1. Introduction
Considering the adverse impacts of cement production on the environment, the construction industry has been looking for substitutes for cementitious binders recently. Many industrial byproducts have been investigated as potential substitutes for Ordinary Portland Cement (OPC), including Basic Oxygen Furnace (BOF) slag and Fly ash (FA) from coal-fired power plants and steel manufacturing, respectively. Promising characteristics of these materials include high pozzolanic activity, enhanced workability, and minimal environmental impact [1–4]. Due to their availability and efficient use, Iron Ore Tailings (IOT), a by-product of the iron mining process, have drawn interest as a potential partial replacement for fine aggregate in concrete in recent years.
EGC, which is made from industrial byproducts like steel slag (BOF slag) and FA [10,11], is one material that holds promise in this regard. EGC is well-known for its superior mechanical qualities and toughness [11]. They are produced through a process called geopolymerization, which involves the reaction of aluminosilicate materials with the alkaline activator solutions [10] thereby forming the inorganic polymers. This geopolymer mechanism is dependent on various factors viz., alkali concentrations, Si-Al ratio, alkali to binder ratio, molarity, etc. which determines the overall strength and performance of the geopolymer concrete/composites [75–78]. Additionally, BOF slag and FA are abundant in aluminosilicates, making them suitable as precursors in the creation of geopolymer (GP) composites [12,15]. Due to its high calcium content, BOF slag, a byproduct of steel production, has been chosen for geopolymer synthesis [14]. Similarly, FA, a byproduct of coal combustion in power plants [61–64], has been extensively researched for its potential in the production of geopolymers owing to its high content in silica (Si) and alumina (Al) [12,15]. Moreover, the resulting incinerated coal from thermal plants comprises about 80% of fly ash and 20% of bottom ash, making the availability of FA in abundance [65].
Past studies have demonstrated that BOF slag and FA have high pozzolanic activity, which qualifies them as cement substitutes in the manufacture of concrete [73]. For instance, Fernández-Jiménez et al. [12] demonstrated that FA could be used to create geopolymers with excellent mechanical properties, and Zhou et al. [14] demonstrated the viability of using steel plant residues in geopolymer synthesis. The utilization of FA towards varied applications apart from geopolymers viz., silica aerogels, zeolites, aggregates, etc. was performed by Das et al. [74]. BOF slag and FA were used as raw materials in the synthesis of geopolymers by Mashifana et al. [1], and the outcomes demonstrated that these materials can be successfully used as precursors in the production of sustainable building materials. Similar findings were reported by Zhuang et al. and Das et al. [2,3] who found that FA-based geopolymer composites have superior mechanical properties to conventional cement-based concrete. However, the combined use of BOF slag and FA in geopolymer synthesis is relatively unexplored. Some minimal studies have shown specific feasible results relating the combined use of FA with other forms of steel slag. For instance, Zhang et al. [16] found that the combination of GGBFS slag and FA could produce geopolymers with improved strength and durability. Similarly, Zhou et al. [13] demonstrated that combining FA with steel slag increases resistance toward acid attack. As a possible partial replacement for fine aggregate in concrete, IOT has also been extensively examined. For instance, George et al. [4] investigated the effect of IOT as a partial replacement of fine aggregate and concluded that the mechanical properties of concrete were not noticeably impacted. Significant amounts of silica and alumina are present in fly ash, a fine-grained coal combustion byproduct, which reacts with calcium hydroxide to produce calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H) gels, the primary cementitious components in concrete [5–9]. Indeed, a significant portion of the calcium oxide found in BOF slag, a byproduct of the steel industry, reacts with silica and alumina to produce C-S-H and C-A-H gels. The amount of cement needed, and greenhouse gas emissions are decreased by using FA and BOF slag in the production of concrete.
In contrast, IOT are byproducts of the iron mining process and are typically dumped in tailings ponds. Additionally, IOT has good pozzolanic activity due to its high iron oxide content, and its use in concrete production reduces the environmental impact of tailings disposal. In conclusion, due to their availability and reduced ecological footprint, BOF slag, FA, and IOT have recently attracted attention as cementitious substitutes and alternatives to fine aggregate.
Additionally, the use of BOF slag and FA in the production of geopolymers is consistent with sustainable development and circular economy principles. By transforming these waste materials into valuable products, the ecological impact associated with their disposal is minimized which has been a major concern leading to severe environmental concerns viz., air, soil and water pollution [18] [66–72,74]. Additionally, compared to OPC, the production of EGC using BOF and FA produces considerably lower CO2 emissions [17,20]. However, the effective use of GP composites demands an in-depth understanding of their characteristics and performance through an evaluation of their mechanical and durability characteristics by assessing their resistance to different environmental conditions [19].
Despite these promising findings, further study is required to fully comprehend the potential of BOF slag, FA, and IOT in the creation of GP-based composites. Also, the concept of effectively utilizing these materials by addressing their disposal issue is motivated by the abundance of these materials, particularly IOT, which is easily accessible. By examining the characteristics of EGC made here from BOF slag, FA, and IOT, this research manuscript aims to address this gap. Nevertheless, no study has dealt with the combination of all these three materials in developing EGC.
The study presented herein investigates the suitability of using BOF slag and FA as major precursors and IOT as a partial replacement for fine aggregate (manufactured sand in this study) in developing EGC, thereby contributing to sustainable construction practices. The manuscript details the characterization of the raw materials involved, namely FA, BOF slag, and IOT. The elemental composition of these materials is determined through X-Ray Fluorescence (XRF) analysis. Additionally, their thermal stability is assessed through Thermal Gravimetric Analysis (TGA), providing insights into their thermal properties. Also, a set of optimum design mix proportions based on the results of the compressive strength development of different mixtures created by varied percentages of BOF slag, FA, and IOT has been discussed. Subsequently, six optimized EGC mixes were obtained from the compression test and subjected to further performance evaluations, including water absorption, flexural strength, impact resistance, and heavy metal leaching analysis. Also, the morphology of the EGC mixes is verified through SEM analysis. The results of this study will add to the body of knowledge on geopolymer technology and its potential advancement toward environmentally friendly and sustainable concrete and composite production methods.
2. Research Significance
Geopolymer concrete (GPC) based on pozzolanic materials as binders have received extensive research attention as an alternative to OPC binders, reducing the significant carbon footprint of OPC concrete. The past studies have extensively reported a wide range of outcomes regarding the same. A recent development over OPC is the use of Engineered Cementitious Composites (ECC), a lightweight composite devoid of coarse material. However, ECC involves higher binder (OPC) volumes, making them less sustainable, thereby urging the need to develop a sustainable composite deriving the principles of ECC.
The research value of this manuscript is in examining the feasibility of using industrial wastes in the development of lightweight geopolymer composites, EGC with BOF slag and FA as major precursors, and the utilization of IOT as a partial replacement for fine aggregate by developing a potential EGC design mix. By merging the materials under consideration, the outcomes of this research could enhance the field of building materials and contribute to sustainable industrial waste management.
