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Article

Influence of Bentonite and Polypropylene Fibers on Geopolymer Concrete

by
Rana Muhammad Waqas
1,
Shahid Zaman
1,
Mohammed K. Alkharisi
2,*,
Faheem Butt
1 and
Eyad Alsuhaibani
2
1
Department of Civil Engineering, University of Engineering and Technology, Taxila 47050, Pakistan
2
Department of Civil Engineering, College of Engineering, Qassim University, Buraidah 52571, Saudi Arabia
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(2), 789; https://doi.org/10.3390/su16020789
Submission received: 14 October 2023 / Revised: 3 January 2024 / Accepted: 9 January 2024 / Published: 17 January 2024
Browse Figures
Versions Notes

Abstract

:
Bentonite is one of the SiO2-rich pozzolanic clay types that has been enormously employed as a cost-effective and eco-friendly supplementary cementitious material in ordinary Portland cement (OPC) concrete. However, the use of bentonite in geopolymer concrete (GPC) has not been explored very widely. Further, the research available on the effect of utilizing treated bentonite in GPC is limited. The practical application of GPC is also very limited due to its significant shrinkage and high brittleness compared to OPC concrete. There are several studies available that have highlighted the use of polypropylene fibers (PPF) in improving the mechanical properties of GPC by reducing drying shrinkage and enhancing ductility. However, the effect of PPF on the durability properties of GPC needs to be addressed. Further, the effect of the combined integration of bentonite and PPF on the mechanical and durability properties of GPC has not been reported yet. The aim of this study is, therefore, to investigate the individual and combined effect of bentonite and PPF on the workability, mechanical properties, and durability of fly ash (FA)-based GPC. Bentonite replaced 10% of FA, and PPF was added at varying proportions (0.5%, 0.75%, and 1%) for raw and treated bentonite. Slump test was used to assess workability, while compressive, tensile, and flexural tests were utilized to evaluate the mechanical properties. Water absorption, acid attack, and abrasion resistance tests were used to evaluate durability. The results showed that bentonite and PPF significantly enhance mechanical properties, especially when combined with treated bentonite, with the highest improvement observed for mixtures with 1% PPF. The compressive strength was improved by an extent of 10% and 18% for raw bentonite-GPC and treated bentonite-GPC, respectively, compared to the control mix without bentonite. The durability test results revealed that water absorption of raw and treated bentonite-GPC mixtures at the age of 90 days was decreased by 16% and 21%, respectively, compared to the control mix (without bentonite). The mass loss of raw and treated bentonite-GPC mixtures in sulphuric acid solution was 5% and 10% lower, respectively, than the control mix (without bentonite). The mass loss of raw and treated bentonite-GPC mixtures in abrasion resistance tests was 6% and 12% lower, respectively, than the control mix (without bentonite). For durability performance, mixtures with 0.5% PPF perform the best, while higher PPF contents negatively impact the GPC durability.

