Integrated Mechanical and Eco-Economical Assessments of Fly Ash-Based Geopolymer Concrete

Abstract

This research evaluates the mechanical properties, environmental impacts, and cost-effectiveness of Hub Coal fly ash (FA)-based geopolymer concrete (FAGPC) as a sustainable alternative to ordinary Portland cement (OPC) concrete. This local FA has not been investigated previously. A total of 24 FAGPC mixes were tested under both ambient and heat curing conditions, varying the molarities of sodium hydroxide (NaOH) solution (10-M, 12-M 14-M and 16-M), sodium silicate to sodium hydroxide (Na₂SiO₃/NaOH) ratios (1.5, 2.0, and 2.5), and alkaline activator solution to fly ash (AAS/FA) ratios (0.5 and 0.6). The test results demonstrated that increasing NaOH molarity enhances the compressive strength (CS.) by 145% under ambient curing, with a peak CS. of 32.8 MPa at 16-M NaOH, and similarly, flexural strength (FS.) increases by 90% with a maximum FS. of 6.5 MPa at 14-M NaOH. Conversely, increasing the Na₂SiO₃/NaOH ratio to 2.5 reduced the CS. and FS. of ambient-cured specimens by 12.5% and 10.5%, respectively. Microstructural analysis revealed that higher NaOH molarity produced a denser, more homogeneous matrix, supported by increased Si–O–Al bond formation observed through energy-dispersive X-ray spectrometry. Environmentally, FAGPC demonstrated a 35–40% reduction in embodied CO₂ emissions compared to OPC, although the production costs of FAGPC were 30–35% higher, largely due to the expense of alkaline activators. These findings highlight the potential of FAGPC as a low-carbon alternative to OPC concrete, balancing enhanced mechanical performance with sustainability. New, green, and cheap activation solutions are sought for a new generation of more sustainable and affordable FAGPC.

1. Introduction

Concrete is a versatile material and the second most utilized substance globally after water. The rapid expansion of infrastructure has led to an increasing demand for ordinary Portland cement (OPC) concrete. However, the production of OPC is highly energy-intensive, generating significant amounts of greenhouse gases, particularly carbon dioxide (CO₂), which is a primary contributor to global warming and the rise in Earth’s surface temperature [1,2,3,4]. Ghafoor et al. [5] reported that the OPC production is projected to increase by 200% from the years 2015 to 2025 due to a surge in the construction industry. On average, the production of 1 ton of OPC generates about 1 ton of CO₂ into the environment [5,6]. This matter has accelerated global warming, resulting in the speedy melting of glaciers, flash floods, and extreme heatwaves.

To address the environmental challenges associated with an increasing OPC production, researchers studied the mechanical properties of cementless alternatives, i.e., geopolymer concrete (GPC), using different precursors such as bagasse ash, fly ash (FA), rice husk ash, metakaolin, and silica fume [1,5,7]. In 1979, Davidovits introduced GPC comprising precursors rich in alumina and silica ions, which chemically react with alkaline activator to form sodium alumina silicate hydrate (N-A-S-H) gel [8,9,10]. GPC’s strength mainly depends on the chemical composition and particle size distributions of FA. Compared to OPC, GPC offers several advantages, including a lower carbon footprint, improved mechanical properties, higher resistance to fire and chemical attacks, and enhanced durability.

FA is a by-product of coal-fired power plants and can act as a precursor in GPC production, so-called FAGPC. Class C FA is high in calcium content, whereas Class F FA primarily comprises oxides of silica and alumina and has low calcium content [1]. The physical and chemical properties of FA vary based on the source of the coal, degree of pulverization, combustion conditions, collection of FA, handling and disposal methods at the plant, synthesis temperature, duration, and molarity of AAS [11,12,13,14]. Therefore, the use of FA in FAGPC requires optimization for a particular source.

In the last two decades, numerous studies investigated the influence of alkaline activator solution (AAS), such as sodium hydroxide (NH) and Na₂SiO₃ (NS), on various mechanical properties of GPC. Raj et al. [15] found that 12-M NH resulted in an optimum compressive strength (CS.) of 56.1 MPa at 28 days. Naenudon et al. [16] reported a CS. of 63.5 MPa at 15 M NH, while Kanwal et al. [1] observed a maximum CS. of 27.8 MPa with 14-M NH in ambient-cured GPC. Several studies, including Amin et al. [17], Rajmohan et al. [18], and Gill and Parveen [19], found that heat-curing at 60–80 °C significantly improved CS., with optimum values reported between 56 MPa and 66.5 MPa using 12-M to 14-M NH. Other studies, including Ghafoor et al. [5] and Wang et al. [20], found similar trends with 14-M NH yielding CS. values around 56 MPa. Elyamany et al. [21] reported the optimum CS. and flexural strength (FS.) of 31.1 MPa and 6.5 MPa, respectively, at 16-M NH solution. Xie and Ozbakkaloglu [22] reported an optimum CS. of GPC cured at 25 °C for 24 h of 48.2 MPa with 14-M NH solution. Fang et al. [23] reported that optimum CS. and FS. of 56 MPa and 2.3 MPa, respectively, were attained at 12-M NH solution. Across these studies, the trend consistently shows that increasing NH molarities (8-M to 16-M) generally resulted in higher CS. in FAGPC, with numerous research studies reporting the optimum CS. at 14-M or 16-M NH. However, higher molarity of AAS is associated with more embodied CO₂ emissions. Therefore, optimizing the performance of FAGPC and embodied CO₂ is an important task.

In addition to NH molarity, numerous studies investigated the influence of Na₂SiO₃/NaOH (NS/NH) ratios, ranging from 1.5 to 2.5, on the mechanical properties of FAGPC. Yazid et al. [24] reported the optimum CS. and FS. of 67.7 MPa and 4.43 MPa, respectively, at a NS/NH ratio of 2.5. Abdulrahman et al. [25] reported an optimum CS. of ambient-cured Class C FAGPC of 38.2 MPa at a NS/NH ratio of 1.5. Hadi et al. [26] investigated varying NS/NH ratios and reported an optimum CS. of 67 MPa at a NS/NH ratio of 1.5. Similarly, Fang et al. [23] found the optimum CS. and FS. of 56 MPa and 2.3 MPa, respectively, at a NS/NH ratio of 1.5. It was observed that the CS. decreased by about 32% with increasing NS/NH ratio from 1.5 to 2.5, as the excessive number of hydroxide ions and silicates hindered the geopolymerization process. Similarly, Deb et al. [27] confirmed an optimum CS. of 51 MPa at a NS/NH ratio of 1.5. Meanwhile, Li et al. [6] investigated the influence of varying NS/NH ratios (0.5 to 3.0) and found a different finding, e.g., the optimum CS. of 56 MPa was observed at a NS/NH ratio of 2.5. In general, most studies indicated that the NS/NH ratio of 1.5 provides the best mechanical performance, while fewer studies reported different optimal ratios.

