Next Article in Journal
Experimental and Simulation Study on Residual Stress of Pure Copper Welded Joint by Laser Shock Peening
Previous Article in Journal
Optimizing Acoustic Performance of Semi-Dense Asphalt Mixtures Through Energy Dissipation Characterization
Previous Article in Special Issue
The Role of Natural Fibers in the Building Industry—The Perspective of Sustainable Development
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Materials
Volume 18
Issue 17
10.3390/ma18174087
materials-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Development of Multifunctional Slag and Bauxite Residue-Based Geopolymers with Heavyweight Aggregate Enhancement

by
Andrie Harmaji
*,
Reza Jafari
and
Guy Simard
Department of Applied Sciences, University of Québec in Chicoutimi (UQAC), 555, boul. de l’université, Saguenay, QC G7H 2B1, Canada
*
Author to whom correspondence should be addressed.
Materials 2025, 18(17), 4087; https://doi.org/10.3390/ma18174087
Submission received: 12 August 2025 / Revised: 25 August 2025 / Accepted: 27 August 2025 / Published: 1 September 2025
(This article belongs to the Special Issue Advances in Function Geopolymer Materials—Second Edition)
Browse Figures
Versions Notes

Abstract

The growing demand for sustainable and multifunctional construction materials, particularly those capable of addressing durability and energy challenges, has motivated the development of conductive and photothermally active geopolymers. This study investigated the use of an Fe-rich spinel aggregate (FSA) as a high-density filler in geopolymers composed of ground granulated blast furnace slag and bauxite residue, with a fixed addition of 1 wt% graphite (binder-based) to enhance electrical conductivity. The effects of different FSA replacement percentages (0–100%) on compressive strength, electrical conductivity, photothermal efficiency, and chemical resistance were evaluated. An increase in the FSA content translated to an increase in the final compressive strength, with 100% FSA replacement achieving the highest value of 45.5 ± 2.5 MPa at 28 days. As the FSA content increased, the electrical resistivity decreased to as low as 42 Ω·m at 100% replacement. Under simulated solar flux conditions (1 kW/m2), photothermal analysis revealed that the 100% FSA mixtures exhibited the highest surface temperature increase of 9.8 °C after 300 s, indicating their superior thermal responsiveness. Furthermore, acid immersion in 10% HCl for 28 days showed mass gain in all geopolymers, with the highest gain observed at 50% FSA (+11.51%). Similarly, the strength increased after acid exposure up to a 75% FSA content. These findings highlight the multifunctional potential of FSA-enhanced geopolymers for high-mechanical-performance, electrically conductive, photothermally active, and chemically durable materials as multifunctional construction materials.

Graphical Abstract

1. Introduction

The advancement of technology, coupled with the increasing global energy demands, has driven a transition away from fossil fuel-based power sources toward cleaner, low-carbon alternatives, such as nuclear energy. Concurrently, the global use of radiation-emitting devices has increased, raising significant health concerns [1]. Heavyweight concrete is commonly utilized for shielding applications owing to its economical aspect and good mechanical properties [2]. This special type of concrete is made using heavyweight aggregates, which are important in improving radiation-blocking performance owing to their good attenuation capacity against photons and neutrons [3]. The common aggregates used to develop heavyweight concrete are magnetite, hematite, ilmenite, and barite [4]. One application of heavyweight concrete is hazardous waste containment [5]. For this purpose, high acid resistance is essential, as acid leaching can degrade the protective barrier, potentially resulting in contaminant leakage or structural failure. However, one main raw material for creating heavyweight concrete is Portland cement, which has poor resistance to an aggressive acidic environment [6]. Moreover, Portland cement production is a significant source of global carbon emissions, as it is responsible for approximately 7–8% of the total global carbon emissions [7].
Owing to these factors, newer and more eco-friendly construction materials are in high demand. One more promising alternative is geopolymers, which utilize the byproducts of various industries, such as ground granulated blast furnace slag (GGBFS) [8] and bauxite residue [9], as the main raw materials. GGBFS is an industrial byproduct that is commonly reused in cement and concrete production, whereas bauxite residue is still largely regarded as waste and disposed of by various industries [10]. Recent studies have highlighted the anticipated benefits of geopolymers over ordinary Portland cement (OPC) concrete [11]. For example, geopolymers have shown the potential to replace OPC concrete by reducing up to 65% carbon emissions with the same strength grades [12]. Expanding on these benefits, Zhang et al. [13] found that the acid resistance of geopolymers can be improved by incorporating siliceous materials. Furthermore, when exposed to acidic environments, geopolymers have demonstrated notable corrosion resistance compared with OPC-based concrete [14]. However, it has been reported that lightweight geopolymer are not durable in sulfate environments, such as oil-well cementing [15].
Fe-rich spinel aggregate (FSA) is a promising conductive aggregate owing to its multifunctional properties and prevalence in high-specific-gravity fillers for heavyweight concrete applications. This material is composed of naturally occurring spinel-type minerals, which are engineered for enhanced performance, including shielding capacity, conductivity, and thermal performance. FSA is characterized by a dark color and relatively low resistivity due to its metallic iron content [16], in addition to its heat transfer capability. Furthermore, it can be tailored to the needs of specific applications, benefiting structural and functional applications, such as energy storage [17]. Originally developed as a photothermal material, its high capability of light absorption enables a conversion to thermal energy, which broadens its range of potential functionalities [18]. Given the already broad applicability of heavyweight concrete in radiation shielding of research facilities, protective barriers, counterweights for cranes, ballast for docks, and underground structures, the development of an FSA-enhanced alkali-activated material represents a sustainable pathway for advanced functional infrastructure.
However, FSA has not yet been investigated as a partial or complete replacement for fine aggregates in alkali-activated systems. Therefore, this study focused on developing a heavyweight composite based on alkali-activated slag and bauxite residue, incorporating FSA to enhance its functional properties. The properties of the geopolymer were evaluated in terms of compressive strength, electrical conductivity, photothermal response, chemical stability, phase composition using X-ray diffraction (XRD), and microstructural characteristics using scanning electron microscopy (SEM).

2. Materials and Methods

2.1. Materials

2.1.1. Aluminosilicate Sources

The bauxite residue used in this study was provided by the Canadian alumina refinery Rio Tinto in Jonquière, QC, Canada. The material was utilized without any prior processing, such as oven drying or neutralization. Therefore, the appearance resembles clay with a particle size of 10–200 µm and coagulated granularity. GGBFS was produced by Lafarge cement in St. Constant, QC, Canada. The oxide compositions of the bauxite residue and GGBFS were determined using X-ray fluorescence (XRF) spectrometry by Bureau Veritas Laboratories, Saint-Laurent, QC, Canada in Table 1.

2.1.2. Alkali Activator

Sodium hydroxide (NaOH) crystals (77.5% Na2O and 22.5% H2O) were obtained from Laboratoire MAT (Québec City, QC, Canada). It was then mixed with tap water to produce a solution of 16 mol/L. The sodium silicate solution (8.9% Na2O, 28.7% SiO2, and 62.4% H2O) was obtained from Univar Solutions (North York, ON, Canada). During the experiment, a sodium hydroxide solution was added to adjust the SiO2-to-Na2O ratios of the sodium silicate solution. The two solutions were combined, thoroughly stirred, and allowed to cool down for 6 h before mixing with aluminosilicate, in accordance with the method used in a previous study [19]. This method was adopted because the mixing of NaOH and Na2SiO3 solutions can release heat, which can induce problems such as efflorescence in the latter stage and reduced compressive strength. The exothermic reaction when mixing NaOH and Na2SiO3 may release heat that can cause flash set [20]. Some alkali may not fully reacted with aluminosilicate and can react with CO2 instead to form a sodium carbonate phase such as Na2CO3 or NaHCO3 [21], some of the known efflorescence compounds. Therefore, controlling the mixing conditions is essential to ensure uniform geopolymerization and maintain material performance.

2.1.3. Aggregates

Natural sand as a fine aggregate was obtained from a quarry in Saint-Henri-de-Taillon, Canada, to produce Portland cement-based concrete and alkali-activated material (AAM) mortar. The absorption coefficient and fineness modulus of the sand were 0.28% and 2.94, respectively. The materials complied with the Canadian Standards Association (CSA) A23.1-19 [22], “Concrete Materials and Methods of Concrete Construction”, requirements. The fine FSA used to produce heavyweight AAM was provided by Magnor Exploration (Saguenay, QC, Canada). It had an absorption coefficient of 1.68% and a specific gravity of 5.0 g/cm3.

2.1.4. Graphite

In this study, 1 wt% (binder-based) graphite was included to promote the electrical conductivity of the geopolymer while enhancing workability and mechanical performance. The graphite (99% purity) was obtained from Fisher Scientific (Ottawa, ON, Canada). Graphite is an inexpensive and commonly used conductive additive that can facilitate the movement of electrons through the matrix to lower electrical resistivity and optimize performance for applications, including indoor radiant heating [23] and snow-melting concrete [24]. Research has demonstrated the conductivity benefits of 1–4 wt% graphite to improve the conductivity of cementitious materials while not excessively compromising their fresh or hardened properties [25]. Excessive dosages (>1 wt%) of graphite can increase the water demand and induce agglomeration, thus compromising the distribution of the conductive agent and mechanical performance of the slurry [26]. Therefore, 1 wt% graphite was determined to be the most practical amount added to optimize electrical performance and workability while maintaining the structural integrity of the geopolymer.