1. Introduction

Climate change is a rising global challenge that is driven by increasing pollution and CO2 emissions. Researchers are collectively working on environmentally friendly methods to protect the environment, including sustainable living practices and the development of eco-friendly technologies. The construction sector is also seeking eco-friendly, low-CO2 emitting materials. Each year, approximately 4 billion tons of ordinary Portland cement (OPC) serve as a binding agent in global construction [1]. However, the production of OPC contributes one ton of CO2 emissions to the environment for every ton it generates, resulting in a significant release of CO2 into the atmosphere, which harms individuals and the natural environment [2,3]. As the demand for infrastructure grows, OPC concrete production continues to expand. Consequently, the construction industry faces the challenge of developing OPC-based material substitutes that meet sustainability and green material criteria [3].
Geopolymer concrete (GPC) is a promising solution to this challenge. GPC is a type of concrete that uses an alkali activator solution to bind aluminosilicate materials, such as materials made entirely from industrial wastes and byproducts [3,4]. On the other hand, OPC, the conventional binder, is a hydraulic binder primarily composed of calcium, silicon, aluminum, iron, and other elements. These raw materials react with water to form a hydrated cement paste, which is the main binding agent in concrete. In contrast, geopolymer concrete, alternatively referred to as geo-cement and eco-friendly concrete, is a type of concrete that does not require ordinary Portland cement (OPC) [5,6]. This eliminates the environmental concerns associated with OPC production, such as CO2 emissions and raw material consumption. GPC is a promising category of green cementitious concrete, which has the potential to produce more efficient, sustainable, and durable concrete that is suitable for all construction applications. Therefore, GPC has the potential to transform the construction industry and safeguard the environment for future generations.
Fly ash (FA), a byproduct of coal power plants, has garnered attention as a promising precursor material for the production of GPC mixtures due to its chemical composition rich in silica (SiO2) and alumina (Al2O3) content [7,8]. The reaction mechanism of GPC is based on the dissolution and polymerization of aluminosilicate materials. Consequently, when FA is mixed with an alkali activator solution, the aluminosilicate compounds in the FA dissolve and form a gel. The gel then undergoes polymerization, which is a process of forming long chains of molecules. These chains of molecules bind together to form a geopolymer matrix. It has been observed that curing GPC mixtures to elevated temperatures further enhances their engineering properties. This is because FA has low reactivity at ambient temperature, and heat curing at temperatures of 80 °C to 100 °C is typically used to activate FA-based GPC [9,10]. Additionally, it has been established that geopolymers cured at elevated temperatures outperform traditional OPC mixtures in most engineering properties [11,12]. The heat curing of GPC at higher temperatures results in improved mechanical properties compared to ambient-cured GPC mixtures [13]. Several research studies have shown that low-calcium FA (Class F FA) has higher mechanical properties after 24 to 48 h of heat curing [13]. However, the ambient curing of FA-based GPC does not yield the desired results due to the limited reactivity of FA at low temperatures and its inability to initiate geo-polymerization at lower temperatures [9,10]. Additionally, the heat curing of GPC for worksite and field applications is not realistically viable [9,10].
The production of GPC has made significant use of FA and slag (a waste product from steel and iron industries), both of which have demonstrated high performance [14,15,16,17,18]. However, concerns remain regarding the reliable supply of high-quality FA, and some regions of the world have experienced FA shortages due to tighter regulations on coal-fired power plants. Additionally, the annual global demand for cement for concrete production is much higher than the annual global supply of slag. As a result, it is not possible to meet the annual global demand for concrete by using FA and slag as the sole source materials for GPC production. To pursue a sustainable alternative to traditional concrete construction, all available alternative raw material options should be thoroughly investigated [19]. One such alternative is low-calcium bentonite, a naturally occurring pozzolana rich in silica and alumina that has been widely used to improve the properties of concrete [20]. Bentonite is an alumino-phyllosilicate clay that is typically formed through the chemical decomposition of volcanic ash in the presence of water and primarily consists of the mineral montmorillonite. Numerous studies have shown that the engineering properties of conventional concrete are improved when bentonite is used as a replacement for cement [20,21,22,23,24,25,26,27,28].
There are two forms of bentonite, which are commonly employed in the industry: calcium and sodium. Calcium bentonite has a significantly lower swelling capacity than other bentonites [20]. Low-calcium bentonite has been used as a partial replacement for OPC in concrete by several researchers [26,27]. A review of the recent literature suggests that low-calcium bentonite has a positive impact on the engineering properties of traditional OPC concrete [21,26,27,29,30,31,32]. It was also reported that mixes with bentonite addition outperformed OPC concrete at different ages when bentonite was used as a 0 to 21% mass replacement of OPC [26]. It was also found that mixtures with bentonite addition enhanced the resistance to acid attacks. Mirza et al. evaluated the performance of Pakistan-origin bentonite as a partial replacement for conventional cement in concrete [27]. The pozzolanic reactivity and durability properties of bentonite-OPC mixes were evaluated to determine the optimal replacement level of bentonite. It was observed that bentonite exhibits pozzolanic behavior in the presence of OPC. However, limited research is available concerning the utilization of bentonite to enhance the performance of GPC mixtures. The bentonite was incorporated as a partial replacement of high calcium fly ash with natural and recycled aggregates [33]. The mechanical, durability, and rheological properties of bentonite-blended GPC mixtures were investigated. It was observed that the incorporation of bentonite (10 wt%) exhibited considerable improvement in the strength and durability characteristics of geopolymer concrete mixes possessing both natural and recycled aggregates.
One of the significant problems in utilizing GPC for structural members is its brittle behavior, with low strength in tension and flexure compared to conventional concrete, so researchers have used fibers as reinforcements to increase its ductility [34,35]. Polypropylene fibers (PPF) are excellent polymer fibers because they are inexpensive, lightweight, have low thermal conductivity, and have a high elastic modulus [36]. Adding PPF to concrete can reduce drying shrinkage and improve tensile, compressive, and flexural strength [37,38]. Studies on the mechanical properties of cement–sand mortars with PPF addition have shown significant improvements in ductility [39]. The mechanical and durability aspects of fiber-reinforced geopolymer were discussed by Farhan et al. [40]. They discovered that the most effective fibers for geopolymer reinforcements were steel and PPF fibers, with glass and a few other fiber materials following. Pioneering studies employed PPF in both FA and metakaolin geopolymers, with initial tests focusing on a slag-FA matrix containing 0.5% and 1% fiber content by volume. However, these attempts revealed that insufficient fiber content yielded minimal improvement in the properties of GPC [41]. However, subsequent studies confirmed the positive impact of PPF on geopolymer properties. Zhang et al. observed increased compressive and flexural strength in metakaolin composites with varying fiber content (0–0.75% by weight) [42]. Moreover, FA-based geopolymers also benefited from PPF reinforcement. Adding PPF to class F FA composites enhanced their mechanical properties over time [43,44]. However, some studies revealed that the addition of the PPF increases flexural strength but causes a decrease in compressive strength [45,46]. The inclusion of PPF results in reducing the water absorption of geopolymer composites, which is primarily because of the improved microstructure and density of the matrix. These properties are directly associated with the fiber content rather than the fiber type [47].
It is obvious from the literature that bentonite, a pozzolanic clay material abundant in SiO2 content, can be used as a supplementary cementitious material in conventional OPC concrete. However, there is limited research available on the performance of bentonite-blended GPC mixtures [33]. It is also reported in the literature that the thermo-mechanical treatment of bentonite can further enhance the performance of bentonite-blended OPC mixes [48]. However, the effect of thermo-mechanical treatment of bentonite on the performance of GPC mixtures is not reported in the literature. The present study aims to investigate the influence of untreated (raw) bentonite and thermo-mechanical treated bentonite on the performance of GPC mixtures.
There are several studies available that have highlighted the use of PPF in improving the mechanical and durability properties of geopolymer concrete by reducing drying shrinkage and enhancing ductility. However, the effect of the combined integration of bentonite and PPF on the mechanical and durability properties of GPC has not been reported yet. The objective of this study is, therefore, to investigate the individual and combined effect of bentonite (treated and untreated) and PPF inclusion on the workability, mechanical properties, and durability of FA-based GPC.

