates the significant impact of incorporating TPU/jute CNF nanocomposites on the mechanical and water absorption properties of geopolymer concrete. The addition of TPU/jute CNFs notably improved the splitting tensile strength by 63.5%, compressive strength by 59%, and reduced water absorption by 0.59% in the composite, indicating a promising route for developing high-performance construction materials. It can be concluded that the optimal amount of TPU nanocomposite addition in geopolymer concrete is 6 wt%, with 100 °C being the optimum curing temperature.

Moreover, the reduction in water absorption with the inclusion of these nanocomposites suggests improved durability, making them ideal for environments with high moisture exposure. The findings highlight the potential of TPU/jute CNF nanocomposites as reinforcement in geopolymer concrete, providing a sustainable and efficient alternative to conventional concrete materials. The use of jute CNFs not only reduces lifecycle CO₂ emissions but also enhances the biodegradability and renewability of the composite. Compared with conventional materials like glass fibers and synthetic polymers, jute CNFs offer an eco-friendlier alternative, making this study highly relevant to ongoing global efforts to achieve net-zero emissions by 2050 and promote sustainable construction practices.

Future research could explore the long-term performance of these composites under various environmental conditions and investigate the scalability of this approach for industrial applications. The development of such materials aligns with the growing demand for sustainable construction solutions, paving the way for innovations in the construction industry.

5. Limitations of Study and Future Recommendation

The outcomes from this study indicate that the incorporation of TPU/jute CNF nanocomposites as reinforcements in geopolymer concrete had positive results in enhancing the water absorption and mechanical properties of the prepared TPU/jute CNF nanocomposites-reinforced geopolymer concrete. The compressive and splitting tensile strengths of the prepared geopolymer/TPU nanocomposites improved significantly and met the minimum requirement of seismic building materials. However, it is suggested that the effect of the shape and size of TPU nanocomposites on the properties of geopolymer concrete can be studied. In addition, it would be a great finding if the effect of seawater exposure on the geopolymer/TPU nanocomposites could be explored. Thus, the application of the prepared concrete can be broadened.