2.2. Methods

2.2.1. Mix Design Proportions

Six mixture proportions were designed to compare standard concrete and the geopolymer. An air-entrained concrete (AEC) that conformed to the CSA A23.1-19 [22] requirements was prepared as the reference sample (Table 2). The AEC consisted of cement, fine and coarse aggregates, Sika Air 60 air-entraining agent, and Sika Viscocrete (high-range water reducing and superplasticizing) admixture. Air entrainment was selected as it is common in Canada because the infrastructure is exposed to some of the worst freeze–thaw conditions, and the entrained air voids provide the necessary resistance to frost damage due to freezing water [27].
The mix design of bauxite residue and GGBFS-based geopolymers, including the curing method, was designed on the basis of previous research [28]. The fine FSA was used as a partial substitute for the fine aggregate in the amounts of 0%, 25%, 50%, 75%, and 100%. The complete mixture is presented in Table 3. A planetary mixer was used to mix the binder, aggregate, graphite, and alkali activator to form a slurry. The slurry was then cast into the mold, followed by ambient curing at room temperature for 7 and 28 days. Each specimen was covered with a hydrophobic film to prevent moisture loss throughout the curing process.

2.2.2. Slump Flow and Bulk Density

The slump of the mortar was measured immediately after mixing using a mini MBE cone, which evaluates the consistency on the basis of the ratio of the solid surface area to the fluid content [29], as shown in Figure 1. A slump cone made of steel plates was filled with fresh geopolymer mortar. The mortar was then poured out freely as the slump cone was raised vertically. The slump is defined as the height differential between the mold and the fresh mortar, and the bulk density was calculated using the weight measurement of the hardened geopolymer.

2.2.3. Compressive Strength Test

A 50 mm × 50 mm × 50 mm cubical sample was made in accordance with the CSA A23.2-9C [22], “Method of Test for Compressive Strength of Hydraulic Cement Mortars”, requirements. After 7 and 28 days of ambient curing, compressive strength tests were performed on the sample. The compressive tests were carried out using a CONTROLS PILOT PRO-EN Automatic Compression Testers machine in Saguenay, QC, Canada with a loading rate of 0.25 ± 0.05 MPa/s that was maintained until the failure of the specimen, with the failure point being the maximum load that the test specimen could withstand before collapsing or fracturing. The mean values were used as representative strengths.

2.2.4. Electrical Property Tests

To measure the electrical properties, the heavyweight geopolymer specimen dimensions were maintained at 50 mm × 50 mm × 50 mm, consistent with the same dimensions used for the compression tests. To ensure reliable contact with the electrodes, the surface was polished with 1000-grit silicon carbide and coated with carbon black powder (≥99% purity; Fisher Scientific, Ottawa, ON, Canada). The two-probe method was used to measure the resistivity of the specimens, as it is a relatively easy way to measure the electrical properties of materials and has been widely used [30]. The specimen was enclosed in a Faraday cage to reduce the effects of external electromagnetic interference. DC conductivity was determined using an HIOKI P5335 Power Meter programmable voltage source (Figure 2).
The samples were subjected to an applied voltage of 0 to 150 V in 30 V steps. The current (I) and voltage (V) were then recorded. The heat flux (Q, in W/m2) was calculated using Ohm’s Law, as in Equation (1):
Q =   V   × I A

2.2.5. Photothermal Performance

To investigate the photothermal conversion performance of the heavyweight geopolymer, the 50 mm × 50 mm × 50 mm cubical samples were exposed to a sunlight simulator (Figure 3). The temperature change was measured at the following points after exposure to a photothermal flux under one solar light intensity (1 KW/m2): 0, 60, 120, 180, 240, and 300 s. This value was adopted in line with previous studies [31] on the photothermal performance of geopolymers.

2.2.6. Chemical Resistance

To determine the degrading behavior of the samples in a solution of acids to simulate a harsh real-life environment, geopolymer samples with similar dimensions for compressive strength (50 mm mortar cubes) were utilized for each mix. The specimens were cast, coated with plastic, and stored at room temperature for a day. Afterward, the specimens were left at ambient temperature for the remaining 24 days before testing for acid resistance. The specimens were initially immersed in 10% hydrochloric acid (HCl) solutions (2.87 mol/L) with a pH of 1  ±  0.1 to simulate intense acid environments, such as the food industry, mining, sewer pipes, and nuclear power plants [32]. This concentration was selected based on previous geopolymer studies where 10% HCl has been employed to evaluate durability [33]. The resistance of the specimens to acid attack was measured in terms of mass and compressive strength loss after a soaking time of 28 days. The mass of the specimens before immersion in the acid solution was measured as the initial mass (mi). After the immersion at 28 days of age, the samples were rinsed with tap water and wiped down using a damp towel. The mass of the samples was recorded as the residual mass (mr). The mass loss percentage due to acid immersion was calculated using Equation (2):
M a s s   l o s s =   m r − m i m i   × 100

2.2.7. XRD Characterization

The chemical behaviors of the AEC and geopolymer were examined using XRD (Rigaku Smartlab, Tokyo, Japan). The analysis was performed with scanning angle from 5° to 80° (2θ) at a rate of 2.5°/min over a total acquisition time of 30 min. The characterization was conducted to 28-day samples of raw materials and hardened samples (AEC and geopolymers) after 28 days. To fit the sample holder, the materials were powdered with a mortar and pestle. Figure 4 reveals the diffractogram of FSA, which is predominantly composed of hercynite (FeAl2O4) with lower amounts of magnetite (Fe3O4) and ilmenite (FeTiO3). The dominance of hercynite with its spinel (AB2O4) structure, together with magnetite, justifies referring to the material as an FSA.
In Figure 5, the bauxite residue indicates the formation of boehmite (Al2O3H2O), gibbsite (Al(OH)3), goethite (FeO(OH)), anatase (TiO2), pseudorutile (Fe2Ti3O9), quartz (SiO2), and hematite (Fe2O3) as the main crystalline phases. Subsequently, the GGBFS is mainly composed of quartz, calcium silicate (Ca2SiO4), and jadeite (NaAlSi2O6). Based on the XRD diffractogram, the GGBFS exhibits a predominantly amorphous phase, indicating its high reactivity and suitability as a raw material for geopolymer synthesis.

2.2.8. SEM Characterization

The morphologies of the AEC and geopolymer were investigated using a scanning electron microscope (SEM; JEOL JSM-7800 F, Tokyo, Japan). SEM observations were performed in high-vacuum mode at a 12 mm working distance, and images were captured at 1000× magnifications to examine the change before and after chemical exposure. For the microstructural analysis, fragments obtained from the compressive strength test were collected, and their cross-sections were prepared for SEM examination.

3. Results and Discussion

3.1. Slump Flow and Bulk Density of Fresh Mortars

Slump flow is a good indicator of the workability and other features of fresh and hardened mortars [34]. The AEC and geopolymer flowability data are displayed in Figure 6. The results show an increasing flow spread with increasing replacement of FSA. The mixture fluidity of FSA0 was measured at 50 mm. As the aggregate replacement increased to 25%, 50%, 75%, and 100%, the slump flow values were recorded at 70, 100, 120, and 140 mm, respectively. The flow spread of 50% FSA replacement was 100 mm, which is well within the range of the properties that resemble AEC mixes.
The increasing flow spread with increased magnetite replacement can be explained by the higher density and lower water absorption of magnetite than those of sand [35,36]. Less water adsorption and a greater density mean less overall surface area and reduced internal friction between the solid particles in the mixture, which translates into enhanced flowability. In addition, the increased density of magnetite allowed the mix to spread out more freely than a sand-only mix under the force of gravity, thereby improving fresh-state performance. This result is in contrast to other research showing that magnetite aggregates decrease the slump of a fly ash, GGBFS, and silica fume-based geopolymer [37].
The bulk density of the geopolymers was measured at different magnetite replacement levels to evaluate the impact of high-density aggregate substitution, as displayed in Figure 7. The results show a significant increase in density as the proportion of FSA increased.
The mix without FSA showed a density of 2216 ± 4.5 kg/m3, whereas replacing 25%, 50%, 75%, and 100% of sand with the heavyweight aggregate resulted in densities of 2543 ± 4.5, 2798 ± 5.5, 2811 ± 2.5, and 3188 ± 1.9 kg/m3, respectively. These results are in line with other research, which revealed that the use of magnetite as a coarse aggregate replacement in a fly ash and GGBFS geopolymer increased its bulk density [37]. In contrast, a typical AEC has a considerably lower mass per unit volume of 1683 ± 3.3 kg/m3, highlighting the substantial mass increase due to the FSA content. The increase in such physical properties can be attributed to the high specific gravity of the FSA, which contains dense mineral phases of magnetite, ilmenite, and hercynite compared with quartz-based sand (approximately 2.65 g/cm3) [38]. As the content increases, the overall particle packing efficiency improves, reducing internal voids and increasing mass per unit volume. The density increase is more pronounced from 0% to 50% FSA but stabilizes between 75% and 100% replacement, suggesting a saturation effect on the particle matrix. This could be due to the limitations in further compacting the matrix, as excess FSA may not significantly alter the internal structure of the mix beyond a certain threshold. The high weight concentration of the FSA-enhanced geopolymer makes it suitable for applications that require enhanced thermal mass, radiation shielding, and improved mechanical stability, while its workability trends must be carefully managed to maintain appropriate construction performance [39]. In addition, the increased density may influence transportation, handling, and structural load considerations, necessitating careful planning in practical applications.