2. Materials and Methods

2.1. Material Description

The workability and mechanical properties of GPC are dependent on the physical properties and chemical composition of the source materials and their components. FA (Superfine ash, Matrixx, Karachi, Pakistan) was utilized as a binder to produce GPC mixtures. For the partial replacement of fly ash, low-calcium bentonite clay was obtained from its natural deposits in Jahangira (Nowshera), Pakistan. The relative density and specific area of bentonite were measured as 2.64 g/cm3 and 4900 cm2/gm, respectively. The comparison of the physical properties of FA and bentonite is presented in Table 1.
The geo-polymerization process begins when the precursors (FA, slag, etc.) react with alkaline activators. To avoid errors in the results due to variations in source materials, all mixes were prepared using materials from the same source. FA was used as the raw material for the geopolymer binder. In the bentonite-GPC mixtures, FA was replaced with bentonite at a weight percentage of 10%. The chemical properties of OPC, FA, and bentonite were determined using X-ray fluorescence (XRF) and are shown in Table 2.
Crushed limestone aggregates, available locally in the size of 10 mm and 20 mm, were used as coarse aggregates. The gradation of coarse aggregate conformed to ASTM-C136-06 [49] and specific gravity satisfied ASTM-C127-07 [50]. The coarse aggregates (CA) were obtained from the Margallah Hills quarry near Taxila, Pakistan. The specific gravity and absorption capacity of CA were measured as 2.66 and 0.7%, respectively.
In addition to the coarse and fine aggregates used in conventional concrete, the precursors and alkaline activator solution (AAS) are the main components of GPC. The most commonly used AAS in GPC production is a mixture of sodium hydroxide (SH), NaOH, and sodium silicate (SS), Na2SiO3. In this study, the AAS was prepared by blending SH and SS in a predefined ratio of 2.0 (SS/SH). The required molarity of SH was achieved by adding water and 98–99% pure SH pellets. Mixing SH pellets in water results in the evolution of heat due to an exothermic reaction, so the SH solution was prepared 24 h before mixing it with the SS solution.
This study employed low-calcium bentonite clay, characterized by particles exhibiting a flaky and spherical morphology. Figure 1 presents a scanning electron microscopic (SEM) image of the bentonite. The scanning electron microscopic (SEM) (VEGA3 TE model, TESCAN, Brno, Czech Republic) imaging of bentonite revealed spherical and thick flake-shaped particles.
PPF with a length of 19 mm were utilized, and their properties are detailed in Table 3. To enhance workability and mitigate the high viscosity of the alkaline solution, a naphthalene-based superplasticizer (SP) was added to make the GPC mixture more cohesive and viscous [51].

2.2. Thermo-Mechanical Treatment of Bentonite

The difference between the raw bentonite and treated bentonite was the treatment process. The raw bentonite was just subjected to mechanical grinding only. However, the treated bentonite was subjected to both mechanical grinding and thermal treatment. The thermo-mechanical treatment of raw bentonite was performed by using the same method, as defined by Rehman et al. [48]. First, the clay that was extracted from the natural source was dried in an oven at 100 °C for 24 h. The clay was pulverized in a Los Angeles abrasion machine after it had dried. Each 5 kg batch of clay was subjected to 4500 revolutions in order to maintain a uniform thickness. The powdered clay was passed through Sieve Number 200, which has a nominal sieve opening of 0.074 mm, and then placed in polythene bags to keep it dry. The mechanical grinding was performed for both types of bentonite, i.e., raw bentonite and treated bentonite. However, the further thermal treatment of bentonite was only performed for the treated bentonite-blended mixes.
The bentonite clay was then further heated to 800 °C for three hours. Before the 5 kg clay batch was placed in the furnace, the furnace was permitted to achieve the target temperature for control and uniform heating. The clay was then placed in the furnace for the required amount of time (three hours). After completing the required time, the furnace was switched off and allowed to cool down to room temperature. It took the furnace 24 h to cool down to room temperature. The sample stayed in the furnace for this time. Once the clay reached room temperature, the sample was taken out and it was sealed off from moisture by placing it within PVC bags.

2.3. Mixture Proportions

Nine mixture proportions were prepared in this study to investigate the influence of untreated (raw bentonite) and treated bentonite, and PPF on the workability and mechanical properties of GPC, as detailed in Table 4 and Table 5. FA was used as the base material for the GPC mixtures, and bentonite clay was incorporated as a replacement for 10% of the FA, in both raw and treated forms. The optimum dosage of bentonite (10% by weight of binder) was used in this study, based on a recent literature review [33]. GPC mixtures with a 0% replacement level of bentonite were taken as the control mixes. The PPF ratio was selected based on recently published research; specifically, Bellum’s study [52] suggests that the optimal content for polypropylene fiber to increase the compressive strength of geopolymer is 2%, while Murthy’s study [53] shows that 1% is the optimum dosage of PPF keeping in view the mechanical and durability properties of geopolymer content.
In our study, PPF fibers were added at three different proportions by volume fraction: 0.5%, 0.75%, and 1%.

2.4. Specimens Preparation

A concrete drum mixer was used to prepare GPC mixtures, as shown in Figure 2a. The AAS in the prescribed ratio was prepared one hour before the mixing operation. For each mix, the mixing process and timing were held constant. All the dry ingredients were introduced in the mixer at first. Fine aggregates were placed first, followed by coarse aggregates and binder ingredients. AAS was added to the dry components that had already been prepared and mixed for another three minutes to establish homogeneity. PPF fibers were incorporated in the end to avoid the clumping of fibers due to more revolutions of the mixer. The specimen molds were then filled with freshly mixed GPC, as shown in Figure 2b.

