Effect of varying molarity and curing conditions on the mechanical and microstructural characteristics of alkali activated GGBS binder

Geopolymer binder offers a more sustainable choice for producing concrete in comparison to traditional ordinary Portland cement (OPC). The substitution of geopolymer binder for construction practices can decrease carbon dioxide emissions by decreasing OPC usage and repurposing industrial waste materials like ground granulated blast furnace slag (GGBS), fly ash, red mud, silica fume. In order to assess the suitability of GGBS as a binding material, it is essential to conduct conventional tests like consistency, setting times, and compressive strength, which are widely employed in cement testing. This study produced alkali activated paste (AAP) from GGBS and an alkaline activator comprising sodium hydroxide at various molarities from 1 M to 8 M. This investigation focused on the compressive strength of alkali-activated GGBS-based AAP under varying alkali activation molarities and curing conditions, including ambient, hot air oven, and humidity chamber curing. Additionally, the end reaction products of AAP showing higher compressive strength were examined for scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), x-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis. The experimental outcomes indicated that GGBS reduced the final setting time of AAP while increasing its compressive strength. Additionally, increasing the quantity of NaOH in the AAP increased its compressive strength. Furthermore, the research findings indicated that the mechanical properties of the alkali-activated GGBS-based material were notably influenced by the chosen curing conditions. Specifically, ambient curing demonstrated superior compressive strength, measuring at 47.06 MPa after 28 days, surpassing the results obtained from hot air oven curing and humidity curing.