3.2. Compressive Strength

The 7- and 28-day compressive strengths of all mixtures in Table 2 and Table 3 are presented in Figure 8. In Portland cement-based mixes, the reference sample has 7- and 28-day strengths of 19.6 ± 2.6 and 34.4 ± 0.1 MPa, respectively. The hydration of the cement in the concrete mix causes the formation of calcium–silicate–hydrate (C-S-H) gel, which accounts for the increase in mechanical properties with age [40].
The mechanical performance of the geopolymer samples improved progressively with the higher FSA content used as a fine aggregate substitute in ambient curing. This method was selected to simulate field conditions and evaluate the practicality of FSA geopolymers without energy-intensive curing. The mix with a 0% FSA content had a compressive strength of 25.2 ± 2.6 MPa at 7 days and increased to 30.1 ± 0.4 MPa at 28 days. In the presence of 25% FSA, the mechanical properties increased slightly to 27.2 ± 0.1 MPa at 7 days and 31.09 ± 0.1 MPa at 28 days, indicating a negligible increase in early-age strength attributed to the increased density and compactness of the mix. Substituting the fine aggregate with 50% FSA had a much better result of a compressive strength of 29.6 ± 0.1 MPa at 7 days and 36.8 ± 1.9 MPa at 28 days, which can be explained by the densification effect of the aggregate replacement particles improving the packing and developing a more refined pore structure, with lower voids leading to higher mechanical properties [41]. Moreover, the very high Fe2O3 content of the FSA may also lead to secondary reaction products that strengthen the interfacial transition zone (ITZ), thus decreasing micro-cracking and increasing the bond between the aggregate and geopolymer gel.
A notable improvement in strength was attained at 75% replacement with FSA, with values reaching 35.1 ± 0.1 MPa at 7 days and 43.1 ± 1.1 MPa at 28 days. The increase in mechanical performance was the largest among all the mixtures in this study. This enhancement may be attributed to the higher specific gravity of the FSA, allowing for more interlocking between aggregates and the binder, thereby increasing load transfer mechanisms [37]. In addition, replacing the aggregate can introduce pozzolanic and thermal effects that may enhance long-term strength by modifying the reaction kinetics of the alkali activation process [42]. At full sand replacement (100%), the mix exhibited a 7-day strength of 32.8 ± 0.2 MPa, which was slightly below the 75% FSA mix, but it achieved the highest 28-day strength of 45.6 ± 2.5 MPa. Although early-age development is somewhat impaired, long-term curing improves the alkali activation process and the development of mechanical properties [43]. However, adding too much FSA could modify porosity and alter binder–aggregate interactions in ways that affect the microstructure of the matrix and the rate of strength gain [44]. While slower strength development and carbonation are possible in ambient curing, compressive strengths above 40 MPa were still achieved at 28 days. In previous studies [45], the incorporation of magnetite as additives into metakaolin-based geopolymers was found to reduce compressive strength as its content increased, which was attributed to a decrease in the dissolution reaction order, indicating the participation of alternative reaction mechanisms distinct from the typical MK dissolution kinetics.
Along with strength, the stiffness of the material, usually described by Young’s modulus, is also an important parameter for having a deeper understanding of the behavior and characteristics of a geopolymer [46]. Although Young’s modulus experimental data were not collected in this study, it is reasonable to assume that the variability in stiffness would follow a similar variation to that of compressive strength since matrix densification and interaction with the aggregate affect both properties of the geopolymer.

3.3. Electrical Performance

The electrical resistivity of the geopolymer varied substantially through the partial replacement of sand with FSA (Figure 9). The observed conductivity results represent a synergistic effect of FSA and graphite.
The AEC mix is not included in the figure because its resistivity was too high to measure accurately within the test range, exceeding 36,000 Ω·m at 30 V. Among the geopolymer samples, the FSA0 mix had the lowest conductivity, which is usually exhibited by conventional geopolymers [47,48]. As the FSA content and applied voltage increased, a significant decrease in resistivity was observed. This change is most likely attributed to the inclusion of FSA, which provides more conductive pathways than insulating sand [49]. The greatest decrease in electrical resistance occurred in FSA100, which ranged from 42 to 57 Ω m. This trend indicates that FSA can aid in charge flow through the matrix of the concrete, possibly by improving particle-to-particle connectivity and reducing impedance [50]. A previous study supports these findings, where the semi-conductivity associated with magnetite-based aggregates translated to improved conductivity of cementitious composites via more interconnected conductive networks [51]. Percolation thresholds have been established for electrically conductive fillers [52]. In FSA-based systems, this is likely governed by the combined effects of electron hopping mechanisms and percolation theory, which also explains the nonlinear increase at higher replacement [53].
An Fe-rich aggregate-based cementitious material has prospective uses in conductive infrastructure applications, such as snow-melting pavements [54] and self-heating concrete [55]. It has been estimated that a surface heat flux between 300 and 550 W/m2 is needed to effectively melt snow and ice in cold climates [56,57]. The measured current, voltage, and surface area of the geopolymer specimens were used as inputs for heat flux calculations (Figure 10).
The calculation in Figure 10 indicated that the FSA0 mix did not achieve sufficient conductivity even at 90 V to meet the threshold required for snow-melting applications. This voltage requirement decreased when FSA was used to substitute the sand, with a power value higher than 300 W/m2 achieved at 90 V (FSA25), 60 V (FSA50 and FSA75), and 30 V (FSA100). A power density of 337 W/m2 was achieved by the latter using just 30 V. The results highlight the benefits of using FSA for the mix, as an increase in the aggregate replacement content tends to lower the voltage requirement for the geopolymer to achieve the power needed for snow-melting applications.
Although these results show encouraging heating performance at the laboratory scale, careful consideration of long-term durability through repeated freeze–thaw and heating–cooling cycles will be needed for real-world deployment, as thermal cycling may develop microcracking and change conductivity pathways [58]. It will equally be necessary to optimize the trade-off between energy input and heat output for energy efficiency, since rising applied voltages increase the heat of concrete [59]. Therefore, field-scale testing with life-cycle energy and cost assessments will be needed to determine the practicality of these applications in real-world contexts. Furthermore, while the two-probe DC method may introduce contact resistance, it was chosen for its simplicity and relevance to bulk electrical performance. Future work should employ four-probe or impedance spectroscopy for more precise characterization.

3.4. Photothermal Conversion Performance

A photothermal experiment was performed on geopolymers formulated with varying proportions of fine aggregate and FSA (Table 3). The temperature change was measured after exposure to a solar irradiance of 1 kW/m2. The difference in temperature (ΔT) is visually presented in Figure 11 and summarized in Table 4.
The AEC mix showed the smallest temperature increase, reaching only 28.1 °C in 300 s. This limited temperature increase can probably be ascribed to the high sand content. Sand, a poor conductor of heat, also has a moderate heat capacity; hence, it resists the transfer of heat from its surface into the bulk and its storage [60]. As heat gets trapped within the material or absorbed very slowly, the temperature will increase rather slowly compared with how it occurs in mixtures containing greater amounts of FSA. This characteristic makes sand-rich mixtures potentially more suitable for applications where thermal insulation is required to reduce heat flow and maintain stable internal temperatures. Intermediate blends with 25–75 wt% FSA showed a moderate increase in temperature, reaching 31.3 °C at 300 s. These mixes depend on the thermal insulating capability of sand rather than the higher thermal conductivity of the FSA components, such as magnetite, to give an intermediate thermal response. Hence, the implication is that by varying the FSA content, the thermal behavior of the geopolymer can be adjusted. These intermediate compositions may be advantageous in applications where moderate heat absorption is beneficial, such as energy-efficient building materials that help maintain stable indoor temperatures without excessive heat gain.
In contrast, the mix composed of 100% FSA had the highest photothermal response, with a surface temperature increase of +9.8 °C after 300 s as compared to its initial temperature. This increase is considerably greater than that of the control AEC mix, indicating that FSA plays a significant role in improving solar energy absorption. Temperature increases of similar magnitudes have been reported based on conductive or darker colored cementitious composites that include magnetite or carbonaceous fillers [61,62]. The temperature of the material appeared to reach approximately 26.2 °C within the first 60 s and then continued heating steadily, suggesting the potential utility of this mix for systems that require thermal mass or a thermal storage capacity, such as passive solar heating.