3. Materials
The proposed development of the reinforced EGC involves the utilization of multiple materials. The primary precursor materials are BOF slag and FA. The BOF slag (Fig. S1a) is the residual matter sourced from the JSW steel plant. FA (Fig. S1b), on the other hand, is a by-product of coal combustion in thermal power plants, and it was obtained from Neyveli Lignite Corporation (NLC). The FA involved was Class F, identified by ASTM C618 [21]. Additionally, IOT were introduced as a partial replacement to conventional fine aggregate (Fig. S1c). The IOTs were supplied by Gogga minerals and Chemicals and subjected to various tests, including specific gravity measurement [22], thermal behavioural analysis by TGA, and XRF analysis. Manufactured sand (M-Sand) was the primary fine aggregate utilized in the study for the EGC production and was partially replaced by IOT as a weight percentage. The specifications of the manufactured sand, including specific gravity, bulk density, and water absorption, were determined according to IS:2386- Part 1&3-1963 [23,24]. The physical properties of fine aggregate (M-sand and IOT) are detailed in Table S1. Moreover, commercially available brass-coated steel fibers were used as a reinforcement, which were of hooked-end orientation and deployed in discrete form. The geometry of the fibers was 0.2 mm in diameter and a length of 13 mm (Fig. S1d). These fibers exhibited a tensile strength of 3230.737 MPa and were coated with a proprietary bonding agent for improved adhesion to the geopolymer matrix. The aspect ratio (length/diameter) was (~62). The particle size distribution data by dynamic light scattering (DLS) provided: BOF ranging from 0.1 to 1000 microns. FA being 8 to 20 μm. While both IOT and M-sand ranged from 0.075 (75 μm) to 4.75 mm (Fig. 1). The combined grading curve provided valuable information regarding the material’s particle size and morphology, indicating a diverse distribution for BOF slag, FA, and IOT. The combination of fine and coarse particles in BOF slag and IOT allows for improved particle packing and interlocking. In contrast, the fine particle size of FA promotes better reactivity and matrix densification, which can positively influence the development of EGC. Incorporating these materials into the EGC formulation can enhance strength, durability, and sustainable construction practices.
3.1. Alkaline Activator
An alkaline activator, consisting of a blended solution of sodium hydroxide (NaOH) and sodium silicate liquid (Na2SiO3) in the ratio (Na2SiO3/NaOH = 0.5), was chosen as the dissolving agent. The anhydrous sodium silicate having a specific gravity of 1.52, was composed of Na2O and SiO2 in weight ratios of 50.46% and 47.24%, respectively. The activator’s modular ratio (Ms) (Ms = SiO2/Na2O) was 0.94. The alkaline molarity used was kept constant at 12M throughout by using NaOH pellets (98% purity) having a specific gravity of 1.32 alongside normal tap water. This solution was prepared 24 hours prior, and the subsequent addition of silicate liquid was done 1 hour before the casting.
4. Mix Profile and Experimental Investigation
The study aimed to optimize the steel fiber reinforced EGC incorporating BOF, FA, and IOT as a partial replacement for manufactured sand, based on their compressive strength assessment. The mix parameters viz., the volume percentage of steel fiber (5 combinations), percentage replacement of IOT as fine aggregate (5 combinations) by weight, and binder combination, FA-BOF (6 combinations), were varied to obtain about 150 (= 5 x 5 x 6) trial mix combinations. The combinations involving the volume percentage of steel fiber (SF) and the percentage of IOT (to be replaced with M-Sand) are outlined in Table S2.
Six distinct combinations of binders were evaluated to optimize the mixture for each binder type by entirely replacing cement. These binder combinations consisted of varying proportions of BOF slag and FA, as presented in Table S3. Overall, a total of 150 trial mix combinations were developed to attain a single optimized mix for each binder combination (Table 1). The optimized mixes have undergone further testing to assess their water absorption, flexural, and impact resistance properties. Additionally, an excess volume of 20% was incorporated to account for potential losses during casting. The proposed design mix specifications for EGC are detailed in Tables S4 and Fig. S2. The molar ratio between NaOH and Na2SiO3 was assumed to be 2.5, while a sand-to-binder ratio (s/b) of 0.3 was utilized in this study. The selection of the molar ratio was based on previous research findings which have shown optimal geopolymerization for a ratio of 2.5 [31]. Similarly, the (s/b) of 0.3 was inferred from studies indicating a well-balanced mix with good workability and strength [32–33]. Based on the research conducted by Bellum et al. [25], an alkali-to-binder ratio of 0.45 and (Na2SiO3 / NaOH) molarity ratio of 0.5 was selected to ensure an optimal geopolymerization rate. Na2SiO3/NaOH ratio in the range of 0.5–2.5 can result in a well-balanced mix with good workability and strength [92]. The molar concentration of NaOH was chosen as 12M based on extensive past research findings showing high strength characteristics of geopolymers [92]. Also, the proposed 12M NaOH helps in the effective dissolution of SiO2 and Al2O3, which is considered essential for the geopolymerization process [93].
For each variation and binder combination, the mixing procedure was performed in a tabletop mortar mixer (Fig. S3). The Initial mix involves FA and BOF being weighed and thoroughly mixed along with an alkaline activator for 180 seconds at 200 rpm ensuring uniform distribution, followed by further mixing at 300 and 400 rpm speed for 180 seconds each. Upon obtaining the initial Geopolymer (GP) binder /paste, subsequent addition of calculated quantities of sand was done. Finally, the fibers were added slowly within a 60-second interval at a stir. Once the fibers were added, the mix was subjected to further mixing at 200 rpm speed for 180 seconds each to achieve a homogeneous paste, thereby ensuring proper dispersion of steel fibres. For the development of the proposed EGC, 50 x 50 x 50 mm cube molds were used following ASTM C109 /C109M - 16a [26] to make six duplicates for each composition, grouping three specimens each for seven and 28-day compressive strength assessment respectively, for a total of 900 samples. Cube specimens were demoulded within 24 hours and kept under oven curing at a constant temperature of 60° C for an initial 24 hours to attain high early strength [94], followed by ambient laboratory curing conditions (i.e., 25±1°C) in sealed condition using plastic wrap to prevent moisture loss [95] until their respective strength attainment of 7 and 28-days (Fig. S4).
4.1. Compressive Strength
The compressive strength of all the EGC trial mixes prepared was evaluated according to ASTM C109 on 50 mm cube specimens after 7 and 28 days of curing [26]. Three specimens were prepared and tested for each material variable listed in Table 1 (overall mix analogy), thereby allowing the calculation of standard deviations and the mean strength. The experimental tests were performed by applying pressure with a constant loading rate of 1800 N/sec.
4.2. Water Absorption Test
Water absorption is a vital aspect to verify the permeability aspect of the proposed EGC, as it directly influences the durability and long-term performance of the material. The water absorption test for the optimized EGC mixes was conducted following ASTM C642-97 [27] standard guidelines. Cube specimens (50 x 50 x 50 mm – 3 no.s per EGC variant) upon 28-day strength attainment are considered to determine the percentage of water absorption. The specimen’s constant dry mass was measured upon initial oven drying for 24 hours. The specimens were immersed in water for 48 hours, and subsequent saturated mass measurements were taken upon removal in surface dry condition to obtain an absorption rate. The increase in weight due to water absorption was then measured and expressed as a percentage of the dry weight.
4.3. Flexural Strength Test
The flexural strength of various mixes was obtained using the three-point load method (Fig. S5) according to ASTM C78/C78M-18 [28]. Prismatic specimens of dimensions 50 x 50 x 200 mm were to validate the ultimate load and the corresponding flexural strength upon attaining their 7 and 28-day strength period.