2.5. Testing Methods

2.5.1. Workability and Mechanical Properties

The workability of concrete is commonly assessed using various test methods to determine its ease of handling, placing, and compacting. One of the most widely used methods for evaluating concrete workability is the slump test by using the slump cone apparatus, so it was implemented in this study, as per the ASTM C143 procedure [54]. The slump value indicates the workability of the concrete, with higher values representing more workable mixes and lower values indicating stiffer mixes. The compressive strength, flexural strength, and tensile strength tests were conducted on a universal testing machine (UTM) with a 4000 kN capacity. The compressive strength of GPC mixtures was measured at 28 days using three identical 150 mm cubes of each mix, in accordance with BS EN 12390 [55] of each mix. Split tensile strength was determined on 150 mm × 300 mm cylinders according to ASTM C496 [56]. Flexural strength was assessed on 100 mm × 100 mm × 500 mm beams following ASTM C78 [57].

2.5.2. Durability Performance

Water absorption is one of the factors considered when assessing the durability performance of GPC, which is an important indicator of a GPC’s ability to resist moisture ingress, which can lead to various durability issues over time. In this study, water absorption of GPC was determined according to the ASTM C642 with a specimen size of 100 mm in diameter and 50 mm in height [58]. To assess the acid attack resistance of GPC mixtures, a mass loss test in a 5% sulfuric acid solution (H2SO4) was conducted. The acid attack resistance of GPC mixtures was evaluated using a mass loss test in a 5% sulfuric acid solution (H2SO4). The samples were submerged in the acid solution for 28 and 90 days. The weight loss of the samples was measured after the test to determine the acid attack resistance. The abrasion resistance of the GPC samples was evaluated using a Los Angeles abrasion test in accordance with ASTM C131 [59] and the previously employed method used by Haq et al. [60] and Ullah et al. [61]. The test was conducted on cylindrical specimens measuring 100 mm in diameter and 150 mm in height. The initial weight of the specimens was recorded. Then, the specimens were placed in the Los Angeles machine and subjected to 300 revolutions at a speed of 30 revolutions per minute, without the use of steel balls. The specimens were then removed from the machine and weighed again.

3. Results and Discussion

3.1. Workability

The incorporation of bentonite, whether in its raw or treated form, has an adverse effect on the workability of the bentonite-GPC mixtures, as demonstrated in Figure 3. It can be observed from Figure 3 that Mix-2 and Mix-6 displayed lower slump values as compared to the control Mix-1 (without bentonite). This is likely due to the higher specific surface area of bentonite compared to FA particles. A corresponding trend was also observed in prior studies [25,26,27]. The decline in workability is more noticeable in raw bentonite than in treated mixes. Bentonite particles have a greater specific surface area, which requires more water or solution to thoroughly wet the particle surfaces, making the mixture more difficult to work with. Despite the reduced slump values due to bentonite, the cohesion of the mixture is still maintained. SP was used to compensate for the lower slump values and produce the desired slump. Previous studies have also found a similar decreasing trend in slump values due to bentonite addition in concrete mixes [35,54]. However, the workability of the treated bentonite mix (Mix-6) was observed on the higher side as compared to the mix with raw bentonite (Mix-2). This may be attributed to the decreased surface area of treated bentonite particles compared to raw bentonite [62,63,64]. It is also reported in the literature that the heat treatment of bentonite reduces its surface area [62,63,64], which decreases the water demand of treated bentonite-GPC mixtures that in turn results in increasing the workability. It was further observed that the addition of PPF to the mixtures significantly decreased their workability. This trend of decreasing workability was uniform for both raw bentonite and treated bentonite-blended mixes. PPF increases the internal friction of the mixtures, even when the water content is kept constant. This is because PPF fibers have a high surface area and tend to adhere to cement paste, which reduces the fluidity of the mixture. It can be observed from Figure 3 that with an increase in fiber content, the slump values of mixes decrease gradually. The decrease in slump values was calculated as 7%, 25%, and 37% for Mix-3 (0.5%PPF), Mix-4 (0.5%PPF), and Mix-5 (0.5%PPF), respectively, compared to the raw bentonite-blended GPC mix without polypropylene fibers (Mix-2). Compared to the treated bentonite-blended GPC mix without polypropylene fibers (Mix-6), the decrease in slump values was estimated as 12%, 17%, and 27% for Mix-7 (0.5%PPF), Mix-8 (0.5%PPF), and Mix-9 (0.5%PPF), respectively. A similar trend was also reported in the literature [65]. The amount of water absorbed on the surface of the fibers increases along with the number of fibers in the unit concrete mixture as the fiber volume percent increases [65]. As a result, fibers’ obstructive effect on fresh concrete’s flowability increases significantly. However, some potential strategies for mitigating the reduced workability caused by bentonite and PPF include the incorporation of workability enhancers or increasing the dosage of SP and optimizing the overall particle size distribution. These approaches can minimize segregation, enhance the flowability of GPC, improve the dispersion of particles, and reduce internal friction, thereby facilitating easier placement and consolidation of the GPC.