Previous studies also investigated the influence of varying AAS/FA ratios, ranging from 0.3 to 0.7, on the properties of FAGPC. Yazid et al. [24] stated the optimum CS. and FS. of FAGPC of 67.7 MPa and 4.4 MPa, respectively, at an AAS/FA ratio of 0.5. Abdulrehman et al. [25] reported an optimum CS. of 38.2 MPa at an AAS/FA ratio of 0.3. Aliabdo et al. [28] investigated the influence of varying AAS/FA ratios (0.3–0.45) on the CS. and FS. of heat-cured FAGPC (50 °C for 48 h) and noted the optimum CS. and FS. of 32 MPa and 2.7 MPa, respectively, at an AAS/FA ratio of 0.4. Meanwhile, Hadi et al. [26] observed an optimum CS. of 35 MPa at a higher AAS/FA ratio of 0.6. Ghafoor et al. [5] reported that the FS. increased by about 20.2% as the AAS/FA ratio was increased from 0.4 to 0.5, while FS. decreased when further increasing the AAS/FA ratio from 0.5 to 0.6. As can be seen, various research studies reported different optimal AAS/FA ratios. Hence, it is important to determine the optimal AAS/FA ratios for various FA sources in Pakistan for the large-scale production of GPC.

Previous studies also reported the influence of curing conditions on the mechanical properties of FAGPC. Sajan et al. [29] reported an optimum CS. of 36.2 MPa at 60 °C when varying the curing temperature from 20 °C to 60 °C. Cui et al. [30] reported an optimum CS. of 46.2 MPa cured at 80 °C for 24 h. Amin et al. [17] reported an optimum CS. of 66.5 MPa cured at 80 °C for 24 h. Meanwhile, attempting higher temperatures, Chen et al. [31] varied the curing temperatures (60 °C, 75 °C, 90 °C, and 105 °C for 24 h) and found an optimum CS. of 110.2 MPa at 75 °C. Similarly, Rajmohan et al. [18] considered the curing temperatures between 60 °C and 100 °C and found an optimum CS. of 56.18 MPa at 60 °C. On the other side, Chithambaram et al. [32] reported that the strength of FAGPC increased with curing temperature within the investigated range. These studies suggested different optimum curing temperatures ranging from 60 °C to 80 °C, which require further studies to unveil the universal suggestion for curing temperature as well as customize the curing condition for a new source of FA, i.e., HUB coal plant in Pakistan.

To facilitate the large-scale production of GPC made of FA sourced from the HUB coal plant in Pakistan, this study investigates the mechanical properties of HUB-FAGPC to determine the optimal mix design constituents (NH, NS/NH, and AAS/FA) and curing conditions. This work is a part of the bigger project sponsored by the China–Pakistan Economic Corridor Collaborative Research Grant to examine the optimal mix design for FAGPC of different sources, e.g., Sahiwal coal power plant (CPP), Port Qasim CPP, Hub CPP, and Thar CPP. Investigations on FA sourced from Sahiwal CPP and Port Qasim CPP were documented in Ghafoor et al. [5] and Khan et al. [4]. Even though using a similar technology, FA from these sources is different in chemical composition and reactivity. For example, FAGPC developed using FA from Sahiwal CPP with Punjab region aggregates and FA from Port Qasim CPP with Sindh region aggregates lead to different optimal mix designs as reported in our earlier studies (Ghafoor et al. [5]; Khan et al. [4]). This study explores the effects of varying NH molarities, NS/NH ratios, AAS/FA ratios, and different curing regimes, ratios, and curing conditions on GPC developed using FA sourced from HUB CPP with Baluchistan aggregates.

It is pertinent to note that significant gaps remain in understanding the specific performance of regionally sourced FA, such as HUB CPP, which is influenced by distinct combustion conditions and chemical compositions. Moreover, 1835 tons of FA sourced from the Hub CPP are produced as industrial waste. This research study investigates the sustainable use of Hub-sourced FA as a 100% replacement of OPC in GPC. Furthermore, most existing studies have not comprehensively examined both the environmental and economic implications of GPC production in a localized context. This study uniquely fills these gaps by analyzing the mechanical properties of 100% FA-based GPC under ambient and heat curing at 110 °C, while also conducting a detailed eco-economical assessment as a case study. The durability assessment of GPC is beyond the scope of this research investigation.

2. Materials and Methods

This section provides detailed information about the experimental program, material sources and testing, and curing and testing of FAGPC specimens.

2.1. Experimental Program

To study the mechanical properties of FAGPC, a total of 24 FA-based GPC mixes comprising 288 GPC specimens with varying NaOH (NH) molarities (10-M to 16-M), NS/NH ratios (1.5 to 2.5) and AAS/FA ratios (0.5 and 0.6) selected based on the extensive review of literature, authors research group studies and pilot test results were prepared. The GPC specimens were cured under different regimes and tested (see

Table 1. Test matrix.

Name	Mix ID	FA (kg/m3)	Coarse Aggregate (kg/m3)	Fine Aggregate (kg/m3)	NaOH Sol. (kg/m3)	Na2SiO3 Sol. (kg/m3)
Mix A	GPC-10-1.5-0.5	368	1294	554	73.6	110.4
Mix B	GPC-10-2.0-0.5				61.3	122.6
Mix C	GPC-10-2.5-0.5				52.5	131.4
Mix D	GPC-10-1.5-0.6	345	1294	554	82.8	124.2
Mix E	GPC-10-2.0-0.6				69	138
Mix F	GPC-10-2.5-0.6				59.1	147.8
Mix G	GPC-12-1.5-0.5	368	1294	554	73.6	110.4
Mix H	GPC-12-2.0-0.5				61.3	122.6
Mix I	GPC-12-2.5-0.5				52.5	131.4
Mix J	GPC-12-1.5-0.6	345	1294	554	82.8	124.2
Mix K	GPC-12-2.0-0.6				69	138
Mix L	GPC-12-2.5-0.6				59.1	147.8
Mix M	GPC-14-1.5-0.5	368	1294	554	73.6	110.4
Mix N	GPC-14-2.0-0.5				61.3	122.6
Mix O	GPC-14-2.5-0.5				52.5	131.4
Mix P	GPC-14-1.5-0.6	345	1294	554	82.8	124.2
Mix Q	GPC-14-2.0-0.6				69	138
Mix R	GPC-14-2.5-0.6				59.1	147.8
Mix S	GPC-16-1.5-0.5	368	1294	554	73.6	110.4
Mix T	GPC-16-2.0-0.5				61.3	122.6
Mix U	GPC-16-2.5-0.5				52.5	131.4
Mix V	GPC-16-1.5-0.6	345	1294	554	82.8	124.2
Mix W	GPC-16-2.0-0.6				69	138
Mix X	GPC-16-2.5-0.6				59.1	147.8

).

