An experimental approach for the determination of the physical and mechanical properties of a sustainable geopolymer mortar made with Algerian ground-granulated blast furnace slag

1.1 Research motivation

In Algeria, the annual production of granulated blast furnace slag (GBFS), crystalline slag, and steel slag is estimated at about 2 and 1.5 million tons per year. The iron and steel complex of El Hadjar in eastern Algeria produces large quantities of a GBFS, which is mainly used as a mineral additive (up to 30%) in the production of ordinary cement. GBFS brings many environment and economic benefits when used as a cement replacement, leading to sustainable solutions in construction [1]. Because its production cost is estimated to be zero except for the embodied energy of the grinding process, estimated to be 0.3 MJ/kg, which represents 7% of the embodied energy of cement [2]. Furthermore, Portland cement concrete (PCC) has a lower economic index (0.320) compared to all geopolymer concretes (GPCs) produced from industrial waste [3].

The economic index for GPC made from ground-granulated blast furnace slag (GGBFS) is estimated to be 0.545 [3]. Experimental data indicate that the total production cost for M30 grade PPC is $109.59, with cement comprising the largest portion of the cost at 53.38% [3]. On the other hand, the production cost for M30-grade GPC, activated with sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) solutions in a 2.5:1 ratio of Na₂SiO₃ to NaOH at a 12 molar NaOH concentration, is approximately $79.22 [3]. In this case, sodium hydroxide solution accounts for 37.26% of the total production cost [3]. In summary, the production cost per cubic meter of M30 grade of GGBFS-GPC is 27.71% lower than that of PCC of the same grade [3].

In this context, the current research aims to exploit the locally produced GBFS to develop a sustainable eco-friendly geopolymer mortar (GPM).

1.2 Research background

Sustainability and green development are central topics of global discussions across various sectors. Notably, the construction industry is a principal contributor to CO₂ emissions, exacerbating climate emergencies and increasing global temperatures [1,4,5]. The cement industry is a substantial contributor to global warming, releasing approximately 0.9 ton of CO₂ for every 1 ton of cement produced, making it the second largest contributor to this issue [1]. Moreover, the manufacture of ordinary Portland cement (OPC) ranks third in terms of the most energy-intensive building material after the manufacture of steel and aluminium, requiring ∼120 kW h per ton [6]. Despite this, the need for OPC to manufacture concrete, the world’s main binder in the construction sector, remains increasing. The increasing need for cement worldwide requires large production of clinker (CaO), which requires substantial amounts of natural materials such as shale, lime, silica, and calcium. This process leads to significant CO₂ emissions into the atmosphere and the overexploitation of natural resources [6]. Moreover, 2.75 tons of precursor materials is required to produce one ton of OPC [6]. Research forecasts predict that OPC concrete production will increase by 200% by 2025 due to the development of global infrastructure [1]. Furthermore, the production of OPC accounts for about 8% of global greenhouse gas emissions [1]. The adverse environmental impact and high-energy requirements of cement manufacturing require an environmentally friendly and sustainable alternative, such as GPC [6], a versatile class of materials with numerous potential applications [7,8]. GPM and concrete are revolutionizing modern construction and sustainability efforts by offering durable, environmentally friendly alternatives to traditional cement-based materials [4,5,6,7,8,12]. Their applications span diverse industries, including construction, infrastructure, and environmental management, driven by their exceptional resistance to environmental degradation and fire, as well as their lower carbon footprint [9,10,11,12,13]. Global projects highlight the potential of these materials on a large scale. India’s GPC Project demonstrates their use in general construction, Australia’s Sustainable Construction Project integrates them into eco-friendly building practices, the UK is exploring their utility in infrastructure repair, and the Netherlands is advancing 3D printing with GPM [10,11,12,13].

Despite their promise, challenges such as securing high-quality raw materials, maintaining production consistency, and optimizing curing conditions must be addressed to unlock their full potential and pave the way for widespread adoption in the future of green building [10,11,12,13,14].

GP materials are synthesized from aluminosilicate sources, such as fly ash (FA) and slag, which, when combined with an alkaline solution under high pH conditions, form Si–O–Al–O polymer bonds [1,2,4,6,9]. The chemical reaction (geopolymerization) and rheological and mechanical properties of GPC depend on several factors, the most important of which are the alkaline solution, the amount of activator used, the fineness of the raw materials, and the curing time [1,9,11,12,15]. GPs were observed to exhibit higher apparent activation energies (Ea) compared to portland cement I (37.5 kJ/mol for α = 0.2) and GGBFS (75.8 kJ/mol for α = 0.2) [16]. This increased Ea slows the reaction at lower temperatures, resulting in extended setting times [16]. However, it also enhances the durability and chemical resistance of GPs by promoting the formation of a denser and more stable structure [16]. The Ea is influenced by several factors, including the chemical composition of the material (such as slag), the type and concentration of activators, curing conditions (such as temperature), and the water-to-binder ratio [16].

Due to their exceptional heat resistance, GPs have been extensively utilized in the production of thermal insulation materials and energy storage concretes [17]. Moreover, GP coatings on steel have proven effective in preserving structural integrity under high-temperature conditions [17]. Duan et al. [18] reported that the heat resistance of GPC, synthesized through the alkaline activation of a combination of fluidized bed FA and metakaolin (MK) with a mass ratio of 1.0 in alkali silicate solutions, is evident in its ability to resist both instability and shrinkage. Notably, the mass loss of GPC is significantly lower compared to that of OPC concrete after exposure to 600°C, primarily due to the decomposition of Ca(OH)₂ in OPC concrete [18]. Ye et al. [19] reported that tailing/slag-based GP experiences a significant strength reduction when exposed to a fire at 1,000°C, losing more than 40 MPa, which corresponds to a 66.67% reduction. However, it demonstrates a strength gain after heating at 1,200°C, attributed to sintering and densification processes. In another study, Sarker et al. [20] observed that OPC concrete suffered severe spalling after exposure to 800 and 1,000°C fires, whereas GPC exhibited superior fire resistance, showing only minor surface crackling. This is because the high-density and low-porosity nature of GPM further contributes to its resistance to cracking, as it reduces the movement of water and air that can lead to volume changes [21]. The authors of previous studies [10,12,20,21] also emphasized that GPMs and GPC exhibited better performance at elevated temperatures in comparison with control cement mortar mixture. For instance, Nkwaju et al. [22] found that lateritic soil-based geopolymer showed a significant reduction in thermal conductivity, decreasing from 0.77 to 0.55 W/mK, with the incorporation of 7.5 wt% sugarcane bagasse fibres, highlighting its potential as a sustainable insulation material. GPM typically exhibits less shrinkage compared to conventional cement mortar, primarily due to its distinct chemical structure and low hydration-based shrinkage [23]. Similarly, Kong and Sanjayan [24] reported that FA-based geopolymer exhibited a shrinkage of approximately 1% between 200 and 300°C, with an additional shrinkage of 0.6% occurring between 700 and 800°C. Furthermore, Yang et al. [25] found that alkali-activated slag and FA-blended recycled aggregate concrete achieve a freeze–thaw resistance grade of F300, with minimal internal damage and accurate damage models based on mass loss and elasticity. In comparison with traditional mortar, the workability and setting time of GPM can be easily modified by altering the components of the GP mixture [4,5,6,7,8,14]. GPC also offers chemical stability compared to ordinary and high-performance OPC-based concrete, which cures and chemically decomposes upon increasing temperatures [7,12,18,20,23]. The slag-based GPC, with a strength of 40 MPa, demonstrated a 33% reduction in strength after 1 year of exposure to an acetic acid solution (pH = 4), which was notably lower than the 47% reduction observed in ordinary OPC concrete [21]. In a 2% H₂SO₄ solution, the GPC experienced an 11% loss in strength, whereas OPC concrete exhibited a significantly greater reduction of 36.2% [21]. In addition, fly ash – ground granulated blast furnace slag-based GPs with high Ca content in their precursors exhibit excellent resistance to both acid and sulphate attacks [26,27]. Moreover, GPC (such as fly ash-based geopolymer concrete) can create a protective ferric oxide layer on the steel reinforcement, which prevents corrosion by maintaining an alkaline environment [12]. This leads to a lower corrosion rate compared to OPC concrete [12]. GPMs also demonstrate significantly greater resistance to alkali–silica reaction (ASR) compared to mortars based on OPC [10,28]. This superior performance is largely attributed to their unique chemical composition and the limited presence of calcium hydroxide, which is a critical contributor to the ASR process [28]. A study by Mahanama et al. [28] examined the influence of GGBFS content on ASR in GPs through accelerated mortar bar tests. The findings revealed that while ASR expansion in GPs increased with higher GGBFS content, it consistently remained lower than that in OPC mixtures. These results highlight the need to carefully consider key factors, particularly the type and molarity of the alkali solution, when incorporating GGBFS into GP systems. The availability of GGBFS worldwide due to the growth and expansion of iron and steel production encourages the use of GGBFS as an alternative to conventional OPC [29]. In addition, global stainless-steel consumption will increase by 3.6% by 2023, with a consequent growth in the amount of slag [29]. Therefore, the use of GGBFS in GPM significantly contributes to sustainability through multiple mechanisms, leveraging its role as an industrial by-product and its chemical properties [29]. By incorporating GGBFS, waste destined for landfills is effectively reduced, promoting waste management practices that align with circular economy principles [26,27,29]. This repurposing minimizes environmental pollution and supports resource efficiency by replacing natural raw materials such as limestone and clay, which are traditionally extracted for cement production [9,12]. Furthermore, GPM made with GGBFS is a low-carbon alternative to PC, producing 60–80% fewer CO₂ emissions due to its reduced reliance on energy-intensive clinker production [4,10,26]. Turner and Collins [30] proved that PC was by far the most significant contributor to emissions, contributing to 76.4% of CO₂-e for OPC concrete. However, the alkali activators expend significant energy during manufacture and the contribution of the geopolymer binder (FA + sodium silicate + sodium hydroxide) is 201 kg CO₂-e/m³ compared with OPC 269 kg CO₂-e/m³ (25.2%) [30]. The total emissions from the OPC and GPC comparison mixes used in this report were estimated as 354 and 320 kg CO₂-e/m³, respectively, showing 9% difference [30]. Moreover Al-Fakih et al. [31] found that the maximum CO₂ footprint of GGBFS-based GPC was estimated at 276.1 kg CO₂-e/m³. A further consideration would be to compare GPC with CO₂-e arising from blended FA or slag cements that comprise the partial replacement of OPC [31]. Although calculations have not been made on specific mixes of mixed cement concrete, CO₂ emission reductions of 19–29% are quite possible [30] and would provide lower CO₂ emissions than GP binders [30].

In addition to its environmental benefits, GGBFS enhances the performance of GPMs by providing improved resistance to chemical attacks, reducing permeability, and increasing durability [4,10,14,15,16]. These properties extend the lifespan of structures, lowering the frequency of repairs and replacements and reducing overall material consumption and associated environmental impacts. Through these combined benefits, GGBFS serves as a pivotal material in advancing sustainable construction practices. Moreover, research indicates that GPMs produced from industrial by-products such as FA and rice husk ash offer sustainable and cost-effective alternatives, contingent on local availability and specific application needs [23]. However, GGBFS-based-GPMs demonstrate superior performance in terms of strength, durability, and fire resistance [10,12,16,27,31]. In the study conducted by Tran et al. [14] for the development of sustainable ultra-high-performance concrete containing GGBFS and glass powder, they found that all mixes had lower manufacture costs than the PCC mix; differences among them were trivial. Experimental results demonstrated that the ecological enhancement of normal-strength mortars is both viable and feasible. By replacing 70–90% of the cement with GGBS, energy consumption for producing 1 tons of binder can be reduced by 67–86%, while the performance energy decreases significantly from 25.4 (kW h/t)/MPa to a range of 6–8 (kW h/t)/MPa [32]. The energy research conducted by Verma et al. [10] revealed that GPC based on FA and GGBFS embodied energy less compared to the OPC concrete. The GPC production generally uses FA, GGBFS, NaOH, sodium silicate, aggregate (fine and coarse), and water [14]. The FA and GGBFS are solid industrial wastes produced by the thermal power plant and steel plant [1,6,11,20]. The FA is directly used in the mix, but the GGBFS requires grinding before use in the mix [10]. The embodied energy for the grinding process is calculated as 0.3 MJ/kg [2]. NaOH production’s embodied energy is 20.5 MJ/kg, and sodium silicate production is 5.37 MJ/kg [2]. There is zero embodied energy in FA and water [2]. The coarse aggregate and fine aggregates’ embodied energies are 0.22 and 0.02 MJ/kg, respectively [10]. The embodied energy of sulfonated naphthalene formaldehyde (SNF) superplasticizers is reported to be 12.6 MJ/kg [10]. The total embodied energy of the OPC concrete is 1897.86 MJ/m³, whereas the embodied energy of the GPC is 1749.21 MJ/m³ [10]. The GPC’s embodied energy is less compared to the OPC concrete [10]. Studies have proven that the performance of POC is lower compared to alkali-activated slag concrete [6,11,12,13]. The geopolymerization of GPC was expedited under ambient curing conditions with the incorporation of GGBFS, leading to a shorter setting time, better workability, and increased mechanical strength (MS) at an early age [10,14,15,27,28,29]. This phenomenon is attributed to the rapid hydraulic reaction of GGBFS, which results from its large surface area [33]. Furthermore, the particle size significantly influences the reactions occurring in GGBFS [33]. For instance, other researchers have observed that a finer particle size (4,170 cm²/g) led to better workability and compressive strength [27].

Alternatively, Hasnaoui et al. [8] observed that the optimal mechanical and rheological properties were achieved with a mixture having a GGBFS/MK ratio of 1, a GGBFS + MK/activator ratio of 3, and a SiO₂/Na₂O ratio ranging between 1.6 and 1.8. On the other hand for the SiO₂/Na₂O molar ratio of 1.16, GP consisting of 95% GGBFS and 5% MK recorded compressive strengths of 47.84 MPa at 24 h [15]. Moreover, Hasnaoui et al. [8] found that for GPs with 50% GGBFS and 50% MK, all mixes with a GGBFS + MK/activator ratio lower than 3 are characterized by higher mechanical resistances than that of PCM (48.4 and 6.4 MPa for compressive and flexural strengths, respectively), whatever the SiO₂/Na₂O ratio. On the other hand, the tensile strength results clearly demonstrated the potential of GPM as a new alternative repair material [20,29]. In a previous study, after 24 h, the GPM exhibited a split tensile strength of 2.95 MPa, which was almost 10 times greater than that of OPC mortar (0.32 MPa) [15]. Scanning electron microscopy/energy dispersive analysis of X-rays, X-ray diffraction (XRD), and FTIR analyses conducted by Prusty [4] on GGBFS-activated gels revealed that the enhanced mechanical properties of GPC mixtures are primarily due to the reaction products formed in alkaline-activated slag and FA mixtures. These products are predominantly governed by the C–(A)–S–H series of gels, with the presence of C–S–H and N–A–S–H gels. Furthermore, XRD spectra indicate that increasing GGBFS substitution promotes the formation of N–(C)–A–S–H gel, which significantly contributes to the higher compressive strength of GPC [4]. Similarly, Dai et al. [5] reported that the coexistence of both N–A–S–H and C–S–H phases in GGBFS and FA mixtures treated at room temperature leads to superior mechanical properties compared to alkali-activated FA systems, which predominantly produce N–A–S–H gels. However, despite the good properties of GGBFS-based GP cement and its availability in large quantities in most parts of the world, GGBFS-based GP has not been widely used due to a lack of adequate studies. The main aim of most of the available studies has been to examine the effects of GGBFS addition on the chemical, mechanical, and microstructural properties of GP concrete. For this reason, conducting further investigation on the chemical, mechanical, and microstructural properties of GGBFS GPs to better understand the behaviour of this type of cement is imperative to popularize and promote the use of Algerian GGBFS as a cement material. The present work is devoted to the physico-chemical properties and durability of geopolymeric binders synthesized by the alkaline activation of GGBFS through an optimized formulation approach. This study looks at how the sand-to-GGBFS mass ratio affects the mechanical and rheological properties of the mortars, as well as how the SiO₂/Na₂O molar ratio in alkaline solution influenced these properties. Additionally, microstructural investigations, including ATR, XRD, and scanning electron microscopy – energy dispersive X-ray spectroscopy (SEM-EDS) analyses, were carried out. Water porosity and efflorescence tests were also performed to enhance the durability of the concrete, as these are key factors in concrete deterioration over time [34].

1.3 Research contribution

The novelty of the research consists of the adoption of an experimental approach to determine the physical and mechanical properties of a sustainable GPM made with Algerian GGBFS. The study is devoted to the physico-chemical properties and durability of geopolymeric binders synthesized by the alkaline activation of GGBFS through an optimized formulation approach.

2.1 Characterization of the materials used

Fine aggregates, GGBFS as a binder, and water and alkaline solution as catalysts used in this study were detailed in the following. The GBFS was purchased from the Siderurgica Steel Industry Complex (El-Hadjar, Algeria) (Figure 1a) and consisted of 10 mm maximal size of spherical grains of a light grey colour. It has therefore been previously ground before its use in cement systems. The GBFS characteristics obtained after grinding are as follows: 4,100 ± 100 cm²/g Blaine for specific surface area and (D10 = 1.807 µm, D50 = 15.489 µm, D90 = 54.939 µm) for the diameters (Figure 1b). A density of 2.9 g/cm³ was obtained according to the American society for testing and materials (ASTM) C204 standard [35]. Microstructural investigations (XRD, SEM, X-ray fluorescence [XRF]) revealed that the slag is amorphous with the slight presence of calcite (CaCO₃) (Figure 11). The amorphous nature of Algerian (GGBS) offers several advantages in geopolymerization compared to its crystalline counterparts. Its disordered atomic structure enhances reactivity, facilitating the release of essential ions such as silica, alumina, and calcium during alkali activation, which leads to a denser and stronger GP network [6,7,34]. Additionally, it ensures better chemical homogeneity, resulting in consistent performance and improved mechanical properties, including faster setting times and greater durability [7,12]. Unlike crystalline forms, amorphous GGBS does not require extra energy-intensive treatments to enhance its reactivity, making it a sustainable and efficient choice for geopolymer applications [7]. The chemical composition of GGBFS (Table 1) was analysed by XRF using a model S2 PUMA (XY) machine, BRUKER brand. The GGBFS of El Hadjar is essentially composed of lime (CaO = 42.11%), silica (SiO₂ = 36.85%), alumina (Al₂O₃ = 8.29%), and magnesia (MgO = 3.8%), which are crucial oxides during the hydration process to produce C–(A)–S–H gels, enhancing strength and durability [4]. Based on chemical analysis, Algerian GGBFS is highly suitable for GPM due to its high calcium oxide content (up to 42.11%), which supports the formation of both C–S–H and C–A–S–H gels, enhancing early strength and material densification [4,7,14,23]. This high calcium content enables hybrid-binding phases of C–A–S–H and N–A–S–H gels, which improve GP performance [4,10,25].

Figure 1

Experiment materials: (a) GBFS of the steel complex of El-Hadjar (Algeria) and (b) GGBFS.

The GGBFS from Algeria has adequate levels of silicon dioxide (36.85%) and aluminium oxide (8.29%), facilitating the formation of N–A–S–H, the primary binding phase in GPMs [4,25]. Additionally, low sulphur trioxide (SO₃) and magnesium oxide (MgO) contents reduce the risk of deleterious reactions, such as expansion or cracking due to delayed ettringite formation (DEF) or magnesia hydration [36]. The loss on ignition indicates high purity, ensuring better reactivity for alkali activation [37]. The amorphous structure of the slag (Figure 11), formed through rapid cooling, is highly reactive under alkaline conditions, aiding efficient geopolymerization [7,8,12,27,33,37]. Finally, a suitable alkali content facilitates the dissolution of the silica and alumina in the slag during the activation process [7,8,10,17,31,37]. If the slag contains trace alkali elements such as Na₂O and K₂O, these can enhance reactivity under alkaline conditions [12]. This is what is available in this slag, as it contains trace K₂O, estimated at 0.86%. The chemical composition of Algerian GGBFS is notable for its lack of sulphates, which, when reacting with excessive alkalis, typically produce salts that migrate to the surface and result in significant efflorescence [34]. The XRF results are consistent with those of EDX analysis (Figure 2). The strong absorption bond of the GGBFS at 872 cm⁻¹ is attributed to an elongation of the C–O bond (Figure 3), corresponding to the presence of calcite CaCO₃ phases [6], while the weak band of 1,428 cm⁻¹ is attributed to the elongation of the inorganic C–O bond [4]. The presence of amorphous AlO₆ octahedra is detected at 696 cm⁻¹, which is assigned to Si–O and Si–O–Al vibrations [4]. To characterize the composition of the slag, a quality coefficient (Mq) and basicity coefficient (Mb) are defined separately in Eqs. (1) and (2):

Figure 2

Morphology of Algerian GGBFS performed with SEM-EDS.

Figure 3

Attenuated total reflectance – Fourier transform infrared spectroscopy (ATIR-FTIR) spectra of Algerian GGBFS.

The Mq and Mb coefficients, equal to 1.46 and 1.11, respectively, indicate that GGBFS is classified as neutral and basic. The hydraulic activity index using the Keil method is given by

where S _b is the Blaine specific area (S _b = 4,100 cm²/g) and F the percentage of slag fines (friability < 80 µm in our case, F = 100%). The hydraulic activity index of the GGBFS of El-Hadjar (= 410) makes it a very active slag. An activator solution comprising commercially available sodium silicate (SS) and sodium hydroxide (SH) was utilized in the formulation of geopolymers. SH and SS are critical components in the geopolymerization process, each playing distinct yet complementary roles. SH ensures effective dissolution of aluminosilicate precursors and acts as a catalyst to accelerate chemical reactions [38]. Meanwhile, SS promotes gelation and enhances the structural integrity of the geopolymer matrix [38]. Their interaction is crucial for achieving a well-balanced geopolymerization process, resulting in strong and durable GPC [38]. This selection was made considering that the most used activators for GGBFS-based GPM are alkaline solutions such as sodium silicate, sodium hydroxide, potassium hydroxide, or their combinations [38]. Additionally, calcium hydroxide and sodium carbonate are frequently employed [38]. These activators enhance the mortar’s performance by improving compressive strength, adjusting setting time, and enhancing durability [38]. The combination of sodium silicate and sodium hydroxide, in particular, provides high strength and reduces porosity, while sodium carbonate enables controlled setting [38]. However, excessive alkalinity may compromise workability or durability [4,38]. The selection of an optimal activator ensures a balance of strength, workability, and longevity for specific applications. Therefore, in this study, the chosen solution (SS) had a density of 1,357 g/l, a molar ratio SiO₂/Na₂O of 3.35, and a composition by weight of 8% for Na₂O, 26.4% for SiO₂, and 61.6% for H₂O to generate varied molar ratios, and 99% pure (SH) pellets were added to the Na₂SiO₃ solution. To make GPMs, the mixture was allowed to cool to ambient temperature and then stored. In this investigation, the aggregates used for preparing mortar specimens were standardized CEN EN 196-1 sand, with a granular size of 0–2 and a siliceous composition. Table 2 presents the physical characteristics. To ensure purity and prevent contamination from mineral salts, distilled water was used to prepare and adapt the fluidity of the mortars.

2.2 Synthesis of GPMs

Ten mixtures were prepared by varying the sand/GGBFS mass ratio and the SiO₂/Na₂O molar ratio of the alkaline solution. In both steps, the water/solid geopolymer binder (W/C) ratio and that of GGBFS to the activator were set to 0.36 and 3, respectively. The effect of the sand/GGBFS mass ratios of 1.5, 2, 2.5, and 3 was also studied. All properties of the mixtures are gathered in Table 3. The selection of the optimum (sand/GGBFS) mass ratio was based on the results of flow ability, setting time, MSs, and water absorption. The optimal sand/GGBFS mass ratio determined in the first step was used to examine the impact of varying the following SiO₂/Na₂O molar ratios: 1.11, 1.14, 1.17, 1, 20, 1.23, 1.26, and 1.29, on the rheological and mechanical properties, water absorption, and efflorescence. The GP mortar was prepared in three steps after preliminary experiments to determine the most effective mixing method for Algerian GGBFS. It was found that mixing the alkaline solution first for 1 min ensures complete geopolymerization of the aluminosilicate. Adding sand to the paste followed by water for 2 min produces a new slurry. The mixture is then poured into metal moulds to create prismatic samples measuring 40 mm × 40 mm × 160 mm and 40 mm × 40 × 40 mm (Figure 4). Before assembly, the inside of the moulds was lightly oiled to aid easy demoulding of the mortars. The moulds were then securely assembled and placed on an impact table, where they were filled evenly with the slurry up to the halfway point. The first layer of slurry was compressed for 60 s (1 impact per second) to ensure proper compaction. The second layer of slurry was added and again compressed with 60 impacts to eliminate any air bubbles trapped during pouring. Once prepared, the specimens were sealed in special plastic oven bags to prevent water loss during the curing process. After 24 h in a humid environment at 20 ± 2°C, all specimens were demoulded and stored under the same conditions until testing. Because ambient curing is slower, it contributes to improved long-term strength and durability [7,38]. The choice of curing time and method should align with the specific application needs, ensuring a balance between early strength and long-term performance [1,7,38]. On the other hand, heat treatment and steam curing accelerate the process, promoting faster strength development and higher early strength, although they require careful monitoring to avoid the risk of thermal cracking [1,7,10,36,37,38].

Figure 4

Casting of GP mortar samples 40 mm × 40 mm × 160 mm.

2.3 Testing methods

2.3.1 Workability

The workability of GP mortars was measured using the modified flow table method of ASTM C1437-07[39], which is an effective method for determining the workability of cement paste and hydraulic cement mortars (Figure 5). This method relies on the determination of the average diameter from four diameters of flow measured symmetrically on two axes if the cone is lifted vertically immediately after pressing each layer of two layers 20 times with a pestle and dropping the flow table 25 times in 15 s. The setting time was determined by a standardized needle with VICAT apparatus according to the ASTM C191 [40].

Figure 5

Measuring the flow of the fresh GP mortar.

3.1 Fresh properties

3.1.1 Workability

3.1.1.1 Effect of sand/GGBFS mass ratio

Figure 6a illustrates the effect of the sand-to-GGBFS mass ratio on the workability of GP mortar.

Figure 6

(a) Flow diameter of GGBFS GP mortars for different sand/GGBFS mass ratio and (b) effect of SiO₂/Na₂O molar ratio on flow diameter of GGBFS mortars.

The overall results exemplified that increasing the sand-to-mass ratio reduced the flow diameter of the mortars because formulations with a low sand-to-GGBFS mass ratio contain more fine particles favourable to mortar workability. The reduction in the flow diameter was approximately 4.42, 5.72, and 14.16% when the sand-to-GGBFS mass ratio was raised from 1.5 to 2, 2 to 2.5, and 2.5 to 3, respectively. The observed trend can be explained by the fact that a higher sand-to-GGBFS ratio increased the water demand, as the sand in the mixture absorbed more water, including capillary water. Consequently, with a higher sand-to-GGBFS ratio, the aggregates absorbed more water, which impeded the flow of the GP mortar and decreased its workability [44].

The workability of GP-based GGBFS mortars can be described as very stiff, stiff, moderate, or high [45]. On the other hand, the flow diameter of all the GGBFS mortars ranged between 148.5 and 216 mm. The value of the flow diameter varying between 140 and 160 mm is considered appropriate for the workability of geopolymer materials for easy placement in moulds [45].

These optimal values of workability correspond to sand/GGBFS mass ratios of 2.5 and 3 (Figure 6a). As a general conclusion, the development trend of GP-based GGBFS mortars was comparable to that of PC mortars.

3.1.1.2 Effect of SiO₂/Na₂O molar ratio

The rheological behaviour of geopolymers is strongly influenced by the SiO₂/Na₂O molar ratio [5,8,15,44]. Figure 6b presents the test results on how the SiO₂/Na₂O molar ratio affects the workability of the evaluated mortars. The data show that the flow diameter increased from 123 to 161 mm and from 161 to 215 mm as the SiO₂/Na₂O molar ratio rose from 1.11 to 1.20 and from 1.20 to 1.29, respectively. The main causes of this increase are the reduction in SH concentration and the increased presence of free water, which cause the alkaline solution’s viscosity to drop when the SiO₂/Na₂O molar ratio rises. In other words, the high amount of silica in the activated solution (31.83%) caused a decrease in pH (12.14) and an increase in the viscosity of the solution [7]. These results are consistent with those reported by others [5].

The GP-based GGBFS mortars had flow diameters that were high, moderate, and stiff. The average diameters were 148.5, 161, and 179 mm, which were limited to SiO₂/Na₂O molar ratios of 1.17, 1.20, and 1.23, respectively.

3.1.2 Setting time

3.1.2.1 Effect of sand/GGBFS mass ratio

Figure 7a illustrates the impact of the raw materials’ composition on the initial and final setting times of GP-based GGBFS. The data indicate that both the initial and final setting times are extended when the slag content is decreased. The samples with a sand/GGBFS mass ratio of 1.5 show an initial setting time of 93 min, which is reduced to 71, 64, and 53 min as the sand/GGBFS mass ratio increases to 2, 2.5, and 3, respectively. However, the final setting time decreases from 146 min to 125, 111, and 107 min. The large amount of sand present in the mixture absorbs a lot of water and consequently leads to reduced setting times. Previous studies have shown that a high slag content in GP-based GGBS mixtures leads to reduced setting times [5,8], due to the high reactivity of the slag under alkaline conditions, where the rate of formation of reaction products and the setting processes are accelerated. This is because the slag’s Ca, Si, and Al units dissolve relatively quickly and are present in large quantities in the solution, particularly calcium.

Figure 7

(a) Effect of sand/GGBFS mass ratio on setting times and (b) effect of SiO₂/Na₂O molar ratio on setting times.

3.1.2.2 Effect of SiO₂/Na₂O molar ratio

Figure 7b shows the effect of the SiO₂/Na₂O ratio on the initial and final setting times of GP mortar-based GGBFS, which shows a faster time in comparison with PC. Their initial setting time is between 98 and 32 min, while the final setting time varies between 204 and 121 min.

Furthermore, the time difference between the initial and final setting times was not long in all cases due to the fast reactivity of GGBFS, which made all final times short, where the longest final time recorded was 204 min. Figure 7b shows that the increase in the SiO_2/Na₂O molar ratio in the mixture has a remarkable impact on the acceleration of the setting time. Integration of a low amount of sodium silicate into the activator solution (i.e., SiO₂/Na₂O molar ratio = 1.11) extended both the initial and final setting times to 121 and 204 min, respectively. A decrease in sodium silicate content in the activator solution resulted in a lower pH (as shown in Table 3) and a reduction in the amount of soluble silica available in the medium. Extended setting times observed with low amounts of soluble silicates are linked to the slow release of Ca²⁺ from GGBFS and its subsequent reactions with silicates [5]. Longer setting times for the intermediate SiO₂/Na₂O molar ratio value (1.17, 1.20) were also recorded in GGBFS mixtures. On the other hand, further increases in the amount of sodium silicate to reach the SiO₂/Na₂O molar ratio value of 1.23 caused a significant decrease in the initial and final setting times to 107 and 53 min, respectively. This was due to the lower pH of the activator solution, which contributes to the increase of soluble silica in the mixtures, resulting in an acceleration of Ca²⁺ release from the silica-reacting GGBFS [8]. The initial setting time of GGBFS-based GPM with a SiO₂/Na₂O molar ratio of 1.29 was very fast (32 min), according to ASTM C1157 [46]. This was due to the availability of additional silica (31.83%), causing a less alkaline medium, which accelerates the solidification process through rapid polycondensation to form the C–S–H core [5]. Figure 7b and Table 3 show the direct effect of the Na₂O content of the activator solution on the setting times of GGBFS mortars: while shorter setting times were obtained for Na₂O contents of 6.5 and 6.34%, longer times were recorded for Na₂O contents of 7.32 and 7.15%. A general assessment indicates that the setting times of alkali-activated GGBFS mixtures are more influenced by the composition of the GGBFS and the SiO₂/Na₂O ratio of the activating solution.

3.2 MS

3.2.1 Effect of sand/GGBFS mass ratio

Figure 8 illustrates the evolution of the MS of GPM by studying the effect of raw material (GGBFS) content on MS at different ages (3, 7, 28, and 90 days).

Figure 8

Effect of the sand/GGBFS mass ratio on compressive strength (left) and flexural strength (right).

Overall, the results showed that CS increased by 48, 28, and 33% after 3–7, 7–28, and 28–90 days of treatment, respectively. In fact, the significant and dramatic improvement in strength after 90 days of curing was primarily due to the ongoing polymerization process, the completion of chemical reactions, and the reduction of porosity, which develops at a low rate during the first few days and gradually increases up to 90 days. In addition, the high calcium oxide content in the GGBFS composition, which reacts slowly with the alkaline solution, produces more gels (C–A–S–H and/or C–S–H). This reaction contributes to the improved CS of the GP-matrix, as reported in several studies [4,5,7,14,25,27]. From Figure 8, it can be concluded that the minimum curing time required to achieve optimal performance for all GGBFS mixtures cured at room temperature is at least 7 days to attain the desired properties. This result is consistent across all types of geopolymer cements [45]. However, curing for 24–48 h may be adequate under elevated temperatures [45]. In general, it is crucial to balance curing time and temperature to ensure optimal performance [1,7,38,45]. In addition, there is an inverse correlation between the MS and the sand/GGBFS ratio where both CS and FS decrease with increasing the sand/GGBFS ratio. The CS reduction is on average 4.74, 2.42, 2.74, and 2.84%, while the FS reduction is 21.95, 15.34, 2.01, and 4.16% when the sand/GGBFS ratio increases by 2, 2.5, and 3, respectively. The findings suggest that a low sand/GGBFS ratio implies a greater presence of reactive species provided by the dissolution of GGBFS, which favours the development of more bonds, thus improving MS. On the other hand, MS of the samples increases by the addition of fine GGBFS particles that provide a large contact surface for hydraulic interactions. They also create a strong geopolymer bond by filling the space between the structures, which results in a transition zone with a denser interface encircling all the sand grains [5,29,33]. The ideal sand/GGBFS mass ratio for enhanced mechanical performance is typically 1.5; however, a ratio of 3 was chosen for two main reasons: (i) for rheological properties and (ii) for economic considerations aimed at reducing the use of raw materials. This choice also facilitates the comparison of strength results found in GP-based GGBFS mortars with OPC according to the recommendations described in the European Standard EN196-1.

3.2.2 Effect of SiO₂/Na₂O molar ratio

Previous research has indicated that the SiO₂/Na₂O molar ratio significantly influences the MS of GP products [8,15,44]. As depicted in Figure 9, MS increases proportionally with curing time. Specifically, the results indicate that MS continues to rise from 3 to 90 days of curing when using low SiO₂/Na₂O molar ratios (1.17–1.23). However, after 90 days of curing, the MS slightly decreases with higher molar ratios (1.29). This can be explained by the fact that lower SiO₂/Na₂O molar ratios result in higher SH concentrations, which enhance the dissolution of GGBFS aluminosilicates and promote the formation of long-chain GPs over time [7,38,44]. A significant increase in compressive strength at 28 days (approximately 22.28%) was found when the SiO₂/Na₂O molar ratio was raised from 1.11 to 1.23, and a similar trend was noted for FS. This enhancement in both CS and FS is associated with the increased development of the sodium aluminosilicate hydrate (N–A–S–H) gel phase with each increment in the SiO₂/Na₂O molar ratio [38]. The N–A–S–H gel serves as the primary binding phase in GP systems [4,5,38]. In addition, a higher molar ratio promotes optimal chemical reactions between silica and alkali activators, reducing the presence of unreacted particles and preventing the formation of excess silica that could negatively affect the strength of the material [44]. Additionally, this ratio leads to reduced porosity and a more refined pore size distribution, resulting in improved mechanical performance [8]. Samples prepared with SiO₂/Na₂O molar ratios of 1.11 and 1.14 showed high early strength compared with samples with other molar ratios. At the same time, these samples showed a lower ultimate strength than those prepared with SiO₂/Na₂O molar ratios of 1.17, 1.20, 1.23, and 1.26. This explains why the presence of a high concentration of SH accelerated the dissolution of SiO₂ and Al₂O₃ thus hindered polycondensation by suppressing [4,44]. Sample G-8 (94.04 MPa) exhibits higher compressive strength compared to G-9, G-4, and G-5 and have more N–A–S–H in their structures (Figure 12), but compared to sample G-13, they have less calcium carbonate. The high MS of GGBFS-based GPs is mainly due to the high-quality calcium oxide of GGBFS, which is the basis for the strength improvement due to the formation of C–(A)–S–H gel [4,5,26,27,45]. Moreover, significant amounts of dissolved SiO₂ increase when the sodium silicate content in the medley increases. The latter effectively supports the development of long-chain gel phases (N–A–S–H), which enhance MS [15,44]. These results are consistent with the microstructural analysis using SEM-EDX (Figure 13c), which shows that SiO₂/Na₂O molar ratios of 1.17 and 1.23 result in a denser and more cohesive structure. The decrease in compressive strength when the molar ratio is increased to 1.29 is due to the increase in the SiO₂ content by 31.83%, which leads to a decrease in the pH of the solution to a value of 12.41, which negatively affects the reaction kinetics. Figure 9 shows that the flexural strength of GGBFS GPMs for different SiO₂/Na₂O molar ratios varied between 7.67 and 13.03 MPa. The results show that no noticeable changes in the overall trend of the flexural strength compared to the compressive strength were observed. For GBFS-GPMs, the optimal SiO₂/Na₂O molar ratio typically ranges between 1.17 and 1.23. Within this range, the balance between silica content and alkali activators supports robust gel formation and minimizes negative effects such as efflorescence or structural weakening due to excess alkalis. The maximum flexural strengths after 28 and 90 days are, respectively, 11.79 and 13.03 MPa for the G-8 mortar. It is observed that the flexural strength results did not much improve at 90 days of cure time, meaning that they reached their highest values at day 28. The flexural strength of GGBFS mortars in the present study is higher than that reported in previous studies [4,16,29], due to the high fineness of GGBFS resulting from the grinding operation undertaken during preparation. In summary, the flexural strength of GGBFS-based GPM, when tested under the same standard (ASTM C348) and identical conditions, is 73.73% higher than that of Portland cement mortar, which reaches a maximum value of 7.5 Map at 90 days [47]

Figure 9

Effect of the SiO₂/Na₂O molar ratio on compressive strength (left) and flexural strength (right).

