Exploring flexural performance and abrasion resistance in recycled brick powder-based engineered geopolymer composites

Abstract

Background

Due to growing global concerns regarding the management of construction waste, this study investigates the feasibility of creating engineered geopolymer composites by replacing traditional industrial by-products (slag) with construction waste, specifically recycled brick waste powder.

Results

Polyvinyl alcohol fibers were incorporated into the engineered geopolymer composite mixtures. The substitution of slag with recycled brick waste powder was carried out at varying percentages: 0, 20, 40, 60, 80, and 100%, resulting in six different engineered geopolymer composite mixtures. The study evaluated the flexural strength, sorptivity, water absorption, and abrasion resistance of the engineered geopolymer composites, and also, microstructural characterization was conducted using scanning electron microscopy. The findings demonstrated that incorporating recycled brick waste powder into the engineered geopolymer composite mixes resulted in a decrease in flexural strength by 35.59% and a notable increase in midspan deflection by 339% when slag was replaced. Concurrently, there was a significant rise in water absorption and sorptivity by approximately 304 and 214%, respectively, when slag was entirely substituted with recycled brick waste powder. Conversely, abrasion resistance decreased, with the inclusion of recycled brick waste powder resulting in an 84% increase in volume change. The scanning electron microscopy (SEM) analysis showed active geopolymerization of recycled brick waste powder within the engineered geopolymer composite mixtures.

Conclusions

The results of this investigation demonstrate that it is feasible to produce engineered geopolymer composites using recycled brick waste powder instead of slag. The greater ductility and increased midspan deflection point to areas that require further optimization, even in spite of the observed decreases in flexural strength and abrasion resistance. The SEM examination reveals an active geopolymerization, highlighting the potential of recycled brick waste powder to produce environmentally friendly and sustainable construction materials. These results offer a good starting point for further studies that try to maximize the durability and performance of these composites.

1 Background

The remarkable tensile strain capacity of engineered cementitious composite (ECC) sets it apart as a special kind of high-performance cementitious composite, exceeding 2%. Its exceptional quality distinguishes it from conventional concrete, which usually has a tensile strain capacity of only 0.01% [1,2,3]. The components of ECC typically consist of ordinary Portland cement (OPC), silica sand, water, admixtures such as superplasticizers, and discontinuous fibers, often consisting of 2% or less of the volume [4, 5]. However, ECC requires a higher amount of cement, normally 2.5 to 3 times more than traditional concrete, leading to increased production costs and reduced eco-friendliness [6, 7]. The production of 1 ton of OPC is responsible for 800 to 1000 kg of carbon dioxide (CO₂) emission, accounting for about 7% of global carbon dioxide emissions [8].

The growing focus on sustainability and environmental concerns within the construction industry highlights the demand for sustainable alternatives to traditional binders like OPC. Engineered geopolymer composite (EGC) appears as a promising sustainable alternative for ECC. EGC utilizes a binder that is absent from cement and instead relies on geopolymer binder technology.

Geopolymers are produced by activating alumina and silica-rich source (precursor) materials, like industrial waste such as slag, fly ash, and silica fume, with an alkaline activator solution composed of sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃).

Many studies [9,10,11,12] highlight the importance of raw materials in the binder for achieving strain-hardening and superior mechanical performance in EGC. Among these precursor materials, slag and fly ash stand out as the most commonly used components in the production of EGC [13,14,15,16].

Investigation on the feasibility of EGC started with a study by Lee et al. [17]. The research demonstrated the viability of a cement-less fiber-reinforced strain-hardening composite by replacing the OPC binder with a geopolymer binder derived entirely from slag. Three distinct mix compositions were investigated, utilizing two activator combinations and varying water/binder ratios. Reinforcement was provided by oil-coated PVA fibers, constituting 2% of the volume. The specimens underwent standard water curing, initially for 24 h at ambient temperature followed by submersion in water for 28 days. Evaluation tests were conducted to determine density, compressive strength, and tensile behavior. Results indicated promising properties, with densities ranging from 1970 to 2020 kg/m³ and compressive strengths from 19.4 to 30.6 MPa at 28 days. However, the study lacked a detailed analysis of micromechanical parameters like the toughness of the matrix and fiber–matrix interfacial characteristics, which are crucial for understanding the strain-hardening behavior of this composite.

Later, Ling et al. [14] indicated that, as the content of slag increased in EGC composites, notable enhancements were observed in compressive strength, ultimate tensile stress, flexural strength, and elastic modulus. However, higher levels of slag replacement resulted in diminished strain capacity and fracture toughness. Moreover, an escalation in slag quantity led to the formation of wider cracks in the EGC composites. Nevertheless, ground granulated blast furnace slag (GGBS) also has its drawbacks. For instance, GGBS-based geopolymer may suffer from poor workability, rapid setting, and significant shrinkage, all of which can impede its suitability as a construction material [18,19,20].

On the contrary, construction and demolition operations are approximated to produce around 10 billion tons annually of waste, constituting approximately 30% of the total solid waste generated globally [21]. The rapid pace of urban expansion and reconstruction results in the demolition of existing brick-concrete structures, yielding an estimated 52 million tons of clay brick waste yearly. This poses a considerable challenge to the efficient recycling of these wastes [22]. Researchers have looked at the possibilities of utilizing leftover clay brick as a raw material to create a geopolymer binder. For instance, Allahverdi et al. [23] conducted research on the feasibility of producing geopolymer concrete by varying the ratios of concrete and brick waste. And according to their research, waste bricks show better performance in geopolymer reactions compared to concrete wastes, producing encouraging findings. In addition, Komnitsas et al. [24] explored samples curing within a temperature range of 60 to 90 °C and changing the molarity of NaOH from 8 to 14 M. The authors’ results showed that the maximum compressive strength (reaching values close to 50 MPa) was attained at a curing temperature of 90 °C and a NaOH molarity of 8M. Zaharaki et al. [25] explored various geopolymer pastes produced from building and demolition wastes that were cured for 24 h at 80 °C. Some of the combinations included only brick powder. The recycled brick-geopolymer samples were prepared using NaOH solution with 10 M, approaching a compression stress of 40 MPa after 7 days. Furthermore, several studies on pastes and mortars made from waste clay bricks have explored the influence of changing the temperature of curing from 20 to 100 °C and the concentration of NaOH solution [26,27,28,29,30].

Also, Silva et al. [31] conducted research to determine the optimal conditions for geopolymerization using fire clay brick as a raw material. Their findings suggested that under the appropriate manufacturing conditions, a compression stress of 37 MPa might be achieved. Additionally, Lyu et al. [32] investigated the effects of incorporating recycled brick aggregate as a substitution for natural aggregate in the production of highly ductile and sustainable geopolymer composites. Although there was a slight reduction in strength, an improvement in the bridging and bonding capacity between the matrix and fibers was observed. This resulted in a tensile stress of 4.41 MPa and a ductility of 4.27%, indicating potential engineering applications. Notably, even under substantial tensile strain, the microcracks remained relatively small, with an average size of less than 50 μm. This characteristic is anticipated to enhance the durability of EGC, reinforcing its ability to withstand abrasive agents such as water and acids [33].

Durability is a critical factor in the performance of construction materials, directly impacting their structural longevity. Geopolymer structures must withstand various mechanical and environmental stresses over their expected service life. Due to the excellent ductility and energy-absorbing capacity of EGC, it finds applications in street roads and construction projects. Therefore, it is imperative to evaluate and enhance its strength characteristics and abrasion resistance, especially for use in high-traffic areas where surface durability is essential.

Research on brick-based EGC is very limited. Previous studies (e.g., Nematollahi et al. [34]; Humur et al. [35]) have primarily focused on the mechanical properties and durability of EGC with traditional industrial by-products (fly ash and slag). This limitation prompted this study to aim to fill the gap by exploring the potential of utilizing recycled brick waste powder (RBWP) as a substitute for industry by-products such as GGBS in EGC production. This study aims to assess the mechanical properties (compression stress, flexural stress, direct tensile stress, and tensile strain) as well as the durability (water absorption, sorptivity, and abrasion resistance) of the resulting EGC when the RBWP is incorporated as a replacement of GGBS. Additionally, the research involves examining the microstructure using scanning electron microscopy (SEM) to understand the internal morphology and bonding characteristics of the composite materials. The specific aims are to evaluate how changing the proportions of RBWP affects the mechanical performance and durability of EGC, which subsequently identifies the product engineering applications.

2 Research importance

This study holds promise for reducing waste, improving material properties, and fostering sustainability in construction practices. Incorporating recycled brick waste powder into EGC production presents an opportunity to mitigate reliance on traditional industrial wastes and promote circular economy principles. This endeavor not only offers innovative construction solutions but also contributes to eco-friendly and resource-efficient building practices. The results of this study provide valuable insights for engineers involved in the design and construction of sustainable infrastructure. By understanding the mechanical and durability properties of EGC with varying RBWP content, engineers can make thoughtful decisions on material selection and mix proportions for each application. The improved knowledge of abrasion resistance is particularly useful for projects requiring high durability, such as bridge decks, industrial flooring, and other high-wear surfaces. Despite the importance of EGC, the aspect of performance remains relatively underexplored, highlighting the need for further research to unlock its full potential in diverse construction scenarios.

3 Materials and experimental program

In this research mechanical properties, durability properties, and the microstructural properties of EGC were examined. The experimental program included 6 different mixtures, each aimed at evaluating the impact of replacing GGBS with RBWP on the durability and mechanical properties of EGC. The flowchart of the methodology for this research is depicted in Fig. 1.

Fig. 1

Methodology flowchart

3.1 Materials, mix proportions, and basic mechanical properties

The selection of binder material significantly influences the development of mechanical strength, particularly when the binder is rich in silica and alumina. Various studies have focused on identifying suitable binder materials for this purpose. In this particular study, engineered geopolymer composite (EGC) mixes were produced using ground granulated blast furnace slag (GGBS), which conforms to BS6699 standards and was sourced from the Mass Steel plants. Recycled brick waste powder (RBWP) was used as a substitute for GGBS in the mixes. The RBWP was obtained from demolished construction sites and excess material from brick manufacturing plants. Initially, the RBWP underwent a crushing process to lower the size of the particles to less than 10 µm. Subsequently, it was milled into powder using an abrasive machine (Los Angeles) for thirty minutes, followed by sieving. Only particles that passed through sieve #200 (75μm) were collected, ensuring similar fineness to GGBS.

Using X-ray fluorescence (XRF), the chemical characteristics of GGBS and RBWP were examined. The findings are shown in Table 1. The investigation revealed that RBWP contains 33.97% silicate (SiO₂) and 30.82% alumina (Al₂O₃). Notably, RBWP exhibited a higher alumina content compared to GGBS. Furthermore, for the particle size distribution the results of GGBS and RBWP are illustrated in Fig. 2. According to the results, RBWP has a relatively finer particle size distribution than GGBS. Specifically, the median particle diameter, which is represented by the D₅₀ value, for RBWP was determined to be 9.48 μm, whereas GGBS showed a D₅₀ value of 12 μm. This implies that half of the particles in RBWP are less than 9.48 μm, whereas half of the particles in GGBS are smaller than 12 μm. Such fine particle sizes are crucial as per Komnitsas et al. [24], improved compression stress might be achieved when particles with D₅₀ < 15μm are used in alkali-activated binders. Micro-silica sand obtained from DCP-Iraq served as the fine aggregate, with a granular size range of (0.08–0.25) mm and a specific gravity of 2.6. The color of the silica sand varied from light brown to creamy. Table 1 presents the results of the XRF examination detailing the chemical analysis of the silica sand. Moreover, silica sand’s particle size distribution curve has a noticeably coarser texture, consistent with its usual use as a fine aggregate in composite materials. When compared to silica sand, RBWP and GGBS have a smaller particle size distribution, which highlights their potential effectiveness as reactive binders in engineered geopolymer composites, improving their mechanical characteristics and durability.

Table 1 XRF analysis of GGBS, RBWP, and silica sand

Fig. 2

Physical properties (particle size distribution) of GGBS, RBWP, and silica sand

In this study, water glass and NaOH served as the alkaline activators. The 10M molar concentration of NaOH solution was prepared by dissolving locally available 97% pure sodium hydroxide pellets in water. Sodium silicate, supplied by the SCL firm, had a density of 1559 kg/m³ and Ms of 2.65 (where Ms is SiO₂/Na₂O). The sodium silicate composition included 34.4% SiO₂, 13% Na₂O, and 52% H₂O. An alkaline activator solution was created using a ratio of 2.5 Na₂SiO₃/NaOH.

The polyvinyl alcohol fiber (PVA) used in this study had a diameter of 0.04 mm, a length of 12 mm, and constituted 2% of the total volume. According to studies, adding 2% of PVA fibers to geopolymer and cement composites improves their mechanical properties significantly and increases their ductility with a strain-hardening pattern [36, 37]. Other properties of the PVA fiber included an elastic modulus of 31 GPa, a tension stress of 1600 MPa, and a density of 1300 kg/m³. Additionally, a naphthalene-based superplasticizer (second-generation) admixture was employed to attain the desired workability. This superplasticizer was added at a dosage rate of (5.4 to 7) % by weight of binder materials.

Table 2 outlines the mixing proportions for the engineered geopolymer composites (EGCs). The alkali activator/binder ratio remained consistently at 0.4, and the ratio of Na₂SiO₃/NaOH remained at 2.5 for all EGC mixtures. Additionally, to enhance ductility in the mixes, the ratio of sand/binder was maintained at 0.2 across all mixes. The experimental program encompassed 6 mixtures, each designed to assess the influence of substituting GGBS with RBWP on the durability and mechanical properties of EGC. Substitutions were made incrementally at a rate of 20%. For example, the code BS40 signifies a mixture containing 40% GGBS and 60% RBWP.

Table 2 Mix design of GGBS-RBWP-based EGCs

Table 3 presents the compression and tension strengths, along with the ultimate capacity of the tensile strain of EGC mixtures at the 28-day mark [38]. To determine the compressive strength of the EGC mixtures, cubic specimens with 50 × 50 × 50 mm were prepared following ASTM C109 standards. The test was conducted using a universal testing apparatus (Alfa) with a 600 kN capacity. Noticeable decreases in compressive strength were observed with a decrease in slag content in the EGC mixture. Particularly, at a replacement level of 0%, the highest compressive stress of 127.39 MPa was achieved.

Table 3 Mechanical properties of GGBS-RBWP-EGC [38]

To assess the direct tensile performance of the EGC mixtures, dog-bone specimens measuring 330 × 60 × 13 mm were utilized. The testing procedure followed the specifications outlined by the Japan Society of Civil Engineers (JSCE) [39]. The tensile tests were conducted using a displacement control universal testing machine with a displacement rate set at 0.3 mm/min.

Figure 3 illustrates the typical tensile stress–strain curves obtained from the EGC mixtures [38]. These curves highlight the performance differences between mixtures with different ground granulated blast furnace slag (GGBS) and recycled brick waste powder (RBWP) contents. After the initial crack formation, subsequent cracks with narrow widths and closely spaced crack patterns emerged, causing the uniaxial tensile stress to advance at a slower rate.

Fig. 3

Tensile stress–strain responses of GGBS-RBWP-EGC [38]

As indicated in Table 3 and Fig. 3, replacing GGBS with RBWP significantly improved the tensile strain capacity, achieving an increase of 11.39%, which makes it 11 times the tensile capacity of the GGBS-based EGC, when the GGBS was completely substituted with RBWP. Additionally, this replacement led to an approximately 34.5% decrease in tensile stress. Ultimately, all mixes demonstrated post-peak softening criteria.

3.2 Mixing process and curing

According to the targeted molarity (10M), which was advised by [40, 41], the specified quantity of NaOH pellets was first dissolved in water, resulting in an exothermic reaction that caused a significant rise in temperature. Following this, sodium silicate was introduced, and the solution was left to equilibrate at room temperature (20–30 °C) for 24 h to achieve the required chemical balance before being added to the EGC mixture. Second, the dry ingredients of the matrix mixture were mixed for around 5 min. Third, the alkaline activator solution was then incorporated into the mixture and for an additional 2 min was stirred. And to attain the desired workability (fourth step), additional water and water-reducing admixture with naphthalene-based were cautiously introduced. Finally, PVA was added gradually once a homogeneous mixture was achieved and stirring continued for an extra 5 min. Figure 4 depicts the total mixing process for each mix, which took twelve and fifteen minutes. Following this, the mixtures were poured and then compacted. Finally, all freshly cast specimens were stored at room temperature for a 24 h resting period [42] while being sealed in nylon bags to reduce water evaporation. The study’s outcomes [7, 36, 43] recommended that curing with heat develops the ductility of the EGC. For this reason, the heat-curing method was used in this study. As depicted in Fig. 4, the specimens were placed in an oven and subjected to heat curing for 24 h at 70 °C following the resting period. Subsequently, after curing, all specimens were stored for 28 days at room temp.

Fig. 4

Mixing, Pouring, and Curing of EGCs Specimens

3.3 Test procedures

3.3.1 Flexural strength

Previous research suggests that when a material exhibits proper strain-hardening behavior, its tensile strain capacity and deflection during bending are frequently correlated [44]. Hence, the three-point bending test was selected for this investigation to assess the ductility of the specimens [45]. The bent test was carried out in compliance with BS EN 196-1:2005 [46] using a displacement-controlled test system on a machine with a capacity of 50 kN, applying a loading rate of 0.017 mm/s. Prism specimens measuring 40 × 40 × 160 mm were used, with three specimens tested for each EGC mixture. The flexural loading span length was 120 mm, and the load was applied at the center of the span length. A computerized data recording system was utilized to capture the load and midspan deflection during the flexural testing. Linear variable displacement transducer (LVDT) was included in the experimental setup, to gauge the capacity of flexural deflection for the specimens, as depicted in Fig. 5. At peak flexural load, the deformation value of the load–deflection curves was utilized to determine the capacity of flexural deflection for the prismatic EGC specimens.

Fig. 5

Three-point flexure test setup

3.3.2 Water absorptions

Permeability is an essential component in assessing the long-term performance of mortar and concrete. It determines the material’s ability to resist the penetration and passage of substances through its matrix. Water absorption, specifically, refers to the amount of water absorbed under defined conditions, and it can be determined through Eq. (1). It indicates the volume of pore space within the specimen matrix that lets liquid ingress. To conduct the test of water absorption, the cubic specimen needs to be initially dried (at 105 °C for 24 h) until reaching a consistent mass as shown in Fig. 6. Subsequently, it is fully saturated with water, and both the dry and saturated samples are weighed.

$$\% W_{{{\text{ab}}}} = \frac{{W_{{{\text{sat}}}} - W_{{{\text{dry}}}} }}{{W_{{{\text{dry}}}} }} \times 100$$

(1)

where W_sat, W_dry, and W_ab are the saturated weight, dry weight, and absorption present, respectively.

Fig. 6

Water absorption test; a oven dry to 105 °C, b weight determinations

3.3.3 Water sorptivity (absorption rate)

Water sorptivity refers to the capacity of a material to absorb water through suction, which is indicative of its durability as it reflects the penetration of water into the material. In this study, the water sorptivity of EGC was assessed following the ASTM C1585 standard procedure [47]. The water sorptivity of the EGC specimens was assessed using three samples measuring 50 × 50 × 50 mm each. At the age of 28 days, three samples from each mixture were dried at 105 °C in an oven until a consistent mass was achieved. Subsequently, the samples were removed from the oven and sealed with silicone around the sides to facilitate water penetration only from the bottom of the specimen, as illustrated in Fig. 7. For this test, a water level of 4 to 5 mm above the bottom of the EGC specimens was maintained. The increase in the specimen’s mass at various intervals was divided by the density of water and the bottom surface area of the specimen to calculate the wet height of the specimen. The sorptivity index of EGC was determined by analyzing the best-fit line’s slope when against the square root of the time the data points were plotted.

Fig. 7

The sorptivity test: a sorptivity specimens and b schematic diagram

3.3.4 Abrasive resistance

Another crucial aspect of a material’s durability is its resistance to abrasion. While the presence of coarse aggregate notably enhances the abrasion resistance of regular concrete, the abrasion resistance of EGC primarily depends on the bridging ability of fibers and the performance of the matrix. For the abrasion test, specimens measuring 71 × 71 × 71 mm (three samples per mixture) at 28 days of age were examined. Following the steps outlined in standard IS 15658: 2021 (Annex E), the abrasion test on EGC specimens was conducted. Prior to testing, the specimens were dried at 105 ± 3 °C in an oven until a consistent mass was achieved.

The abrasion test utilized the Bohme test apparatus, as depicted in Fig. 8. This apparatus comprises with a 750 mm diameter steel disk rotating at a speed of 30 ± 1 cycles/min, a digital counter, and a lever capable of applying a weight of 294 ± 3 N on the specimens as shown in Fig. 9. To ensure uniform surface grinding, the specimen was clamped and secured to the mount and pressed against the rotating grinding disk. Initially and after every 4 cycles (where each cycle involves the grinding disk completing 22 rotations), the specimens were weighed. A dry state abrading powder (quartz sand) weighing 20 ± 0.5 g was applied on the testing track for each cycle to accelerate deterioration. After each cycle, both the disc and sample surfaces were cleaned with a brush, and the specimens were rotated 90° clockwise. This process was repeated for 16 cycles, totaling 22 × 16 revolutions endured by the samples.

Fig. 8

Bohme abrasion testing machines as specified in IS: 15,658/2021

Fig. 9

Abrasion test specimen under (294 ± 3) N

A digital dial gauge was used to determine the initial and final thickness at five different locations, as illustrated in Fig. 10. Subsequently, the measured values were compared to the maximum permissible limits outlined in IS 1237: 2012 (Annex G), which specify 3.5–4.0 mm for domestic wear applications and 2–2.5 mm for heavy movement.

Fig. 10

Thickness measurement according to IS 1237; a measurement arrangement and b measurement points

The results of the test after 16 cycles indicated the wear of EGC, calculated as the loss of volume (ΔV) of the specimen in mm³ per unit area of 50 cm², by using Eq. (2).

$$\Delta V = \frac{\Delta m}{{{\text{PR}}}}$$

(2)

ΔV is the volume loss after 16 cycles (mm³); Δm is the mass loss after 16 cycles (g); and PR is the density of the sample (g/mm³).

3.3.5 Microstructural observation

Scanning electron microscopy (SEM) was employed to analyze the microstructural characteristics and identify features of the EGC samples. A Quanta 450 instrument was utilized for SEM examination. This technique allowed for the investigation of microstructural-level parameters contributing to the observed differences in strength among the EGC samples.

4 Results

4.1 Flexural performance

The load–deflection curves for the flexural behavior of all investigated EGC mixtures are shown in Table 4 and Fig. 11. In these curves, the initial drop in load, indicating the onset of cracking, is termed as the first cracking load, while the ultimate flexural load is used as an indicator to the flexural strength. The flexural response was evaluated by averaging the results from three specimens for each EGC mixture. Typically, the first cracks emerge on the tension surface of the specimens. Despite the occurrence of initial cracking, there is an increase in resistance to loading, attributed to the development of multiple cracks. Microcracks propagate from the location of the first crack and disperse across the midspan of the samples. Flexural failure of the EGC occurs when the bridging stress of the fiber at the microcracks is approached, resulting in localized deformation at that specific section as the flexural strength is reached.

Table 4 Flexural properties of GGBS-RBWP-EGC

Fig. 11

Flexural load–deflection curves of EGCs

Table 4 shows the impact of substituting GGBS with RBWP on the first cracking load of EGC composites. In general, the first-crack load of EGC mixtures ranged from 2610 to 4350 N.

The effect of replacing GGBS with RBWP on the ultimate flexural strength of EGC is shown in the load–displacement curve in Fig. 11 and Table 4. The flexural strength at 28 days of age varies from 4277 to 6640 N, depending only on the level of replacement for the GGBS with RBWP in EGC.

The flexural deflection at midspan, indicative of the material’s ductility, is illustrated in Fig. 10 and summarized in Table 4. The midspan deflection, as depicted in Fig. 11, is notably influenced by the proportion of RBWP in the EGC mixture. For instance, with the complete substitution of GGBS by RBWP, the midspan deflection increased from 0.98 to 4.3 mm (a 339% increase), reflecting a significant enhancement in ductility. The minimum midspan deflection of 0.98 mm was achieved when the maximum flexural strength was attained with 0% replacement.

4.2 Water absorption

Concrete’s quality can deteriorate over time due to the penetration of corrosive substances and moisture into its network of pores. Concrete’s water absorption capacity may be used to directly assess the material’s durability since water serves as the main transporter of hostile ions in the substance [48]. The impact of replacing GGBS with RBWP on the absorption of water in EGC specimens at 28 days is depicted in Fig. 12. It is evident that water absorption generally increases with higher RBWP content. For instance, water absorption increased by 53 and 304% when GGBS was substituted with 20 and 100% RBWP, respectively.

Fig. 12

Relationship between water absorption and RBWP content

4.3 Sorptivity (absorption rate)

The test of sorptivity was conducted to assess the capillary structure of a porous material, providing insight into its inclination to absorb and transmit water through capillary pores. Factors such as capillary forces, porosity, permeability, and pore structure in concrete significantly influence the rate of absorption (sorptivity) [49]. Figure 13 demonstrates the sorptivity of EGC samples produced with a NaOH molarity of 10M, and Ms (Na₂SiO₃/NaOH) ratio of 2.5.

Fig. 13

Sorptivity result of GGBS-RBWP-EGCs

4.4 Abrasion resistance

The abrasion resistance test was conducted at 28 days of age following IS 15658: 2021 (Annex E) standards. The resistance to abrasion was evaluated based on changes in volume (ΔV) and thickness loss. Figure 14 illustrates the variation in abrasive wear values over grinding cycles for the EGC specimens. Table 5 summarizes the mass loss, percentage volume loss, and average thickness loss of all tested specimens.

Fig. 14

Abrasive wear vs grinding cycles of GGBS-RBWP-EGCs

Table 5 Results of abrasion resistance test for GGBS-RBWP-EGCs

4.5 SEM analysis

In this section, a comprehensive analysis of the microstructural characteristics of the mixtures through SEM analysis was performed, focusing on the blends that exhibited optimal results concerning abrasive wear and flexural strength. SEM examinations offer insights into the morphologies of the generated geopolymer composites. The morphology and microstructure of two engineered geopolymer composite specimens (GGBS-based EGC and RBWP-based EGC) were investigated and are shown in Fig. 15.