3.5. Changes in the Geopolymer Mass and Strength Under Acid Attack

The mass change comparison after 28 days of 10% HCl immersion between AEC and geopolymer incorporating varying levels of FSA (0–100%) as a sand replacement is shown in Figure 12. AEC had a 3.75% mass loss, whereas all the geopolymer mixes showed a mass gain, with the highest increase observed for the 50% FSA replacement.
The weight reduction in AEC reflects its vulnerability to acid attack, primarily due to the breakdown of C-S-H into gypsum and silica hydroxide [63]. Acid exposure leads to the leaching of Ca2+ ions, which in turn leads to the production of soluble salts and decalcification of the matrix [64], thereby contributing to material deterioration and a loss of material. In contrast, all the geopolymer samples exhibited an increase in weight, with the highest gain (11.51%) at 50% replacement of the aggregate. Wang et al. [65] reported a similar gain in mass phenomena by immersing a fly ash-based geopolymer in a 5% sulfuric acid (H2SO4) solution for 56 days, which resulted in a weight gain of 0.8%. The FSA inclusion provides more iron sites that can be exposed for a surface reaction with H+ and Cl− ions to form insoluble products, such as FeCl2, FeCl3, and iron oxide films, which could precipitate in pores and surfaces [66]. Although bauxite residue contains iron oxides, the iron oxides primarily exist in stable phases, such as hematite, which are less reactive than the spinel minerals (hercynite, magnetite, and ilmenite) in the FSA. The result is that the FSA0 mix produces none of the protective pore-filling or passivation effects exhibited in acidic environments. Consequently, the FSA0 mix did not exhibit the same protective pore-filling or passivation effects under acidic conditions. The reaction products formed by the reactive phases of the FSA likely account for the observed weight gain and reduced porosity, as insoluble iron compounds and hydroxides of titanium and aluminum precipitate in pores. This secondary phase formation can refine the microstructure and improve resistance to acid penetration.
To further verify the behavior of the AEC and geopolymer samples subjected to a 10% HCl solution for 28 days, additional strength testing was performed. The average compressive strength results are shown in Figure 13. The AEC and 100% FSA mixes lost strength, whereas partial replacement with 0–75% FSA increased the compressive strength.
The strength reduction in the AEC can be attributed to the acid-induced leaching of calcium phases, which led to structural weakening by increasing porosity [67]. Conversely, geopolymers are more resistant to acid due to their lower calcium content and more chemically stable aluminosilicate gel, which has the tendency to be resistant to leaching in acidic media [68]. The geopolymer samples with lower-to-medium FSA contents (0–75%) showed strength gains after exposure to HCl, with the 0% FSA sample having the highest increase (22.37%). This is perhaps due to continued densification or alkali activation via secondary gel formation upon exposure to a low pH environment. The latter was confirmed by Park et al. [69], who used phosphoric acid (H3PO4) as the main activator in a metakaolin-based geopolymer. The strength gains decreased with higher FSA contents, likely due to the reduced SiO2 available from sand and the increased number of acid-reactive sites, which may compromise the matrix integrity. These intermediate mixes maintain a balance between reactive binder phases and aggregate stability, possibly enabling further alkali activation in acidic media, which improves strength over time [70].
On the other hand, a strength loss of 21.7% was recorded for a 100% FSA replacement, despite the mass gain. This suggests that beyond a certain FSA threshold, acid penetration can compromise the structure instead of reinforcing it. The modified aluminosilicate gel in FSA100 after chloride attack can lead to lower chemical resistance, possibly due to decalcification and the formation of additional mineral products [71], which cannot be overcome through the limited densification of iron-based products. The reduction in strength at 100% FSA loading is not entirely due to a lack of matrix cohesion, and replacing siliceous sand with the Fe-rich spinel aggregate may not only reduce silicon availability for geopolymerization and lower the Si/Al ratio but also result in the limited formation of a continuous aluminosilicate gel network.

3.6. XRD Analysis

The XRD patterns of AEC after 28 days of oven curing (Figure 14) revealed the predominant presence of alite (C3S), belite (C2S), calcite (CaCO3), and gypsum (CaSO4.2H2O), alongside with minor trace amounts of quartz (SiO2), a trend consistent with previous reports [72]. The presence of gypsum could increase the porosity of concrete and makes it susceptible to microcrack propagation [73].
The diffractogram of the geopolymers is shown in Figure 15. It is evident that Portland cement and geopolymers exhibit distinctive reaction products. The patterns of the mixtures indicate the possible presence of zeolitic phases, which may form during the alkali activation process.
The XRD analysis revealed that the geopolymers with different FSA replacement percentages were predominantly amorphous in nature, which is attributed to the low intensity of the ordered mineral structure. On the contrary, FSA0 exhibited clearly defined aluminosilicate gels, such as gismondine (CaAl2Si2O8.4H2O), laumontite (CaAl2Si4O12.4H2O), and ussingite (Na2AlSi3O8OH), along with the presence of quartz from the raw material and sand aggregate. The presence of aluminosilicate likely contributes to enhanced acid resistance and structural stability [74]. These phases can buffer acid diffusion through ion exchange mechanisms and physical densification, reducing permeability and pore connectivity [75]. The formation of gismondine, laumontite, and ussingite indicates the partial zeolitization of the geopolymer matrix. These occurrences typically form when excess aluminosilicate species reorganize themselves into a more thermodynamically stable three-dimensional network structure under alkaline conditions [76]. Importantly, the occurrence of these secondary crystalline phases aligns with the observed strength development in FSA-rich mixes, suggesting that they not only reflect the chemical evolution of the system but could also stabilize the matrix microstructure and enhance its mechanical performance over time.
The increase in the FSA content induced a parallel increase in the overall crystalline phases due to the noticeable appearance of the magnetite (Fe3O4), NaFeO3, and esseneite (CaFeAlSiO6) phases. The latter indicates partial crystallization of the Al-Si network gels into ferrosialate (-Fe-O-Si-O-Al-O-) phases [77], which implies effective geopolymerization and better chemical stability. This is likely to contribute to both mass and strength retention or a gain in geopolymers with an FSA content of up to 75% sand replacement. Fe-rich aluminosilicate phases have also been reported to have superior durability in sulfate exposure [78]. The unusual NaFeO3 formation that is a sodium–iron phases have been reported in the literature, for instance when lateritic aluminosilicate was activated with phosphoric acid and subsequently calcined [79].
On the other hand, the FSA100 sample showed a decline in mechanical performance (approximately 21.7%), despite gaining mass. This can be attributed to the lack of a silica phase, which is crucial for durable alumino-silicate gel formation. Excess FSA might disrupt the formation of a continuous matrix network while also creating more chemically reactive Fe-based sites that could be less stable under acid attack. Moreover, high crystallinity could reduce the flexibility of the matrix to accommodate chemical interactions [80], thereby lowering mechanical performance. In contrast, the AEC showed a 3.75% mass loss and 17.75% strength reduction after immersion in 10% HCl, likely due to the leaching of calcium-rich hydration products (e.g., portlandite and C-S-H) that are vulnerable in acidic environments [81]. The absence of a stable Al-Si network and the dominance of acid-sensitive portlandite and C-S-H gels in OPC-based concrete result in greater susceptibility to chemical degradation. The acid resistance of geopolymers is superior to cement owing to their high alkali content [82]. Meanwhile, the acid resistance of conventional concrete relies heavily on its protective layer [83].

3.7. Microstructural Changes in the Geopolymer Samples Under Acid Attack Through SEM Analysis

The SEM images of the conventional concrete and alkali-activated matrix taken before and after acid immersion are shown in Figure 16. The images taken before exposure displayed a dense network of hydration products in the AEC and the aluminosilicate gel framework of the geopolymer. The microstructure showed clear changes after acid exposure, indicating material degradation. The needle-like ettringite crystals that formed during cement hydration were still observed in the AEC mix even after acid immersion, indicating incomplete decomposition. Their persistence may contribute to structural deterioration and mass loss under acidic conditions. This phase is unstable and decomposes when the pH value decreases more than 11.5–12, which was the case upon acid exposure [84], leading to material degradation.
Meanwhile, the SEM images of the geopolymer present considerable differences in morphologies with different FSA contents. Aluminosilicate-based matrix exhibits more acid resistance than normal OPC hydration products [85]. The geopolymer samples with FSA contents of up to 75% exhibited the formation of bubble-like particles. Soaking the Fe-rich material prepared from bauxite residue and the heavyweight aggregate caused the leaching of Fe2+ [86]. This was followed by a reaction with Cl- ions to develop FeCl3 [87], which could result in a denser framework by sealing the microcracks in the geopolymers prior to acid exposure. Moreover, the incorporation of iron-rich aggregates promotes a finer pore structure, as the FSA has reduced the connectivity of large pores in the mix, which limits the area of concentrated stress under a load. Thus, the stronger mechanical properties observed can be ascribed to a combination of these microstructural and chemical factors, rather than only the densification effect. However, FSA100 exhibited hairline cracks, which also likely to trigger crack propagation and increased porosity at augmented FSA contents, which could makes the material more prone to acid penetration and deterioration [88]. This highlights the importance of optimizing the amount of the heavyweight aggregate to balance the functional benefits with structural durability.