3.3 Durability

3.3.2 Efflorescence

In the case of alkaline activation of GGBFS, cations are physically absorbed onto the surface structure rather than being structurally incorporated, leading to a high extent of efflorescence [34]. Efflorescence in GGBFS-based GPs occurs when soluble salts, such as alkali hydroxides, migrate to the material’s surface and crystallize, forming visible white deposits [8,34]. One of the main causes of this phenomenon is the excessive use of alkali activators, such as sodium hydroxide or potassium hydroxide, in the mix [8,34]. Moreover, the alkali cations that reach the surface can react with atmospheric CO₂, forming alkali carbonates, which further contribute to efflorescence deposits [22,34]. Figure 10 shows the impact of SiO₂/Na₂O ratios on the amount of efflorescence observed in GGBFS-based geopolymers. The sample P13, with a 1.44 SS/SH mass ratio, exhibits the most surface deterioration due to the addition of large amounts of soluble silicates (SiO₂/Na₂O molar ratio = 1.29), leading to severe efflorescence as a result of the lowering of the solution’s pH (pH 12.14). Hasnaoui et al. [8] also explained that when an excessive alkali activator is used, the concentration of alkali cations in the pore solution increases. This makes it easier for the cations to migrate to the surface during drying or curing [34]. As moisture evaporates, the cations crystallize at the surface, resulting in efflorescence [8,34]. Furthermore, the retention of excess moisture within the geopolymer matrix facilitates the movement of soluble salts to the surface [34,37]. The geopolymer produced with a high SS/SH ratio has a lower reaction volume during the early reaction steps, with a shorter initial setting time of 32 min (Figure 7b), and a higher reaction volume during the later reaction steps. According to Liu et al. [34], this occurs when the Na⁺ content is lower (6.34%, Table 3). The efflorescence of samples P11 and P12, which exhibit higher MSs, is greater compared to that of samples P4 and P8. Therefore, an increased amount of alkali activator in the mix raises the concentration of soluble salts, exacerbating efflorescence and affecting both the aesthetic appearance and durability of GGBFS-based GPs [8,34,37].

Figure 10

Effect of SiO₂/Na₂O molar ratio on the amount of efflorescence of GGBFS mortar.

3.4 Microstructural study

3.4.1 XRD

Figure 11 presents the XRD analysis of the GPs (G-4, G-5, G-6, G-7, G-8, G-9, and G-10 of Table 3), cured for 90 days at 20°C. These analyses show the presence of a large bump at 2Ѳ between 20° and 40°; the latter represents the geopolymer phase where the Si–O–Ca and Si–O–Si(Al) bonds are formed and the tobermorite phase (Ca₅Si₆O₁₆(OH)₂-4H₂O) during alkaline activation by dissipating Ca, Si, and Al. From these models, we note that all the samples show strong quartz (SiO₂) peaks, with CaCO₃, Ca(OH)₂ tobermorite, (CaSiO₃), and C–A–S–H observed, which means that Ca²⁺ present in GGBFS reacted to form a C–A–S–H gel.

Figure 11

XRD models of precursor material (GGBFS) and GGBFS mortars for different SiO₂/Na₂O molar ratios.

In the samples G-5, G-8, G-9, and G-10, new peaks of alite are observed, indicating the presence of a high content of N–A–S–H gel in these mixtures, which resulted in a significant improved in the MS of these samples due to the inorganic network structure of the N–A–S–H gel. Note that this gel gives the alkali-activated materials’ excellent MS and durability, such as thermal stability and high resistance to chloride and sulphate attacks, far exceeding OPC [4,18,21,23,34,44]. It forms due to the reaction between aluminosilicate materials (such as FA or slag) and alkali activators [34,48]. The predominant chemical elements involved are silicon (Si), sodium (Na), and aluminium (Al). These elements participate in the geopolymerization process, forming strong Si–O–Al and Si–O–Si bonds, which are fundamental to the development of (N–A–S–H) gel. In this structure, Na⁺ ions help balance the negative charge of Al [44]. The gel creates a highly interconnected network of strong chemical bonds between silicon and aluminium atoms, providing high bonding strength between the material particles [34,44,48]. This dense and durable structure fills voids and reduces porosity, which enhances the material’s resistance to compressive forces [38]. Additionally, N–A–S–H gel is chemically stable and resistant to aggressive environments, such as acidic or alkaline conditions, making it more durable and less susceptible to chemical attacks such as sulphate corrosion or chloride-induced degradation [48]. There is also the formation of (CaOH)₂ in samples G-5, G-8, G-9, and G-10. Due to the dissolution of Ca²⁺ from GGBFS and its interaction with large amounts of OH in the alkaline solution of these mixtures, Ca(OH)₂ subsequently reacts with CO₂ in free atmosphere to form CaCO₃ [49]. The existence of Ca (OH)₂ and CaCO₃ together completely dissipates CaO and generates calcium silicates [49]. Additionally, CaCO₃, whether formed through carbonation or added directly as a filler such as limestone, helps improve performance by contributing to the final material’s strength, workability, and thermal stability, as well as by filling voids and reducing crack formation [4,6,49]. However, for the characterization of amorphous phases, other characterization techniques were used, such as ATR-FTIR, SEM, and EDS, which can provide invaluable information about the nanostructure and composition of the gel.