Introduction
The construction industry relies significantly on OPC as a fundamental binding agent, which serves as a crucial constituent in the formulation of concrete and plaster. It holds the distinction of being the second most extensively utilized material globally, with water being the sole exception [1]. To cater the escalating demands of the construction sector, India produces approximately 500 million tons of OPC annually [2]. Projections indicate that by the end of year 2025, India aims to augment OPC production to an annual output of 550 million tons [3]. Nevertheless, the unregulated production and utilization of OPC have adverse environmental implications. OPC production entails the depletion of renewable resources and results in the emission of substantial volumes of carbon dioxide into the surrounding environment. It is estimated that the production of one metric ton of OPC emits 0.87 metric tons of carbon dioxide, thus making a substantial contribution to overall greenhouse gas emissions [4]. In order to mitigate the negative consequences associated with OPCdriven industries, it is imperative to explore alternative construction materials that significantly reduce cement consumption, thereby enhancing the sustainability of the construction sector. Such alternatives could encompass the incorporation of recycled materials or the implementation of more efficient production methodologies that curtail waste generation and energy consumption [5,6]. By conducting a thorough investigation and embracing the adoption of these viable substitutes, a meaningful contribution can be made towards mitigating the ecological consequences of the construction sector and forging a more sustainable trajectory for the future [7,8].
Geopolymer represents an innovative binding material generated through the polymerization of diverse industrial by-products, including GGBS, fly ash (FA), red mud (RM), metakaolin (MK), pond ash (PA), silica fume (SF), among others, all of which contain ample silicon dioxide (SiO 2 ) and aluminium oxide (Al 2 O 3 ) ions through alkali activation [9,10]. This process necessitates the utilization of an alkali activator solution (AAS) to stimulate the SiO 2 and Al 2 O 3 components present in the aluminosilicate precursors, resulting in the formation of calcium aluminate silicate hydrate and sodium aluminate silicate hydrate gels. This stands in contrast to OPC, which generates calcium silicate hydrate gel within high-alkaline conditions [11].
Conventionally, the AAS employed comprises a fusion of sodium hydroxide (NaOH) and sodium silicate (Na 2 SiO 3 ), or alternatively, potassium hydroxide (KOH) and potassium silicate (K 2 SiO 3 ) solutions. Geopolymer binders exhibit diverse characteristics that are heavily influenced by the type of industrial waste products (aluminosilicate sources) and AAS employed used and the manufacturing environment. Consequently, they exhibit a diverse spectrum of attributes. Some notable distinctive features of geopolymer concrete encompass a marked augmentation in strength properties compared to OPC concrete, remarkable fire resistance, low thermal conductivity, and the ability to withstand acidic environments [12]. In contrast to OPC concrete, geopolymer binders offer enhanced attributes such as superior resistance to both chemical and physical deterioration, rendering them particularly well-suited for deployment in challenging settings [7]. Furthermore, geopolymer binders have been found to engender notably diminished greenhouse gas emissions in comparison to OPC concrete, thus designating them as a more sustainable alternative for construction projects [13,14].
GGBS, or ground granulated blast furnace slag, is an outcome of pig iron in the manufacturing process. It is generated by rapidly cooling the boiling slag from an iron furnace with water [15]. It has a highly cementitious tendency and can hydrate similarly to OPC when crushed to a fine powder. Approximately 320 million tons of GGBS are produced worldwide each year, with an annual increase of 1.6% [16]. GGBS is often deposited in open landfills, where it can accumulate and negatively impact the environment's fertility and underground water. Nevertheless, GGBS can also be utilized as a primary component in the production of geopolymer binder systems. When combined with other aluminosilicate materials, GGBS can exhibit excellent qualities, including significant compressive strength, appreciable flexural properties, and exceptional durability characteristics [17]. The utilization of metallurgical slags in alkali activation can produce low-carbon cementitious binders and minimize carbon dioxide emissions by up to 75% compared to OPC concrete [18]. Additionally, geopolymer binder systems incorporating GGBS demonstrate better resistance to elevated temperatures compared to conventional OPC based cement binder.
By incorporating GGBS into FA blended geopolymer concrete and manipulating the curing parameters, the attainment of optimal setting time, compressive strength, and workability can be achieved [19]. The presence of NaOH exerts a significant influence on the physical attributes of the geopolymeric paste. Through the inclusion of NaOH as an alkaline activator within the geopolymeric paste, the dissolution ratio of SiO 2 and Al 2 O 3 from FA is enhanced, culminating in a geopolymeric paste characterized by enhanced compressive strength and improved rheological attributes [20]. The molar concentration of NaOH within the AAS significantly governs the fresh properties of the geopolymeric matrix [21]. In the absence of FA, GGBS can induce a reduction in the rheological attributes, encompassing setting time and workability, of the geopolymer paste. Nevertheless, an escalated NaOH concentration within the AAS, coupled with an elevated substitution of GGBS for FA, can foster an enhancement in the resultant compressive strength of geopolymer concrete.
The utilization of alkali-activated combinations of FA and GGBS has been ascertained as a feasible alternative for producing a resilient and ecologically sustainable cementitious substance. The rheological behaviour of these amalgamations ranged between 26% and 39%, wherein mixtures richer in GGBS exhibited accelerated setting kinetics and augmented compressive strength of the mortar [22]. Elevating the molar concentration of NaOH and the proportion of GGBS led to a reduction in both rheological consistency and setting time of the alkali-activated compositions, thereby fostering homogeneity and diminishing the setting duration [23]. The ratio of SiO 2 to Al 2 O 3 in the precursor materials emerges as a critical parameter influencing the setting kinetics and the evolution of strength in the resultant geopolymeric matrix. The constituents of the alkali-based activators significantly influence the initial strength at heightened molar concentrations, with aluminum and silicon ions exerting a prominent influence on the stable microstructural configuration of the geopolymer matrix [24]. Investigations have substantiated that the incorporation of GGBS into the preparation of geopolymeric binders enhances their compressive characteristics and the index of polymerization activity across diverse curing periods. Owing to the enlarged specific surface area inherent to GGBS constituents, the presence of calcium hydroxide (Ca(OH) 2 ) within the paste is curtailed, consequently leading to a reduction in the Ca/Si ratio of the hydrated calcium silicate gel within the mortar specimen [18].
The geopolymeric paste specimens exhibit a diminished degree of hydration in comparison to conventional Portland cement. The alkali activators induce the formation of calcium-silicate-hydrate (C-S-H) and an amorphous matrix as their end reactant production from polymerization reaction. The augmentation in calcium content, consequent upon the incorporation of GGBS within the geopolymeric mortar, fosters the progression of an amplified C-S-H gel [25]. This augmentation in the resultant C-S-H phase contributes to the enhancement of compressive properties and a more densely compacted microstructural arrangement within the alkali-activated binder [26]. With the appearance of both C-S-H and calcium-aluminum-silicate-hydrate (C-A-S-H) phases, the introduction of GGBS into the geopolymeric binder has been demonstrated to enhance its flexural and compressive attributes, alongside fostering morphological characteristics. Nonetheless, an elevation in the levels of GGBS substitution may exert an adverse influence on the flow characteristics, potentially necessitating the introduction of supplementary water or superplasticizers to maintain desired workability [27]. The incorporation of a 30% proportion of GGBS into the FA-based geopolymeric mixture resulted in an enhancement of mechanical performance and has triggered the generation of C-S-H gel, thereby resulting in a densification of the microstructure within the geopolymeric concrete matrix. The inclusion of calcium compounds through GGBS serves to engender the formation of both C-S-H and C-A-S-H phases, thereby fortifying the geopolymerization process. However, it is crucial to underscore that calcium compounds exhibit a tendency to reduce the mechanical characteristics of geopolymers under conditions of high-temperature curing [28].
In spite of the rapid strides and advancements achieved in geopolymer concrete research, its practical implementation remains relatively limited. A primary factor contributing to this circumstance is the absence of standardized guidelines for formulating geopolymer concrete, resulting in uncertainties surrounding its compatibility with diverse aluminosilicate source materials and the lack of comprehensive knowledge regarding its fundamental properties [15]. In response to these challenges, this study directs its focus toward two pivotal dimensions. Primarily, it investigates the influence of varied concentrations of NaOH as an alkali activator on the strength and microstructural attributes of GGBS within varying curing conditions. Additionally, beyond the assessment of the feasibility of incorporating GGBS, an industrial by-product, into the manufacturing process of geopolymer paste, this research aims to enhance the comprehension of geopolymer concrete and promote its practical utilization. The primary objective of the ongoing study is to assess the impact of an alkali activator comprising GGBS paste, characterized by diverse NaOH concentrations, on several critical attributes, including initial and final setting times, as well as compressive, flexural, and microstructural properties. These investigations are conducted within the framework of three distinct curing conditions.