3.2. Mechanical Properties

3.2.1. Compressive Strength

The compressive strength of bentonite-GPC mixtures is a critical property for determining the load-bearing capacity of structural members. Several factors, such as source material composition, mineral or chemical admixtures, binder-to-alkaline solution ratio, and sodium hydroxide solution molarity, influence the compressive strength. The compressive strength measurements of all the mixtures are shown in Figure 4 The results showed that the compressive strength of the concrete mixes increased with the addition of PPF, both for raw and treated bentonite-blended mixes. The compressive strength of mix ID 3, 4, and 5 with 0.5%, 0.75%, and 1% PPF were increased by an amount of 8%, 13%, and 14%, respectively, compared to the control mix ID 2 (without PPF) for raw bentonite-GPC mixtures. Similarly, the compressive strength of mix ID 7, 8, and 9 mixes with 0.5%, 0.75%, and 1% PPF were increased by an amount of 5%, 13%, and 15%, respectively, compared to control mix ID 6 (without PPF) for treated bentonite-GPC mixtures.
Compressive strength was improved by an extent of 10% and 18% for raw and treated bentonite-GPC mixtures, respectively, compared to the control mix without bentonite. This increase in strength can be attributed to the bentonite filler and pozzolana reaction properties, which improved the strength and led to a more compacted and refined microstructure in the GPC. The pozzolanic reaction of bentonite, which produces more cementitious chemicals, and the filling qualities of bentonite are both responsible for the increase in strength [25]. Due to these characteristics, the microstructure of the GPC is finer and more densely packed. Treated bentonite specimens outperformed untreated bentonite specimens in terms of compressive strength, consistent with the findings of other studies [25]. This is likely due to the improved pozzolanic reactivity of treated bentonite, which is attributed to the transformation of the clay mineral structure from smectite to illite. The same phenomenon was reported by Rehman et al. for conventional OPC concrete mixes [48]. It was reported that the thermos-mechanical treated bentonite mixes of OPC concrete perform better than mechanically treated bentonite mixes. A lot of clays have pozzolanic qualities, and some may have fixed crystalline structures. A clay mineral’s crystalline structure is changed into an amorphous one after calcination, which also significantly raises the mineral’s pozzolanic reactivity [66]. Clay, shale, and other minerals undergo structural changes when heated, transforming into quasi-amorphous silica or alumina, both of which have favorable pozzolanic reactivities with CH [67].
The addition of PPF further increased the compressive strength of all mixtures by 5–18%, with the greatest increase observed for a mixture with 1% PPF for both raw and treated bentonite blended. The compressive strength of mix ID 3, 4, and 5 (raw bentonite-GPC mixtures with 0.5%, 0.75%, and 1% PPF, respectively) increased by 10%, 13%, and 14%, respectively, compared to the control mix ID 2 (without PPF). Similarly, the compressive strength of mix ID 7, 8, and 9 (treated bentonite-GPC mixtures with 0.5%, 0.75%, and 1% PPF, respectively) increased by 5%, 13%, and 15%, respectively, compared to the control mix ID 6 (without PPF). PPF improves compressive strength by controlling and preventing crack formation, significantly reducing stress at crack tips, and resulting in increased compressive strength. This observation is consistent with previous studies that reported slight improvements in compressive strength when PPF is added to concrete mixes [65,68,69]. Mohammed et al. studied the effect of PPF addition with different fractions (0.5%, 1%, and 1.5%) on the mechanical properties of metakaolin-based geopolymer concrete [69]. It was reported that an increase in compressive strength was observed with an increase in the fiber content of up to 1%. A slight reduction in compressive strength was observed with an increase in the fiber content from 1% to 1.5%. These results support the findings of the present study. The effect of PPF was most noticeable in mixtures with 0.5% and 0.75% PPF, for both raw and treated bentonite specimens. The compressive strength of mix ID 5 and 9 with 1% PPF increased by only 1% and 2%, respectively, compared to mix ID 4 and 8 with 0.75% PPF. This increase in compressive strength at higher PPF concentrations (>0.75%) can be attributed to non-uniform fiber distribution due to decreased workability, fiber fusion, and increased pore formation [32,34].

3.2.2. Tensile Strength

Splitting tensile strength was tested after 28 and 90 days of casting. The results presented in Figure 5 show that all GPC mixtures with 10% bentonite replacement (treated and raw) exhibited an increase in tensile strength compared to the control mix. However, the effect of bentonite on tensile strength was not very significant in terms of percent increment in strength values. The inclusion of bentonite results in a small increase in the splitting tensile strength values. The performance of treated bentonite-GPC mixtures was better than those blended with raw bentonite. Splitting tensile strength of mix ID 2 and 6 was 4% and 10% higher than the control mix, respectively. This improvement in strength can be attributed to the filler and pozzolanic reaction properties of bentonite, which increase the strength and lead to a more compacted and refined microstructure in the GPC. Treated bentonite samples outperformed raw bentonite samples in terms of splitting tensile strength, consistent with the findings of previous studies [25,48]. A similar trend was observed by Rehman et al. for conventional OPC concrete mixes [48]. The splitting tensile strength of untreated bentonite and treated bentonite was compared, and it was found that thermo-mechanically treated bentonite showed significant improvement in splitting tensile strengths compared to raw bentonite due to the increased reactivity of clay minerals in pozzolanic reaction.
The addition of PPF also caused an increase in the splitting tensile strength of all bentonite-GPC mixtures. The splitting tensile strength of mix ID 3, 4, and 5 increased by 9%, 20%, and 32%, respectively, compared to the control mix ID 2. Similarly, the splitting tensile strength of mix ID 7, 8, and 9 increased by 7%, 19%, and 33%, respectively, compared to the control mix ID 6. The same trend of increase in strength with an increase in the fiber content of up to 1% fraction in metakaolin-based GPC mixes was observed by Mohammed et al. [69]. It was found that the splitting tensile strength of GPC mixes increases by an amount of 16% with an increase in the fiber content from 0% to 1%. The increase in splitting tensile strength can be attributed to the coarse texture of PPF, which improves adhesion and bonding between the concrete and the fibers. Additionally, PPF can block crack propagation as soon as the concrete begins to crack due to the bridging action. This reduces the brittleness of concrete and improves its post-cracking behavior [32,36]. Zhang et al. reported the same phenomenon for fly ash-based geopolymer composites [42]. It was reported that PPF can improve the quality of the geopolymer matrix significantly by modifying the basic structure of the geopolymer matrix and self-deformation even during tensile failure.

3.2.3. Flexural Strength

Figure 6 shows the flexural strength of all GPC mixtures, tested after 28 and 90 days of casting. The comparison of flexural strength values between the control mix and bentonite-GPC mixtures showed that bentonite has a positive effect on the flexural strength of all GPC mixtures. Treated bentonite specimens also outperformed raw bentonite specimens. Additionally, the inclusion of PPF increased the flexural strength of all mixtures by 6–29%. The specimens with 1% PPF showed the greatest increase in strength, both for raw and treated bentonite mixtures compared to the specimen without PPF. The flexural strength of mix ID 3, 4, and 5 increased by 8%, 17%, and 27%, respectively, compared to the control mix ID 2. Similarly, the flexural strength of mix ID 7, 8, and 9 increased by 6%, 18%, and 29%, respectively, compared to the control mix ID 6. This demonstrates unequivocally that adding polypropylene fiber increases the flexural strength of all GPC mixes. This finding is in line with other research that found that adding PPF to concrete mixtures led to increases in flexural strength [65,68,69]. The impact of PPF addition at several fractions (0.5%, 1%, and 1.5%) on the mechanical characteristics of geopolymer concrete based on metakaolin was investigated by Mohammed et al. [69]. According to that research, an increase in fiber content from 0% to 1% was linked with an increase in flexural strength. The improvement in the mechanical connection between the fiber and cement paste is what leads to the increase in flexural strength. The increased fiber content in the mixture significantly aids in reducing the crack spreading more widely. Another study by Divya S. Dharan and Aswathy La reported that adding fibers to cement paste increased the flexural strength [70]. Pham et al. witnessed similar results when they tested the flexural tensile strength of polypropylene fiber-reinforced geopolymer concrete specimens [71]. They discovered that the flexural strength ascended as the volume percent of fiber increased from 0.5% to 1.5%. The increase in flexural strength can be attributed to the rough surface of the PPF, which results in a strong connection and bond in the concrete [32,36]. Additionally, PPF can arrest crack propagation as soon as the concrete begins to crack due to their bridging action. This reduces the brittleness of concrete and improves its post-cracking behavior [32,36].

3.3. Durability Properties

3.3.1. Water Absorption

Water absorption is an important factor in determining the durability of GPC, as it measures the amount of water that can penetrate the GPC mixture. This can be harmful, as it allows the ingress of aggressive chemicals and salts that can react with the constituents of concrete and degrade its properties. A water absorption test was performed on all bentonite-GPC mixtures and control mix at the ages of 28 and 90 days. The results, shown in Figure 7, indicate that the addition of bentonite, both raw and treated, has a positive effect on water absorption. A significant reduction in water absorption was observed for all bentonite-GPC mixtures at both 28 and 90 days of age. The water absorption of raw and treated bentonite-GPC mixtures at the age of 90 days was decreased by 16% and 21%, respectively, compared to the control mix (without bentonite). The water absorption of mix ID 2 and 6 decreased by 11% and 15%, respectively, compared to the control mix without bentonite at the age of 90 days. It was also observed that water absorption tended to decrease with age for each individual mixture. The water absorption of mix ID 1, 2, 3, 4, 5, 6, 7, 8, and 9 at the age of 90 days decreased by 7.3%, 5.5%, 5.7%, 5.6%, 5.1%, 7.5%, 5.9%, 6.0%, and 5.5%, respectively, compared to the water absorption values of the same mixtures at the age of 28 days. Previous studies have suggested that the reduction in water absorption of concrete can be attributed to the improved microstructure of the material resulting from the addition of bentonite [25]. Bentonite has smaller particles than FA, which allows for a more uniform distribution in the mixture and a more compact and denser microstructure. Additionally, the pozzolanic reactivity of bentonite may lead to the development of calcium–silicate–hydrate (C-S-H) gel in conjunction with geopolymer gel, which can provide an even denser microstructure of the GPC [72]. It was reported by different studies [33,72,73] that the addition of bentonite to cementitious matrices results in the development of C-S-H phases. It is also reported that CASH phases are developed when bentonite is added to fly ash-based cementitious products. Therefore, it can be concluded that adding bentonite to GPC mixtures is beneficial for reducing the water absorption capacity of the GPC. These findings are in line with the outcomes of the previous studies [25,33].
The addition of 0.5% PPF further decreased the water absorption of bentonite-GPC mixtures. It was observed that the addition of 0.5% PPF resulted in a decrease in the water absorption of both raw bentonite-blended mixes and treated bentonite-blended mixes by 3.4% and 3.7%, respectively, compared to their control mixes (without PPF). However, higher dosages of PPF (0.75% and 1%) led to an increase in the water absorption of raw bentonite mixtures. The water absorption of mix ID 4 (0.75% PPF) and 5 (1% PPF) increased by 2% and 11%, respectively, compared to mix ID 3 with 0.5% PPF at the age of 28 days. On the other side, the mixes of treated bentonite mixes showed different trends. The water absorption of mix ID 8 with 0.75% PPF was decreased slightly as compared to the preceding mix (Mix ID 7) with 0.5% PPF. However, the water absorption of mix ID 9 with 1% PPF increased by 4%, 7%, and 10%, respectively, compared to mix ID 6 (without PPF), 7 (0.5% PPF), and 8 (0.75% PPF). This may be due to the lower adhesion between PPF and the GPC. It has been reported that increasing the PPF concentration increases the likelihood of large pores and intensifies the non-uniformity of the concrete’s microstructure, which increases absorption [36]. Previous studies reported in the literature have shown similar findings [53,74]. Shreyas and Patil studied the durability characteristics of polypropylene fiber-reinforced geopolymer concrete with different fractions of PPF 0.5%, 1%, and 1.5% by mass of binder [53]. It was concluded that a mix with 1% PPF exhibited minimum water absorption. It was also reported that the further increment of fiber beyond 1% resulted in increasing water absorption. The addition of fibers decreases the pores in the concrete matrix, hence reducing water absorption. The water absorption of concrete reinforced with PPF and plain concrete was compared by Yuan and Jia [74]. It was observed that when the volume percentage of PPF was greater than 0.45%, the water absorption of the PPF-reinforced concrete was higher than that of the plain concrete. This is due to PPF’s hydrophobic properties, which lead to poor cement mortar bonding performance. Furthermore, an excess content of PPF is likely to cause the internal structure of concrete to be non-uniform [74].

3.3.2. Abrasion Resistance

The abrasion loss for each of the nine mixtures is displayed in Figure 8. The results show that the addition of bentonite and PPF reduced the abrasion loss of GPC mixtures. The abrasion loss of mix ID 2 and 6 was 6% and 12% lower, respectively, compared to mix ID 1 (without bentonite). The addition of 0.5% PPF further reduced the abrasion loss of GPC mixtures, with mixtures 3 and 7 showing abrasion loss of 7% and 12% lower, respectively, than mixtures 2 and 6. However, at higher PPF dosages (0.75% and 1%), the abrasion loss of GPC mixtures increased. The highest abrasion loss was observed for mixtures with 1% PPF (5 and 9), which was 15% and 20% higher than the mixtures 2 and 6, respectively. The improved abrasion resistance of GPC mixtures in the presence of bentonite and PPF can be attributed to a number of factors. Bentonite can form a protective layer on the surface of concrete, which reduces the wear and tear caused by abrasion. PPF can also improve the abrasion resistance of GPC mixtures by bridging cracks and preventing their propagation. Similar findings were reported by Grdic et al. [75] where abrasion resistance of polypropylene fiber-reinforced concrete was evaluated. It was reported that the addition of PPF has a positive effect and aids in increasing the resistance of concrete to abrasive erosion.

3.3.3. Acid Attack Resistance

Figure 9 shows the mass loss of GPC mixtures containing bentonite and PPF when exposed to sulfuric acid solution for 28 and 90 days. The control mix (mix ID 1) did not contain bentonite or PPF. The other mixtures contained 10% bentonite (2 and 6), 0.5% PPF (3 and 7), 0.75% PPF (4 and 8), and 1% PPF (5 and 9). The results show that the addition of bentonite and PPF reduced the mass loss of GPC mixtures in sulfuric acid solution. The mass loss of mix ID 2 (raw bentonite mix) and 6 (treated bentonite mix) was 5% and 10% lower, respectively, than mix ID 1. A similar effect of bentonite on the acid attack resistance of concrete mixes was observed by Rehman et al. [48].
The addition of 0.5% PPF further reduced the mass loss of GPC mixes, with mixtures 3 and 7 showing mass losses 7% and 12% lower, respectively, than mixtures 2 and 6. However, at higher PPF dosages (0.75% and 1%), the mass loss of GPC mixtures increased. The highest mass loss was observed for mixes with 1% PPF (5 and 9), which was 15% and 20% higher than mixtures 2 and 6, respectively. The improved resistance of GPC mixtures to acid attack in the presence of bentonite and PPF can be attributed to a number of factors. Bentonite has a high pH, which helps to neutralize the acidic environment. Additionally, bentonite can form a protective layer on the surface of concrete, which reduces the penetration of sulfate ions. PPF can also improve the resistance of GPC mixtures to acid attack by bridging cracks and preventing their propagation. However, at higher PPF dosages, the increased porosity [76] and inhomogeneity of the concrete microstructure may lead to increased mass loss [77]. As sulfate ions penetrate the GPC mixture more deeply over time, more weight is lost, leading to increased degradation of GPC with age when exposed to acid.

4. Conclusions

This study has presented the results of a comprehensive experimental investigation aimed at assessing the influence of bentonite and PPF on both fresh and hardened GPC properties. The following significant conclusions can be drawn from this study:
  • The incorporation of bentonite, whether in its raw or treated form, has an adverse effect on the workability of the bentonite-GPC mixtures. The decrease in workability is more pronounced in the case of raw bentonite than in treated bentonite-GPC. Also, the incorporation of PPF results in a further decrease in workability.
  • Bentonite improves the mechanical properties of GPC mixtures, with a 10% replacement yielding better results than the control mixture. Compressive strength was improved by an extent of 10% and 18% for raw and treated bentonite-GPC mixtures, respectively, compared to the control mix without bentonite.
  • The effect of treated bentonite on the mechanical properties of GPC mixtures was more pronounced compared to the raw bentonite-blended mixes.
  • For each mixture, the outcomes of the tensile and flexural strengths show trends that are identical to those of the compressive strength results.
  • The compressive strength of the concrete mixes was increased with the addition of PPF, both for raw and treated bentonite-blended mixes.
  • PPF improves the mechanical properties of GPC mixtures, with the highest improvement observed for mixtures with 1% PPF.
  • Bentonite and PPF improve the durability properties of GPC mixtures, with the best results observed for mixes with 0.5% PPF. Higher PPF contents can have a negative effect on durability properties.
In view of the results of the present study, it is obvious that bentonite is a promising material to be used as a replacement for FA in the production of GPC and providing satisfactory results. The inclusion of bentonite has a positive effect on the mechanical and durability properties of concrete. These positive effects may further be augmented by using treated bentonite in concrete. The utilization of bentonite in the production of GPC will help to cope with shortage issues of the supply of FA and SG faced in some parts of the world. It will also help to improve the longevity and life cycle of geopolymer concrete structures, save natural resources, lessen energy usage, and address environmental issues associated with cement production. The results of this study revealed that the combination of bentonite and PPF (0.5% by volume fraction) imparts more positive effects on the mechanical and durability properties of concrete.
It is recommended that future investigations assess the impact of treated bentonite on other concrete durability attributes, such as carbonation and chloride ingress. The microstructural investigation also needs to be carried out emphasizing the effect of bentonite and PPF on the microstructure of bentonite-blended and PPF-reinforced GPC mixtures. The potential of utilizing bentonite in GPC structural members needs to be further investigated as well.