The designation of test specimens comprised three numbers. The first number represents the NaOH (NH) molarity (10-M, 12-M, 14-M and 16-M), the second number indicates Na₂SiO₃/NaOH (NS/NH) ratio (1.5, 2.0, and 2.5), and the third number reflects AAS/FA ratio (0.5 and 0.6). For example, Mix GPC-10-1.5-0.5 infers a mix cast with 10 M NH solution, NS/NH ratio of 1.5, and AAS/FA ratio of 0.5 (Table 1).

2.2. Materials

2.2.1. Fly Ash

FA sourced from the HUB CPP was used as a precursor (100% replacement of OPC) in the production of GPC. The chemical compositions of FA were determined using XRD analyses [33] and according to ASTM C114-23 [34] (see

Table 2. Chemical composition.

Element	CaO (%)	MgO (%)	SiO2 (%)	SO3 (%)	Al2O3 (%)	Fe2O3 (%)	L.O.I (%)
Fly Ash	5.1	1.05	71.2	1.9	8.1	4.85	5.8

). The sum of SiO₂, Al₂O₃, and Fe₂O₃ was 84% and significantly greater than 70%, the CaO content was 5.1% (<18%), the SO₃ content was 1.9% (<6%), loss on ignition (LOI) was 5.8% (<6%). The FA was classified as Class F according to ASTM C-618-22 [35].

The XRD analysis of FA mainly comprises quartz/silicon dioxide (silica), aluminum oxide (alumina), iron oxide/hematite (Fe₂O₃), and lime (CaO) as illustrated in Figure 1. The FA used in this study displayed a crystalline structure, primarily comprising quartz, hematite, and alumina. It is suggested that Hub FA was expected to be less reactive than similar FAs that have an amorphous structure. In general, FA often contains a mix of amorphous and crystalline components, with crystalline phases commonly observed in FA derived from bituminous coal. The presence of crystalline phases in FA has been reported in the existing research studies [33,36]. High SiO₂, Al₂O₃, and Fe₂O₃ contents increase the sodium alumina silicate hydrate (N-A-S-H) gel formation and hence enhance the compressive strength of the mix [9,33].

2.2.2. Aggregates

Coarse and fine aggregates comprised 77.5% by mass of the GPC mix. The coarse aggregates were obtained from a local vendor in Kuchlak, Quetta, Balochistan. Their size was limited to less than 12.5 mm. The aggregate impact value and crushing value were 18 and 22.3, respectively, as per BS 812-112:1990 [37]. The loose bulk and dry densities of coarse aggregates were 1484 kg/m³ and 1575 kg/m³, respectively, as per ASTM C29-17 [38].

A petrographic analysis of coarse aggregates, performed as per ASTM C295-19 [39], revealed that aggregates mainly comprised calcite/micrite (98%), which were coarsely crystalline and ranged from light brown to yellowish brown. Other minerals included were irregular fine grains of dolomite (1%), fine traces of quartz (0.5%), and iron oxide (0.3). The coarse aggregates exhibited very low potential for alkali silica reactivity.

Fine aggregate (sand) was sourced from Kuchlak, Balochistan. The fineness modulus of fine aggregate was 2.78 as per ASTM C136-19 [40].

2.2.3. Alkaline Activator Solution

The alkaline activator solution (AAS) comprised Na₂SiO₃ (NS) and NaOH (NH). The NS solution and NaOH solids were purchased from local vendors in Lahore, Pakistan. The NS solution comprised 46% solids and 54% water by mass. The ratio of silicon dioxide to sodium oxide in NS was 3:1.

Four NH solutions of different molarities (10-M to 16-M) were prepared by dissolving the required quantity of NH solids in 1000 mL of distilled water. The NH solutions of 10-M, 12-M, 14-M and 16-M comprised 30.5%, 36.6%, 42.75% and 48.85% solids, respectively [4].

2.3. Mix Designs

In this research, factorial design was used to investigate the influence of NH molarity (Factor 1), NS/NH ratio (Factor 2), AAS/FA ratio (Factor 3), and curing regime (Factor 4). A total of 24 GPC mixes comprising 288 GPC specimens (144 cylinders and 144 prisms) with the target CS. of 21 MPa and 28 MPa were cast and tested, see Table 1. Each GPC mix comprised six GPC cylinders (100 mm dia. and 200 mm height), and six GPC prisms (75 mm × 75 mm cross-section and 300 mm length). Half of the test specimens per mix were ambient-cured for 28 days, whereas the other half of the test specimens per mix were heat-cured.

2.3.1. Preparation of Test Samples

The AA solutions were prepared 24 h prior to the casting of GPC specimens. Initially, a dry mix of FA, coarse aggregates, and sand was blended in a pan mixer for 1–2 min (Figure 2a). Afterwards, the AA solution was added to the dry mix along with an additional water, i.e., 15–20% of the mass of FA, to attain the slump value of 50 mm. The mixing process continued for another 1–2 min until a homogenous GPC mix was formed (Figure 2b). The fresh GPC mix was then poured into cylindrical and prism molds in three layers, and each layer was simultaneously compacted using tamping rods and a vibrating table (Figure 2c). The specimens were demolded after 24 h of casting, and the mass and dimensions (diameter and height) of the GPC cylinders were measured to compute the wet density of GPC as per ASTM C-138-17 [41].

2.3.2. Curing

Half the number of GPC specimens were ambient-cured (A/C) (see Figure 3a), with the other half the number of the specimens being heat-cured (H/C) as depicted in Figure 3b. In the A/C, temperatures in the Laboratory at 23 ± 2 °C and relative humidity ≥ 90 were maintained using wet hessian rugs for 28 days. In the H/C, specimens were placed in an oven and the temperature was gradually increased to 110 °C in 90 min, and 110 °C temperature was maintained in the Oven for 24 h, followed by A/C (temperatures in the Laboratory were 23 ± 2 °C and relative humidity ≥ 90 was maintained using wet hessian rugs) for 27 days. To ensure the uniformity of the curing process amongst all the 24 GPC mixes, the record of casting, demolding, A/C, and H/C for all mixes was properly maintained.

The dry density of GPC cylinders was measured after 28 days as per ASTM C-138-17 [41].

2.4. Instrumentation and Testing Procedures

The GPC cylinder specimens were subjected to axial compression using a 1000 kN Shimadzu Universal Testing Machine (UTM) as per ASTM C39-20 [42]. Each cylinder was instrumented with two Linear Variable Displacement Transducers (LVDTs) placed diagonally at 180° apart. Testing was conducted under displacement-controlled loading at a rate of 1 mm/minute (Figure 4a). The flexural strength (FS) was determined using the four-point loading under the same UTM, as per ASTM C78-22 [43], with displacement-controlled loading at 1 mm/minute. The GPC prisms were divided into three equal parts of 90 mm length with 15 mm cantilever on both sides of the supports (Figure 4b).

3. Results and Discussions

The test results of wet density, dry density, CS, and FS are presented in

Table 3. Test details of densities and mechanical properties of A/C and H/C GPC.

Name	Mix ID	Wet Density (kg/m3)		Dry Density (kg/m3)		Compressive Strength (MPa)		Flexural Strength (MPa)
		A/C	H/C	A/C	H/C	A/C	H/C	A/C	H/C
Mix A	GPC-10-1.5-0.5	2397	2395	2357	2376	9.8	11	3.3	3.4
Mix B	GPC-10-2.0-0.5	2414	2446	2389	2420	9.1	9.3	3	3.3
Mix C	GPC-10-2.5-0.5	2433	2401	2408	2357	7.6	7.7	2.3	2.5
Mix D	GPC-10-1.5-0.6	2408	2433	2369	2382	13.1	18.9	3.4	3.9
Mix E	GPC-10-2.0-0.6	2433	2401	2382	2350	12.3	13.7	3.5	3.5
Mix F	GPC-10-2.5-0.6	2408	2414	2395	2362	7.7	8.9	2.8	3
Mix G	GPC-12-1.5-0.5	2412	2408	2363	2376	14.5	15.4	3.9	4.5
Mix H	GPC-12-2.0-0.5	2420	2414	2395	2382	13.9	14.9	3.6	4
Mix I	GPC-12-2.5-0.5	2414	2476	2376	2427	12.3	13.7	3.5	3.8
Mix J	GPC-12-1.5-0.6	2427	2433	2427	2414	25.9	26.3	6.1	6.3
Mix K	GPC-12-2.0-0.6	2433	2452	2395	2433	23.7	22	5.4	6.1
Mix L	GPC-12-2.5-0.6	2420	2439	2369	2414	18.3	15.9	4.5	5.2
Mix M	GPC-14-1.5-0.5	2414	2427	2382	2401	20.3	22.8	4.5	5.2
Mix N	GPC-14-2.0-0.5	2439	2439	2414	2389	16.5	16.5	4.4	4.9
Mix O	GPC-14-2.5-0.5	2433	2433	2389	2409	15.8	15.9	3.9	4.5
Mix P	GPC-14-1.5-0.6	2471	2427	2427	2414	30.8	31.4	6.5	6.7
Mix Q	GPC-14-2.0-0.6	2465	2439	2446	2401	27.5	23.1	5.6	6.5
Mix R	GPC-14-2.5-0.6	2439	2420	2395	2395	20.8	20.3	4.7	5.4
Mix S	GPC-16-1.5-0.5	2452	2439	2412	2433	23.3	23.7	4.7	5.8
Mix T	GPC-16-2.0-0.5	2433	2452	2414	2439	25.6	26.9	4.5	5.6
Mix U	GPC-16-2.5-0.5	2433	2429	2420	2401	21.8	22.4	4.3	5.5
Mix V	GPC-16-1.5-0.6	2477	2452	2435	2433	32.8	25	5.9	6.3
Mix W	GPC-16-2.0-0.6	2459	2408	2427	2389	31.4	22.3	5.5	5.9
Mix X	GPC-16-2.5-0.6	2443	2437	2414	2389	27.4	18.8	4.5	5.0

and discussed in the following subsections.

3.1. Failure of Tested GPC Specimens

The GPC cylinders failed under axial compression in a similar manner to OPC concrete, with cracks initiated at the weaker interfaces of the geopolymer matrix and coarse aggregates. In general, the hairline cracks appeared, which continued towards the middle of the GPC cylinders (Figure 5a). The GPC prisms failed within the middle span (maximum bending region) under four-point loading as shown in Figure 5b.

3.2. Wet and Dry Densities of Ambient-Cured (A/C) and Heat-Cured (H/C) GPC

The densities of GPC specimens remained unaffected by the curing conditions, as both ambient-cured (A/C) and heat-cured (H/C) specimens exhibited similar densities as tabulated in Table 3. The average wet densities of the A/C specimens ranged from 2397 kg/m³ to 2477 kg/m³, while the H/C specimens showed comparable wet densities, ranging from 2395 kg/m³ to 2476 kg/m³ (Figure 6). Similarly, the average dry densities of the A/C specimens ranged from 2357 kg/m³ to 2446 kg/m³, closely aligning with the H/C specimens, which ranged from 2350 kg/m³ to 2439 kg/m³.

Additionally, variations in NaOH (NH) molarities, NS/NH ratios, and AA/FA ratios did not considerably affect the wet or dry densities of either the A/C or H/C GPC specimens, confirming the findings reported by Ghafoor et al. [5].

3.3. Compressive Strength (CS)

The variation in the compressive strengths (CS) of 24 GPC mixes with varying Na₂SiO₃/NaOH (NS/NH) ratios, NaOH (NH) molarities, and alkaline activator solution to fly ash (AAS/FA) ratios cured under either ambient (A/C) or heat (H/C) conditions is presented in Figure 7.

3.3.1. Influence of NS/NH Ratios

The CS. of both A/C and H/C GPC specimens reduced with an increase in the NS/NH ratios, with only one exception of GPC-16-2.0-0.5 (see Figure 8). For A/C GPC, an increase in the NS/NH ratios from 1.5 to 2.0 led to a 8.1% reduction in the average CS., with a further increase from 2.0 to 2.5 resulting in an additional 18.5% reduction. Likewise, the average CS. of H/C GPC decreased by about 15.9% when the NS/NH ratio increased from 1.5 to 2.0, followed by a 17.1% reduction as the ratio increased to 2.5. Both the A/C and H/C specimens exhibited optimal CS. at a NS/NH ratio of 1.5. Increasing the NS/NH ratio from 1.5 to 2.5 lowered the CS. due to the reduced presence of NH solution, and consequently fewer hydroxide ions (OH⁻), which are essential in the formation of primary three-dimensional networks of sodium alumina silicate hydrate (N-A-S-H) gel that imparts strength to GPC.