Experimental procedure 2.1. Aluminosilicate precursor material
The objective of this investigation was to generate an alkali-activated paste mixture by utilizing GGBS as the fundamental component. The GGBS utilized in this research was procured from JSW cements, Vellore. To achieve this objective, varying concentrations of NaOH solution were used under three different curing conditions. Table 1 displays the findings of x-ray fluorescence (XRF) analysis performed on GGBS to ascertain its composition of oxides.
The data from table 1 revealed that GGBS consists primarily of SiO 2 (36.93%), CaO (37.65%) and Al 2 O 3 (15.14%). While the composition of GGBS may differ based on the materials employed in iron manufacturing, it typically consists of a substantial amount of SiO 2 , Al 2 O 3 , and CaO, rendering it a favorable source material for alkali activated synthesis. In addition, the high CaO content in GGBS can enhance the simultaneous development of C-S-H matrix and alkali activator at an earlier stage.
Microstructural investigation was employed using SEM to analyze the morphological features of raw GGBS powder sample, which revealed that GGBS substances were sharp and angular (uneven) in shape, as depicted in  ions from the aluminosilicate source material, enabling them to polymerize and generate stable end products. The particle size distribution of GGBS, as presented in figure 3, clearly indicates that over 50% of the particles exhibit dimensions below 100 microns. This characteristic underscores the potential for enhanced reactivity during the polymerization process, attributed to the larger surface area inherent to such fine particles. Furthermore, table 2 summarizes the physical characteristics of GGBS. NaOH flakes was utilized as the AAS in this investigation to extract SiO 2 and Al 2 O 3 ions from the precursor material (GGBS). The AAS was prepared using commercially available sodium hydroxide flakes obtained from Astraa Chemicals, Chennai, constituted with a purity level of 98%. Typical values for the molar mass and specific gravity of the sodium hydroxide flakes used in the study were 40.0 g mol −1 and 2.14, respectively. The highpurity sodium hydroxide flakes ensured the consistent quality of the alkaline activator solution, which is vital for reliable results in the production of alkali-activated paste. NaOH solution is more effective in dispersing Al 2 O 3 and SiO 2 ions in comparison with KOH solution. The alkali content is critical in managing the disintegration of both silica and alumina from GGBS substances during polymerization, which determines the mechanical properties of the geopolymer matrix. Increased NaOH content can increase the dispersion of SiO 2 and Al 2 O 3 fractions in raw materials, resulting in enhanced compression performance. Therefore, achieving the desired strength in geopolymer concrete depends on the critical factor of NaOH concentration.