4. Conclusions

This study investigated the durability, microstructural evolution, and multifunctional performance of a GGBFS–bauxite residue-based geopolymer incorporating FSA as a partial-to-full replacement for silica sand. The results demonstrate that an intermediate amount of heavyweight filler replacement achieved the most favorable balance between early and long-term compressive strength and microstructural integrity. At these levels, the coexistence of reactive aluminosilicate phases and heavyweight aggregates contributed to densification and strength development.
The FSA-enhanced geopolymer exhibited promising electrical performance by significantly reducing resistivity with an augmented FSA content, reaching as low as 42 Ω·m at 100% sand replacement, supporting the prospect of using FSA-enhanced geopolymers in electrically conductive infrastructure. The photothermal performance assessment further highlighted the role of Fe-rich phases in enhancing the thermal response of geopolymers. Higher FSA contents enhanced solar heat absorption and retention under simulated sunlight, enabling tailored thermal behavior.
Acid exposure testing revealed that in addition to functional properties, the geopolymers exhibited outstanding acid resistance at an FSA content up 75%, where the mixtures in fact gained mass rather than losing it, differing from the conventional AEC, which experienced material degradation. The weight increase in the alkali-activated system was most likely due to the acid reaction, FSA surface reactions forming passive iron compounds, and ionic bonding, which help in densification. These beneficial effects were observed only within an optimal range of heavyweight fillers. The XRD analysis confirmed the formation of aluminosilicate crystalline phases, such as gismondine, laumontite, and ussingite, in the geopolymers, which are presumably responsible for strength retention and chemical resistance to degradation. However, while 100% FSA replacement provided optimal electrical and thermal performance, it also showed reduced acid resistance. This highlights the need to balance the FSA content to achieve multifunctionality without compromising long-term structural stability.
The use of the bauxite residue and GGBFS not only provides functional and durability advantages but also provides significant sustainability benefits. By referring to sustainable waste material, this approach reduces disposal issues and simultaneously lowers the reliance on Portland cement. This research focused on the engineering properties of the waste-derived materials, but a full life cycle assessment is beyond the scope of this study. However, the combined benefits of waste valorization and avoiding emissions related to clinker production indicated that FSA-enhanced geopolymers may be considered among the alternative solutions for sustainable construction. Overall, this study validates the potential of these systems as multifunctional construction materials. These composites provide a sustainable pathway for advanced infrastructure applications, such as snow-melting pavements and self-heating concrete. Future research should therefore include a direct measurement of Young’s modulus to provide a more comprehensive understanding of the mechanical performance of FSA-enhanced geopolymers, examine their long-term performance under real environmental conditions, and investigate scalability for field applications to further unlock their infrastructure readiness and environmental benefits.

Author Contributions

A.H., R.J. and G.S. contributed to the conceptualization of the project. A.H. curated the data and carried out the visualization. Formal analysis and methodology were conducted by A.H. and R.J. The investigation was carried out by A.H. and R.J. Project administration, funding acquisition, resources, and supervision were provided by R.J. and G.S. Validation of the results was conducted by A.H. and R.J. A.H. wrote the initial draft of the manuscript, while R.J. reviewed and edited the final version. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Acknowledgments

We thank Rio Tinto, Centre Universitaire de Recherche sur l’Aluminium (CurAl) UQAC, Centre de recherche sur l’aluminium (REGAL), Chaire institutionnelle de recherche en matériaux antigivre innovants (CiMAGI) UQAC, PSDRC 2024–2025 Volet 5—Soutien à la publication des travaux de recherche et de création UQAC, and FUQAC for the support for this research. We gratefully acknowledge our colleagues Mohammad Bakhtiari and Alhassane Sylla for their support with the photothermal characterization and electrical tests, respectively.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
FSAFe-rich Spinel Aggregate
GGBFSGround Granulated Blast Furnace Slag
AECAir-entrained Concrete
OPCOrdinary Portland Cement
XRDX-ray Diffraction
SEMScanning Electron Microscopy
XRFX-ray Fluorescence
CSACanadian Standards Association
CSHCalcium–Silicate–Hydrate
HClHydrochloric Acid