Consistent findings have been reported in the literature. For instance, Deb et al. [27] noted a 19% reduction in CS. as the NS/NH ratio increased from 1.5 to 2.5, identifying an optimal CS. at 1.5. Similarly, Fang et al. [23] reported a decline in the CS. with an increasing NS/NH ratio over the same range. Poloju et al. [44] noted an average CS. reduction of approximately 5.6% as the NS/NH ratio was raised from 1.5 to 2.5. In agreement, Pavithra et al. [45] found 46 MPa as the optimal CS. at a NS/NH ratio of 1.5, observing reduced CS. at higher ratios. Nath et al. [46] attributed a 10% decrease in CS. at higher ratios due to the excessive concentration of OH⁻ and silicate (SiO₃)⁻² ions within the matrix, which impedes the formation of robust geopolymer bonds during geopolymerization.

The exceptional performance of A/C and H/C GPC-16-2.0-0.5 can be attributed to the synergistic effect of high NH molarity (16-M) and a moderate NS/NH ratio (2.0). The elevated NH concentration enhances the dissolution of silica and alumina ions from fly ash, leading to robust N-A-S-H gel formation, which strengthens the matrix. At the same time, the NS/NH ratio of 2.0 provides sufficient silicate ions without oversaturating the system, maintaining an optimal Si/Al ratio for geopolymerization. These effects are consistent across both curing conditions and likely result in a denser, more homogeneous microstructure with fewer unreacted fly ash particles or microcracks. This unique combination enhances the compressive strength beyond the expected trend, making GPC-16-2.0-0.5 an outlier in the study.

3.3.2. Influence of NaOH (NH) Molarities

The NH molarity showed a significant influence on the CS. of both A/C and H/C GPC, with three exceptions of H/C mixes GPC-16-1.5-0.6, GPC-16-2.0-0.6, and GPC-16-2.5-0.6 as presented in Figure 9. For the A/C GPC specimens, the average CS. increased by about 81.7%, 22.6% and 26.7%, respectively, as the NH molarity rose from 10 to 12M, 12 to 14M, and 14 to 16M. Similarly, H/C GPC specimens showed an increase in CS of approximately 59.4%, 21.2% and 36.0%, respectively, as the NH molarity rose from 10 to 12M, 12 to 14M, and 14 to 16 M. The increase in NH molarity promoted greater dissolution of silica (Si) and alumina (Al) ions, promoting N-A-S-H gel formation. This results in a denser geopolymer microstructure and improved CS. due to stronger mechanical interlocking [15].

Consistent findings are reported in the literature. Khan et al. [4] reported that increasing NH molarity from 10-M to 16-M improved the dissolution of silica (Si) and alumina (Al) ions, which produced a maximum CS. of 57 MPa at 12 M. Similarly, Farhan et al. [47] noted that the CS. of GPC increased with rising NH solution from 12 to 14-M due to enhance geopolymer gel (N-A-S-H) formation. In line with these observations, Ghafoor et al. [5] reported that the CS. of FA-based GPC increased by about 106% with increasing molarity of NH solution from 8-M to 14-M.

The three exceptions observed in H/C mixes GPC-16-1.5-0.6, GPC-16-2.0-0.6, and GPC-16-2.5-0.6 can be attributed to the combined effects of high NH molarity (16 M) and an increased AAS/FA ratio (0.6). While higher molarity typically improves geopolymerization, the elevated AAS/FA ratio increases the concentration of OH⁻ and SiO₃⁻² ions, which can lead to ion saturation and hinder the reaction process. This congestion reduces the dissolution efficiency of silica and alumina ions, resulting in incomplete N-A-S-H gel formation and a more porous microstructure. Additionally, heat curing at 110 °C may have worsened the situation by causing microcracks or partial decomposition of hydration products, further compromising the compressive strength in these specific mixes.

3.3.3. Influence of AAS/FA Ratios

The CS. of both A/C and H/C GPC increased with the AAS/FA ratio (0.5 and 0.6), with two exceptions of H/C Mixes GPC-16-2.0-0.6 and GPC-16-2.5-0.6 as presented in Figure 10. For the A/C specimens, the average CS increased by about 42.3% as the AAS/FA ratio rose from 0.5 to 0.6. This increase can be attributed to a higher Si content and elevated SiO₂/Al₂O₃ ratio in the matrix, which promotes the formation of stronger Si-O-Si bonds, and hence enhances the CS. of GPC. Similarly, the H/C GPC specimens showed an average increase in CS. of approximately 38.0% as the AAS/FA ratio increased from 0.5 to 0.6. The maximum CS. of 32.8 MPa (A/C) and 31.4 MPa (H/C) were attained at an AAS/FA ratio of 0.6.

Contrary findings are reported in the literature, where studies by Pavithra et al. [45] and Ghafoor et al. [5] indicated a decrease in the CS. of about 19% and 21%, respectively, as the AAS/FA ratio rose from 0.5 to 0.6. This reduction was attributed to the early precipitation of Si and Al ions before the geopolymerization process could fully proceed. Moreover, Aliabdo et al. [28] investigated the AAS/FA ratio ranging from 0.30 to 0.45 and observed a maximum CS. of 32 MPa at an AAS/FA ratio of 0.4. Beyond this ratio, the CS. decreased by approximately 26% due to an excess of OH⁻ ions in the solution, which slowed down the rate of geopolymerization.

The two exceptions in H/C Mixes GPC-16-2.0-0.6 and GPC-16-2.5-0.6 can be explained similarly to the previous observation, where the combined effect of high NH molarity (16-M), increased NS/NH ratios, and AAS/FA ratio (0.6) led to ion congestion, resulting in incomplete geopolymerization, and consequently reducing the compressive strength. Even though there is a general trend for the NS/NH ratio, NH molarity, and AA/FA ratio, this trend may have exceptions when a high molarity of 16 M is used, indicating the combined and complex influences of these factors on GPC performance.

3.4. Flexural Strength

The variation in the flexural strength (FS.) of all 24 GPC mixes with varying Na₂SiO₃/NaOH (NS/NH) ratios, NaOH (NH) molarity, and alkaline activator solution to fly ash (AAS/FA) ratios cured under either ambient or heat conditions is presented in Figure 11.

3.4.1. Influence of NS/NH Ratios

Figure 12 presents the influence of varying NS/NH ratios, NH molarities, and AAS/FA ratios on the FS. of both A/C and H/C GPC specimens. An increase in the NS/NH ratio from 1.5 to 2.0 reduced the average FS. of A/C GPC by approximately 8.1%, and a further increase from 2.0 to 2.5 resulted in a more substantial reduction in FS. of about 14.1%.

Similarly, for H/C GPC, an increase in the NS/NH ratio from 1.5 to 2.0 reduced the average FS. by approximately 5.7%, with a further increase of NS/NH ratio to 2.5 decreasing the FS. by an additional 12.5%. These results suggested that higher NS/NH ratios from 1.5 to 2.5 resulted in lower FS. for both A/C and H/C GPC mixes. This reduction in FS. is attributed to a decrease in OH⁻ in the AAS, which limited the development of the N-A-S-H gel network, crucial for achieving high FS. The optimal FS. of 6.7 MPa for H/C GPC was attained at a NS/NH ratio of 1.5.

3.4.2. Influence of NaOH (NH) Molarities

The effect of varying NH molarities (10-M–16-M) on the FS. at an AAS/FA ratio of 0.5 is presented in Figure 13. For GPC with the AAS/FA ratio of 0.5, the FS. increased with increasing molarity of NH solution regardless of the curing condition. For example, the FS. of A/C GPC increased by approximately 47.5% as NH molarity rose from 10-M to 12-M, followed by a more moderate increase of 10.6% and 5.7% as molarity further increased from 12-M to 14-M and 14-M to 16-M, respectively. For H/C GPC, a similar trend was observed, with FS. rising by about 52.4% from 10 M to 12 M, then by 12.2% and 16.0% as molarity increased from 12-M to 14-M and 14-M to 16-M, respectively. The incremental increase in FS. with higher NH concentrations is attributed to enhance dissolution of silica (Si⁺⁴) and alumina (Al⁺³) ions in the solution, which strengthens the monomer linkages and promotes a robust polycondensation reaction within the matrix. Ghafoor et al. [5] observed a comparable pattern, noting gradual FS. improvements up to 16-M. Similarly, Aliabdo et al. [28] reported an optimum FS. of 2.7 MPa at 16 M; however, they observed that beyond 16 M, FS. declined by about 32%, indicating a possible saturation effect in ion dissolution and bond formation.

It is pertinent to note that for the AAS/FA ratio of 0.6, the FS of GPC first increased with an increasing molarity of NH solutions from 10-M to 14-M however, the FS decreased with further increase in molarity from 14-M to 16-M under both A/C and H/C conditions. This could be attributed to the congestion of silicate and hydroxide ions in the matrix, resulting in a reduction in the geopolymerization reaction.

3.4.3. Influence of AAS/FA Ratios

The FS. of GPC increased with increasing AAS/FA ratio, regardless of the curing condition, as presented in Figure 14. The average FS. of A/C GPC increased by about 26.2% as the AAS/FA ratio was increased from 0.5 to 0.6. Likewise, the average FS. of H/C GPC increased by approximately 23.7% as the AAS/FA ratio was enhanced from 0.5 to 0.6. The optimal FS. of 6.5 MPa (A/C) and 6.7 MPa (H/C) was attained at an AAS/FA ratio of 0.6.

Even though general trends exist for the influence of the NS/NH ratio, NH molarity, and AAS/FA ratio on the FS., these trends exhibited exceptions at a high molarity of 16-M, highlighting the complex and interdependent effects of these factors on GPC performance. Similar exceptions were also observed incase of CS. results as found for the FS., and the reasons can be attributed to the same factors discussed earlier, including ion congestion, incomplete geopolymerization, and microstructural defects resulting from the combined influence of high NH molarity (16-M), increased NS/NH ratios, and high AA/FA ratio (0.6).

3.5. Comparison of Ambient Versus Heat Curing

In general, the CS. and FS. of the GPC mixes increased with the curing temperature. The maximum CS. and FS. of 31.4 MPa and 6.7 MPa, respectively, were achieved at 14-M NH solution under H/C conditions. Higher temperatures reduced the unreacted FA particles in solution because of increased dissolution of aluminosilicate particles during the geopolymerization process, which contributed to greater CS. and FS. of GPC. Heat curing also accelerates the formation of monomers and polymers during geopolymerization. However, when temperatures exceed an optimal limit, the development of N-A-S-H gel ceases, and it resulted in a porous microstructure with micro cracks in GPC.

Rajmohan et al. [18] reported an optimal CS. at a curing temperature of 60 °C, while higher temperatures, up to 100 °C, decreased the CS. Amin et al. [17] reported that FA-based GPC achieved a peak CS. of 66.5 MPa with a 14 M NH solution when oven-cured at 80 °C for 24 h. Similarly, Sajan et al. [29] found that a 14 M NH solution produced optimal CS. at a curing temperature of 60 °C, noting that temperatures above 100 °C did not enhance the mechanical properties of GPC.

3.6. Scanning Electron Microscopy (SEM) Analysis of GPC

SEM analysis was performed to examine the internal microstructure of ambient-cured (A/C) GPC, e.g., Mix A (GPC-10-1.5-0.5), Mix G (GPC-12-1.5-0.5), Mix S (GPC-16-1.5-0.5), and Mix V (GPC-16-1.5-0.6), as well as heat-cured (H/C) GPC, e.g., Mix P (GPC-14-1.5-0.6) and Mix V (GPC-16-1.5-0.6). SEM images showed that the microstructure of GPC consisted of N-A-S-H gel and microcracks to explain the observations in Section 3 above.

A comparative analysis of Mix A, Mix G, and Mix S (which have different molarities while other factors remained unchanged) revealed that Mix A with the lowest molarity displayed a reduced N-A-S-H gel compared Mix G and Mix S due to lower OH⁻ ions availability in the matrix, resulting in limited N-A-S-H gel formation (Figure 15). In contrast, Mix S (highest molarity of 16 M) demonstrated a denser N-A-S-H gel compared to Mix G (12-M) and Mix A (10-M). However, micro-cracks were also observed in Mix-S, which could be attributed to an increase in OH⁻ ions without an increase in silicate ions. In general, this increased molarity enhanced the dissolution of silica (Si⁴⁺) and alumina (Al³⁺) ions, intensifying the geopolymerization process and, consequently, promoting the formation of monomers and polymers.

Similar observations were also found for H/C GPC, i.e., heat curing significantly enhanced the degree of geopolymerization, leading to the development of denser monomers and polymeric chains of N-S-A-H gel (Figure 15e,f). For H/C GPC mixes, the unreacted silica and alumina ions in the solution were reduced because of the increasing dissolution of ions during geopolymerization. However, increased exposure to 110 °C for 24 h caused excessive microcracks and partial decompositions of hydration products in some mixes, which led to a comparative reduction in the CS. compared to ambient curing [18,30]. In contrast, the SEM image of A/C Mix V demonstrated a denser N-S-A-H gel matrix with fewer unreacted FA particles, indicating that A/C can also yield a compact microstructure under optimized conditions.

