Cost-Effective Perspective of Fiber-Reinforced Geopolymer Concrete Under Different Curing Regimes

Abstract

Composite geopolymer concrete (CGPC), is receiving growing attention in the construction sector for its sustainable nature, environmental benefits, and its valuable role in promoting efficient waste utilization. The strategic incorporation of reinforcing fibers into geopolymer concrete (GPC) matrices is critical for enhancing mechanical performance and meeting the durability requirements of high-performance construction applications. Although substantial research has focused on strength enhancement of fiber-reinforced geopolymer concrete (FGPC) individually, it has neglected practical considerations such as energy use for curing and life-cycle assessments. Thus, this study investigates the cost-effective aspects of FGPC cured under different regimes. Different cementitious binders were incorporated, i.e., fly ash (FA) and ground granulated blast-furnace slag (GGBS), in addition to alkaline activators (a combination of sodium hydroxide and sodium silicate), hooked-end steel fibers (HESFs), basalt fibers (BFs), and polypropylene fibers (PPFs), as well as aggregates (gravel and sand). The effect of different geopolymer-based materials, reinforcing fibers, and different curing regimes on the mechanical, durability, and economic performance were analyzed. Results showed that the applied thermal curing regimes (oven curing or steam curing) had a considerable impact on durability performance, compressive strength, and flexural strength development, especially for GPC mixes involving high FA content. Cost analysis outcomes suggested that the most affordable option is GPCM1 (100% FA without fibers), but it demonstrates low strength under ambient curing conditions; RGCM4 (100% GGBS and 0.75% HESF) provided the best strength and durability option but at higher material cost; RGCM7 (50% FA, 50% GGBS, and 0.75% HSF) exhibited a balanced choice since it offer satisfied strength and durability performance with moderate cost compared to other options.

1. Introduction

The construction sector has experienced a dramatic increase in cement demand, driven by the continuing expansion of urban populations [1,2]. Ordinary Portland cement (OPC), which is the primary binder in conventional concrete, has a highly energy intensive production process [3]. It was reported that the average global CO₂ emission factor for OPC is approximately 0.86 kg of CO₂ per kilogram of cement, attributed specifically to limestone calcination [4]. Furthermore, it was also stated that the cement industry accounts for an estimated 5–8% of total global CO₂ emissions, with roughly 40% resulting from the combustion of fossil fuels and the remaining 60% derived from the chemical breakdown of limestone [5]. Moreover, OPC has some issues regarding durability and permeability that may introduce corrosion-induced deterioration of infrastructure and civil engineering structures. Despite distinguished advancement in the development and application of corrosion protection technologies, this weakness is still a prevalent and persistent global issue, as it can adversely affect structure age and the service life of reinforced concrete [6]. Recent publications have further established that the severity of steel reinforcement corrosion is highly associated with the concrete type in addition to its microstructural characteristics, i.e., permeability, porosity, and pore connectivity which involve a serious influence on the rate and extent of corrosion processes [7].

In response to increasing environmental concerns and durability issues, the construction industry has been actively pursuing environmentally sustainable alternatives [8]. Among these, geopolymer concrete (GPC) has emerged as a promising alternative to OPC, owing to its environmental benefits, superior hardened properties, and cost effectiveness [9,10]. GPC, synthesized through the alkali activation of aluminosilicate-based materials, has emerged as a promising low-carbon alternative [5,11]. The environmental profits of GPC compared to OPC are particularly distinguished in terms of its significantly lower CO₂ emissions [12,13]. Fly ash (FA), ground granulated blast-furnace slag (GGBS), and silica fume (SF) are commonly utilized precursors in GPC, associated with substantially reduced CO₂ outputs during production when compared to OPC [14]. For instance, it was indicated that the carbon emissions resulted by producing FA-based GPC can be up to 80% lower than those from OPC-based concrete [15]. Additionally, the application of these industrial byproducts can contribute to mitigating environmental concerns associated with waste management, as FA has been commonly disposed of in landfills, posing risks of environmental contamination [16,17]. Incorporating GP-based materials into concrete mixes not only diverts considerable quantities of waste from landfills but also diminishes the reliance on the extraction of natural resources [18,19].

The polymerization of alkali-activated constituents results in a hardened material that demonstrates comparable or superior properties to OPC. One of the key attributes of GPC is its high early age strength due to the rapid hardening characteristics of alkali-activated materials. Additionally, the compressive, flexural, and tensile strengths of GPC can be further improved through the combination of fibers, i.e., steel fibers, basalt fibers, carbon fibers, glass fibers, and polypropylene fibers [20]. The synergistic effects of fiber-reinforced geopolymer concrete (FRGPC) are particularly obvious in environments that require high durability. The enhanced hardened properties make FRGPC an ideal material for applications in harsh environments, such as marine structures and high-rise buildings, as well as bridges [14].

The performance of FRGPC is highly affected by the applied curing regime [21]. The curing systems have an essential impact in determining the hardened and durability properties of GPC, as it extremely affects the extent of geopolymerization. Unlike OPC-based concrete, GPC often demands elevated temperature curing to realize optimal performance, specifically when synthesized from low-calcium aluminosilicate sources. Common curing regimes include ambient cuing and heat curing at temperatures ranging from 40 °C to 90 °C for durations of 6 to 48 h. Typically, oven or steam curing accelerates the polymerization reactions in addition to enhancing initial strength development, but at different rates depending on the temperature and its duration [22,23]. Consequently, selecting an appropriate curing system is vital for modifying the microstructure and performance characteristics of GPC to meet satisfied structural and environmental requirements [24]. Though, the application of such curing regimes can increase energy consumption and construction budgets. Hence, an optimal balance between curing conditions and mechanical performance, as well as sustainability needs to be attained.

While extensive investigation has been conducted on FRGPC, comprehensive studies that inspect the combined effects of different GP-based materials, fiber types, and curing regimes in the context of cost efficiency and sustainability are still scarce. Specifically, there is a notable gap in integrated investigations that concurrently evaluate hardened properties, durability characteristics, environmental impact, and economic feasibility. Moreover, the present literature has predominantly highlighted strength enhancement, often overlooking some critical practical aspects, i.e., curing energy consumption in addition to life-cycle environmental assessments. Thus, this study aims to evaluate the cost effectiveness and sustainability perspectives of FRGPC produced with different proportions of FA, GGBS, and fibers, cured under different curing regimes to deliver recommendations for practical applications based on performance, cost, and durability considerations.