References

  1. Miller, A.B.; Sears, M.E.; Morgan, L.L.; Davis, D.L.; Hardell, L.; Oremus, M.; Soskolne, C.L. Risks to Health and Well-Being from Radio-Frequency Radiation Emitted by Cell Phones and Other Wireless Devices. Front. Public Health 2019, 7, 223. [Google Scholar] [CrossRef] [PubMed]
  2. Onaizi, A.M.; Amran, M.; Tang, W.; Betoush, N.; Alhassan, M.; Rashid, R.S.M.; Yasin, M.F.; Bayagoob, K.H.; Onaizi, S.A. Radiation-shielding concrete: A review of materials, performance, and the impact of radiation on concrete properties. J. Build. Eng. 2024, 97, 110800. [Google Scholar] [CrossRef]
  3. Ban, C.C.; Khalaf, M.A.; Ramli, M.; Ahmed, N.M.; Ahmad, M.S.; Ali, A.M.A.; Dawood, E.T.; Ameri, F. Modern heavyweight concrete shielding: Principles, industrial applications and future challenges; review. J. Build. Eng. 2021, 39, 102290. [Google Scholar] [CrossRef]
  4. Khalaf, M.A.; Ban, C.C.; Ramli, M. The constituents, properties and application of heavyweight concrete: A review. Constr. Build. Mater. 2019, 215, 73–89. [Google Scholar] [CrossRef]
  5. El-Sayed, T.A. Performance of heavy weight concrete incorporating recycled rice straw ash as radiation shielding material. Prog. Nucl. Energy 2021, 135, 103693. [Google Scholar] [CrossRef]
  6. Damion, T.; Chaunsali, P. Evaluating acid resistance of Portland cement, calcium aluminate cement, and calcium sulfoaluminate based cement using acid neutralisation. Cem. Concr. Res. 2022, 162, 107000. [Google Scholar] [CrossRef]
  7. Belaïd, F. How does concrete and cement industry transformation contribute to mitigating climate change challenges? Resour. Conserv. Recycl. Adv. 2022, 15, 200084. [Google Scholar] [CrossRef]
  8. Li, L.; Xie, J.; Zhang, B.; Feng, Y.; Yang, J. A state-of-the-art review on the setting behaviours of ground granulated blast furnace slag- and metakaolin-based alkali-activated materials. Constr. Build. Mater. 2023, 368, 130389. [Google Scholar] [CrossRef]
  9. Hertel, T.; Pontikes, Y. Geopolymers, inorganic polymers, alkali-activated materials and hybrid binders from bauxite residue (red mud)–Putting things in perspective. J. Clean. Prod. 2020, 258, 120610. [Google Scholar] [CrossRef]
  10. Harmaji, A.; Jafari, R.; Simard, G. Valorization of Residue from Aluminum Industries: A Review. Materials 2024, 17, 5152. [Google Scholar] [CrossRef]
  11. Awoyera, P.; Adesina, A. A critical review on application of alkali activated slag as a sustainable composite binder. Case Stud. Constr. Mater. 2019, 11, e00268. [Google Scholar] [CrossRef]
  12. Neupane, K. Evaluation of environmental sustainability of one-part geopolymer binder concrete. Clean. Mater. 2022, 6, 100138. [Google Scholar] [CrossRef]
  13. Zhang, W.; Yao, X.; Yang, T.; Liu, C.; Zhang, Z. Increasing mechanical strength and acid resistance of geopolymers by incorporating different siliceous materials. Constr. Build. Mater. 2018, 175, 411–421. [Google Scholar] [CrossRef]
  14. Rawat, R.; Pasla, D. Evaluating the corrosion resistance and LCA of lightweight geopolymer concrete integrating sintered fly ash aggregate as coarse aggregate. Constr. Build. Mater. 2024, 449, 138370. [Google Scholar] [CrossRef]
  15. Adjei, S.; Abdelaal, A.; Elkatatny, S.; Abdelfattah, A.M. Durability of lightweight oil-well geopolymer system in sulfate environment. J. Pet. Explor. Prod. Technol. 2023, 13, 439–448. [Google Scholar] [CrossRef]
  16. Esteves, D.S.; Melo, A.; Alves, S.; Durães, N.; Paiva, M.C.; Sequeiros, E.W. Magnetic Field-Assisted Orientation and Positioning of Magnetite for Flexible and Electrically Conductive Sensors. Micromachines 2025, 16, 68. [Google Scholar] [CrossRef]
  17. Tetuko, A.P.; Sebayang, A.M.S.; Fachredzy, A.; Setiadi, E.A.; Asri, N.S.; Sari, A.Y.; Purnomo, F.; Muslih, C.; Fajrin, M.A.; Sebayang, P. Encapsulation of paraffin-magnetite, paraffin, and polyethylene glycol in concretes as thermal energy storage. J. Energy Storage 2023, 68, 107684. [Google Scholar] [CrossRef]
  18. Alexandrovskaya, Y.M.; Pavley, Y.R.; Grigoriev, Y.V.; Grebenev, V.V.; Shatalova, T.B.; Obrezkova, M.V. Thermal behavior of magnetite nanoparticles with various coatings in the range 30–1000 °C. Thermochim. Acta 2022, 708, 179120. [Google Scholar] [CrossRef]
  19. Harmaji, A.; Kirana, M.C.; Jafari, R. Machine Learning to Predict Workability and Compressive Strength of Low- and High-Calcium Fly Ash–Based Geopolymers. Crystals 2024, 14, 830. [Google Scholar] [CrossRef]
  20. Kwek, S.Y.; Awang, H.; Cheah, C.B. Influence of Liquid-to-Solid and Alkaline Activator (Sodium Silicate to Sodium Hydroxide) Ratios on Fresh and Hardened Properties of Alkali-Activated Palm Oil Fuel Ash Geopolymer. Materials 2021, 14, 4253. [Google Scholar] [CrossRef]
  21. Harmaji, A.; Jafari, R.; Simard, G. Durable bauxite-residue geopolymers: Enhancing scaling resistance with fly ash and waste glass powder. Constr. Build. Mater. 2025, 491, 142661. [Google Scholar] [CrossRef]
  22. CSA A23; Concrete Materials and Methods of Concrete Construction/Test Methods and Standard Practices for Concrete. CSA Group: Toronto, ON, Canada, 2024. Available online: https://www.csagroup.org/store/product/2701210/?srsltid=AfmBOooGNMxnZylys79gZooMEt02t7qQ3FD9nV2hkaK6F2eCxiyB9PI9 (accessed on 26 August 2025).
  23. Mingqing, S.; Xinying, M.; Xiaoying, W.; Zuofu, H.; Zhuoqiu, L. Experimental studies on the indoor electrical floor heating system with carbon black mortar slabs. Energy Build. 2008, 40, 1094–1100. [Google Scholar] [CrossRef]
  24. Li, C.; Ge, H.; Daquan, S.; Xuhong, Z. Novel conductive wearing course using a graphite, carbon fiber, and epoxy resin mixture for active de-icing of asphalt concrete pavement. Mater. Struct. 2021, 54, 48. [Google Scholar] [CrossRef]
  25. Frąc, M.; Szołdra, P.; Pichór, W. Smart Graphite-Cement Composites with Low Percolation Threshold. Materials 2022, 15, 2770. [Google Scholar] [CrossRef]
  26. Luo, T.; Wang, Q. Effects of Graphite on Electrically Conductive Cementitious Composite Properties: A Review. Materials 2021, 14, 4798. [Google Scholar] [CrossRef]
  27. Gao, J.; Huo, Y.; Liu, Y.; Yang, Y. Absorbing effect of air entraining agent on early frost expansion of mortar: Calculation model and application. Constr. Build. Mater. 2024, 415, 135100. [Google Scholar] [CrossRef]
  28. Harmaji, A.; Ekaputri, J.J. The Application of Microbes to the Fly Ash-Based Alkali-Activated Material Performance Containing Slag; Springer Nature: Singapore, 2023. [Google Scholar]
  29. Zhao, Z.; Remond, S.; Damidot, D.; Xu, W. Influence of fine recycled concrete aggregates on the properties of mortars. Constr. Build. Mater. 2015, 81, 179–186. [Google Scholar] [CrossRef]
  30. Anis, M.; Abdel-Raheem, M. A Review of Electrically Conductive Cement Concrete Pavement for Sustainable Snow-Removal and Deicing: Road Safety in Cold Regions. Transp. Res. Rec. 2024, 2678, 50–71. [Google Scholar] [CrossRef]
  31. Pan, D.; Deng, X.; Ge, Y.; He, Y.; Cui, X. Preparation of a low-cost electric compensation photothermal evaporator for all-weather-available and efficient evaporation. Sol. Energy 2023, 259, 151–164. [Google Scholar] [CrossRef]
  32. Aygörmez, Y.; Canpolat, O. Long-term sulfuric and hydrochloric acid resistance of silica fume and colemanite waste reinforced metakaolin-based geopolymers. Rev. Construcción 2021, 20, 291–307. [Google Scholar] [CrossRef]
  33. Aygörmez, Y.; Canpolat, O.; Al-Mashhadani, M.M. Assessment of geopolymer composites durability at one year age. J. Build. Eng. 2020, 32, 101453. [Google Scholar] [CrossRef]
  34. Zhang, P.; Zheng, Y.; Wang, K.; Zhang, J. A review on properties of fresh and hardened geopolymer mortar. Compos. Part B Eng. 2018, 152, 79–95. [Google Scholar] [CrossRef]
  35. Pundienė, I.; Pranckevičienė, J.; Kligys, M.; Girskas, G. Study of the Course of Cement Hydration in the Presence of Waste Metal Particles and Pozzolanic Additives. Materials 2022, 15, 2925. [Google Scholar] [CrossRef]
  36. Nabiun, R.; Nasajpour-Esfahani, N.; Jasim, D.J.; Mirvalad, S.; Garmestani, H.; Toghraie, D. Laboratory evaluation of mechanical properties and noise absorption in magnetite-enhanced concrete for pavement applications. Case Stud. Constr. Mater. 2024, 21, e03570. [Google Scholar] [CrossRef]
  37. Valizadeh, A.; Aslani, F.; Asif, Z.; Roso, M. Development of Heavyweight Self-Compacting Concrete and Ambient-Cured Heavyweight Geopolymer Concrete Using Magnetite Aggregates. Materials 2019, 12, 1035. [Google Scholar] [CrossRef]
  38. Ali, S.I.A.; Lublóy, É. Fire resistance properties of heavyweight magnetite concrete in comparison with normal basalt- and quartz-based concrete. J. Therm. Anal. Calorim. 2022, 147, 11679–11691. [Google Scholar] [CrossRef]
  39. Sas, Z.; Sha, W.; Soutsos, M.; Doherty, R.; Bondar, D.; Gijbels, K.; Schroeyers, W. Radiological characterisation of alkali-activated construction materials containing red mud, fly ash and ground granulated blast-furnace slag. Sci. Total Environ. 2019, 659, 1496–1504. [Google Scholar] [CrossRef]
  40. Liu, X.; Feng, P.; Chen, J.; Liu, Q.; Yu, X.; Cai, Y.; Zhu, H.; Qing, L.; Hong, J. A critical review on the interaction between calcium silicate hydrate (C-S-H) and different ions. Constr. Build. Mater. 2024, 413, 134931. [Google Scholar] [CrossRef]