Sample preparation
A 1 M sodium hydroxide (NaOH) solution can be created by dissolving 40 gm of NaOH flakes in one liter of water. For other concentrations of NaOH, the flakes were diluted in distilled water and left to sit for 24 h [26]. The molarity of the alkaline solution added to the solid components, including GGBS, was crucial for the strength characteristics AAP. Mixing was carried out thoroughly for about a minute in a pan mixer, followed by wet-mixing for 4 min. The resulting mixture was manually compacted and cast into cubes using 50 mm×50 mm×50 mm moulds with varying molarities ranging from 1 M to 8 M in increments of 1 M. The cubes were then subjected to different curing conditions, including hot air oven, humidity curing, and ambient curing. After being removed from the moulds, the AAP samples were left to cure at ambient temperature until the day of testing.
The intent to explore a range of NaOH concentrations ranging from 1 M to 8 M in the context of this study is to understand how the influence of varying activator concentrations impacts the mechanical properties of the geopolymer binder and its suitability for construction related purposes. This insight has the potential to guide the formulation of geopolymers endowed with precisely customized attributes suitable for distinct applications, thereby driving forward the progress of eco-friendly construction materials. Preliminary investigations reveal that elevating the NaOH concentration beyond 8 M leads to a progressive reduction in the initial setting time. Additionally, an efflorescence phenomenon manifested in alkali-activated paste samples when the molar concentration exceeded 8 M. Consequently, for the present study, the scope of NaOH molarity was varied within the range of 1 M to 8 M.

Different curing methods
During hot air oven curing, the AAP samples were exposed to high temperatures in a hot air oven for more than an hour, which resulted in the enhancement in compressive property [29]. While longer curing durations typically lead to stronger samples, the improvement in strength becomes negligible beyond 24 h [30]. The AAP GGBS based specimens were initially oven cured for eight hours at 50°C. Following that, they were permitted to drop down to ambient temperature prior to proceeding with traditional room temperature curing. The compressive strength variations of hot air oven cured specimens was examined at the end of 1, 7, and 28 days.
Under ambient curing circumstances, the specimen moulds were covered using a vacuum sealed film to prevent the evaporation of water. Following twenty-four hours of ambient curing, the test specimens were demoulded and exposed to the ambient environment until testing. The study observed that the performance of the  alkali-activated paste (AAP) cured at room temperature was equivalent to that of OPC, even though the incorporation of GGBS influenced the setting time, workability, and initial strength. According to Al-Majidi et al [31], incorporation of GGBS resulted in the development of internal heat, which accelerated geopolymerization during ambient curing, leading to improved early strength development. Fly ash and slag were used as binders for geopolymer concrete, which was cured using three methods. Ambient curing produced better results [32].
During humidity chamber curing, the specimens were exposed to a controlled environment inside a humidity chamber maintained at a temperature of 35°C and a relative moisture level of 65% for a period of 8 h. Subsequently, the specimens were transferred to an oven and allowed to cool to room temperature. The cube samples were then further cured at normal room temperature, and the compressive strength of the alkaliactivated paste (AAP) cubes was measured after 1, 7, and 28 days of curing. The specimens that underwent high relative humidity hardening were compared to those cured in water, with the latter resulting in higher mechanical strength. Hardening geopolymer specimen at high relative humidity was found to reduce water porosity and enhance mechanical properties.
The selected curing methodologies and their respective durations offer to provide a comprehensive understanding of how varying curing conditions influence the development of geopolymer binders. The employment of a humidity chamber aims to replicate conditions optimal for curing, while the utilization of a hot air oven seeks to investigate the potential for accelerated curing. In parallel, ambient curing conditions reflects practical world applications. This multifaceted approach is designed to facilitate a comprehensive evaluation of the behaviour of the material, thereby furnishing valuable guidance to both researchers and practitioners, aiding them in making well-informed decisions concerning the selection of appropriate curing strategies for geopolymers.

Setting time
To ascertain the viability of GGBS-based alkali activator paste for on-site utilization, it is imperative to establish both the initial and final setting time. In accordance with the IS-4031 (Part V) specifications, the initial and final setting time of the GGBS based AAP samples were evaluated.

Compressive strength
The assessment of compressive strength outcomes for alkali-activated GGBS paste specimens encompassing diverse molar concentrations (ranging from 1 M to 8 M) across three distinct curing regimes (humidity, hot air oven, and ambient conditions) was conducted in adherence to ASTM C 109 standards at the end of 3, 7, and 28 days [33].
The analysis of compressive strength was executed on cubic alkali-activated GGBS paste samples measuring 50 mm×50 mm×50 mm, using a compression testing machine (CTM) with a load capacity of 2000 kN, as illustrated in figure 4. Each measurement was conducted on a set of three specimens, and the arithmetic mean of the outcomes was recorded.

Flexural strength
Following the guidelines outlined in ASTM C496/C496M standards [34], the assessment of the flexural strength characteristics of alkali-activated GGBS paste sample was determined. For this purpose, prismatic specimens with dimensions of 40 mm×40 mm×160 mm were casted and subjected to flexure test for each mix proportion. The flexural strength evaluation procedure was executed utilizing a universal testing machine (UTM) possessing a maximum capacity of 50 kN as depicted in figure 5. To ascertain the average flexural strength, a total of three samples from each mixture were subjected to testing after 7 and 28 days of humidity curing.