3.7. Energy-Dispersive X-Ray Spectroscopy (EDX) Analysis of GPC

The elemental analysis was conducted using EDX spectrometry to examine the elemental composition and behavior of GPC with varying AAS concentration and curing conditions. Figure 16 represents the atomic percentage of key elements in the A/C GPC mixes, i.e., Mix A (GPC-10-1.5-0.5), Mix G (GPC-12-1.5-0.5), Mix S (GPC-16-1.5-0.5) and Mix V (GPC-16-1.5-0.6), as well as the H/C GPC mixes, i.e., Mix P (GPC-14-1.5-0.6) and Mix V (GPC-16-1.5-0.6), see

Table 4. Atomic percentages of elements in FAGPC.

Elements	O	Si	Al	Na	Mg	K	Ca	Si/Al
	%	%	%	%	%	%	%
Mix A	63.42	4.93	2.45	4.97	0.38	0.04	0.23	2.01
Mix G	61.73	4.23	2.63	6.43	0.3	0.04	0.25	1.61
Mix S	53.61	3.44	2.65	3.47	0.73	0.03	0.47	1.30
Mix V	63.4	5.83	2.25	5.5	0.32	0.04	0.14	2.59
Mix P (heat-cured)	60.45	4.33	1.78	5.16	0.32	0.03	0.29	2.43
Mix V (heat-cured)	62.56	4.58	2.12	5.71	0.24	0.04	0.12	2.16

. The primary elements identified include oxygen (O), silicon (Si), aluminum (Al), and sodium (Na) with varying atomic weight percentages, along with trace amounts of magnesium (Mg), iron (Fe), and calcium (Ca).

The Si and Al ions in FA, serving as the precursor, are mainly responsible for the development of N-A-S-H gel through monomer and polymer development. The Si/Al ratios, computed using the atomic weight percentages of Si and Al, were recorded as follows: Mix A (2.01), Mix G (1.61), Mix S (1.3), Mix V (2.43), Mix P (2.59), and Mix V (2.16). An increase in Si/Al ratio from 1.3 to 2.59 was associated with enhanced monomer and polymer formation in the GPC matrix, contributing to greater N-A-S-H gel formation, particularly in Mix V (GPC-16-1.5-0.6). A higher Si/Al ratio resulted in denser networks of N-A-S-H gel, which bolstered the CS. of GPC. This observation aligns with Tawatchai et al. [48], who reported that GPC mixes with a Si/Al ratio exceeding 2.5 exhibited the development of a denser network of N-A-S-H gel structure, further enhancing mechanical properties.

3.8. Environmental Assessment and Cost Analysis

This study conducted an environmental assessment of all GPC mixes with varied NH molarities, NS/NH ratios, and AAS/FA ratios, comparing them against conventional OPC concrete (OPCC) in terms of embodied CO₂ (e-CO₂) emissions. The e-CO₂ emissions data of different construction materials such as FA, coarse and fine aggregates, varying NH solutions, NS solution, water and OPC were opted from previous studies by Turner and Collins. [49], Abdullah et al. [50], Khan et al. [51], and Yaseri et al. [52] as tabulated in

Table 5. e-CO₂ emissions of concrete ingredients.

Materials	FA	CA	FA	NS Sol.	NaOH Solution				Water	OPC
					10-M	12-M	14-M	16-M
e-CO2 (kg/kg)	0.009	0.0459	0.0139	0.6964	0.5841	0.7009	0.8187	0.9355	0.0003	0.8300

. The e-CO₂ of NaOH solids and Na₂SiO₃ solids are 1.915 kg/kg and 1.514 kg/kg, respectively. The 10-M, 12-M, 14-M and 16-M NaOH solutions comprised 30.5%, 36.6%, 42.75%, and 48.85% solids, respectively. The e-CO₂ emissions of a mix were determined by multiplying the fly ash, coarse and fine aggregates, NaOH solution, Na₂SiO₃ solution, and water contents given in kg/m³ in Table 1 with their corresponding e-CO₂ emission value given in Table 5.

The e-CO₂ emissions of 24 GPC mixes varied from 190 kg/m³ to 234 kg/m³ (

Table 6. e-CO₂ emissions and cost analysis of 24 GPC mixes.

Name	Mix ID	e-CO2 (kg/m3)					Total e-CO2	Cost
		FA	Aggregates	NaOH	Na2SiO3	Water	kg/m3	$/m3
Mix A	GPC-10-1.5-0.5	3.31	67.09	42.99	76.88	0.015	190.3	44.6
Mix B	GPC-10-2.0-0.5			35.80	85.44	0.013	191.6	46.2
Mix C	GPC-10-2.5-0.5			30.72	91.50	0.011	192.6	47.3
Mix D	GPC-10-1.5-0.6	3.10	67.09	48.36	86.49	0.017	205.1	47.3
Mix E	GPC-10-2.0-0.6			40.30	96.10	0.014	206.6	49.1
Mix F	GPC-10-2.5-0.6			34.52	102.99	0.012	207.7	50.4
Mix G	GPC-12-1.5-0.5	3.31	67.09	51.58	76.88	0.014	198.9	45.6
Mix H	GPC-12-2.0-0.5			42.96	85.44	0.012	198.8	47
Mix I	GPC-12-2.5-0.5			36.86	91.50	0.010	198.7	48
Mix J	GPC-12-1.5-0.6	3.10	67.09	58.03	86.49	0.016	214.7	48.3
Mix K	GPC-12-2.0-0.6			48.36	96.10	0.013	214.6	50
Mix L	GPC-12-2.5-0.6			41.42	102.99	0.011	214.6	51.1
Mix M	GPC.-14-1.5-0.5	3.31	67.09	60.25	76.88	0.013	207.5	46.5
Mix N	GPC.-14-2.0-0.5			50.18	85.44	0.011	206	47.8
Mix O	GPC.-14-2.5-0.5			43.06	91.50	0.009	204.9	48.7
Mix P	GPC.-14-1.5-0.6	3.10	67.09	67.78	86.49	0.014	224.4	49.4
Mix Q	GPC.-14-2.0-0.6			56.49	96.10	0.012	222.8	50.9
Mix R	GPC.-14-2.5-0.6			48.38	102.99	0.010	221.6	51.9
Mix S	GPC-16-1.5-0.5	3.31	67.09	68.85	76.88	0.011	216.1	47.5
Mix T	GPC-16-2.0-0.5			57.34	85.44	0.009	213.2	48.6
Mix U	GPC-16-2.5-0.5			49.20	91.50	0.008	211.1	49.4
Mix V	GPC-16-1.5-0.6	3.10	67.09	77.45	86.49	0.013	234.1	50.5
Mix W	GPC-16-2.0-0.6			64.50	96.10	0.011	230.8	51.8
Mix X	GPC-16-2.5-0.6			55.28	102.99	0.009	228.5	52.7