3.4.2 ATR-FT-IR Spectra

Figure 12 displays the ATR-FT-IR spectra for GGBFS and GP with compositions G-4, G-5, G-6, G-7, G-8, G-9, and G-10 from Table 3. Notable observations include prominent peaks at 2,361 and 3,376 cm⁻¹ in the geopolymer spectra, and a peak at 1,650 cm⁻¹ in the GGBFS spectra. These peaks correspond to the stretching and bending vibrations of O–H bands from adsorbed water, which results from the absorption of atmospheric moisture by the GGBFS or geopolymer samples [38]. It is also possible to observe that the strong band at 696 cm⁻¹ present in the ATR spectra of GGBFS, which is assigned to the asymmetric stretching vibration of bending Al–O–Si bonds, moved slightly to lower wave numbers, ranging from 389 to 461 cm⁻¹ of Si–O–Si(Al) bond in the spectra of GP mixtures [50,4]. It describes the generation of N–A–S–H geopolymer gel because of the geopolymerization of raw materials containing calcium carbonate, specifically in the form of calcite. This result agrees with those of Liu et al. [34]. They showed that the displacement of the N–A–S–H gel band results from the substitution of the Si atom by Al in the Si–O–Si(Al) bond, thus decreasing its angle and shifting the main band of the geopolymer system to a lower wavenumbers because the bond strength constant of the Si–O–Si bond is greater than that of the Al-O-Si bond. The principal bond localized at 872 cm⁻¹ and those at 2,109 and 1,428 cm⁻¹ in the IR spectrum of GGBFS correspond to the vibration of CO₃ groups exhibited in carbonate minerals [4,6,50]. This is attributed to carbonization, either from the absorption of atmospheric CO₂ by diopside during the grinding of GGBFS or from the natural dissolution of CO₂ as the magma cools [44]. Additionally, the XRD pattern of GGBS also revealed the presence of CaCO₃. A peak around 483 cm⁻¹ is also observed in the IR spectra of the G-6, G-7, G-4, G-5, G-8, and G-9 samples. This band is assigned to the flexing vibrations of O–Si–O groups, indicating the formation of C–S–H gel, which will transform into C–A–S–H gel over time [50]. While samples G-7 does not reveal the presence of such vibration groups that confirm the formation of C–S–H gel; therefore, this shows that the reaction was stopped, due to a decrease in pH. According to Liu et al. [34], C (N) A S H gels forms as a stable product when pH is high in synthetic blends. This observation of microstructure correlates well with the compressive and tensile flexural strengths found and may explain the low values relative to the G-10 mixture. The GP samples G-8 and G-9 showed additional asymmetric Si–O–Si(Al) bond extension vibration groups at 1,041 and 1,190 cm⁻¹, indicating the dense formation of N–A–S–H type gels [50], which increases its density (reducing porosity) and improving its mechanical properties. Moreover, the bands 1,944–1,988 cm⁻¹ and 1,971–1,973 cm⁻¹ are observed for the samples G-4, G-10, G-6, G-7, and G-9, which are the result of asymmetric C–O stretching due to the presence of CaCO₃ [50]. Produced as a result of the atmospheric carbonation of leftover sodium ions during the geopolymerization process [44]. All GGBFS samples show peaks in the 2,000–2,200 cm⁻¹ range corresponding to the C–O bond vibration due to carbonate phases (CaCO₃, Na₂CO₃) [50]. This large amount of CaCO₃ indicates the prevalence of carbonation in all GGBFS mortars previously observed in the efflorescence (Figure 10). On the other hand, the samples G-4, G-5, and G-8 exhibited the presence of groups of Si–O–Al bending vibrations between 540 and 577 cm⁻¹, which reveals the development of N–(C)–A–S–H gel [50]. This large amount of amorphous and nanocrystalline phases (C–(N, A)–S–H and N–A–S–H) forms high-density microstructures that contribute to improved mechanical properties [34,48].

Figure 12

ATR-FTIR spectra of GGBFS mortars for different SiO₂/Na₂O molar ratios.

3.4.3 SEM-EDS

SEM combined with EDX analysis was used to investigate the microstructure of the selected GP samples after 90 days of curing. Figure 13(a–d) presents the morphology, with three different magnifications, and the chemical composition of the GP matrix for samples G-6, G-4, G-8, and G-10, all cured at 20°C. Specimens studied by SEM-EDS show that all final products of GGBFS GP are in the form of a dense continuous bulk geopolymer matrix, indicating a homogeneous and compact microstructure with small pores and cracks. Obtaining a dense and consistent microstructure after a long curing period indicates that the geopolymer bond coated all the samples with less pronounced pore spaces [5]. Figure 13b–d shows that even after the mortar breaks, there is strong adhesion between the aggregate and the geopolymer matrix, and the difference in the transition zone between the geopolymer matrix and the aggregate is small, which can be explained by the effect of mechanical loading during compression testing or shrinkage during solidification due to water evaporation. The presence of pores is due to residual air bubbles, polymerization reactions, or water retention in the geopolymer during moulding [26]. Small quantities of ettringite, a key hydration product in cement, contribute significantly to the early strength development of concrete. However, excessive formation can result in DEF, potentially causing expansion and cracking [36,37,38,39,40,41,42,43,44,45,46,47,48,49,50]. The main elements detected according to the EDS results (Figure 13b–d) are O, Ca, and Si. GP gels are primarily composed of silicon (Si) and oxygen (O), forming the silicate networks that define their structure [1,25,48]. These elements create a silica-rich gel phase, essential to the material’s properties. Additionally, sodium (Na) and calcium (Ca) play crucial roles; sodium, often introduced through activators such as sodium hydroxide, facilitates the geopolymerization process, while calcium, typically sourced from materials such as blast-furnace slag, enhances setting, material’s MS, and workability [7,12,15,34,48]. It can influence the formation of calcium-silicate-hydrate (C–S–H) phases, which are associated with improved compressive strength and resistance to cracking [4,14,49]. The presence of these elements, identifiable through EDS, serves as a hallmark of GP formation [7,25,38,48]. The presence of chemical compounds such as Si, O, Al, and Na (Figure 13–d) is vital for the geopolymerization reaction, resulting in the formation of strong Si–O–Al and Si–O–Si bonds. These components are crucial for producing sodium aluminosilicate hydrate gel (N–A–S–H), with Na⁺ added to balance the negative charge from Al⁻ [48]. This gel gives the matrix a dense and continuous microstructure after condensation, which can explain the enhancement in MS and durability of the G-10, G-8, and G-4 mixtures [25,48]. Moreover, the presence of adequate Ca in all samples promotes the transformation of N–A–S–H into C–A–S–H. At low pH, this phase behaves like a zeolite, engaging in ion exchange where Ca displaces Na until the available Ca is depleted [34]. The presence of Ca strengthens the bonds between particles, contributing to a more cohesive and durable matrix [48]. The formation of C–A–S–H gel in GGBFS cement plays a crucial role in enhancing the material’s properties [25,48]. Its ability to increase density, reduce porosity, and improve strength and toughness is due to the stable, dense nature of the gel that fills voids [25,48]. The variation in mechanical and physical properties of geopolymer composites with different SiO₂/Na₂O molar ratios can potentially be explained by these results, which show the difference in the microstructure of GP.

Figure 13

(a) SEM micrographs with EDS analysis of GP mortar corresponding to G-6 mix after 90 days of curing at 20°C, (b) SEM micrographs with EDS analysis of GP mortar corresponding to G-4 mix after 90 days of curing at 20°C, (c) SEM micrographs with EDS analysis of GP mortar corresponding to G-8 mix after 90 days of curing at 20°C, and (d) SEM micrographs with EDS analysis of GP mortar corresponding to G-10 mix after 90 days of curing at 20°C.