  41. Abdullah, M.A.H.; Rashid, R.S.M.; Amran, M.; Hejazii, F.; Azreen, N.M.; Fediuk, R.; Voo, Y.L.; Vatin, N.I.; Idris, M.I. Recent Trends in Advanced Radiation Shielding Concrete for Construction of Facilities: Materials and Properties. Polymers 2022, 14, 2830. [Google Scholar] [CrossRef] [PubMed]
  42. Aslani, F.; Asif, Z. Properties of Ambient-Cured Normal and Heavyweight Geopolymer Concrete Exposed to High Temperatures. Materials 2019, 12, 740. [Google Scholar] [CrossRef] [PubMed]
  43. Castillo, H.; Collado, H.; Droguett, T.; Sánchez, S.; Vesely, M.; Garrido, P.; Palma, S. Factors Affecting the Compressive Strength of Geopolymers: A Review. Minerals 2021, 11, 1317. [Google Scholar] [CrossRef]
  44. Sikora, P.; Abd Elrahman, M.; Horszczaruk, E.; Brzozowski, P.; Stephan, D. Incorporation of magnetite powder as a cement additive for improving thermal resistance and gamma-ray shielding properties of cement-based composites. Constr. Build. Mater. 2019, 204, 113–121. [Google Scholar] [CrossRef]
  45. Wu, X.; Zhou, X.; Guo, C.; Kang, D.; Zhang, W.; Lan, J.; Fang, Z. Behavior of hematite, magnetite, and reduced iron powder in geopolymers: Effects of mechanical properties and reaction mechanism. J. Clean. Prod. 2024, 444, 141178. [Google Scholar] [CrossRef]
  46. Nguyen, K.T.; Ahn, N.; Le, T.A.; Lee, K. Theoretical and experimental study on mechanical properties and flexural strength of fly ash-geopolymer concrete. Constr. Build. Mater. 2016, 106, 65–77. [Google Scholar] [CrossRef]
  47. Fiala, L.; Pommer, V.; Böhm, M.; Scheinherrová, L.; Černý, R. Self-heating alkali activated materials: Microstructure and its effect on electrical, thermal and mechanical properties. Constr. Build. Mater. 2022, 335, 127527. [Google Scholar] [CrossRef]
  48. Vlachakis, C.; Perry, M.; Biondi, L. Self-Sensing Alkali-Activated Materials: A Review. Minerals 2020, 10, 885. [Google Scholar] [CrossRef]
  49. Gomaa, M.M. Electrical properties of hematite and pure sand synthetic homogeneous mixture. Appl. Water Sci. 2022, 13, 43. [Google Scholar] [CrossRef]
  50. Ren, Z.; Zeng, H.; Zeng, X.; Chen, X.; Wang, X. Effect of Nanographite Conductive Concrete Mixed with Magnetite Sand Excited by Different Alkali Activators and Their Combinations on the Properties of Conductive Concrete. Buildings 2023, 13, 1630. [Google Scholar] [CrossRef]
  51. Omana, L.; Chandran, A.; John, R.E.; Wilson, R.; George, K.C.; Unnikrishnan, N.V.; Varghese, S.S.; George, G.; Simon, S.M.; Paul, I. Recent Advances in Polymer Nanocomposites for Electromagnetic Interference Shielding: A Review. ACS Omega 2022, 7, 25921–25947. [Google Scholar] [CrossRef]
  52. Qureshi, N.; Dhand, V.; Subhani, S.; Senthil Kumar, R.; Raghavan, N.; Kim, S.; Doh, J. Exploring Conductive Filler-Embedded Polymer Nanocomposite for Electrical Percolation via Electromagnetic Shielding-Based Additive Manufacturing. Adv. Mater. Technol. 2024, 9, 2400250. [Google Scholar] [CrossRef]
  53. Al-Ghamdi, A.A.; Al-Hartomy, O.A.; Al-Solamy, F.R.; Dishovsky, N.; Malinova, P.; Atanasova, G.; Atanasov, N. Conductive carbon black/magnetite hybrid fillers in microwave absorbing composites based on natural rubber. Compos. Part B Eng. 2016, 96, 231–241. [Google Scholar] [CrossRef]
  54. Kim, H.-S.; Jang, C.; Kim, H.G.; Woo, B.-H. Snow-melting performance of the thermally conductive concrete pavement–experimental evaluation in field application. Constr. Build. Mater. 2024, 411, 134508. [Google Scholar] [CrossRef]
  55. Wang, Z.; Bai, E.; Huang, H.; Wang, T.; Sun, H. Study on the electromagnetic property and microwave heating efficiency of concrete with magnetite aggregate. Constr. Build. Mater. 2022, 342, 128080. [Google Scholar] [CrossRef]
  56. Dehghanpour, H.; Yilmaz, K.; Afshari, F.; Ipek, M. Electrically conductive concrete: A laboratory-based investigation and numerical analysis approach. Constr. Build. Mater. 2020, 260, 119948. [Google Scholar] [CrossRef]
  57. Sassani, A.; Ceylan, H.; Kim, S.; Arabzadeh, A.; Taylor, P.C.; Gopalakrishnan, K. Development of Carbon Fiber-modified Electrically Conductive Concrete for Implementation in Des Moines International Airport. Case Stud. Constr. Mater. 2018, 8, 277–291. [Google Scholar] [CrossRef]
  58. An, M.; Huang, H.; Wang, Y.; Zhao, G. Effect of thermal cycling on the properties of high-performance concrete: Microstructure and mechanism. Constr. Build. Mater. 2020, 243, 118310. [Google Scholar] [CrossRef]
  59. Wu, T.; Huang, R.; Chi, M.; Weng, T. A study on electrical and thermal properties of conductive concrete. Comput. Concr. 2013, 12, 337–349. [Google Scholar] [CrossRef]
  60. Xu, B.; Han, J.; Kumar, A.; Li, P.; Yang, Y. Thermal storage using sand saturated by thermal-conductive fluid and comparison with the use of concrete. J. Energy Storage 2017, 13, 85–95. [Google Scholar] [CrossRef]
  61. Şahan, N.; Fois, M.; Paksoy, H. Improving thermal conductivity phase change materials—A study of paraffin nanomagnetite composites. Sol. Energy Mater. Sol. Cells 2015, 137, 61–67. [Google Scholar] [CrossRef]
  62. Gao, H.; Yin, T.; Ma, J.; Zhou, Y.; Li, K.; Bao, J. Research Progress of Photothermal Superhydrophobic Surfaces for Anti-Icing/Deicing. Molecules 2025, 30, 1865. [Google Scholar] [CrossRef]
  63. GivKashi, M.R.; Tohidloo, M. The effect of freeze-thaw cycles and sulfuric acid attack separately on the compressive strength and microstructure of 3D-printed air-entrained concrete. Constr. Build. Mater. 2024, 440, 137411. [Google Scholar] [CrossRef]
  64. Yang, H.; Che, Y.; Leng, F. Calcium leaching behavior of cementitious materials in hydrochloric acid solution. Sci. Rep. 2018, 8, 8806. [Google Scholar] [CrossRef] [PubMed]
  65. Wang, Y.; Cao, Y.; Zhang, Z.; Huang, J.; Zhang, P.; Ma, Y.; Wang, H. Study of acidic degradation of alkali-activated materials using synthetic C-(N)-A-S-H and N-A-S-H gels. Compos. Part B Eng. 2022, 230, 109510. [Google Scholar] [CrossRef]
  66. Wattanasiriwech, D.; Yomthong, K.; Wattanasiriwech, S. Adsorption efficiency and photocatalytic activity of fly ash-based geopolymer foam mortar. Ceram. Int. 2021, 47, 27361–27371. [Google Scholar] [CrossRef]
  67. Pan, Z.; Wang, S.; Liu, Y.; Li, B.; Jia, Z.; Zhang, Y.; Wang, J. The hydration, pore structure and strength of cement-based material prepared with waste soaking solution from acetic acid treatment of regenerated aggregates. J. Clean. Prod. 2019, 235, 866–874. [Google Scholar] [CrossRef]
  68. Wang, Y.; Cao, Y.; Zhang, Z.; Zhang, P.; Ma, Y.; Wang, A.; Wang, H. Intrinsic sulfuric acid resistance of C-(N)-A-S-H and N-A-S-H gels produced by alkali-activation of synthetic calcium aluminosilicate precursors. Cem. Concr. Res. 2023, 165, 107068. [Google Scholar] [CrossRef]
  69. Park, J.A.; Pimenta, M.M.; Bezerra, A.C.d.S. Acid Activation in Low-Carbon Binders: A Systematic Literature Review. Buildings 2024, 14, 83. [Google Scholar] [CrossRef]
  70. Bingöl, Ş.; Bilim, C.; Duran Atiş, C.; Durak, U.; İlkentapar, S.; Karahan, O. An investigation of resistance of sodium meta silicate activated slag mortar to acidic and basic mediums. Rev. Construcción 2020, 19, 127–133. [Google Scholar] [CrossRef]
  71. Damion, T.; Soda, P.R.K.; Gupta, S. Enhanced acid resistance of geopolymer containing recycled masonry wastes as high-volume replacement of natural sand. J. Build. Eng. 2025, 105, 112396. [Google Scholar] [CrossRef]
  72. de Souza Oliveira, A.; Dweck, J.; Fairbairn, E. de M.R.; Gomes, O. da F.M.; Toledo Filho, R.D. Crystalline admixture effects on crystal formation phenomena during cement pastes’ hydration. J. Therm. Anal. Calorim. 2020, 139, 3361–3375. [Google Scholar] [CrossRef]
  73. Jin, B.; Wu, S.; Fan, S.; Hang, F.; Chen, H. Strength and Expansion of LHEC with Different Gypsum Contents Under Thermal Curing. Materials 2024, 17, 5766. [Google Scholar] [CrossRef]
  74. Shamsudin, M.A.; Mustapha, F.; Abdullah, M.N.; Mustapha, M. Sustainable Aluminosilicate Coatings from Palm Oil Waste for Enhanced Thermal and Microstructure Properties. Materials 2025, 18, 821. [Google Scholar] [CrossRef]
  75. Zuo, Y.; Ye, G. Pore Structure Characterization of Sodium Hydroxide Activated Slag Using Mercury Intrusion Porosimetry, Nitrogen Adsorption, and Image Analysis. Materials 2018, 11, 1035. [Google Scholar] [CrossRef] [PubMed]
  76. Shao, Z.; Ma, B.; Wang, J.; Cai, Q.; Jiang, J.; Qian, B.; Cheng, G.; Hu, Y.; Ma, F.; Sun, J.; et al. The influence of ZSM-5 waste on the properties of fly ash-based foamed geopolymer. J. Clean. Prod. 2022, 355, 131800. [Google Scholar] [CrossRef]
  77. Davidovits, J.; Davidovits, R. Ferro-Sialate Geopolymers. Geopolymer Institute Library. 2020. Available online: https://www.geopolymer.org/wp-content/uploads/Ferro-sialate.pdf (accessed on 10 July 2025).
  78. Patrisia, Y.; Law, D.W.; Gunasekara, C.; Wardhono, A. Long-term durability of iron-rich geopolymer concrete in sulphate, acidic and peat environments. J. Build. Eng. 2024, 97, 110744. [Google Scholar] [CrossRef]