), while OPCC mixes with target CS. of 21 MPa and 28 MPa exhibited e-CO₂ emissions of 310 kg/m³ and 358 kg/m³, respectively (Figure 17). On average, GPC mixes with 10-M and 12-M NH solutions (with corresponding CS of ~21 MPa) demonstrated 36% and 33% lower e-CO₂ emissions, respectively, than OPCC at a CS of 21 MPa. Similarly, GPC mixes with 14-M and 16-M NH solutions (with corresponding CS. of ~ 28 MPa) yielded 40% and 38% lower e-CO₂ emissions, respectively, than OPCC with a CS of 28 MPa. Notably, the GPC mixes developed in this research achieved an average 37% reduction in e-CO₂ emissions compared to OPCC. A significant portion of the e-CO₂ emissions in GPC was attributed to NH and NS solutions [49,51]. Therefore, when a more sustainable (lower e-CO₂ emissions) activation solution is developed in the trend of this research direction, a new generation of GPC is expected to show significantly lower e-CO₂ emissions.

Moreover, this research evaluated the cost of 24 GPC mixes (Table 6) and conventional OPCC (

Table 7. e-CO₂ emissions and cost analysis of OPC concrete.

Concrete	Cement	Coarse Aggregates	Fine Aggregates	Water	e-CO2	Cost
	kg/m3	kg/m3	kg/m3	kg/m3	kg/m3	$/m3
OPC (21 MPa)	303	1028	830	205	310	31.9
OPC (28 MPa)	361	1028	817	205	358	36.4

). The cost of GPC and OPC concrete mixes was calculated based on the actual market prices of the construction materials. The cost of 24 GPC mixes per cubic meter varied from USD 44.6 to USD 52.7 as depicted in Figure 18. The average cost of GPC mixes with 10-M and 12-M NH solutions was 48.7% and 51.4% greater than that of OPCC with a target CS of 21 MPa, respectively. Similarly, the average cost of GPC mixes with 14-M and 16-M NH solutions was 35.1% and 37.4%, respectively, greater than that of OPCC with a target CS of 28 MPa. These higher costs of GPC mixes were attributed to the higher costs of NH and NS solutions, which suggests a research motivation to bring this GPC to practice. A use of lower molarities of NH (12-M/14-M), NS/NH (1.5), and AAS/FA (0.5) can reduce the overall production cost of GPC. Moreover, H/C can accelerate the geopolymerization process, and higher strengths can be attained at lower AAS and hence reduced production costs compared to the A/C GPC. Furthermore, the bulk purchase in the case of field production of GPC will also reduce the production cost.

This research evaluated the cost of 24 GPC mixes (Table 6) and conventional OPCC (Table 7). The cost of ingredient materials used in producing GPC and OPCC mixes was calculated based on current local construction industry prices as of Spring 2025. The cost of 24 GPC mixes per cubic meter varied from USD 44.6 to USD 52.7 as depicted in Figure 18. The average cost of GPC mixes with 10-M and 12-M NH solutions was 48.7% and 51.4%, respectively, greater than that of OPCC with a target CS of 21 MPa, respectively. Similarly, the average cost of GPC mixes with 14-M and 16-M NH solutions was 35.1% and 37.4%, respectively, greater than that of OPCC with a target CS. of 28 MPa. These higher costs of GPC mixes were attributed to the higher costs of NH and NS solutions, which suggests a research motivation to bring this GPC to practice.

4. Conclusions

This study comprehensively investigated the mechanical properties, i.e., compressive strength (CS.) and flexural strength (FS.) of GPC mixes, with varying NaOH (NH) molarities, Na₂SiO₃/NaOH (NS/NH), and AAS/FA ratio under both the ambient-cured (A/C) and heat-cured (H/C) conditions. Moreover, an environmental assessment and cost analysis were conducted for all the GPC mixes. The following conclusions are drawn:

• Effect of NaOH molarity: for A/C GPC, increasing NH molarity from 10-M to 16-M enhanced CS. by 145%, with the maximum CS. of 32.8 MPa achieved at 16-M NaOH. The FS. also increased by 90% with increasing NH molarity raised from 10-M to 14-M, reaching an optimal FS of 6.5 MPa at 14 M.
• Influence of NS/NH ratio: increasing the NS/NH ratios from 1.5 to 2.5 decreased the CS. of A/C GPC mixes by 12.5% and the FS. by 9.1%. The maximum CS. of 32.8 MPa and FS. of 6.5 MPa were obtained at a NS/NH ratio of 1.5.
• Impact of AAS/FA ratio: both A/C and H/C GPC showed increased CS. with increasing AAS/FA ratios from 0.5 to 0.6. The average CS. of A/C GPC improved by 42.3%, reaching a maximum CS. of 32.8 MPa at an AAS/FA ratio of 0.6. For FS., increases of 3% to 56.4% were observed across various NH molarities, with the maximum FS. of 6.5 MPa achieved at the same ratio.
• Results for heat-cured GPC: CS. of H/C GPC rose by 66% with NH molarity increased from 10-M to 14-M, achieving the maximum CS. of 31.4 MPa at 14-M, NS/NH ratio of 1.5, and AAS/FA ratio of 0.6. FS. also improved by 47.2%, reaching an optimum FS. of 6.7 MPa under these conditions.
• Microstructural observations: The SEM analysis showed that a higher NH molarity (16-M) resulted in increased AA solution availability, facilitating greater leaching and dissolution of silica and alumina ions from FA, thereby enhancing N-A-S-H gel formation.
• Environmental and cost assessment: GPC mixes demonstrated e-CO₂ emissions that were 35–40% lower than those of OPC concrete, although GPC costs were approximately 30–35% higher due to the expense of AA components (NH and NS solutions).

In summary, this study highlights the potential of GPC as an environmentally favorable substitute to OPC concrete, offering substantial reductions in CO₂ emissions despite its higher initial costs. Optimal mix designs of Class F FA, particularly with a NH molarity of 14–16 M, NS/NH ratio of 1.5, and AAS/FA ratio of 0.6, offer balanced performance in terms of strength, cost, and environmental impact. Based on both the environmental assessment and cost analysis, GPC Mix-M with an AAS/FA ratio of 0.5, NH molarity of 14 M, and NS/NH ratio of 1.5 has been recommended for Hub CPP FA-based GPC.