2. Experimental Program

The experimental program included fourteen different mixes: five geopolymer concrete mixes (GPCMs) excluding fibers and the rest incorporating different percentages of hooked-end steel fibers (HESFs), polypropylene fibers (PPFs), or basalt fibers (BFs). All mixes were cured under ambient, oven, or steam curing conditions. The mechanical properties, i.e., flexural and compressive strengths, were measured, along with durability tests including water absorption.

2.1. Materials

The utilized binders were fly ash (Class F), with low calcium content less than 10%, byproduct supplier and ground granulated blast-furnace slag (GGBS) was sourced from a steel industry byproduct supplier from Iran; it was rich in calcium oxide, approximately 40% (Figure 1). For more details,

Table 1. Chemical composition of FFA and GGBS.

Compound	FFA	GGBS
SiO2	6.80%	38.6%
Al2O3	0.32%	21.3%
Fe2O3	14.63%	2.0%
CaO	0.96%	39.8%
SO3	12.53%	2.7%
MgO	0.11%	14.2%
K2O	0.19%	0.36%
TiO2	0.22%	0.57%
LOI	41.1%	<1%

includes the chemical composition of FFA and GGBS. Also, an alkaline activators solution (AAS), a combination of sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃), was prepared with a weight ratio (SiO₂/Na₂O) of 2.0. NaOH was used in pellet form to prepare an 8 M molarity solution [25]. The chemical composition of the sodium silicate solution included 32% SiO₂, 14% Na₂O, 60% H₂O, and 44% Na₂SiO₃. Also, coarse aggregate (gravel) and fine aggregates (sand) were utilized with maximum particle size of 4 mm and 20 mm, respectively.

Table 2. Physical properties of sand and gravel.

Property	Sand	Gravel
Max Particle Size	4 mm	20 mm
Specific Gravity	2.62	2.64
Bulk Density	1550 kg/m3	1630 kg/m3
Water Absorption	2.3%	1.7%

includes the physical properties of the utilized aggregates. Also, three types of fibers were utilized in this paper, i.e., hooked-end steel fibers (HEFs), basalt fibers (BFs), and Polypropylene Fibers (PPFs) (Figure 1), their physical and mechanical properties are included in

Table 3. Physical and mechanical properties of HES, BF, and PPF.

Physical Properties
Property	HESF	PPF	BF
Length	30 mm	20 mm	20 mm
Diameter	0.5 mm	0.5 mm	0.02 mm
Aspect Ratio (L/D)	60	40	1000
Density	7850 kg/m3	900 kg/m3	2700 kg/m3
Mechanical properties
Property	HESF	PPF	BF
Tensile Strength	1800 MPa	650 MPa	470 MPa
Modulus of Elasticity	200 GPa	5 GPa	110 GPa

.

2.2. Mix Proportions

In this study, the experimental program comprised a total of fourteen concrete mixes; five geopolymer concrete mixes (GPCMs) excluded fibers and included different percentages of FA and GGBS. Conversely, the rest of the geopolymer concrete mixes incorporated different percentages of hooked-end steel fibers (HEFs), polypropylene fiber (PPFs), or basalt fibers (BFs). All mixes were formulated using FFA and GGBS as the principal binders. Fine aggregate (sand) and coarse aggregate (gravel) were utilized with a maximum particle size of 4 and 20 mm, respectively, in addition to alkaline activator solution consisting of D-grade sodium silicate solution (DSS) and sodium hydroxide (NaOH). The NaOH solution was prepared at a concentration of 8 molarity (8 M), based on previous research findings to ensure adequate workability and mechanical strength [25]. Sand and gravel contents were consistently maintained at fixed values of 750 kg/m³ and 900 kg/m³, respectively, throughout all concrete mixtures. Similarly, the water-to-binder ratio, the alkaline activator-to-cementitious material ratio, and NaOH-to-D-grade sodium silicate ratio were maintained at constant values of 0.23, 0.36, and 2, respectively. Detailed mix proportions are presented in

Table 4. Mix proportions.

Mix No.	Mix ID	FFA Kg/m3	GGBS Kg/m3	Utilized Fibers	V%	AAS Kg/m3	SP Kg/m3
1	GPCM1	600	0	0	0	210	3.7
2	GPCM2	0	600	0	0	210	8.5
3	GPCM3	450	250	0	0	210	5.7
4	GPCM4	250	450	0	0	210	7.7
5	GPCM5	300	300	0	0	210	7
6	FRGCM1	600	0	HSF	0.75%	210	7.5
7	FRGCM2	600	0	BF	0.75%	210	8.3
8	FRGCM3	600	0	PPF	0.5%	210	5.5
9	FRGCM4	0	600	HSF	0.75%	210	12.7
10	FRGCM5	0	600	BF	0.75%	210	13.7
11	FRGCM6	0	600	PPF	0.5%	210	11.2
12	FRGCM7	300	300	HSF	0.75%	210	9.9
13	FRGCM8	300	600	BF	0.75%	210	10.7
14	FRGCM9	300	600	PPF	0.5%	210	8.8

.