4.4. Impact Resistance Test
The impact resistance was measured by conducting a low-velocity impact test using the drop hammer procedure outlined in ASTM D7136/D7136M-15 [29]. Upon attaining 28-day strength, cylindrical specimens of 200mm diameter and 100mm height were subjected to a sudden impact load, and the energy absorbed by the samples before failure was measured. The total weight of the hammer was 20 kg, and the drop height was set at 400mm, intended to the centre of the specimen until it fails and does not withstand any further blows (Fig. S6). Based on the record the impact force (Ft) and velocity (Vt), the absorbed energy (Et) can be calculated according to ASTM D7136-05 [30], which is expressed in Eq. (1), (2) and (3). Using the conventional method for impact test, energy can be calculated as in Eq. (4).
where
Vi and δi are the initial velocity and position, respectively. g and m are the acceleration of gravity and applied mass, respectively.
where
4.5. Heavy Metals Leaching Analysis
The leaching analysis of heavy metals from the EGC paste samples comprising the industrial wastes viz., FA, BOF slag, and IOT, was performed by the Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) method, also termed ICP-OES. A total of six EGC paste samples were prepared following the binder combinations (Table S3) with an alkaline to liquid to binder ratio of 0.45 and cured until 28 days following ambient curing conditions (25±1°C). The subsequent leaching concentrations from the proposed paste samples within 28d were carried out following a modified version of the tank analysis defined by NEN 7375 guidelines [116].
4.5.1. Acid digestion
The analysis was carried out following the acid digestion procedure. Fine powdered samples (1–2 g; passing 125 μ sieve) are combined with 5mL of a 1:1 nitric acid solution in a digestion tube. The mixture is then heated at a temperature range of 90–95°C for a maximum duration of 15 minutes. Following this, 2.5 mL of concentrated nitric acid is added and the sample is refluxed for 30 minutes. Upon cooling, an additional 2.5 mL of concentrated nitric acid is added and the sample is refluxed for another 30 minutes. Subsequently, 0.5 mL of 30% hydrogen peroxide is added to the cooled sample, and the sample is heated again until no further effervescence is observed. This process is repeated three more times, with the addition of 1.0 mL of hydrogen peroxide each time, and heating in between each addition. Subsequently, 5mL of concentrated hydrochloric acid is added and the sample is heated for 15 minutes. Finally, the sample is allowed to cool down and diluted with deionized water up to a total volume of 50 mL [117–118].
4.5.2. Tank analysis
The treated EGC samples are immersed in acetic acid solution (pH 7), having a solid to liquid ratio of 1:3 and being maintained at 22±2°C. The immersion was performed for three different stages (7,14 and 28d) and their corresponding heavy metal leachates were assessed. The quantification involved major heavy metals viz., Cd, Cr, Cu, CN, Pb, Zn, and As. The measured leaching concentrations were compared with the Indian standard specifications for the discharge of environmental pollutants in water bodies by the pollution control board of India [119].
5. Results and Discussion
This research article presented a feasibility report on developing a steel fiber reinforced EGC utilizing the industrial by-products viz., BOF slag, FA, and IOT. The properties of each material were assessed through XRF and TGA analysis. The compressive strength development was evaluated for various combinations of the resultant EGC mixes, considering different levels of IOT replacement over M-Sand and varying SF percentages. Based on the compressive strength results, six optimized mixes were obtained, representing different binder combinations of the precursors used. In addition, the six optimized mixes of EGC are subjected to further investigation to validate their performance by assessing their water absorption properties, flexural strength, and impact resistance properties. The findings demonstrate the suitability and potential of BOF slag, FA, and IOT in developing lightweight EGC. The incorporation of steel fibers further enhanced the mechanical properties of the composites.
5.1. Characterization and Analysis
This section presents the specifications and results of X-ray fluorescence (XRF) analysis, Thermogravimetric analysis (TGA), and particle size analysis for the materials of concern, viz., BOF slag, FA, and IOT. The particle size distribution (size ranging from nanometer to micrometer scales) was characterized by the Dynamic light scattering (DLS) method rather than the conventional sieve analysis methodology to compare the results with other binder materials used in this study.
5.1.1. X-ray fluorescence (XRF) analysis
The XRF analysis was performed to determine the chemical composition using an X-Ray Florescence spectrometer involving the (FP) oxide cups method. The X-ray fluorescence (XRF) analysis was conducted to determine the chemical composition of BOF, FA, and IOT, and the results are shown in Table S5.
5.1.1.1. Basic oxygen furnace (BOF) slag
The chemical composition of BOF slag is commonly composed of oxides like SiO2, CaO, Al2O3, Fe2O3, MgO, and others. Also, BOF contain high concentrations of silicon dioxide (SiO2) and calcium oxide (CaO) in specific. These oxides suggest that BOF can be a great source of the alkaline activators necessary for the geopolymerization process. Additionally, incorporating BOF into EGC can help with waste reduction, sustainability, and exploiting the steel industry byproduct.
5.1.1.2. Fly ash (FA)
Fly Ash typically comprises SiO2, Al2O3, Fe2O3, CaO, MgO, and other oxides. FA is renowned for having high alumina (Al2O3) and silica (SiO2) concentrations. These oxides can potentially be used to develop EGC and other sustainable construction methods since they are significant in forming geopolymeric linkages, demonstrating their suitability as a vital component for geopolymerization.
5.1.1.3. Iron ore tailings (IOT)
The chemical composition of IOT revealed majorly, Fe2O3, SiO2, Al2O3, CaO, MgO, and other oxides. IOT has a high concentration of iron oxide (Fe2O3), making it a desirable material for EGC development. Iron oxide can potentially result in the formation of robust geopolymeric linkages, which enhances the mechanical properties of the composites. Also, the presence of other oxides, such as Silica (SiO2) and alumina (Al2O3), further supports the geopolymerization process. Utilizing this mining waste material efficiently upon integrating it into EGC promotes sustainable resource management.
The XRF analysis revealed that the principal oxides present in the BOF slag were silicon dioxide (SiO2), calcium oxide (CaO), and aluminium oxide (Al2O3), with traces of other oxides present in small amounts. However, FA exhibited high silica (SiO2) and alumina (Al2O3) content. Along with these oxides of silica and alumina, IOT indicated the presence of Iron oxide (Fe2O3). The findings demonstrate that essential elements like Si, Ca, Al, Fe, and other minor elements are present in the precursor materials.
According to the findings of the XRF research, all three materials —BOF, FA, and IOT—display chemical compositions that are acceptable for the production of EGC. These materials offer vital elements, such as calcium oxide (CaO) and iron oxide (Fe2O3), along with the major oxides required for geopolymerization, including silica (SiO2) and alumina (Al2O3). Numerous advantages, such as increased sustainability, waste reduction, and improved mechanical characteristics, can be attained by using these materials in EGC. As a result, the results of the XRF study strongly suggest that BOF, FA, and IOT are appropriate for the development of EGC, which will eventually result in the implementation of sustainable building materials.