  79. Tome, S.; Nana, A.; Tchakouté, H.K.; Temuujin, J.; Rüscher, C.H. Mineralogical evolution of raw materials transformed to geopolymer materials: A review. Ceram. Int. 2024, 50, 35855–35868. [Google Scholar] [CrossRef]
  80. Zhou, Y.; Liang, Y.; Wu, Z.; Wang, X.; Guan, R.; Li, C.; Qiao, F.; Wang, J.; Fu, Y.; Baek, J.-B. Amorphous/Crystalline Heterostructured Nanomaterials: An Emerging Platform for Electrochemical Energy Storage. Small 2025, 21, 2411941. [Google Scholar] [CrossRef]
  81. Pouyanne, A.; Boudache, S.; Hilloulin, B.; Loukili, A.; Roziere, E. Experimental Investigation on the Effects of Mineral Water Composition on the Leaching of Cement-Based Materials. Materials 2024, 17, 1548. [Google Scholar] [CrossRef]
  82. Ma, Q.; Yang, W.; Duan, Z.; Liu, H.; Hua, M.; Deng, Q. Influence of Alkali-Activators on Acid Rain Resistance of Geopolymer-Recycled Pervious Concrete with Optimal Pore Size. Materials 2022, 15, 8368. [Google Scholar] [CrossRef]
  83. Yang, W.; Zhu, P.; Liu, H.; Wang, X.; Ge, W.; Hua, M. Resistance to Sulfuric Acid Corrosion of Geopolymer Concrete Based on Different Binding Materials and Alkali Concentrations. Materials 2021, 14, 7109. [Google Scholar] [CrossRef]
  84. Santhanam, M.; Cohen, M.D.; Olekm, J. Sulfate attack research—whither now? Cem. Concr. Res. 2001, 31, 845–851. [Google Scholar] [CrossRef]
  85. Matalkah, F.; Salem, T.; Soroushian, P. Acid resistance and corrosion protection potential of concrete prepared with alkali aluminosilicate cement. J. Build. Eng. 2018, 20, 705–711. [Google Scholar] [CrossRef]
  86. Liu, J.; Peng, C.; Jiang, J.; Zhang, X.; He, D.; Zhou, K.; Chen, W. Recovery of iron and aluminum from iron-rich bauxite residue by an integrated phase reconstruction approach. Sci. Total Environ. 2023, 904, 166702. [Google Scholar] [CrossRef] [PubMed]
  87. Yoosuk, P.; Suksiripattanapong, C.; Hiroki, G.; Phoo-ngernkham, T.; Thumrongvut, J.; Sukontasukkul, P.; Chindaprasirt, P. Performance of polypropylene fiber-reinforced cellular lightweight fly ash geopolymer mortar under wet and dry cycles. Case Stud. Constr. Mater. 2024, 20, e03233. [Google Scholar] [CrossRef]
  88. Ninan, C.M.; Ramaswamy, K.; Sajeeb, R. Influence of acid replenishment and concentration on the degradation kinetics of cement mortar exposed to acetic acid. Mater. Today Proc. 2022, 65, 908–914. [Google Scholar] [CrossRef]
Materials 18 04087 g001
Figure 1. Mini-MBE cone test for measuring the slump of mortar.
Figure 1. Mini-MBE cone test for measuring the slump of mortar.
Materials 18 04087 g001
Materials 18 04087 g002
Figure 2. Schematic illustration of the experimental setup for determining the electrical conductivity of the geopolymer.
Figure 2. Schematic illustration of the experimental setup for determining the electrical conductivity of the geopolymer.
Materials 18 04087 g002
Materials 18 04087 g003
Figure 3. Schematic illustration of the experimental setup for determining the photothermal conversion performance.
Figure 3. Schematic illustration of the experimental setup for determining the photothermal conversion performance.
Materials 18 04087 g003
Materials 18 04087 g004
Figure 4. XRD patterns of the Fe-rich spinel aggregate (H, hercynite; I, ilmenite; M, magnetite).
Figure 4. XRD patterns of the Fe-rich spinel aggregate (H, hercynite; I, ilmenite; M, magnetite).
Materials 18 04087 g004
Materials 18 04087 g005
Figure 5. XRD patterns of the bauxite residue and GGBFS (Si—calcium silicate, Q—quartz, Ja—jadeite, He—hematite, Gi—gibbsite, Go—goethite, Bo—boehmite, Ti—anatase, Ps—pseudorutile).
Figure 5. XRD patterns of the bauxite residue and GGBFS (Si—calcium silicate, Q—quartz, Ja—jadeite, He—hematite, Gi—gibbsite, Go—goethite, Bo—boehmite, Ti—anatase, Ps—pseudorutile).
Materials 18 04087 g005
Materials 18 04087 g006
Figure 6. Influence of the replacement of sand with FSA on slump flow.
Figure 6. Influence of the replacement of sand with FSA on slump flow.
Materials 18 04087 g006
Materials 18 04087 g007
Figure 7. Effect of the replacement of sand with FSA on bulk density.
Figure 7. Effect of the replacement of sand with FSA on bulk density.
Materials 18 04087 g007
Materials 18 04087 g008
Figure 8. Impact of FSA as a partial replacement for the fine aggregate on the compressive strength of the geopolymer.
Figure 8. Impact of FSA as a partial replacement for the fine aggregate on the compressive strength of the geopolymer.
Materials 18 04087 g008
Materials 18 04087 g009
Figure 9. Influence of FSA as a partial replacement for fine aggregate on the electrical resistivity of the geopolymer.
Figure 9. Influence of FSA as a partial replacement for fine aggregate on the electrical resistivity of the geopolymer.
Materials 18 04087 g009
Materials 18 04087 g010
Figure 10. Consumed heat flux at different applied voltages for different specimens.
Figure 10. Consumed heat flux at different applied voltages for different specimens.
Materials 18 04087 g010
Materials 18 04087 g011
Figure 11. Photothermal performance of the geopolymer after exposure to solar irradiance of 1 kW/m2.
Figure 11. Photothermal performance of the geopolymer after exposure to solar irradiance of 1 kW/m2.
Materials 18 04087 g011
Materials 18 04087 g012
Figure 12. Mass changes of the AEC and geopolymer samples subjected to 28 days of 10% HCl immersion (positive, gain; negative, loss).
Figure 12. Mass changes of the AEC and geopolymer samples subjected to 28 days of 10% HCl immersion (positive, gain; negative, loss).
Materials 18 04087 g012
Materials 18 04087 g013
Figure 13. Strength changes of the AEC and geopolymer samples immersed in 10% HCl for 28 days (positive, gain; negative, loss).
Figure 13. Strength changes of the AEC and geopolymer samples immersed in 10% HCl for 28 days (positive, gain; negative, loss).
Materials 18 04087 g013
Materials 18 04087 g014
Figure 14. XRD patterns of air-entrained Portland cement-based concrete (A—alite, B—belite, C—calcite, G—gypsum, and Q—quartz).
Figure 14. XRD patterns of air-entrained Portland cement-based concrete (A—alite, B—belite, C—calcite, G—gypsum, and Q—quartz).
Materials 18 04087 g014
Materials 18 04087 g015
Figure 15. XRD patterns of the geopolymer with different percentages of FSA as a sand replacement (E—esseneite, G—gismondine, L—laumontite, M—magnetite, N—NaFeO3, Q—quartz).
Figure 15. XRD patterns of the geopolymer with different percentages of FSA as a sand replacement (E—esseneite, G—gismondine, L—laumontite, M—magnetite, N—NaFeO3, Q—quartz).
Materials 18 04087 g015
Materials 18 04087 g016aMaterials 18 04087 g016b
Figure 16. Microstructural changes in the AEC and geopolymer samples before and after exposure to 10% HCl for 28 days.
Figure 16. Microstructural changes in the AEC and geopolymer samples before and after exposure to 10% HCl for 28 days.
Materials 18 04087 g016aMaterials 18 04087 g016b
Table 1. Chemical compositions of the bauxite residue and GGBFS.
Table 1. Chemical compositions of the bauxite residue and GGBFS.
Oxide (%)Bauxite ResidueGGBFS
SiO211.133.8
Al2O317.29.9
Fe2O344.41.1
MgO0.055.9
CaO2.6146.2
Na2O6.440.6
K2O0.06-
LOI11.9-
Table 2. Mix proportions of the air-entrained concrete that conformed to CSA A23.1 [22].
Table 2. Mix proportions of the air-entrained concrete that conformed to CSA A23.1 [22].
SamplePortland
Cement
(kg/m3)		Fine
Aggregate (kg/m3)		Coarse
Aggregate (kg/m3)		Water
(kg/m3)		Sika
Air 60
(kg/m3)		Sika
Viscocrete (kg/m3)
AEC420950804.31570.153.12
Table 3. Mixtures of the geopolymer samples with different aggregates.
Table 3. Mixtures of the geopolymer samples with different aggregates.
SampleBauxite
Residue (kg/m3)		GGBFS (kg/m3)Fine
Aggregate (kg/m3)		Fine
Magnetite (kg/m3)		NaOH (kg/m3)Na2SiO3 (kg/m3)Graphite (kg/m3)
FSA0384576192001122249.6
FSA2538457614404801122249.6
FSA503845769609601122249.6
FSA7538457648014401122249.6
FSA100384576019201122249.6
Table 4. Changes in the geopolymer temperature at different time intervals under photothermal exposure.
Table 4. Changes in the geopolymer temperature at different time intervals under photothermal exposure.
SampleTmin
(°C)		Tmax (°C)ΔT
(°C)
AEC23.528.14.6
FSA023.529.66.1
FSA2523.530.26.7
FSA5023.7317.3
FSA7523.731.37.6
FSA10024.133.99.8
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Harmaji, A.; Jafari, R.; Simard, G. Development of Multifunctional Slag and Bauxite Residue-Based Geopolymers with Heavyweight Aggregate Enhancement. Materials 2025, 18, 4087. https://doi.org/10.3390/ma18174087

AMA Style

Harmaji A, Jafari R, Simard G. Development of Multifunctional Slag and Bauxite Residue-Based Geopolymers with Heavyweight Aggregate Enhancement. Materials. 2025; 18(17):4087. https://doi.org/10.3390/ma18174087

Chicago/Turabian Style

Harmaji, Andrie, Reza Jafari, and Guy Simard. 2025. "Development of Multifunctional Slag and Bauxite Residue-Based Geopolymers with Heavyweight Aggregate Enhancement" Materials 18, no. 17: 4087. https://doi.org/10.3390/ma18174087

APA Style

Harmaji, A., Jafari, R., & Simard, G. (2025). Development of Multifunctional Slag and Bauxite Residue-Based Geopolymers with Heavyweight Aggregate Enhancement. Materials, 18(17), 4087. https://doi.org/10.3390/ma18174087

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop