Mechanical Characteristics and Durability of Metakaolin-Based Self-Compacting Geopolymer Concrete as A Function of Recycled Aggregate and Steel Fiber Contents

Concrete, second only to water in terms of usage, necessitates a significant amount of Portland cement. Portland cement production entails significant energy consumption and results in substantial emissions of carbon dioxide (CO₂) and other detrimental greenhouse gases. Therefore, it is crucial to identify an alternative to the current one. Geopolymer is regarded as a substitute for cement that helps mitigate environmental issues [1-3]. It offers the benefits of minimal CO₂ emissions during manufacture and the utilization of significant amounts of industrial waste, in contrast to Portland cement. Hardened geopolymers possess an intricate three-dimensional network structure composed of interconnected tetrahedrons [1]. These substances can exist in an amorphous state, meaning they lack a well-defined crystal structure, or in a quasi-crystalline state, where they have some degree of order. The primary tetrahedrons included silicon oxide (SiO₄), aluminum oxide (AlO₄), and phosphorus oxide (PO₄). Geopolymerization is the chemical reaction that forms geopolymers. It involves an aluminosilicate source (rich in aluminum and silicon) and an activator solution. Typical aluminosilicate sources include: (i) an industrial byproduct of coal combustion, contains aluminosilicate minerals) fly ash) (ii) a glassy byproduct of steel manufacture that may also contain aluminosilicates (slag). Activators are usually alkaline solutions, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH). Acidic activators are also being studied, but alkaline activation is more prevalent. During geopolymerization, the activator dissolves the aluminosilicate source, allowing aluminum and silicon to react and form new, strong covalent connections. These linkages form a three-dimensional network structure like some natural zeolites [4]. Depending on the aluminosilicate supply and activator utilized, several geopolymers can be generated. These changes can affect the material's ultimate qualities. Polysialates are the most prevalent type of geopolymer, created with alkaline activators and characterized by Si-O-Al-O bonding units. Sialate siloxanes and disiloxanes: These have a greater Si/Al ratio (more silicon than aluminum) than polysialates, which is produced by employing certain activator concentrations or changing the aluminosilicate supply. Geopolymers are three-dimensional network structures that are formed through a unique chemical activation procedure. As a result, these materials have many uses as binders, including construction, industrial heavy metal adsorption, traffic reconstruction projects, radioactive material treatment, and more [5]. Additionally, geopolymers typically have great anti-corrosion properties, high-temperature stability, and immediate excellent strength. Improper compaction and high viscosity of geopolymer concrete may result in its failure, owing to its elevated viscosity [6]. In order to address this concern, the notion of Self-compacting geopolymer concrete was proposed. This unique type of concrete does not require manual compaction. Instead, it naturally flows and compacts due to its own weight [7-9].

Metakaolin is a newly developed pozzolanic cementitious ingredient. It differs from other available cementitious additives since it is manufactured in a controlled environment [10]. Due to its high reactivity and the appropriate proportion of alumina to silica for geopolymerization, metakaolin has gained significant attention as a potential replacement material for geopolymer binder production over the past decade [11]. Metakaolin could potentially serve as a viable precursor material in the geopolymer manufacturing process due to its consistent properties and propensity for reactivity during both its synthesis and progression [12]. The potential of metakaolin geopolymer as an alternative to cement binder was assessed by researchers. It has been disclosed that geopolymer concrete (GPC) exhibited enhanced mechanical conductivity while maintaining a higher porosity than cement concrete. The use of 15% MK in concrete enhances its compressive and flexural strength [13, 14].

Despite their strength, geopolymers are extremely brittle [15, 16]. To overcome this, there are three main approaches: (a) modifying substrates chemically or physically, (b) reinforcing internally, and (c) applying external constraints. Among the three approaches, the last two are typically more effective and economical. Adding fibers to the geopolymer substrate is a method to enhance brittleness by increasing its density and roughness [17]. There are numerous types of fibers readily accessible for commercial use, including either synthetic or natural fibers, glass, nylon, and steel. One crucial factor influencing the absorbing of material energy is the manner in which fibers and cement matrix are bonded. One of the most advantageous attributes of fibers is their capacity to inhibit the spread of fractures. The use of steel fibers is found to result in an increase in the compressive strength of concrete [18].

In the realm of construction industry environmental sustainability, recycled aggregates serve as a viable alternative to natural aggregates when incorporated into concrete formulations [19-21]. As a result, the incorporation of waste materials into concrete mixtures is gaining traction globally. However, the compressive strength of geopolymer composites containing recycled concrete aggregates was found to be poor in comparison to those comprising natural coarse aggregates, according to study [22]. Thus, RCA included geopolymer concretes are needed to be modified [23].

In simple terms, the blending and merging of composites can result in significant and intricate variations in the functional and mechanical characteristics of GPC [24]. As a result, the use of GPC composites in applications pertaining to engineering is restricted. Furthermore, the specific action mechanisms involved in GPC composites at both the mesoscopic and microscopic levels are still poorly understood and require a thorough examination. The present study aims to examine the impact of utilizing MK and steel fibers in conjunction with RCA as a complete replacement for coarse aggregate on the mechanical characteristics and functionality of SCGPC. Mechanical and shrinkage properties of the fabricated samples were systematically investigated. Furthermore, the effect of attacking acids and salts on geopolymer concrete was observed the morphological changes, weight and compressive strength after exposure to salts and acid were investigated. The authors state that there is limited information on the use of steel fibers and RCA together in metakaolin-based SCGPC. This research is considered innovative because of this lack of existing knowledge.

Existing research on steel fiber-reinforced self-compacting geopolymer concrete has mostly utilized natural coarse aggregates [25]. There is a scarcity of data on how RCA interacts with steel fibers in a metakaolin-based geopolymer binder. This information gap impedes the development of sustainable construction materials based on recycled components. This study examines the impact of using MK, steel fibers, and RCA as a complete replacement for natural coarse aggregate (Impact of combined utilization). This work expands on earlier research by investigating how RCA interacts with steel fibres in a metakaolin geopolymer matrix. This combined effect has not been fully studied, particularly in terms of mechanical qualities and durability [26, 27]. The study assessed how this combination affects the mechanical qualities of the SCGPC, including as compressive and flexural strength, among others (Mechanical performance). The study tested the SCGPC's durability by subjecting it to severe environments such as acidic or salty situations. Following this exposure, we assessed changes in weight and compressive strength, as well as look for any morphological changes (changes in microscopic structure) (Durability assessment). This study added to the development of sustainable construction materials by investigating the potential of incorporating fully recycled coarse aggregate and steel fibre reinforcement into a geopolymer concrete formulation. This mixture provides a more environmentally friendly alternative to ordinary concrete.

1.1 Materials

The metakaolin was produced through the process of calcination of the kaolin. The collection of kaolin clay was conducted at Al-Anbar, Iraq. The kaolin was initially ground and subsequently heated in a furnace at a temperature of 700℃±20℃ for an hour, resulting in the formation of metakaolin Figure 1(a). The mineral composition of metakaolin employed in the study was determined using X-ray diffraction (XRD) spectral analysis. Figure 1(b) shows the largest quantity of SiO₂. Table 1 shows the chemical and physical characteristics of metakaolin. To activate the SCGPC mixtures, an alkaline solution was utilized. This solution was composed of sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃). The alkaline solution was prepared in advance of its use by 24 hours. A superplasticizer, which was formulated from a polycarboxylic-ether mixture, was employed in all compositions to achieve the required slump value for fresh SCGPC. The superplasticizer had a specific gravity of 1.08±0.02 and a pH value of 7±1. RCA was obtained locally from a demolished structure Figure 1(c). Its particular physical features and size distribution are described in Table 2. The used coarse natural aggregate is seen in Figure 1(e). Fine steel fibers, shown in Figure 1(g), were used in the experimental process to improve SCGPC flexibility.

Table 1. Physical properties and chemical composition of metakaolin

Table 2. Physical properties aggregate

Table 3. Properties of steel fiber

The fibers properties are illustrated in Table 3. The mixture used natural sand as the fine aggregate, with a maximum particle size of 4.75 mm. The specific gravity and water absorption were measured at 2.63 and 1.5%, respectively, indicating conformity with the BS.882 criteria. SCGPC production process showing in Figure 1 (h).

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Figure 1. (a) Metakaolin; (b) XRD pattern of metakaolin; (c) Coarse recycled aggregate; (d) Grading of fine aggregate; (e) Coarse natural aggregate; (f) Grading of natural and recycled aggregate; (g) Steel Fiber; (h) Fresh mixture preparation process

1.2 Mix proportions

The research systemically integrated RCA into the sample at different weight percentages (0%, 25%, 75%, and 100%) in place of natural coarse aggregate. Then, varying amounts of SF (0%, 0.5%, 1%, and 1.5%) were mixed with each RCA replacement amount. The total amount of binder used in each batch of SCGPC mixes was 500 kg/m³, sodium hydroxide concentrations of 12M while curing occurs in lab temperature, 8% superplasticizer, 24% extra-water. The benchmark was a reference concrete mixture made entirely of natural coarse aggregate with no SF added. A comprehensive analysis of all examined concrete mix proportions incorporating RCA and SF is provided in Table 4. Dry ingredients, including aggregates and metakaolin, are combined in a pan mixer for a maximum of 2.5 minutes to initiate the preparation process. Then, a distinct liquid mixture is introduced, which has been pre-mixed and pre-shaken to guarantee uniformity. Typically, this liquid constituent comprises an alkaline solution, a superplasticizer, and extra water. After an additional three minutes of mixing, the mixture is brought into complete uniformity. After verifying the functionality of the recently prepared SCGPC mixture, it is poured directly into molds without undergoing any mechanical compaction. This is due to the self-weight of the mixture being adequate to occupy any empty spaces or compartments. After concrete formation, specimens were covered with a plastic wrap for 24 hours to avoid alkaline solution evaporation. The next day after casting, the samples were taken from their moulds and stored in a curing room with an ambient, controlled temperature until the testing day.

Table 4. Mix proportions of SCGPC

3.1 Fresh properties of SCGPC

Slump flow of SCGPC

The ability of SCGPC to smoothly and completely fill complex shapes without requiring vibration is crucial. This mostly relies on the quantity of water in the activating agent solution and the quantity of Superplasticizer used [29]. The slump flow of SCGPC mixtures is represented in Figure 4(a). Among the five mixes, RCA0SF0 sample has the highest recorded value for slump flow (741 mm). This suggests that the mixture has good workability and can easily flow and fill complex forms. The elevated slump value indicates that the mixture was affected by a superplasticizer, which improves its capacity to flow. RCA0SF1.5 has a slump value of 661 mm, which is somewhat lower suggesting that introducing SF may result in a reduction in workability and flowability when compared to the initial mixture. The fine SF particles cover voids between bigger ones, decreasing internal friction and enhancing packing density. However, they also absorb some water required for lubrication. Regarding 0% RCA vs. 100% RCA, the reduction in slump flow from 741 mm to 665 mm indicates some adverse effect caused by the addition of 100% RCA. Possible causes include: RCA has a higher water absorption rate than natural aggregates, which decreases the amount of water available for lubrication and impedes flow. RCA's rougher surface texture and angular shape may enhance internal friction and impede flow compared to smooth, spherical natural aggregates. Overall, all tested SCGPC mixtures showed satisfactory flow and ease of handling.

T₅₀₀ test for SCGPC

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Figure 4. The effect of RCA and SF on (a) slump flow; (b) T₅₀ duration; (c) V-funnel duration; (d) L-box height ratio

3.2 Hardened properties of SCGPC

Compressive Strength

Figures 5 (a) and (b) show the impact of SF and RCA quantities on the compressive strength of SCGPC at different curing times. It is discovered that the compressive strength of SCGPC at 7 days about 70% of that at 28 days, suggesting that early curing days are where strength development mostly happens because to a quick hydration reaction. SCGPC mixes with RCAs showed reduced strength compared to mixes without RCAs. SCGPCs made with 100% RCA had a compressive strength that was 50%-55% lower after 7 days and 50%-60% lower after 28 days when compared to mixtures made without RCA, respectively. The reduced strength of concrete mixtures with RCA in comparison to those without RCA is consistent with preceding research on plain concrete. This is attributed to the presence of weak interfacial transition zones (ITZs) in the microstructure of concrete with RCA, which occur among the aggregate and the past mortar that was included to the RCA, as well as between the fresh mortar and the aged mortar [35]. In general, when RCA contents rise, SCGPC’s compressive strength tends to drop, while SF contents rise [18]. SCGPC mixes labeled RCA0SF1.5, RCA50SF1.5, and RCA100SF1.5 with 1.5% steel fiber showed a 26%, 28%, and 41% increase in 7-day compressive strength and a 27%, 37%, and 45% increase in 28-day compressive strength compared to equivalent mixes without steel fiber. There is an improvement in compressive strength of SCGPC’s that occurs when SF is added, often falling within the range of 30-40%. This rise is most likely because to the It can be concluded that the fibers could bridge across the cracks and arrest the crack face separation during the crack propagation [36]. The improvement in compressive strength is minimal when the SF content increases from 0% to 0.5%, but becomes significant when the SF ratio grows from 0.5% to 1.5% as shown in the Figure 3. This finding is aligned with other reported researches [26, 37, 38]. These findings show that even at a 100% substitution rate, the strength decrease of the concrete caused by replacing natural course aggregate NCA with RCA may be compensated for by appropriately constructing mixes with steel fiber.