5.1.2. Thermogravimetric analysis (TGA)
To ascertain the thermal stability and weight loss characteristics of the BOF slag, FA, and IOT, thermogravimetric analysis (TGA) was conducted. The materials are heated to high temperatures to examine the weight loss observed as a function of temperature and gain insight into the material's thermal stability and decomposition patterns. A thermogravimetric analyzer with a temperature range of 20°C to 1000°C is used to obtain the TGA data. DTA (Differential Thermal Analysis), DTG (Differential Thermogravimetry), and TG (Thermogravimetry) are the three curves in the graph. The temperature ranges at which major thermal events occur can be determined by examining the DTA curve. The DTG curve aids in determining the material's thermal stability and decomposition behaviour. Understanding the thermal stability, volatile content, and composition of the materials can be made by the TG curve, which depicts the change in weight of the material as a function of temperature.
5.1.2.1. Basic oxygen furnace (BOF) slag
The TGA curves for BOF slag are shown in Fig. S7, which showed a weight loss of 2.7%. The DTA curve of BOF slag (shown in green) displayed a dispersed pattern with noticeable changes at various temperature ranges. The initial negative values show that moisture and volatile substances in the slag have been released. The upward trend from 150°C to 450°C points to an exothermic reaction or crystallization process in this temperature range. The phase transformation of the slag or structural alterations may be responsible for the dip in the curve at 730°C. An endothermic process or the release of gases during additional thermal deterioration is suggested by the sharp increase at 900°C. Overall, the thermal behaviour of the BOF slag was complex, with possible reactivity and contributions to the EGC geopolymerization process.
BOF's DTG curve (shown in red) showed pronounced spikes with ups and downs, indicating multiple thermal events occurring within the material. As volatiles and moisture are released, the initial negative readings agree. The considerable spike at about 650°C indicates a quick exothermic or combustion event. Also, the subsequent dip shows a decrease in reaction rate or thermal deterioration. The overall thermal behaviour of BOF slag signifies its potential role in developing EGC with enhanced mechanical properties through the geopolymerization process.
The steady weight loss of the BOF slag, as seen by the TG curve (shown by the blue hue), was predominantly caused by the evaporation of moisture and volatiles at lower temperatures. The tests revealed a controlled weight loss, which proves the stability of the slag and its compatibility with high-temperature applications. The smallest weight loss (0.7%) seen up to 1000°C implies that key components were retained, which strengthens the utility of BOF slag in EGC formulations.
5.1.2.2. Fly ash (FA)
The TGA plot of FA is shown in Fig. S8. The DTA curve for FA (shown in green) displayed a haphazard pattern with both positive and negative values. It can be shown that up to 100 degrees Celsius, FA shows a linear decrease in heat flow, reaching approximately −6uV. This pattern persists up to 200 degrees Celsius, showing negligible deflections signifying the release of moisture and volatiles. Around 180 degrees Celsius, there is a noticeable shift as the curve begins to progressively climb, peaking at 430 degrees Celsius (−2uV). This can be an indication of an endothermic process or the breakdown of carbonates or other substances in FA. The ensuing decline in the curve from 430 to 800 degrees Celsius, with a value of around −4.5uV at 800 degrees Celsius, illustrates the thermal degradation of organic components and subsequent reactions. The curve displayed a steady pattern up to 850 degrees Celsius and exhibited a small spike at 900 degrees Celsius (−3.8uV) and 1000 degrees Celsius (−3.2uV). The potential reactivity and contribution to the geopolymerization process in EGC are highlighted by the overall thermal behaviour of FA.
Sharp spikes and shifts in the DTG curve of FA (shown in red) indicated multiple thermal events occurring within the material. The curve initially exhibits a linear upward trend, from −1ug/min to 2ug/min, up to 100 degrees Celsius, consistent with volatile release and organic matter combustion. Short temperature ranges, such as between 100 and 110 degrees Celsius (2 ug/min to 12 ug/min) and between 150 and 170 degrees Celsius (−1 ug/min to 23 ug/min), showed sharp spikes and dips in the plot. After 170°C, the behaviour appears to be very stable, which suggests reduced reactivity or thermal stability of FA with fluctuations between 0 and 2.5ug/min. Nevertheless, the rise between 600°C and 650°C indicates the presence of reactive components in this temperature range. These thermal properties demonstrate FA's potential for geopolymerization processes and its contribution to the enhancement of EGC's properties.
FA's TG curve (indicated in blue color) displayed thermal stability up to high temperatures, with minimal weight loss (maintaining a steady value of 17.25mg up to 1000 degrees Celsius). The controlled weight loss recorded during the testing confirms the stability of FA and its suitability for high-temperature applications. The DTA curve demonstrates that FA undergoes a significant exothermic reaction at 430°C, which could be related to a phase change. The DTG curve draws attention to different volatile FA components, causing the thermal decomposition to fluctuate. The retention of vital components is highlighted by the relatively minimal weight loss (0.46%) up to 1000°C, thereby establishing FA as a promising component in the development of EGC.
5.1.2.3. Iron Ore Tailings (IOT)
Fig. S9 shows the TGA plot for the IOT demonstrating the different curves. The heat flow is shown to linearly decline up to 70 degrees Celsius on the DTA curve (shown in green colour) for IOT, reaching a minimum of −7.2 uV. After this initial drop, the range remains comparatively steady between −7.2 uV and −6.8 uV up to 150 degrees Celsius. The curve then gradually continues to rise, peaking at 1 uV at 400 degrees Celsius. Subsequently, the curve rapidly drops from 400 to 550 degrees Celsius, until it reaches −3 uV at 550 degrees Celsius. It is comparatively steady between 550 and 650 degrees Celsius at 0 uV and maintains this value up to 750 degrees Celsius. The graph shows only minor changes above 750 degrees Celsius, with the curve slightly increasing to 0.2 uV at 1000 degrees Celsius.
Sharp spikes and variations are visible on the DTG curve of IOT (shown in red), which is reminiscent of a stock chart. The curve initially shows a linear upward trend, ranging from 48ug/min to 62ug/min, up to 50 degrees Celsius. A sudden dip is observed from 50 to 120 degrees Celsius, and the DTG value reduces from 62 to 12 ug/min. The DTG values oscillate between 5 and 15 ug/min with nonlinear fluctuations between 120 and 380 degrees Celsius. Then, at a temperature of 400 degrees Celsius, a substantial rise peaks at 42 ug/min, followed by a sudden drop to 15 ug/min at 420 degrees Celsius. The curve shows a sharp rising trend above 420 degrees Celsius, peaking at 110 ug/min at 550 degrees Celsius. The DTG curve displays a straight decrease trend from 550 to 1000 degrees Celsius, gradually dropping to 0 ug/min.
The weight loss properties of IOT are revealed by the TG curve (marked in blue colour) during the TGA study. Up to 30 degrees Celsius, the sample retains its original weight (17.88 mg). After then, a non-linear exponential curve showing gradual weight degradation is seen. The weight decreases to 17.55 mg at 100 degrees Celsius and 17.35mg at 500 degrees Celsius. The weight loss stabilizes above 700 degrees Celsius, retaining a constant value of 16.8mg at 1000 degrees Celsius. Moreover, the maximum weight loss measured was approximately 5.6% throughout the test.
The aforementioned findings suggest that IOT is appropriate for inclusion in EGC owing to their strong thermal stability (up to 1000°C) and minimal weight loss. Natural resource preservation and environmental waste reduction can be benefitted by including IOT in the development of EGC.
The TGA analysis graph offers essential information on the weight loss and thermal stability traits of the materials under investigation. It was possible to get important insights into the thermal behaviour, decomposition temperatures, and weight loss patterns of the respective materials (BOF slag, FA, and IOT) by analyzing their DTA, DTG, and TG curves. This information is found feasible for their suitability in developing the proposed EGC mixes.
5.2. Compressive Strength
The compressive strength test results for the FA: BOF based EGC mixtures are presented in Fig. S10. The corresponding graph illustrates the relationship between the compressive strength (CS) and the three variable parameters considered (SF%; FA%; IOT%), being differentiated by colour shades (Fig. S11). The highest strength achieved, 41.77 MPa, was observed in the mix with EGC-FA: BOF-60:40, 1.5% SF, and 45% IOT. This result is consistent with the trend depicted in the main effects plot (Fig. S12), which shows a significant linear increase in compressive strength with increasing IOT content. Specifically, at 35% weight replacement of M-sand with IOT, the average strength increased by 60% compared to mixes without IOT replacement. However, a dip in strength was observed at 45% replacement, which may be attributed to increased shrinkage caused by IOT particles, thereby confirming their higher water absorption % in contrary to M-Sand. The physical characteristics of IOT particles contribute to this phenomenon. IOT particles are typically small and have irregular shapes, which gives them a large surface area. Additionally, their porous and irregular micro-structures allow them to absorb a lot of water. When added to the EGC matrix at high replacement levels (45% in this study), the IOT particles absorb a significant amount of water from the mixture. This increases the overall water demand of the EGC mix and reduces the amount of water available for the geopolymerization reaction between the FA: BOF and the activated alkaline solutions. As a result, the geopolymerization process is incomplete, leading to weaker geopolymer bonds and reduced compressive strength development and shrinkage [38–40].
Furthermore, three interesting inferences were observed:
The implementation of IOT replacement exhibited a linear trend being followed, as the increment in IOT produced further strength gain up to its 45% replacement with M-Sand for EGC-FA: BOF-70:30 and up to 35% for EGC-FA: BOF-90:10;80:20;60:40 and 50:50 irrespective to the binder combinations and the steel fibre variations, whereas EGC-FA: 100 denoted maximum strength gain of 33.32MPa corresponding to 1.5% SF only and the other combinations recorded strength development only up to 20% IOT. This behaviour may be attributed to the absence of the additional binding material, BOF slag, thus making the gelling of IOT difficult towards the full FA mix.
The addition of brass-coated steel fibers enhanced the compressive strength only up to a maximum of 1.5% addition, but generally, an increase in SF composition showed a decreasing trend in strength [47–48]. Moreover, the addition of fiber content did not follow any particular trend, as the mixes corresponding to EGC-FA: BOF-90:10 and EGC-FA: BOF-70:30 showed enhanced strength at 0.8 and 1 % SF, respectively, while the remaining types exhibited their maximum strength attainment at 1.5%. In general, the enhancement in compressive strength achieved through fiber reinforcement is attributed to the crack-bridging mechanism revealed by the fibers, which effectively restricts crack growth and propagation. This mechanism also influences the failure mode of Geopolymer Composites (GPCs), transforming it from a brittle nature to a more ductile behavior. Considering the specimens with SF% (1.5%) at which strength development was achieved, all EGCs experienced significant deformation and splitting mode of failure causing columnar vertical cracks at failure (Fig. S13 v to ix). Whereas, Fig. S13 (iii and iv) denotes the EGCs corresponding to FA: BOF-90:10 and 70:30, respectively, which reported considerable strength attainment at 0.8% and 1% SF, respectively. However, EGCs with pure M- Sand (MS) mix experienced brittle failure, which can be attributed to the significance of IOT and its binding nature towards the precursors in contrary to M-Sand (EGC-FA: BOF-MS) (Fig. S13 i and ii).
Lastly, adding Basic Oxygen Furnace (BOF) Slag led to a sudden increase in strength compared to using FA as the sole binder. This can be attributed to the angular and tough texture of BOF slag and its high alkalinity. The increased strength of the EGC can be attributed to the angular and tough texture of BOF slag and its high alkalinity. Previous studies have also shown that higher slag content leads to enhanced strength [41–43]. The angular and tough texture of BOF slag improves the interlocking effect within the EGC matrix, which enhances load transfer and overall strength development [44]. Additionally, BOF slag has a high alkalinity due to its chemical composition, particularly the presence of calcium oxide (CaO) [45]. This high alkalinity promotes the geopolymerization reaction, creating a favorable environment for the activation of FA and aiding in the dissolution of silica and alumina from FA particles. This results in the formation of geopolymeric gels that contribute to the strength development of the EGC [46]. 40% FA replacement with BOF slag demonstrated an average strength of 39% greater than 100% FA mix. However, the strength development observed at 10% replacement diminished thereafter.
Comparing the 7-day and 28-day compressive strengths (CS-7 and CS-28) of the EGC specimens (Fig. 2), it can be observed that they have comparable values, as the R-square value accounts for (~0.706) aligning closely with the x=y line. This is consistent with the well-known property of geopolymer concrete (GPC) to exhibit high early compressive strengths [36–37]. The frequency distribution of strengths (Fig. S14) reveals that compressive strengths ranging from 10.8 to 15.5 MPa are the most frequently observed in the test results.
As inferred from the mix specifications (Table S4), the density of EGC mixes according to the various percentages of SF addition (Table S2) is shown in Fig. S15. The densities of the FA: BOF based steel fibre reinforced EGC averaged around 2.07 g/cm3.
Subsequently, the EGC mix proposed exhibited densities lower than the density of concrete (Geopolymer / OPC), usually around 2.40 g/cm3. Nevertheless, in the case of FA: BOF based EGCs, out of all 150 mix combinations considered, a minimum of one mix per binder combination, irrespective of the SF and IOT % was able to exceed the compressive strength of conventional concrete (i.e., 30 MPa) (Fig. S10). Thereby, six optimized mixes above this range were obtained (Table 2). This underscores the remarkable strength-to-weight ratio exhibited by these materials.
It is important to emphasize that the EGC with the highest compressive strength (referred to as EGC-FA: BOF-60:40-MS: IOT-55:45-SF-1.5), attaining 41.8 MPa can be categorized as an effective and sustainable alternative to conventional concrete material based on the IS456:2000 specifications (i.e., 25 MPa > f’c < 55 MPa). Notably, this EGC demonstrates a density of 13.75% lower than that of conventional concrete, further highlighting its beneficial lightweight properties.
To optimize the composition of the proposed EGC mix involving BOF slag, FA, IOT, and SF, an extensive experimental study was performed, in which a total of 150 combinations, resulting in 900 cubes, were cast and tested for compressive strength at seven-and twenty-eight-day intervals. The collected data was then analyzed and interpreted using the Minitab software to identify the optimal mixes and understand the relationship between the independent and dependent variables.
The experimental results demonstrate that the developed steel fiber reinforced EGC exhibits promising compressive strength performance. Among the six variants of FA: BOF combinations, one mix per combination that exhibited the highest twenty-eight-day compressive strengths was identified as the optimum mix. Notably, the highest strength was achieved for the mix: FA: BOF-60:40, 1.5% steel fiber, and 45% IOT (EGC-FA: BOF-60:40-MS: IOT-55:45-SF-1.5).