2.3. Specimen Preparation and Testing Procedure

The effect of different types and content of geopolymer-based materials and embedded fibers on fresh concrete properties were evaluated in terms of workability. Slump tests were employed to assess the flow characteristics of different geopolymer concrete mixtures (GPCMs) following the guidelines of ASTM C143/C143M [26]. Also, the influence of three curing systems, i.e., ambient curing, oven curing, and steam curing on the mechanical concrete properties, durability performance, and cost effectiveness of blended concrete and fiber-reinforced geopolymer concrete (FRGPC) was also investigated. The preparation of geopolymer concrete specimens (GPCSs) commenced with the setup of standardized molds. Initially, a dry mixing process was carried out for 2 min on solid constituents, i.e., geopolymer binders, fine aggregates (sand), gravel, and fibers (if any were used). Subsequently, the wet mixing phase began by gradually adding the alkaline activator solutions (AASs) and SP into the dry mixture using an electric mixer. During this stage, the alkaline solutions initiated the geopolymerization process by binding the unreacted constituents, i.e., aggregate and fibers, with binding materials (FFA and GGBS) forming geopolymer concrete mortar (GPCM). This process continued for 5 min to produce a homogeneous paste with appropriate consistency. Both the dry and wet mixing procedures were performed at an ambient temperature of 23 °C. The freshly prepared GPCMs were then cast into the designated molds, followed by compaction using a vibrating table for 3 min to attain proper placement of the mortar within the molds and to minimize air voids. Then, GPC specimens were covered with plastic sheets for 24 h at an ambient temperature of 23 °C to maintain internal moisture levels and prevent undesirable interactions between the green geopolymer concrete specimens (GPCSs) and airborne contaminants. After this initial curing stage, GPCSs were demolded and divided into three groups. The first group was cured continuously at an ambient temperature until the day of testing. The second group was subjected to oven curing temperatures at 60 °C for a duration of 24 h. The third group was subjected to steam curing; the chamber was maintained at 80 °C for 12 h. Following thermal treatment, the GPCSs were stored at room temperature until the designated testing date. Hardened properties were assessed in terms of compressive strength, flexural strength, and water absorption. Compression tests were carried out on cylindrical specimens (150 mm in diameter and 300 mm in height), following ASTM C39/C39M [27]. Flexural strength was assessed using three-point bending tests on prismatic specimens with dimensions of 100 mm × 100 mm × 500 mm, conducted at 7, 14, and 28 days in accordance with ASTM C1609/C1609M [28]. The reported compressive and flexural strength values represent the average of three specimens per mix. To assess the long-term performance of the GPCSs, durability tests were conducted in terms of water absorption, which is a vital indicator of the concrete’s resistance to aggressive environments. Water absorption was measured following ASTM C642, where oven-dried specimens were immersed in water, and the increase in mass was recorded to determine pore connectivity [29]. All GPCSs used in durability testing were cured under controlled conditions and aged for 28 days prior to testing to ensure consistent maturity levels.

3. Results and Discussion

3.1. Fresh Properties

The outcome of the slump tests is illustrated in Figure 2. The slump results of GPC mixes demonstrated the impact of binder composition and fiber type on the workability and the corresponding requirement for superplasticizer (SP). Among all involved mixes, GPCM1, comprising 100% FFA, showed the highest slump of 180 mm with a relatively low SP quantity of 3.7 Kg/m³. Basically, the spherical shape and smooth surface of FA particles serve to reduce internal friction within GPC mixture [30]. GPCM2, produced with 100% GGBS, recorded a low slump value of 109 mm, requiring 8.5 Kg/m³ of SP. Essentially, GGBS includes more irregular and angular particles, which tend to potentially lessen the flowability of GPCMs and raise water demand [31]. Moreover, GGBS exhibits a higher degree of early phase reactivity in comparison to FA, which can shorten the setting time and constrain the workable stage of fresh GPC [32]. GPCM5, containing equal proportions of FFA and GGBS, showed a moderate slump value of 137 mm, representing the balancing impact of FA in offsetting the workability reduction resulted by GGBS. On the other hand, the incorporation of fiber into GPCMs significantly influenced its workability. For instance, the addition of hooked-end steel fibers (0.75% HESFs) in FRGCM1 reduced the slump to 130 mm, raising the SP demand to 7 Kg/m³. Conversely, FRGCM4, with the same content of 0.75% HESFs but 100% GGBS, presented a sharper decrease in the slump value down to 73 mm and required 12.7 Kg/m³ of SP. This comparison highlights the compounded negative effect of HEF and GGBS on workability. The addition of BSFs resulted in the lowest slump values of 120 mm (FRGCM2), 67 mm (FRGCM5), and 90 mm (FRGCM8) compared to other utilized fibers (HESFs and PPFs). This reduction belongs to the physical and surface characteristics of BSFs. More specifically, the rough texture and high surface area of BSFs increase the fiber entanglement and agglomeration within GPCMs, and that impedes uniform dispersion and increases internal friction. Conversely, GPCMs produced with polypropylene fibers (PPFs), i.e., FRGCM3, FRGCM6, and FRGCM9, presented slump values of 152, 90, and 114 mm, respectively, which were considerably higher than those of GPCM-incorporated BSFs and HESFs, representing the slight interference of PPFs with the fluidity of the mixture due to their finer and flexible morphology.

3.2. Compressive Strength

3.2.1. Effect of Geopolymer-Based Materials

Figure 3 represents the effect of geopolymer-based materials on the GPC’s compressive strength results. It can be indicated that the compressive strength of GPC under ambient curing conditions is significantly influenced by the type and proportion of aluminosilicate binder materials. More specifically, geopolymer concrete specimens (GPCSs) produced with GPCM1 (100% FA) exhibited low 7-, 14-, and 28-day compressive strengths of 4.2, 7.5, and 15.8 MPa, respectively. The limited strength development can be attributed to the slow pozzolanic reaction at room temperature, which delays the development of calcium silicate hydrate (C–S–H) and this can lead to higher porosity due to unreacted FA particles which decreases GPC matrix density and strength [33]. GPCSs produced with GPCM2 (100% GGBS) presented higher compressive strength developments of 84%, 92%, and 99% at 7, 14, and 28 days, respectively, compared to those of GPCM1 at the same specified ages. The superior early age performance even under ambient conditions can be attributed to the high calcium oxide content of GGBS, which facilitates faster matrix formation. GPCSs made with blended GPCMs such as GPCM3 (75% FA and 25% GGBS) demonstrated higher compressive strength results of 18%, 20%, and 23% at 7, 14, and 28 days relative to GPCM3 (100% FA), and that indicates the beneficial influence of incorporating even small proportions of GGBSs into FA-based geopolymers. Moreover, GPCSs produced with the GPCM4 mix, which was composed of 75% GGBS and 25% FA, achieved considerably higher compressive strength results across all ages compared to those of blended mixes (GPCM3 and GPCM5). For instance, the compressive strength enhancements of GPCSs made with GPCM4 at 7, 14, and 28 days were 68%, 73%, and 79%, respectively, compared to those produced with GPCM1 and this enhancement can be attributed to the dominance of GGBS’s calcium-rich reactivity. GPCSs formed with GPCM5 (50:50 FA-GGBS) presented balanced behavior by showing satisfactory compressive strength gains of 6.2, 11.3, and 22.5 MPa at 7, 14, and 28 days, respectively, achieving approximately up to 37% compressive strength enhancement greater than that of GPCM1 (100% FA). From the above, it can be indicated that increasing the GGBS content consistently enhances the rate of compressive strength development under ambient curing. These observations align with previous conclusions which emphasize the role of GGBS in accelerating geopolymerization and improving mechanical performance in low-temperature environments [34,35].