Splitting tensile strength

The fluctuation in splitting tensile strength of SCGPC’s follows a pattern similar to compressive strength, with a notable increase due to SF content (Figure 5). The 7-day splitting tensile strengths of RCA0, RCA50, and RCA100 mixtures with 1.5% steel fiber were 36%, 38%, and 48% higher, respectively, compared to the peer unreinforced mixtures. For 28 days splitting tensile strengths of RCA0, RCA50, and RCA100 mixtures with 1.5% steel fiber were 35%, 37%, and 39% respectively. The SF percentage has a considerable effect on the splitting tensile strength of SCGPC due to the SF crack-bridging effect, particularly with elevated SF contents. Introducing SF greatly enhances the splitting tensile strength of SCGPC, with the impact becoming more pronounced as the SF percentage increases. Furthermore, the inclusion of RCA reduces the splitting tensile strength of SCGPC’s. At a RCA value of 50%, the rate of change decreases gradually, ranging from11.52% to15.5% for 7 days curing and 4.5% to 9% for 28 days curing. It should be noted that the reducing rate is lower for SCGPC consisting higher content of SF. Studies have demonstrated that replacing natural aggregate with RCA in concrete mixes consistently results in lower splitting tensile strength, regardless of whether the concrete is conventional, self-compacting, or reinforced [37, 39, 40]. This is because using RCAs weakens the interface transition zone (ITZ) between the aggregate and geopolymer pasta, lowering the concrete's total tensile strength. To put it simply, the strength of a composite material is determined by the amount of each component used: density, recycled aggregate, sand, steel fiber, and water. Any modifications to these amounts will cause the compressive strength to fluctuate. The compressive strength is an indirect measure of the composite's other qualities. Figure 5(e) depicts the relationship between the compressive and tensile strengths of concrete formed from recycled aggregate and metakaolin. The chart shows a significant association, with a regression coefficient near to one (83.76%). This validates the statistical analysis.

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Figure 5. (a, b) Compressive strengths; (c, d) Splitting strength of different mixes at 7 and 28 days; (e) Statistical analysis of the relationship between compressive and tensile strength using regression analysis

3.3 Flexural behavior

Load-deflection curves

Figure 6 displays the load-deflection characteristics of SCGPC samples with varying proportions of RCA and SF. Each curve depicts the mean response of test specimens assessed after 28 days of the curing process. The findings demonstrate that the SCGPC samples saw a substantial load reduction after peak load. Mixes that contained more RCA content experienced a greater load decline after peak load in comparison to those with less RCA ratio. Mixes with 1.5% steel fiber experienced a less noticeable load drop than those with 0.5% steel fiber. Mixtures with a higher RCA may have higher load drops due to RCAs' poor strength, whereas mixes with a higher SF may have lower load drops due to steel fibers' fracture bridging ability [41]. After peak load, the tooth-shaped curve shows that the steel fibers are slowly being drawn out of the paste matrix [42]. An increase in SF ratio resulted in a rise in the residual flexural load of concrete, as shown in the Figure 6(b), for a given RCA ratio. Additionally, it is demonstrated that for a SF, the residual load of the concrete was reduced by adding RCA and reduced further by increasing RCA. Table 5 comprehensively analyses the primary flexural mechanical characteristics obtained from these graphs, including peak load, maximum deflection, residual strength, and related toughness. To explore the behavior of SCGPC subsequent to its attainment of maximum flexural strength, this research presents three distinct residual flexural loads. The loads are designated as PL/300, PL/150, and PL/100, which are equivalent to 1, 2, and 3 millimeter deflections, respectively. This methodology is in accordance with the guidelines specified in the ASTM C1609/C1609M-2012 code [43]. The flexural toughness of a substance denotes its resistance to flexural deformation and fracture. The quantity is determined by taking the integral of the load-deflection graphs until a specific deflection value is reached [44]. Figure 6(a) shows that the load-deflection graphs of SCGPC can be separated into two stages: the rising branch and the falling branch. SCGPC specimens without SF show only the beginning, rising part of the load-deflection curve in Figure 6(b). The load quickly declines to zero at the apex, indicating fragile behavior. In contrast, SCGPC specimens with SF exhibit a definite falling apart after the apex, indicating more ductile behavior. Figures 6(b) and (c) shows that SF and RCA content substantially impact the flexural behavior of SCGPC. As the RCA content increases, the slopes of the first ascending parts of the load-deflection curves and peak loads decrease for all SCGPC samples. Interestingly, for SCGPC with 0.5% SF, the initial ascending branches overlap for all RCA contents. However, the descending branches show significant variances. This shows that increasing RCA concentration diminishes SCGPC ductility, whereas increasing SF content reverses this effect, considerably improving peak and residual loads.

Table 5. Main flexural mechanical properities of SCGPC

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Figure 6. Load-deflection graphs of (a, b) Impact of RCA ratio; (c) Impact of SF ratio

Load and deflection properties

Figures 7 and 8 display the maximum flexural strength and peak deflection of SCGPC with different amounts of SF and RCA. Clearly, the flexural strength and peak deflection of SCGPC are reduced when the RCA content increases in the majority of cases. In SCGPC without SF, when the RCA content is between 0% and 100%, the peak strength is negatively affected by 4.34%, 8.72%, 11.5%, and 16.67%, while the peak deflection is negatively affected by 0.798%, -2.33%, -4.86%, and -8.91%. The percentages for peak strength and peak deflection for SCGPC with 1.5% SF content are -5.19%, -7.32%, -17.06%, and 12.86%, respectively. R100S1.5 has a more uniform structure with only one type of RCA inclusion, which contributes to its higher peak strength compared to R75S1.5. The percentages for peak strength and peak deflection for SCGPC. An increase in the proportion of SF from 0% to 1.5% results in corresponding increases of 8.72%, 10.61%, and 15.81% in peak strength. On the contrary, there is a more substantial alteration in the peak deflection, denoting the extent to which the material can be bent before failure; it rises by 24.23%, 12.65%, and 18.42%, respectively.

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Figure 7. Flexural strength of SCGPC's (a) Impact of RCA content; (b) Impact of SF ratio

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Figure 8. Flexural deflection of SCGPC’s (a) Impact of RCA content; (b) Impact of SF content

Flexural toughness

Toughness is a valuable measure for assessing the ability of materials and structures to absorb energy when exposed to external forces. Figure 9 demonstrates the impact of the contents of RCA and SF on the flexural toughness of RCA. The fluctuation tendency of SCGPC’s toughness is in accordance with that of its flexural strength and deflection. The influence of RCA and SF contents on Ω_L/300 is determined to be minimal due to the presence of identical ascending branches of SCGPC. For a given RCA, SCGPC mixes including 1.5% steel fiber generated a first crack load that was roughly 4% and 5% higher, respectively, than similar companion blends containing 0.5% steel fiber. The influence of several descending branches of SCGPC becomes evident for Ω_L/150 and Ω_L/100. Furthermore, when the RCA percentage is less than 50%, the disparities between them are minimal due to the restricted harm caused by RCA at low concentrations. Comparable findings are also evident for SCGPC containing 0.5% and 1.5% SF. The observation illustrates that the inclusion of SF in RCA only improves the structural integrity of the concrete. While the fibers do help bridge cracks, they also introduce initial flaws in the RCA, such as aggregating of fibers, fragile fiber-matrix ITZs, and holes that ultimately harm the SCGPC’s matrix.

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Figure 9. Flexural toughness of RCA. (a) Impact of RCA content; (b) Impact of SF content