The influence of the independent variables on the compressive strength was examined, revealing that an increase in the percentage of Iron Ore Tailings (IOT) led to a nearly linear increase in strength. Additionally, including BOF slag as a replacement for FA proved successful, resulting in a significant strength improvement even at a 10% replacement level. Thereby, BOF slag as a binder significantly enhanced the strength as opposed to FA alone (EGC-FA100).
The SF content of 1.5% yielded the highest compressive strength values. However, increasing the fiber content beyond this point had a negative effect on the compressive strength.
Statistical analysis showed that all independent parameters, except for the interactions, were statistically significant in influencing the compressive strength of the EGC. Further, IOT was the most influential parameter on the compressive strength, followed by Steel Fibers and BOF slag replacement.
Nonetheless, the significance of particle size distribution has been established in many studies in the realm of geopolymer research. The particle size distribution of the precursors has played a crucial role in the geopolymerization process. It is to be noted that fine particles with greater surface area and reactivity, contribute towards compressive strength development in geopolymers. This signifies that the particle size distribution could be an effective method to optimize the strength attributes apart from the parametrical variation of precursors and alkaline activators of geopolymers [96–97].
Moreover, the grinding of precursors to achieve fineness has also been studied emphasizing the particle size distribution. Activation by mechanical means enhanced the reactivity of FA by increasing the surface area and reducing particle size. The fine ground FA resulted in higher compressive strength compared to the untreated FA owing to their increased dissolution rates [98].
In this study, the precursors involved being raw, and as mentioned earlier, the DLS distribution of FA ranged from 8 to 20 μm and BOF slag was 0.1 to 1000 microns. Despite involving untreated precursors, the proposed EGC mixes were able to attain a maximum compressive strength of up to 41.8 MPa (EGC-Mix-5). Also, the average compressive strength of all the optimized mixes was>30 MPa. The findings suggest that the compressive strength along with other mechanical characteristics of EGC could potentially be optimized further by considering the particle size distribution of the precursor materials.
5.3. Water Absorption Test
A significant correlation was found between water absorption and compressive strength of the six different EGC mixes (Fig. 3). It was determined that mix-5 (EGC-FA: BOF-60:40-MS: IOT-55: 45-SF-1.5), which exhibited the highest compressive strength, had the lowest moisture absorption percentage of 1.8%. This indicates that mix-5 created a denser composite that contained fewer voids, leading to lower moisture absorption. In contrast, mix-4, which had the lowest compressive strength, showed the highest water absorption percentage of 2.6%. According to the specific binder and aggregate composition, the mix had a higher porosity, which led to increased water absorption. A similar trend was also observed among other mixes, with mix-2 (EGC-FA: BOF-90:10-MS: IOT-80: 20-SF-0.8) and mix-3 (EGC-FA: BOF-80:20-MS: IOT-65:35-SF-1.5) both showing lower water absorption percentages of 2.1% and 2.2%, respectively, in accordance with their higher compressive strengths. Water absorption percentages for mix 1 (EGC-FA100-MS: IOT-65:35-SF-1.5) and 6 (EGC-FA: BOF-50:50-MS: IOT-55:45-SF-1.5) were 2.45% and 2.5%, respectively, resembling their lower compressive strengths.
Nonetheless, water absorption in geopolymer composites is closely related to their surface porosity [105–106], which determines their permeability characteristics. Generally, the water absorption properties of geopolymers are directly proportional to their porosity [105–107]. The relationship between water absorption and porosity in geopolymers is influenced by several factors. One of these is the alkaline liquid-to-binder ratio used in the geopolymer synthesis, which directly influences the micro and macro capillaries of the geopolymer matrix [108]. The water absorption characteristics of the optimized EGC mixes involving varied combinations of slag and sand proportions behave accordingly based on the morphology of each mix having distinct porosity.
Additionally, structural pores in geopolymers also play a crucial role in water absorption. These pores can be categorized viz., large intergranular spaces and smaller network pores [105,108]. The EGC mixes could be possibly related to the same showing relatively lesser water absorption levels for mixes with maximum strength denoting their highly dense microstructure and relatively lesser pores preventing free water intrusion.
There is a significant correlation between water absorption and compressive strength of the six different Engineered Geopolymer Composites (EGC) mixes.
The findings provide valuable insights into the porosity, surface pores, and permeability of the EGC mixtures.
The findings aid in determining the durability of the EGC mixes in terms of its water resistance, which ensures their feasibility for various applications.
5.4. Flexural Strength Test
The results of the flexural strength test from the sustained flexural load show a complex interaction between the suggested EGC mixtures. Flexural strength ranges from 4.48 to 5.78 MPa for the proposed mixes (Fig. 4). The measured flexural strengths were mix-1 (4.48 MPa), mix-2 (5.44 MPa), mix-3 (5.09 MPa), mix-4 (3.88 MPa), mix-5 (5.78 MPa), and mix-6 (4.46 MPa). The resistance to bending forces of the EGC mixes followed similar patterns relating to their resistance to compression. Mix-5 (EGC-FA: BOF-60:40-MS: IOT-55:45-SF-1.5) had the highest flexural strength measuring 5.78 MPa. This strength was connected with the highest compressive strength previously determined for the same, thereby improving its resistance to bending forces [109]. Mix-4 (EGC-FA: BOF-70:30-MS: IOT-55:45-SF-1), on the other hand, exhibited the lowest flexural strength (3.88 MPa), indicating a relatively lesser proportion of BOF slag content and SF%, the fact that may have a detrimental effect on their flexural strength [110]. Interestingly, mix-2 (EGC-FA: BOF-90:10-MS: IOT-80:20-SF-0.8) also showed a greater flexural strength (5.44 MPa), demonstrating that the higher FA content and MS: IOT may compensate for the lower SF% in terms of flexural strength [111]. Alternatively, mix-3 (EGC-FA: BOF-80:20-MS: IOT-65:35-SF-1.5) exhibited a flexural strength of 5.09 MPa, indicating that increasing the BOF slag content and retaining a modest MS: IOT ratio can still provide high flexural strength, especially at higher SF percentages [112]. Additionally, mix-6 (EGC-FA: BOF-50:50-MS: IOT-55:45-SF-1.5) demonstrated a flexural strength of 4.46 MPa. This implies that a high flexural strength may not always be achieved, upon maximum replacement of IOT or slag especially at higher SF% levels [113].
5.5. Correlation Between Compressive and Flexural Strength
The correlation between flexural strength and compressive strength is proposed as a result of regression analysis of data generated by destructive testing in the laboratory. It is observed that a reasonably good relationship between these two strengths exists (Fig. 5).
The regression analysis followed a trendline, represented by the FS= 0.1845 CS - 1.6899 equation, (FS - Flexural Strength; CS - Compressive Strength). This equation suggests a positive linear relationship between the compressive strength and flexural strength of the EGC mixes. As the compressive strength increases, the flexural strength increases, albeit at a slower rate, as indicated by the slope of (0.18448 ± 0.03638). The R-square value obtained from this linear regression analysis is 0.865, indicating that approximately 86.5% of the variation in flexural strength can be explained by the variation in compressive strength. This high R-square value suggests a strong correlation between the two variables. The adjusted R-square value of 0.83168 further confirms the robustness of the model, taking into account the number of predictors in the model. The residual sum of squares, a measure of the discrepancy between the data and an estimation model, is relatively low at 0.33668, indicating a good fit of the model to the data. Finally, Pearson’s r value of 0.93024 signifies a strong positive correlation between the compressive strength and flexural strength of the EGC mixes. This high correlation coefficient further validates the strong relationship between these two variables, reinforcing the reliability of the regression model.
In conclusion, the correlation study provides a positive relationship between the compressive strength and flexural strength of the optimized EGC mixes. This correlation is essential to apprehend the behavior of these mixes under different loading conditions, thereby aiding in the design and structural application of EGC [34–35].
5.6. Impact Resistance Test
The impact resistance measurements showed a substantial association with the mix composition, and the results of this study provide valuable insights into the toughness and durability of EGC mixes. The number of blows sustained, and the amount of energy absorbed during the impact loading varied between the mixtures, demonstrating the impact of the binder, steel fibre, and fine aggregate ratios on the overall performance of the EGC (Fig. 6). Mixes 1 and 6, which had the highest and lowest FA: BOF ratios, respectively, sustained the least number of blows (2) and absorbed the least energy (156.96 Nm). This shows that extreme FA to BOF ratios, such as 100% FA or an equal distribution of FA and BOF, could not offer the best impact resistance [114]. Despite having varying binder ratios, both mix-2 and mix-4 sustained three blows and exhibited the same levels of energy absorption (235.44 Nm). This could be attributed to the higher SF% (0.8%) and MS: IOT (80:20). The mixes with the highest impact resistance, mix-3 & mix-5, sustained four blows each. Despite mix-5 having a higher SF content (1.5%) and a lower MS: IOT (55:45), they both absorbed the same amount of energy (313.92 Nm). This suggests that while the binder ratio may influence the number of blows endured, the energy absorption capacity may be more influenced by the SF% and the fine aggregate ratio.
Considering the impact resistance obtained for the optimized EGC mixes, the proposed FA: BOF based EGC could be an effective alternative to conventional geopolymer concrete (GPC), as the latter exhibit inherent brittleness and poor cracking resistance [115].
5.7. Effect of SF Aspect Ratio
The aspect ratio, defined as the ratio of length to diameter of the fiber, is a crucial parameter that can significantly influence the mechanical properties of the geopolymer composite. The aspect ratio (length/diameter) of the SF used in this study (~62), is an essential parameter that influences the performance of discrete fibre reinforced composites [79]. Higher aspect ratios result in enhanced dispersion of fibres and distribution within the composite matrix. It also improves the fibre-matrix bond and crack bridging ability of the fibres under compressive and flexural loads [79–81].
Studies have proven that higher aspect ratios lead to improved strength properties of steel fibre reinforced composites [82]. Also, reinforced composites with aspect ratios between 50–100 exhibited higher flexural strength and toughness compared to those with lower aspect ratios in the range 20–40 [83–85]. The compressive strength and impact resistance of steel fibre reinforced concrete increased with increasing aspect ratio up to a value of 65 [83,86–87]. The elongated brass coated fibres were able to disperse well and bridge cracks more effectively within the composite matrix [79–80]. Subsequently, the high aspect ratio of 62 used in this study could be associated with their contribution to the mechanical performance of the EGC being developed [88–91].
The aspect ratio of steel fibers (SF), which is ~62 in this study, plays a vital role in enhancing the strength aspects of the proposed EGC mixes. This high aspect ratio is associated with the denser EGC matrix observed in mix-5, leading to minimal water absorption. Furthermore, it significantly enhances the flexural strength properties of the EGC mixes 2 and 5. In addition to these, the high aspect ratio also improves the impact resistance of the EGC mixes 3 and 5, as evidenced by the enhanced impact strength observed at higher aspect ratios up to 65 [86–87]. Thus, the aspect ratio of SF proves to be a crucial factor in optimizing the performance of EGC mixes across various strength parameters.
5.8. Scanning Electron Microscopy (SEM)
To obtain a comprehensive understanding of the relationship between the morphology and the experimental results of the EGC, Scanning Electron Microscopy (SEM) has been performed. The SEM images of the various raw materials (FA, BOF slag, and IOT) are depicted in (Fig. S16 a to c), while the SEM images of the optimized EGC mixes are detailed in (Fig. S17 a to f). Fractured EGC samples were used to capture the microstructural SEM images by treating the samples with platinum before their analysis.
The SEM images of the raw materials viz., FA, BOF slag, and IOT reveal their distinct characteristics on their morphology. FA particles are mostly spherical, which contributes to the workability of the EGC mix [88]. Also, this spherical orientation allows better distribution of particles within the matrix, leading to a cohesive and more uniform structure (Fig. S16a). Contrarily, BOF slag particles are angular and irregular, showing good interlocking properties and in turn, contributing to the overall strength and durability of the EGC [102–103] (Fig. S16b). IOT displayed a rough surface texture, which can enhance the bond strength of the matrix, resulting in a robust and durable composite (Fig. S16c) [103–104].
The optimized EGC mixes 1–6 showed an overall homogeneous microstructure, indicating uniform mixing, and reaction between the materials [99]. The reactions of the precursors can be differentiated as the pozzolanic reaction of FA and the hydraulic reaction of BOF slag [99–100]. Mix 1 (Fig. S17a), comprising FA alone as the precursor, showed a relatively porous microstructure owing to the spherical orientation of FA powder with evident voids and gaps denoting unreacted FA particles [100]. This can be associated with their comparatively lower compression values than the mixes comprising BOF slag [88]. As inferred from previous studies, the presence of unreacted FA in a geopolymer microstructure averages to 58%. Nevertheless, the presence of unreacted FA and BOF slag indicates potential for further strength development [101]. Mix 2 (Fig. S17b) and Mix 3 (Fig. S17c) comprising 10–20% BOF slag displayed an irregular and rough texture due to the presence of angular BOF particles. Also, the mechanical interlocking was enhanced in these mixes, correlating well with their mechanical characteristics of enhanced compression and flexural resistance [88].
Mix 4 (Fig. S17d) with 30% BOF slag showed some unreacted FA spheres surrounded by the geopolymer gel being formed as a result of polymerization. This could be possibly due to the reaction of higher proportions of slag with FA [99–100]. Mix 5 (Fig. S17e) comprising 45 % BOF slag showed a dense microstructure with minimal voids, correlating well with the experimental results showing high strength characteristics. Additionally, mix 5 (Fig. S17e) depicts a well-defined interfacial transition zone (ITZ) (the region located between the geopolymer matrix and the aggregate volume), indicating a strong zone for geopolymers. This lies in good correlation with the observed results showing enhanced bonding and strength characteristics [101].
Also, the SEM images indicate the distribution and orientation of the SF in the EGC matrix. Mixes with high SF% (Mix 1, 3, 5, and 6) showed a well-distributed matrix oriented in multiple directions, which could effectively resist tensile and flexural loads (Fig. S17a to f). This observation correlates with the experimental results, exhibiting enhanced flexural strength. Contrarily mixes with lower SF % (Mix 2 and 4) showed fewer fibers in the EGC matrix, associated with their lower flexural resistance being recorded. Also, the SEM images reveal that the IOT particles are well-bonded with the matrix, contributing to the overall strength attainment of EGC.
This microstructural analysis involving SEM correlates well with the strength properties. The dense microstructure exhibited for mixes 1, 5, and 6 resulted in high compressive strength. The irregular and angular textures of mixes 2 and 3 improved their flexural resistance. Additionally, the porous structure of IOT facilitated the increase in energy absorption under impact loading, as seen in mix 5.
The SEM analysis provided valuable insights into the morphological and microstructural aspects of the proposed raw materials and the optimized EGC mixes, showing a good correlation with the trends in mechanical characteristics obtained experimentally. This highlights the importance of microstructural analysis to apprehend the effectiveness of FA, BOF, and IOT in developing sustainable EGC.
The SEM images confirm the enhanced strength attainment and densification achieved by the addition of BOF slag and IOT as a partial replacement for fine aggregate.
Upon correlating the SEM images with the experimental results, a comprehensive understanding of the relationship between the microstructure and the performance of the EGC is achieved.
This demonstrates the feasibility and effectiveness of FA, BOF slag, and IOT in developing sustainable EGC.
5.9. Heavy Metals Leaching Analysis
Initially, the overall composition of heavy metals (mg/kg) in the materials (FA, BOF slag, and IOT) is determined as detailed in table S6. Subsequently, to determine the intensity of heavy metals leaching from the proposed EGC pastes, the various concentrations of leaching during the tank analysis test (28d) were recorded. The leaching concentration for all the mixes, irrespective of the heavy metal considered decreased distinctly with the increase in the immersion period (Table S7). Notably, the presence of heavy metals was negligible and immeasurable upon reaching 28d. This could be associated with the phenomenon of heavy metals being able to dissolve in the EGC matrix and be expelled into the acidic solution during the initial stages of immersion. Nonetheless, as the period increases, the dissolving and expelling of heavy metals subsides and becomes almost negligible, owing to the dense and homogenous EGC matrix [120].
The soluble levels of heavy metals in the EGC paste samples are recorded as a measure of their respective leaching concentrations for a total duration of 28d. It is evident from the test results (Table S7) that almost all mixes have relatively less or negligible traces than the standard limitations (GSR 801 (E), EPA) [119], considering the environmental emissions in the inland surface water category [121].
Nevertheless, the leaching concentrations for mix 5 and 6 having the highest proportions of BOF slag (up to 50%) corresponding to Cr and Cd were marginally higher than the standard values. Considering the higher limit prescribed by other categories of emissions, the aforementioned deviation is presumed to cause a comparatively lower risk of pollution by the EGC mix involving FA, BOF, and IOT [121–122]. However, prolonged exposure of FA: BOF: IOT based EGC to the various environmental water sources, might initiate contamination corresponding to Cd and Cr.
5.10. Cost Analysis of EGC
The production cost of EGC and other similar cementitious composites available in the market might exhibit substantial variation due to the distinct materials and procedures employed. For EGC, the principal cost is linked with the alkaline activators, which are commonly a combination of sodium hydroxide and sodium silicate [49]. These chemicals are often more expensive than the water used in the hydration process for OPC based composites. However, the usage of industrial by-products like BOF slag, FA, and IOT, which are considered waste resources, can contribute to significant cost savings owing to their abundant availability [50–52]. The cost of BOF slag and IOT might vary based on the region and their grade. Similarly, the price of FA can range from roughly ₹500 to ₹2,000 per ton in India and around $93 per metric ton in the US [53]. The usage of these by-products not only adds to cost savings but also aids in trash management and environmental sustainability [54–56]. On the other hand, OPC involves the hydration of cement, which largely requires water. The price of water may fluctuate based on the geographical area and accessibility, although it is typically more affordable compared to the alkaline solution utilized in EGC [57–59]. However, OPC manufacture is energy-intensive and contributes to large CO2 emissions [58]. For instance, for standard concrete, the cost of OPC concrete is 11% greater than EGC mix of similar strength. This is mostly due to the exploitation of industrial by-products such as BOF slag and FA and the non-availability of coarse aggregate in EGC [60]. Although the initial expenses involved in switching from OPC based composites and concrete to EGC may be more due to factors such as equipment adaptation and employee education, the long-term advantages in terms of cost reduction, environmental effects, and sustainability establishing EGC as a feasible substitute for OPC. The use of industrial wastes like BOF slag, FA, and IOT not only adds to these cost savings but also aids in waste management and environmental sustainability.
Overall, although the initial expenses involved in manufacturing EGC may be high, the long-term advantages in terms of cost reduction from utilizing industrial by-products, environmental impact, and sustainability make EGC a practical substitute for OPC. It should be observed that these expenses can vary considerably based on aspects such as geographical location, accessibility of resources, and particular manufacturing methods.
6. Conclusions
This study aims to investigate the feasibility of using BOF slag and FA as primary precursor materials and IOT as a partial replacement to fine aggregates in the development of steel fibre reinforced Engineered Geopolymer Composites. The following conclusions may be drawn from the performed study:
The findings demonstrate the suitability and potential of BOF slag, FA, and IOT in developing lightweight EGC, supported by XRF, TGA, and DLS analysis. The varied particle sizes can improve particle packing, reactivity, matrix densification, and ultimately, the strength and durability of EGC, promoting sustainable construction.
The experimental results indicate that the EGC-FA: BOF-60:40-MS: IOT-55:45-SF-1.5 mix achieved the highest compressive strength (41.8 MPa), flexural strength (5.78 MPa), high impact resistance (313.92 Nm), and lowest water absorption (1.8%) among the tested variants, demonstrating the potential of steel fiber reinforced EGC in structural applications. Optimum compressive strength in the EGC was achieved with a steel fiber content of 1.5%, while the most influential parameters on compressive strength were found to be IOT, followed by steel fibre and BOF slag replacement.
Flexural resistance results of the EGC mixes demonstrate their strong correlation with the compressive strength data.
The water absorption results of the EGC are well associated with their surface porosity, structural pores, and permeability characteristics. This relationship is essential for optimizing the properties of the proposed sustainable FA: BOF based EGC for various structural applications.
The findings on the impact resistance highlight the evolution of EGC as a prominent material in the construction industry upon a notable shift from conventional geopolymer concrete (GPC). This is primarily associated with the brittle nature and inconsiderable resistance towards cracking in GPC, while the same is effectively addressed by the EGC. The findings of this study on the impact resistance of FA: BOF based EGC mixes further highlight the potential of EGC towards more sustainable and resilient infrastructures.
The SEM images and experimental results correlate well and confirm the strength enhancement and densification of the EGC matrix upon the addition of BOF slag and IOT in partial replacement levels as a binder and fine aggregate respectively. This evaluates the potential of FA, BOF slag, and IOT in developing sustainable EGC.
The leaching concentrations of major heavy metals from the FA: BOF: IOT based EGC were minimal and considered safe. The concentrations of Cr and Cd exceeding the standard emission levels by marginal value are considered to pose a relatively lower risk of contamination for the EGC mixes 5 and 6. Despite the lower probability of instantaneous risk, enduring impacts on the environment need attention.
Overall, the proposed study contributes to the evolution of sustainable building materials by establishing the potential of FA, BOF slag, and IOT. Also, upon optimizing these materials, sustainable EGC mixes, with high compressive and flexural strengths are achieved. Furthermore, the study unveils the feasibility of utilizing these industrial residues in various construction applications, thereby contributing to eco-efficiency.
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