3.2.2. Effect of Imbedded Fibers

The compressive strength performance of geopolymer concrete (GPC) under ambient curing conditions is also influenced by the type and dosage of reinforcing fibers. Figure 4 shows the impact of incorporation of 0.75% hooked-end steel fibers (HESFs), 0.75% basalt fibers (BFs), and 0.5% polypropylene (PPF) on GPC compressive strength at 7, 14, and 28 days. More specifically, the addition of 0.75% HESF into the GPC mixture composed entirely of 100% FA (FRGCM1) resulted in a slight compressive strength development up to 7% at 28 days, compared to that without fibers (GPCM1). In the same way, BF did not contribute to strength development in 7 days and had a slight increase up to 4%. This slight improvement can belong to the role of HSF and BS in improving crack-bridging efficiency and enhanced internal stress distribution. Similarly, the addition of 0.5% PPFs resulted in compressive strength enhancement up to 8% after 28 days. Basically, the addition of PPFs aids in enhancing the residual strength and toughness of FA-based composites and that can improve unconfined compressive strength [36,37]. In the same way, the addition of 0.75% HESF, 0.75% BF, or 0.5% PPF into GPC mixes produced with 100% GGBS showed minor strength developments up to 9% (FRGCM4), 6% (FRGCM5), and 12% (FRGCM6), respectively, compared to those of GPCM2 at 28 days. Blended reinforced geopolymer mixes, containing 50% FA and 50% GGBS (FRGM7, FRGM8, and FRGM9) exhibited the same trend of FRGPC specimens produced with 100% FA and 100% GGBS. More specifically, the inclusion of HSF, BS, and PPF into blended mixes led to compressive strength enhancements up to 8%, 5%, and 10% at 28 days, respectively.

3.2.3. Effect of Different Curing Regimes

It was indicated that the applied curing regime had a significant influence on the compressive strength development of GPC, as seen in Figure 3, Figure 4, Figure 5 and Figure 6. More specifically, under ambient curing, GPCS produced with mixes involved high FA content, i.e., GPCM1, FRGCM1, FRGCM2, and FRGCM3, showed relatively slow compressive strength development. With 7-day strength, they reached only 23–29% of their 28-day values due to the low calcium content of FA. However, a modest gain in compressive strength was observed at 28 days, but the final compressive strength remained comparatively low at 15.8 MPa (GPCM1), 16.9 MPa (FRGCM1), 16.4 MPa (FRGCM2), and 17.1 MPa (FRGCM3). In contrast, GPCS made with GPCM included 100% GGBS, i.e., GPCM2, FRGCM4, FRGCM5, and FRGCM6 achieved about 30–35% of its 28-day compressive strength by day 7 and continued to develop through 14 and 28 days. Blended GPC mixes, i.e., GPCM3, GPCM4, GPCM5, FRGCM7, FRGCM8, and FRGCM9 showed a trend of improved early age strength with increasing GGBS content. For instance, the 7-day compressive strengths of GPC specimens produced with GPCM3 (75% FA and 25% GGBS), GPCM4 (75% GGBS and 25% FA), and GPCM5 (50% FA and 50% GGBS) were 4.9, 7.0, and 6.2 MPa, respectively. For more details, Figure 3 and Figure 4 illustrate the effects of ambient curing conditions on the compressive strength development of GPCSs produced with different GPCMs. In contrast, under oven curing at 60 °C for 24 h, GPCM1, FRGCM1, FRGCM2, and FRGCM3 demonstrated significant enhancement in 7-day compressive strength up to approximately eight times of their values under ambient curing (Figure 5). However, compressive strength gains beyond 14 days plateaued in GPCSs produced with these mixtures and that could be due to the early reaction saturation. On the other hand, GPCS made with mixes including high percentages of GGBS, i.e., GPCM2, FRGCM4, FRGCM5, and FRGCM6, responded less dramatically to oven curing relative to 100% FA mixes due to its inherently high early reactivity and enhancement was around five times higher relative to their peers under ambient curing. However, compressive strength improvements of GPCSs produced with these mixtures did not show an important growth after 14 days and that could be due to the early reaction fullness. GPCSs produced with blended mixes, i.e., GPCM3, GPCM4, GPCM5, FRGCM7, FRGCM8, and FRGCM9, showed early compressive enhancement about seven higher times relative to their peers under ambient curing conditions. It was also indicated that GPCSs produced with GPCMs of high GGBS content consistently outperformed GPCMs included FA even though they showed less compressive strength development relative to GPCSs produced with high FFA content and that can highly belong to the differences in their chemical composition and reactivity. For more information, Figure 5 illustrates the effect of oven curing temperature on compressive strength of GPCSs made with different GPCMs.

Steam curing followed a similar pattern of oven curing in developing the compressive strength of involved GPCSs, but its effect was more evident in early stages than oven curing especially with GPCSs produced with GPCMs including high percentages of FA (Figure 6). For instance, GPCSs produced with GPCM1, FRGCM1, FRGCM2, and FRGCM3 and subjected to steam curing of 80 °C for 12 h recorded a 7-day compressive strength improvement roughly eleven times that compared to their peers under ambient curing conditions and 24–28% relative to oven curing conditions. Early strength development can be attributed to sustained reaction kinetics. GPCSs, produced with high GGBS content, i.e., GPCM2, FRGCM4, FRGCM5, and FRGCM6, showed additional compressive strength enhancement up to 15% compared to that of the oven curing regime. The significant enhancement achieved by steam curing compared to ambient curing is highly attributed to its ability to maintain both elevated temperatures and high humidity levels. This combined effect is attributed to accelerating the geopolymerization process by promoting the dissolution of aluminosilicate precursors and facilitating the formation of a denser gel matrix. Also, the presence of moisture in steam curing prevents premature drying and shrinkage, which can occur in dry oven systems and result in incomplete reactions or microcracks. Consequently, steam curing results in better structural enhancement and interparticle bonding within the GPC matrix, leading to developed early and long-term compressive strength compared to oven curing and ambient curing [38].

3.3. Flexural Strength

3.3.1. The Effect of Geopolymer-Based Materials

According to Figure 7, it can be indicated that flexural strength development of GPC is influenced by the type and ratio of precursor materials. More specifically, the flexural strength of GPCSs produced with mix included 100% FA and cured under ambient conditions was typically low, registering baseline values at 7 and 14 days of 0.38 and 0.63 MPa, respectively. Basically, the low outcomes are attributed to the slow geopolymerization and the absence of calcium, which delayed matrix densification and bonding [39,40]. By 28 days, GPCM1 showed a modest flexural strength value of 1.20 MPa which is considered a modest improvement if compared to flexural strengths of 7- and 14-day values. Conversely, GPCMs which included 100% GGBS demonstrated a satisfied strength gain with an increase of approximately 78%, 85%, and 90% at 7, 14, and 28 days compared to those of GPCM1 (100% FA) at the same testing ages. The strength improvements achieved by GPCM2 belong to the high calcium content of GGBS as well as latent hydraulic reactivity, which accelerate matrix formation and enhance cohesion. Hybrid mixes revealed intermediate trends of flexural strength development. Namely, the blend mix (GPCM3), which contained 75% FA and 25% GGBS showed 7-, 14-, and 28-day flexural strength enhancements at ambient curing conditions of roughly 15%, 17%, and 20% relative to those of GPCM1 (100% FA). The results of GPCM2 can highlight the positive contribution of GGBS even in minor quantities. On the other hand, GPCM4, which involved 75% GGBS and 25% FA, exhibited higher early and long-term strengths with a flexural strength enhancement of 60, 77%, and 82% at 7, 14, and 28 days in comparison with the strength results of GPCM1. It was indicated that the performance of GPCM4 closely matches that of GPCM2 (100% GGBS), while GPCM5, which contained 50% FA and 50% GGBS, resulted in moderate flexural strength gains approximately 40%, 50%, and 65% at 7, 14, and 28 days in ambient conditions. This mixture reflected a balanced contribution from both precursors (GGBS and FA). Overall, flexural strength correlated positively with the increase in GGBS content due to its ability to promote rapid matrix hardening and superior interfacial bonding, which makes GGBS a vital factor in optimizing the flexural performance of GPC under ambient curing.

3.3.2. The Effect of Imbedded Fibers

Figure 8 represents the effect of different fiber types, i.e., HESFs, BSFs, and PPFs, on flexural strength development. Based on Figure 8, it is obvious that the flexural strength of GPC is significantly sensitive to the type and proportion of both the binder materials, i.e., FA, GGBS, or their blends, and the reinforcing fibers. In 100% fly ash-based GPCS (GPCM1), flexural strength was generally limited at early ages due to the slow rate of polymerization under ambient curing conditions. However, the inclusion of 0.75% HESF resulted in a maximum flexural strength enhancement of approximately 35% (FRGCM1) at 28-days compared to that of GPCM1 (without fibers). This enhancement is primarily due to the effect of HESFs in bridging cracks, redistributing load, and improving post-cracking behavior. The addition of 0.75% BSFs also improved the flexural performance at 28 days up to 23% (FRGCM2) under ambient curing conditions, relative to that of GPCM1. The flexural strength growth achieved by BSFs can be attributed to the impact of the fiber in improving the bonding and bridging capabilities of a concrete matrix, though BSFs are slightly less effective than HESFs due to the differences in their physical and mechanical properties. The incorporation of 0.5% PPF into 100% fly ash-based GPCS resulted in more modest gains of 9% (FRGCM3) under ambient curing conditions, as their low modulus limits their ability to resist flexural stresses, though they still contribute to crack resistance and toughness. For 100% GGBS-based GPC specimens, the flexural strength growths were more pronounced at initial ages due to rapid reaction kinetics from the high calcium content. Furthermore, the addition of 0.75% HESF into 100% GGBS-based mixes led to significant flexural strength improvements of approximately 53% (FRGCM4) at 28 days, with noticeable improvements of 26%, and 15% at 14 and 7 days, respectively. On the other hand, the inclusion of 0.75% BSFs into 100% GGBS-based GPC showed a similar trend to that of HESFs but a slightly lower flexural strength development of 30%, 14%, and 8% (FRGCM5) at 28, 14, and 7 days, respectively, compared to those of GPCM2. The addition of PPFs resulted in the lowest flexural strength gains of 4%, 7%, and 11% relative to flexural strengths of GPCM2. On the other hand, fiber-reinforced blended mixes, i.e., FRGCM7, FRGCM8, and FRGCM9 offered balanced flexural strength performance. More specifically, the incorporation of 0.75% HESFs, 0.75% BSFs, and 0.5% PPFs into blended GP mixes, which included 50% GGBS and 50% FA, led to maximum 28-day flexural strength improvements of 34%, 20%, and 9% under ambient conditions, relative to flexural strength results of GPCM5. These findings align with most of the previous literature, which underlined the importance of imbedded fibers in improving bond strength and the flexural capacity of GPC composites, particularly in low-calcium systems like FA-based mixes where ductility and toughness are crucial [20,41,42,43].

3.3.3. Effect of Different Curing Regimes

The flexural strength performance of GPC is significantly influenced by the applied curing regime, as seen in Figure 7, Figure 8, Figure 9 and Figure 10. Under ambient curing, GPCM1 (100% FA) exhibited relatively poor flexural strength at 7 days, typically achieving only 32% of its potential 28-day strength due to limited early age geopolymer gel formation. After 28 days, modest improvements of 1.2 MPa were observed, but the value remains inferior compared to that of the GGBS-based mixture (Figure 7). GPCM2 (100% GGBS) demonstrated a moderate early flexural strength of 0.7 MPa at 7 days under an ambient curing regime. This enhancement attained by day 7 was due to rapid calcium-mediated matrix formation. Under ambient curing conditions, blended mixes, i.e., GPCM3, GPCM4, and GPCM5, showed moderately enhanced early and later-age flexural performance in correlation with GGBS content. For instance, the flexural strength results of GPCM4 (75% GGBS and 25% FA) were nearly close to those of GPCM2 in almost all testing ages, as can be seen in Figure 7.

According to Figure 9, oven curing (60 °C for 24 h) had a significant impact on the development of flexural strength, particularly GPC specimens produced with 100% FFA. For instance, GPCSs produced with GPCM1 recorded significant early flexural strength enhancement up to eight times of its peers under ambient curing conditions. On the other hand, GPCSs made of GGBS-rich mixes showed more initial strength gains of 37% at 7 days under oven curing compared to GPCM1 (100% FFA), this additional strength growth was due to already accelerated early reactions and exhibited 552% compared to its peers under ambient conditions. Conversely, GPCSs formed with blended mixes, i.e., GPCM3, GPCM4, and GPCM5, demonstrated 7-day flexural improvements of around 700%, 590%, and 650%, respectively. The strength enhancement was highly dependent on the blending percentage of FA and GGBS, while the addition of fibers further enhanced flexural strength performance. For instance, FRGCM1 (100% FA and 0.75% HESF) resulted in a 7-day flexural strength improvement of up to 36% under oven curing conditions relative to that without fibers (GPCM1) under the same curing conditions. The strength’s growth can be attributed to the excellent crack-bridging and load-transfer capacity of HESFs. Also, FRGCM2 (100% FA and 0.75% BSFs) had a 7-day strength gain of 20% under ambient curing which was less than that of specimens which included 0.75% HESF by 6%. Furthermore, FRGCM3 (100% FA and 0.5% PPFs) recorded 10% flexural strength improvement at 28 days due to lower stiffness and bond strength. However, FRGCM4 delivered the highest flexural strength outcomes across all curing regimes, with high early age performance even under ambient curing, and the application of oven curing regime added additional strength to reach 6.75 MPa at 28 days. Blended GPCMs with fibers (FRGCM7, FRGCM8, and FRGCM9) showed balanced strength enhancements less than those of GGBS-based reinforced mixes and higher than FFA-based reinforced mixes (FRGCM1, FRGCM2, and FRGCM3) in all involved regimes, as shown in Figure 8, Figure 9 and Figure 10. Also, their flexural strength enhancements were highly influenced by the applied regime. More specifically, it was indicated that steam curing was more effective for all involved mixes, especially with FA-rich mixes. For instance, the flexural strength results of GPCSs produced with GPCM1 under ambient curing, oven curing, and steam curing were 0.38, 3.43, and 4.24 MPa, respectively. The same trend was obtained for the rest of the FFA-based reinforced mixes cured under the steam curing regime. Fiber-reinforced GGBS and blended fibered specimens under steam curing recorded less flexural strength improvements but their strengths were still higher than those of 100% FFA-based mixes in all curing ages, as shown in Figure 7, Figure 8, Figure 9 and Figure 10.

3.4. Durability Properties

Durability properties were evaluated in terms of water absorption, since it is a primary indicator of concrete porosity and long-term durability. Based on Figure 11, it can be indicated that water absorption of GPC was influenced by precursor type and fiber inclusion, as well as the applied curing regime. Under ambient curing conditions, GGBS-based GPCSs produced with GPCM2 typically exhibited the lowest water absorption values of 7.4% among the involved GPCMs, with a reduction of 25% at 28 days compared to that of GPCM1 (100% FA). That can be attributed to faster setting and higher calcium-induced matrix densification of GGBS-based concrete [44]. GPCM1 showed higher water absorption of 9.9% because of the low reactivity of FA at room temperature, which resulted in incomplete geopolymerization and a more porous matrix. In terms of blended mixes, FRGCM4 (75% GGBS and 25% FA) showed the lowest water absorption percentage of 8.0% under ambient curing conditions among the hybrid mixes (FRGCM3 and FRGCM5). GPCM5 exhibited moderate durability, with a reduction in water absorption up to 14% at 28 days compared to that of GPCM1. Under ambient curing conditions, the addition of fibers into GPC mixes had an adverse impact on the durability enhancement of all involved GPCSs but in different percentages. For instance, the inclusion of HESFs slightly increased water absorption in all linked geopolymer mixes, i.e., 6% (FRGCM1), 3% (FRGCM4), and 5% (FRGCM7) compared to those of GPCM1, GPCM2, and GPCM5, respectively, due to potential micro voids and weak interfaces around the utilized fibers [45]. However, GPCSs produced with mixes involved 0.75% BF, i.e., FRGCM2, FRGCM5, and FRGCM8 demonstrated the highest water absorption percentages of 11%, 5%, and 8% compared to those of GPCM1, GPCM2, and GPCM5, respectively. Basically, the rough, porous surface and hydrophilic nature of basalt fibers could be the main reason behind the increase in water absorption, since it can promote moisture uptake and capillary action [46]. Though, some previous studies reported a marginal reduction in water absorption and improvements in durability due to crack-bridging effects [47]. On the other hand, the application of heat curing, i.e., oven curing at 60 °C for 24 h or steam curing at 80 °C for 12 h, significantly reduced the water absorption value of GPCSs especially for fly ash-based mixes, since heat curing has a substantial effect on improving the microstructure of GPCSs. In thermally cured fly ash geopolymer concrete specimens produced with GPCM1 (100% FA), water absorption decreased to 6% under over curing and 5% under steam curing. GPCSs made of GPCM2 (100% GGBS) showed a reduction in water absorption to 4.5% under oven curing and 3.8% under steam curing, as they were already reactive under ambient conditions.

Similarly, water absorption values of GPCSs produced with hybrid mixes and reinforced geopolymer mixes also decreased with the application of heat curing in comparison with those under ambient curing. Under oven or steam curing systems, the addition of HESFs into GPCMs, i.e., FRGCM1, FRGCM4, and FRGCM7, resulted in significant reduction in water absorption compared to their peers under ambient curing conditions. This enhancement can be attributed to the temperature effect upon triggering geopolymerization. That leads to better fiber–matrix bonding in GPC systems, reduced capillary absorption, and enhanced durability of GPCS. While some studies report that the addition of basalt fibers results in a reduction ranging from 20% to 30% under oven or steam curing due to the enhanced microstructure densification, other research indicates that under ambient curing, BSFs can increase water absorption as a result of fiber clustering and poor workability, which introduces additional voids [48]. However, the current paper indicates that the incorporation of 0.75% BSFs into GPCMs, i.e., FRGCM2, FRGCM5, and FRGCM8 led to significant improvement of 46% in water absorption under steam curing compared to those under ambient curing. The addition of 5% PPFs into GPCMs demonstrated a reduction in water absorption in thermal-cured GPCS produced with FRGCM3, FRGCM6, and FRGCM9, because of the enhancement of the matrix’s resistance to moisture ingress. However, the addition of fibers into GPCMs led to a marginal increase in water absorption percentages even with the application of thermal curing compared to their peers without fibers, but it was less than those cured under ambient curing conditions. Overall, heat curing enhances the degree of geopolymerization and matrix densification, resulting in significantly lower porosity and improved durability, particularly in FA-based geopolymer systems [49]. Specially, steam curing in all GPCM systems showed potential for further durability enhancement in terms of water absorption reduction and the promotion of rapid and complete geopolymerization.

3.5. Cost Analysis

This section includes the compared material costs, curing energy costs, and the overall economic feasibility of GPC produced with different geopolymer mix designs.

3.5.1. Materials Cost Analysis

Material costs were calculated for all materials involved, i.e., fly ash, GGBS, sand, gravel, alkaline activator solutions, and fibers in USD. For more details,

Table 5. The details of material costs.

Mix No.	Mix ID	FA Cost (USD/m3)	GGBS Cost (USD/m3)	Fiber Cost (USD/m3)	AAS Cost (USD/m3)	Sand Cost (USD/m3)	Gravel Cost (USD/m3)	SP Cost (USD/m3)	Total Cost (USD/m3)
1	GPCM1	42	0	0	82.5	15	13.5	7.4	160.4
2	GPCM2	0	72	0	82.5	15	13.5	17	182
3	GPCM3	31.5	30	0	82.5	15	13.5	11.4	164
4	GPCM4	17.5	54	0	82.5	15	13.5	15.4	184
5	GPCM5	21	36	0	82.5	15	13.5	14	172
6	FRGCM1	42	0	70.88	82.5	15	13.5	15	217
7	FRGCM2	42	0	53.25	82.5	15	13.5	16.6	198
8	FRGCM3	42	0	12.75	82.5	15	13.5	11	159
9	FRGCM4	0	72	70.88	82.5	15	13.5	25.4	253
10	FRGCM5	0	72	53.25	82.5	15	13.5	27.4	234
11	FRGCM6	0	72	12.75	82.5	15	13.5	22.4	194
12	FRGCM7	21	36	70.88	82.5	15	13.5	19.8	235
13	FRGCM8	21	36	53.25	82.5	15	13.5	21.4	217
14	FRGCM9	21	36	12.75	82.5	15	13.5	17.6	177

shows the total cost of materials per cubic meter (USD/m³) for each mix design, based on the assumed unit prices. According to what is revealed in Figure 12 and Table 5, GPCM1 (100% FFA) included the lowest cost material of 146 USD/m³. GPCM2 was recorded as the highest cost material of 182 (USD/m³) among mixes without fibers since the production cost of GGBS is higher than FFA. The addition of fibers significantly increased the material cost involved in the production GPC.

3.5.2. Cost Analysis for Different Curing Regimes

According to Figure 13, the ambient curing regime is the most cost-effective curing method, requiring no additional energy or specialized equipment, making it ideal for large-scale applications in hot climates as it demands no active heating and therefore incurs negligible energy costs. However, its effectiveness in developing concrete strength is limited, especially in early stages, and it is expected to be even worse in frost environments or when initial strength gain is significant in specific applications. Conversely, thermal curing methods increased the cost of concrete processing up to approximately 38% compared to that of ambient curing due to higher energy demands. Steam curing, for instance, can consume up to 164 kWh per cubic meter, resulting in a cost of approximately $9.84/m³ at an electricity rate of $0.06 per kWh. Previous estimates suggest an increase of approximately 25% in energy costs above ambient curing plus considering additional related expenses, i.e., steam generation and safety, the overall bump could be approximately 15% above ambient curing. Then, the estimated total cost could rise to 40% above that of ambience curing. However, oven curing at 60 °C is more energy efficient, typically requiring around 40 kWh/m³, or about USD 4.00/m³ at USD 0.10 per kWh [50]. The oven curing regime requires extra costs such as maintaining the electricidal oven at the required temperature for a full day with continuous heating; this can add around 10% above ambient curing to the unit cost of the concrete. In addition to labor/time cost, extended curing in controlled conditions, incurs overheads for monitoring and equipment use. Conservative estimates drive the increase to about 15% above ambient curing. Then, the estimated total cost increased approximately from 20 to 25% above ambient curing. That means that steam curing can cost approximately twice as much as oven curing costs per cubic meter of concrete. Basically, the cost difference is primarily due to the mechanisms involved in heat transfer. Steam curing relies on the generation and transfer of latent heat through saturated vapor, which often results in energy loss through boiler inefficiencies and heat dissipation. Oven curing, however, delivers dry heat more directly and efficiently to the concrete mass. Additionally, steam curing systems often require more complex infrastructure and tighter environmental control, further increasing operational expenses [51]. As a result, while both methods accelerate early strength gain, oven curing systems offer a more economical solution than steam curing, particularly when energy efficiency and cost control are priorities.

3.5.3. Feasibility Evaluation for Strength and Durability Under Different Curing Regimes

It is also essential to consider the feasibility aspect of the designed GPCs cured under curing systems in terms of cost effectiveness besides strength and durability performance. According to current paper results, GPC mixes incorporated high GGBS percentages exhibited superior mechanical properties and durability performance, even under ambient curing, due to the calcium content which promotes early strength growth [52]. Among these, GGBS combined with HESFs, i.e., FRGCM4, demonstrated enhanced flexural strength and compressive strength, but the addition of HESFs elevated material costs. GGBS mixes, including BSFs (FRGCM5) and PPFs (FRGCM6), showed moderate improvements in strength and durability. More specifically, BSFs provided a favorable balance between cost and performance, whereas PPFs, although economical, offered limited structural enhancement represented by flexural strength. On the other hand, FFA-based mixes, while less costly and more environmentally sustainable, required elevated curing temperatures to attain comparable mechanical properties to GGBS, which can offset initial material savings. The addition of fiber reinforcement into FFA-based mixes further improved mechanical properties, especially flexural strength. For instance, the addition of HESFs resulted in the highest flexural strength performance compared to the addition of BFs and PPFs, albeit at a higher cost, while basalt and PP fibers offered more economic options but less noticeable enhancements. Conversely, the hybrid mixes of FFA and GGBS improved the strength and durability of GPC by over GPCM1 (100% FFA), offering a more cost-effective alternative under ambient curing conditions compared to GPCM1. Overall, the GGBS-based systems, particularly when combined with 50% FFA or blended with cost-efficient fibers like BSFs, provided an optimal balance of strength, durability, and cost effectiveness, especially under ambient curing which minimizes energy input. For more clarification,

Table 6. The summary of the mechanical properties’ evaluation for each mix design.

Mix ID	Compressive Strength			Flexural Strength			Water Absorption
	Ambient Curing	Oven Curing	Steam Curing	Ambient Curing	Oven Curing	Steam Curing	Ambient Curing	Oven Curing	Steam Curing
GPCM1	Poor	Good	Very Good	Poor	Fair	Fair	Poor	Good	Very Good
GPCM2	Good	Excellent	Excellent	Fair	Good	Good	Good	Excellent	Excellent
GPCM3	Poor	Good	Very Good	Poor	Fair	Fair	Poor	Good	Excellent
GPCM4	Good	Excellent	Excellent	Fair	Good	Good	Good	Excellent	Excellent
GPCM5	Good	very Good	Excellent	Poor	Fair	Good	Fair	Very Good	Excellent
FRGCM1	Poor	Good	Very Good	Poor	Very Good	Excellent	Poor	Good	Very Good
FRGCM2	Poor	Good	Very Good	Poor	Good	Very Good	Poor	Good	Very Good
FRGCM3	Poor	Good	Very Good	Poor	Fair	Fair	Poor	Good	Very Good
FRGCM4	Good	Excellent	Excellent	Fair	Excellent	Excellent	Good	Excellent	Excellent
FRGCM5	Good	Excellent	Excellent	Fair	Very Good	Very Good	Good	Excellent	Excellent
FRGCM6	Good	Excellent	Excellent	Fair	Good	Good	Good	Excellent	Excellent
FRGCM7	Good	Very Good	Excellent	Poor	Very Good	Excellent	Fair	Very Good	Excellent
FRGCM8	Good	Very Good	Excellent	Poor	Very Good	Excellent	Fair	Very Good	Excellent
FRGCM9	Good	Very Good	Excellent	Poor	Good	Good	Fair	Very Good	Excellent

involves the summary of the mechanical properties’ evaluation for each mix design.

4. Conclusions

In conclusion, this research paper demonstrated the effectiveness of different curing regimes applied to different geopolymer concrete specimens produced with different GPC mixes and fiber-reinforced geopolymer mixes. The conclusion of this paper is as follows:

• The compressive and flexural strength developments of GPC under ambient curing conditions are significantly influenced by the type and proportion of aluminosilicate binder materials. Thus, mixes including 100% FA offer limited early strength under ambient curing conditions due to FA’s low calcium content and the inherently slow geopolymerization kinetics at room temperature. The GPCMs produced with 100% or 75% of GGBS showed satisfied development under ambient curing conditions due to GGBS’s high calcium oxide content, which facilitates faster matrix formation.
• Thermal curing regimes (oven curing or steam curing) have a substantial influence on the compressive and flexural strength developments of GPC, especially for GPC mixes involving high FA content. Thermal curing demonstrates significant enhancement in 7-day compressive strength compared to their peers under ambient curing. GPC mixes including high percentages of GGBS respond less dramatically to thermal curing relative to 100% FA mixes.
• The incorporation of HESF, BSFs, or PPFs into GPCM results in slight initial compressive strength developments, especially under ambient curing conditions. The inclusion of 0.75% HESF leads to the highest flexural strength enhancement in all curing regimes and all utilized binder materials. The addition of 0.75% BSFs into GPCMs also improves the flexural performance but is less effective than HESFs due to the differences in physical and mechanical properties. Conversely, the incorporation of 0.5% PPFs results in more modest gains, as their low modulus limits their ability to resist flexural stresses.
• The application of low fiber proportions and heat-assisted curing systems into GPC significantly improves the water absorption performance of geopolymer concretes, particularly with FFA and hybrid systems, highlighting the importance of optimizing both fiber content and curing regimes for durability enhancement.
• The most affordable option is GPCM1 (100% FA without fibers), but it demonstrates low compressive and flexural strengths under ambient curing conditions. RGCM4 (100% GGBS and 0.75% HESF) can be considered the best strength and durability option, as it provides the highest strength performance but results in higher material costs. RGCM7 (50% FA, 50% GGBS, and 0.75% HSF) can be regarded as a balanced option since it offers satisfied flexural strength, compressive strength, and durability performance with moderate cost compared to other options.