Fig. 15

SEM micrograph of EGCs of a GGBS-EGC, and b RBWP-EGC

The microstructure of the GGBS-EGC sample, which consists entirely of GGBS with no RBWP, is illustrated in Fig. 15a. It has a comparatively compact microstructure with fewer microcracks and a dense structure with few unreacted particles. Pores in the GGBS-EGC sample ranged from a few micrometers to about 10–20 µm. This dense microstructure probably contributed to its improved compressive strength and less sorptivity and water absorption. The microstructure that was examined suggests that during the alkali treatment, the aluminosilicates that were present in the slag beginnings experienced a substantial dissolution. Cross-linked silicon and aluminum tetrahedrons were produced as a consequence of this procedure, which produced strong covalent connections between silicon, oxygen, and aluminum [50].

Figure 15b illustrates the micrograph of the SEM of the RBWP-EGC. The sample displayed indications of some uniformity with irregularly shaped crystalline structures and a few numbers of unreacted RBWP particles in the EGC matrix. Pores in RBWP-EGC appear relatively larger, ranging from a few micrometers to about 30–40 µm, a factor in its decreased mechanical strength and increased porosity and water absorption. Similar observations were revealed by Komnitsas et al. [24] that investigated materials with a chemical composition similar to the brick used in this research. The authors have found that the brick-based geopolymer matrix showed a certain degree of homogeneity, which contributed to the total strength of the produced geopolymer. In addition, the SEM images demonstrate the disillusion of aluminosilicate materials during the RBWP’s alkaline activation, resulting in the creation of a three-dimensional tetrahedral polymer network dispersed across the sample’s surface [51].

In addition, figures from the SEM reveal that large particles do not completely react during the phase of alkaline activation, suggesting that particle size distribution is essential to the completion of the geopolymerization process. Larger particles only seem to experience a partial reaction, whereas smaller particles seem to undergo total dissolution and reaction [52].

In summary, the images of SEM show a homogeneous and compact structure that is uniformly distributed, with robust interconnections within the matrix. Particularly, the GGBS-EGC specimen exhibited a dense structure characterized by a more compact microstructure and less unreacted particles. This structural configuration likely influenced its increased mechanical strength and abrasion resistance.

5 Discussion

5.1 Flexural performance

The impact of substituting GGBS with RBWP on the first cracking load of EGC composites in Table 4 shows that the first-crack load of EGC mixtures ranged from 2610 to 4350 N, corresponding to the level of GGBS replacement with RBWP. The first cracking load decreased with the increase of RBWP content. The highest reduction was recorded in the complete replacement of GGBS, where the first cracking load decreased by approximately 40%.

The impact of substituting GGBS with RBWP on the ultimate flexural strength of EGC, which is shown in the load–displacement curve in Fig. 11 and Table 4, varies from 4277 to 6640 N, depending only on the level of replacement for the GGBS with RBWP in EGC. Notably, all EGC mixtures with RBWP showed lower flexural strength than the GGBS-EGC. For example, the ultimate flexural strength decreased by 12.35% when 20% of GGBS was replaced, and 35.59% reduced when the GGBS was entirely substituted with RBWP. Ling et al. [14] utilized GGBS to replace the FA at varying contents up to 30% (by weight). A similar finding was observed that the flexural strength of the EGC increased with the increasing slag content.

The high strength can be attributed to the rapid reaction of the soluble silicon (Si) and aluminum (Al) contents, which likely occurred due to the presence of additional calcium oxide (CaO) in GGBS, which according to the XRF analysis listed in Table 1, the calcium oxide in GGBS and RBWP is 38.68 and 17.29%, respectively. This facilitated the creation of calcium aluminosilicate hydrate (C-A-S-H), contributing to the early strength development [53]. The trend of GGBS replacement on flexural strength mirrors that of the compressive strength test results, wherein the compression stress of the mixtures with RBWP was consistently less than that of GGBS-EGC. Similarly, the highest compressive strength obtained was at 0% replacement of GGBS, reaching 127.39 MPa, as shown in Table 3.

The flexural deflection at midspan is notably influenced by the proportion of RBWP in the EGC mixture, reflecting a significant enhancement in ductility. This improvement in strain-hardening behavior is primarily attributed to the high micro-hardness and amorphous structure of RBWP. In this scenario, the bridging stress between the matrix and the fiber is strengthened, leading to the development of maximum stress and strain [54].

Although direct tensile testing is frequently thought to be the most reliable technique to assess strain-hardening behavior, earlier research has shown a significant relationship between the direct tensile strain capacity and the deflection capacity seen during bending [55, 56]. Figure 16 depicts a strong correlation between deflection capacity and direct tensile strain capacity for all EGC mixtures. The analysis reveals a linear rise in deflection corresponding to an increase in direct tensile strain (with an R-squared value of 0.87). This implies that the deflection capacity of the EGC increases proportionately to the increase in tensile strain capacity. With an R-squared of 0.87, the linear model fits the data quite well; 87% of the variance in deflection capacity can be determined by the direct tensile strain capacity. According to this linear correlation, greater direct tensile strain capacities result in larger deflection capabilities in the EGC. Comprehending the behavior of EGC mixtures under tensile stresses is crucial for applications that need higher durability and flexibility.

Fig. 16

Relationship between direct tensile and flexural deflection capacities

A single, wide crack, as shown in Fig. 17a, was indicative of a semi-brittle failure in the GGBS-EGC specimen. This suggests that the material was limited in its ability to absorb energy before breaking suddenly and catastrophically. On the other hand, the RBWP-EGC specimen exhibited a ductile failure characterized by the inclusion of multiple tightly spaced cracks as shown in Fig. 17b. This suggests a ductile tendency, meaning that before failing, the material may absorb and sustain a sizable amount of energy. Numerous tiny cracks show that the material can bend and transfer the load more uniformly throughout its structure, avoiding a sudden breakdown.

Fig. 17

Three-point bending test specimens failure; a GGBS-EGC, b RBWP-EGC

The examination of the specimens revealed the presence of numerous microcracks with narrow average widths and fine spacing, as illustrated in Fig. 17b. Interestingly, an increase in RBWP content appeared to enhance ductility by promoting the formation of multiple cracks with limited width, rather than a single wide crack leading to sample failure. Brick waste powder exhibits high pozzolanic activity, and when combined with an alkaline activator, it reacts with calcium hydroxide to generate additional binding phases. These phases likely contribute to the formation of the geopolymer gel, thereby enhancing the strength of the geopolymer composite.

Substituting slag with waste brick powder could potentially lead to beneficial modifications in the microstructure of the geopolymer composite. This substitution may result in a composite with lower strength but increased flexibility. Hence, further investigations are necessary to have a better understanding of the underlying mechanisms. Techniques of microstructural analysis, such as scanning electron microscopy (SEM), as described in Sect. 4.5, would be particularly valuable for this purpose.

5.2 Water absorption

As it is noted in Fig. 12, the water absorption generally increases with higher RBWP content. The reason behind the increment of water absorption in high RBWP content in the mixture of EGC might be due to the porous surface of unreacted RBWP [57, 58]. Consequently, water may be rapidly absorbed by the porous surface. Alzeebaree et al. [59] reported that the highest absorption of water occurred in the mixtures where fine soil powder was replaced with clay brick powder, particularly in those with a replacement ratio of 100%. Moreover, similar outcomes were documented by Migunthanna et al. [60], that when looking into replacing some of the binder in geopolymers with FA and/or slag with waste clay brick. Also, comparable results were noted by Zawrah et al. [53], where the incorporation of GGBS into geopolymer systems of waste clay bricks typically results in an increase in density and a reduction in porosity and water absorption of the specimens, which in turn positively affects compressive strength. The development of new phases inside the geopolymer network may be the cause of this phenomenon. Furthermore, it was found that the characteristics of the resultant hardened geopolymer were significantly influenced by the calcium oxide (CaO) level in the precursor of slag [61].

Additionally, there appears to be a strong correlation (with R² = 0.9) between water absorption and the RBWP content in the EGC mixture. Furthermore, the water absorption of all EGC specimens ranged from 3.17 to 12.82%. Notably, EGC mixtures containing RBWP exhibited low absorption characteristics, with values generally below 10%, except for the mixture with 100% RBWP replacement.

5.3 Sorptivity

As depicted in Fig. 13, the sorptivity curves exhibit a steeper slope during the initial 20 to 40 min compared to the subsequent duration of the test. This phenomenon arises from the progressive filling of both small and large voids at early stages. Furthermore, Fig. 13 illustrates that the minimum sorptivity coefficient, recorded at 0.051, was observed for the GGBS-EGC specimen. Subsequently, the sorptivity coefficient consistently increased with the rise in RWBP percentage, culminating in the highest recorded sorptivity coefficient in this study of 0.160 for the RWBP-EGC specimen. In essence, when the slag was entirely replaced with RWBP in the EGC mixture, the sorptivity coefficient increased by a factor of 3.14, attributable to higher porosity and a less efficient alkali activation process. A similar finding was reported by Alzeebaree et al. [59], the highest absorption of water and sorptivity was achieved in the clay brick mortar samples. Furthermore, a linear correlation between the sorptivity coefficient and the RWBP percentage was observed with R² = 0.96 as shown in Fig. 18.

Fig. 18

Relationship between sorptivity coefficient and RBWP content

5.4 Abrasion resistance

When RBWP was used as a replacement for GGBS in EGC and tested for abrasion resistance, the results were consistent with the compressive and flexural strengths. In comparison with the control mixture (GGBS-EGC), all EGC mixtures with RBWP exhibited lower performance in abrasion resistance in terms of ΔV. The increase in ΔV loss compared to GGBS-EGC ranged from 11.31 to 84.68%. The percentage of loss increases correlating with the decrease in compressive strength of the mixture. Additionally, the thickness loss for the EGC specimens was recorded, with all values falling below the limit specified in IS 1237–2012 (4 mm). The highest thickness loss was 3.86 mm, observed when the GGBS was replaced by 100% with RBWP. Figure 19 illustrates the effect of replacement on the change in volume and loss in thickness. Table 5 and Fig. 19 demonstrate a clear trend: EGCs with higher compressive and flexural strengths generally exhibited lower abrasion values. This suggests that both compressive and flexural strengths are significant factors influencing the abrasion resistance of EGC [62, 63]. Indeed, as highlighted by Andrews-Phaedonos [64], various factors can influence abrasion resistance, including binder type, compressive strength, binder content, water content, aggregate properties, porosity, and surface finish. However, there is limited literature available on the abrasion resistance of waste brick-based EGC. Therefore, further comprehensive studies on this aspect are recommended to provide a deeper understanding of its abrasion resistance characteristics.

Fig. 19

Effect of RBWP content on abrasion resistance of GGBS-RBWP-EGCs

5.5 Correlations between abrasion resistance and mechanical strength of EGC

The research aimed to establish an equation for predicting abrasion based on the experimental data collected. Initially, the correlation between wear of abrasive (ΔV) and compressive strength, as well as flexural strength, was analyzed, as depicted in Figs. 20 and 21. These figures illustrate that higher compressive and flexural strengths correspond to increased abrasion resistance in EGC. In other words, greater strength (both compressive and flexural) tends to result in lower ΔV. Additionally, it was observed that flexural strength shows a stronger correlation with abrasion resistance compared to compressive strength, with correlation coefficients (R²) of 0.92 and 0.91, respectively. The results reported by Kabay et al. [63] align with the current study’s findings, indicating that flexural strength shows stronger correlations with abrasion compared to compressive strength in concretes containing basalt fiber. Although compressive stress is typically considered the primary factor affecting abrasion resistance in EGC, the presence of fibers in EGC suggests that wear of abrasive is predominantly affected by flexural strength more than compressive strength. Consequently, flexural strength was employed to predict abrasion (Eq. (3)).

$${\text{Abrasion }}(\Delta V) = \, - {3}.{\text{8358U}}_{{{\text{FL}}}} + { 36}0{81}$$

(3)

where U_FL is the ultimate flexure load. The suggested equation could be utilized in determining the abrasive wear (according to the Böhme test) of EGC with GGBS replaced with RBWP.

Fig. 20

Relationship between abrasive wear and compressive strength of GGBS-RBWP-EGCs

Fig. 21

Relationship between abrasive wear and flexural strength of GGBS-RBWP-EGCs

6 Conclusion

In conclusion, this study investigated the effects of incorporating rice bran waste powder (RBWP) as a replacement for ground granulated blast furnace slag (GGBS) in the production of engineered geopolymer concrete (EGC). Our findings indicate that the presence of RBWP can significantly influence the mechanical and durability properties of EGC.

Conceptually, the declination in flexural strength by 35.59% with full replacement of GGBS by RBWP raises the need for optimizing the RBWP content to balance strength and ductility, because the amount of RBWP in the EGC mixture had a notable effect on midspan deflection. With the entire replacement of GGBS the midspan deflection increased approximately 339%, which suggests that RBWP enhances ductility that is beneficial for applications requiring energy absorption and deformation capacity. Furthermore, water absorption consistently increased with the RBWP percentage in the EGC mixture, reaching a 304% increase with full substitution of GGBS. Sorptivity of the EGC specimens also increased with the replacement level of GGBS, with the maximum sorptivity coefficient observed in fully brick-EGC, reaching 3.14 times that of GGBS-EGC. The increase in water absorption and sorptivity increases the importance of addressing potential long-term durability issues related to moisture ingress when designing RBWP-based EGC.

Moreover, the presence of RBWP in EGC reduced abrasion resistance, particularly with a 100% replacement of GGBS resulting in an 84% decrease in abrasion resistance (ΔV) and a 75% increase in thickness loss. Abrasion resistance and ductility have a trade-off that needs to be properly considered. This finding is significant for applications in civil engineering where abrasion resistance is critical, such flooring and bridge decking.

Also, our study revealed significant correlations between compressive and flexural strength and abrasion resistance in EGC, with higher compressive and flexural strength significantly enhancing EGC abrasion resistance. Lastly, the SEM images provided further insights, showing that the addition of RBWP to slag resulted in a remarkable reaction with the activator, producing uniform, compact, dense samples, albeit with few unreacted particles in the matrix, highlighting the importance of particle size in the geopolymerization process.

In summary, the results suggest that integrating recycled brick waste powder (RBWP) in place of industrial by-product GGBS for engineered geopolymer composites (EGCs) fabrication is viable and holds promise. Future studies could delve into optimizing this substitution, assessing long-term durability, and evaluating the scalability of this method, opening the way for improvements in sustainable construction materials.

6.1 Limitations

This research offers significant contributions to the understanding of the feasibility of substituting ground granulated blast furnace slag (GGBS) with recycled brick waste powder (RBWP) in engineered geopolymer composites (EGCs). However, it’s important to recognize a few limitations:

This study did not evaluate the long-term performance and durability of RBWP-based EGC under different environmental conditions. Future research should concentrate on chemical resistance, freeze–thaw cycles, and long-term aging to make sure the material is suitable for practical uses.

The process’s scalability to industrial production has not been assessed, as the experiments were carried out on a laboratory scale. Large-scale problems with curing, mixing, and quality control must be solved in order to promote wider use.

6.2 Recommendations

The present study’s results and limitations have led to the following recommendations for further research and practical applications:

Future research needs to evaluate the RBWP-based EGC’s long-term efficacy and durability in varied environments. To make sure the material is appropriate for use in practical applications, this involves assessing its resistance to aging, freeze–thaw cycles, and chemical exposure. Also, it is necessary to carry out a thorough analysis of both the economic and environmental impacts of utilizing RBWP in EGC. This involves analyzing the material’s cost-effectiveness, the decrease in building waste, and the advantages of sustainability overall.