Author Contributions

Conceptualization, R.M.W. and M.K.A.; Methodology, S.Z. and E.A.; Validation, F.B. and S.Z.; Investigation, M.K.A. and E.A.; Supervision, R.M.W. and F.B.; Resources, M.K.A. and E.A.; Project administration, S.Z.; Writing—original draft, R.M.W. and S.Z.; Writing—review and editing, M.K.A. and E.A.; Visualization, F.B.; data curation, R.M.W. and S.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data present in this study are available upon request.

Acknowledgments

The authors acknowledge the support provided by the University of Engineering and Technology Taxila. The researchers would like to thank the Deanship of Scientific Research, Qassim University for funding the publication of this project.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. SEM image of low-calcium bentonite used in this study.
Figure 1. SEM image of low-calcium bentonite used in this study.
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Figure 2. Preparation of GPC mixes (a) Preparation of concrete in drum mixer (b) Prepared GPC samples.
Figure 2. Preparation of GPC mixes (a) Preparation of concrete in drum mixer (b) Prepared GPC samples.
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Figure 3. Slump cone test results of GPC mixtures.
Figure 3. Slump cone test results of GPC mixtures.
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Figure 4. Compressive strength values of GPC mixtures.
Figure 4. Compressive strength values of GPC mixtures.
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Figure 5. Split tensile strength values of GPC mixtures.
Figure 5. Split tensile strength values of GPC mixtures.
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Figure 6. Flexural strength values of GPC mixtures.
Figure 6. Flexural strength values of GPC mixtures.
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Figure 7. Water absorption of GPC mixtures.
Figure 7. Water absorption of GPC mixtures.
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Figure 8. Percentage of mass loss of GPC mixtures in abrasion resistance test.
Figure 8. Percentage of mass loss of GPC mixtures in abrasion resistance test.
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Figure 9. Percentage of mass loss of GPC mixtures in sulfuric acid solution.
Figure 9. Percentage of mass loss of GPC mixtures in sulfuric acid solution.
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Table 1. Physical properties of materials.
Table 1. Physical properties of materials.
PropertyFABentonite
Memon et al. [26]The Present Study
Relative density (g/cm3)1.82.812.64
Specific surface area (cm2/gm)325048004900
Loss on ignition (%)0.575.49.6
Table 2. Weight percentage of the chemical composition of materials.
Table 2. Weight percentage of the chemical composition of materials.
Chemical CompositionFABentonite
Memon et al. [26]The Present Study
Silicon Dioxide (SiO2)35.854.552.8
Aluminum Dioxide (Al2O3)20.220.216.4
Ferric Dioxide (Fe2O3)11.48.65.8
Magnesium Dioxide (MgO)1.804.51.4
Calcium Dioxide (CaO)14.37.34.6
Sodium Dioxide (Na2O)1.201.30.62
Potassium Dioxide (K2O)2.23.60.7
Table 3. Properties of PPF.
Table 3. Properties of PPF.
PropertyValues
Tensile strength at breaking (MPa)31–41
Flexural strength (MPa)41–55
Elongation at break (%)100–600
Tensile modulus (MPa)1137–1551
Specific gravity0.9–0.91
Table 4. Percentage of mixture proportions.
Table 4. Percentage of mixture proportions.
Mix IDFARaw BentoniteTreated BentonitePPF
1100%---
290%10%--
390%10%-0.5%
490%10%-0.75%
590%10%-1%
690%-10%-
790%-10%0.5%
890%-10%0.75%
990%-10%1%
Table 5. Quantities of mixture proportions (kg/m3).
Table 5. Quantities of mixture proportions (kg/m3).
Mix IDFine Agg.Coarse Agg.FABentoniteSHSSSP
16401201400-531076
2643120636040531076
3646121236040531076
4652122036040531076
5643120636040531076
6646121236040531076
7644120836040531076
8647121436040531076
9655122536040531076
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Waqas, R.M.; Zaman, S.; Alkharisi, M.K.; Butt, F.; Alsuhaibani, E. Influence of Bentonite and Polypropylene Fibers on Geopolymer Concrete. Sustainability 2024, 16, 789. https://doi.org/10.3390/su16020789

AMA Style

Waqas RM, Zaman S, Alkharisi MK, Butt F, Alsuhaibani E. Influence of Bentonite and Polypropylene Fibers on Geopolymer Concrete. Sustainability. 2024; 16(2):789. https://doi.org/10.3390/su16020789

Chicago/Turabian Style

Waqas, Rana Muhammad, Shahid Zaman, Mohammed K. Alkharisi, Faheem Butt, and Eyad Alsuhaibani. 2024. "Influence of Bentonite and Polypropylene Fibers on Geopolymer Concrete" Sustainability 16, no. 2: 789. https://doi.org/10.3390/su16020789

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