Ultrasonic pulse velocity test
To assess the quality of the alkali activated paste samples fabricated using GGBS under diverse molar concentrations of NaOH solution, the Ultrasonic Pulse Velocity (UPV) technique is employed. This nondestructive method involves the propagation of electronic waves through the concrete to evaluate its integrity. The assessment is performed at the end of a 28-day ambient curing period, following the guidelines outlined in ASTM C597-16 standards and the experimental set up for the UPV tests are visually depicted in figure 6.

Microstructural characterization
The morphological and the elemental constitution of the alkali activated GGBS paste samples subjected to three distinct curing regimes were investigated employing high-resolution scanning electron microscopy (EVO/18 research, Carl Zeiss) in conjunction with energy dispersive x-ray spectroscopy (EDX-Oxford Instruments).
The application of a Fourier-transform infrared (FTIR) spectrophotometer was utilized to identify the absorbance patterns and distinctive functional groups formed throughout the polymerization process when GGBS was employed as the principal source material for the preparation of AAP. The characteristic peaks present in the final constituents of the AAP samples were ascertained with the utilization of an IR-affinity 1, Shimadzu Japan FT-IR Spectrophotometer.
The x-ray Diffraction (XRD) profiles of the alkali-activated paste samples derived from GGBS, subjected to three distinct curing conditions, were acquired utilizing a Bruker D8 advanced powder x-ray diffractometer of German origin. The XRD apparatus operated at 2.2 kW, employing monochromated Cu-Kα radiation (λ=1.541874 Å). The scanning span (2θ range) and incremental interval (step size) were set at 0-90 and 0.02, respectively.

Setting time of GGBS based AAP
In order to assess the suitability of GGBS-based alkali activator paste (AAP) for in situ applications, it is crucial to determine both the initial and final setting times. The initial setting time (IST) refers to the duration starting from when GGBS comes into contact with the alkaline activator until it loses its plasticity and transforms into a paste. On the other hand, the final setting time (FST) is the period between the blending of GGBS with the alkaline liquid and the point at which it gains structural strength and loses its fluidity. The IS-4031 (Part V) provides a specified test method for measuring the setting time using the Vicat needle application for hydraulic cement. In this method, a 1-mm Vicat needle is inserted into the AAP at regular intervals, allowing for the determination of both the IST and FST based on specific criteria.
The FST is the duration from the initial GGBS and alkaline activator solution contact until the needle can no longer penetrate the paste. The IST and FST of GGBS-based alkali activator paste (AAP) can vary depending on the NaOH molarity. In this case, the range of  figure 7 illustrate that the inclusion of NaOH flakes in the alkaline activator at different molarities has the potential to decrease the setting time [21].
When the molarity of NaOH in the AAS is increased, the FST of the GGBS-based alkali activator paste (AAP) is reduced. This is because the higher concentration of hydroxide ions (OH) in the solution accelerates the disintegration of silica (Si) and alumina (Al) ions from the precursor materials, promoting the geopolymerization reaction and resulting in a faster setting time [23,35].  conditions and over distinct curing periods. The concentration of the NaOH solution is an important factor in controlling the strength property of AAP. Higher concentrations of NaOH lead to a more alkaline environment, which increases the dissolution of reactive alumino-silicates in GGBS, resulting in a higher degree of polymerization reaction and thus increased the strength of AAP paste. Furthermore, the presence of calcium oxide content in the GGBS aided the strength development by allowing the formation of C-S-H and C-A-S-H end components.

Compressive strength variation of AAP
The compressive strength property of GGBS based AAP gradually improves with the curing age. Longer curing periods provide more time for the polymerization process to occur, resulting in a denser and stronger structure thereby enhancing the compressive strength. At 28 days strength, the compressive strength ranged from 12.8 MPa to 41.6 MPa under humidity chamber curing, from 9.3 MPa to 36.6 MPa under hot air oven curing, and from 15.73 MPa to 47.06 MPa under ambient curing. Furthermore, the ambient curing process resulted in better strength than the hot air oven curing and humidity chamber curing. This property of AAP was found to increase when the alkaline activator constituted a higher molarity of NaOH solution. Figure 11 illustrates the experimental outcomes of a flexural strength assessment conducted on prism specimens of alkali-activated GGBS paste. The investigation encompassed a spectrum of molar concentrations of NaOH  The results indicate that the gradual increment in the NaOH molarity resulted in the enhancement in flexural properties of alkali-activated GGBS paste. This is because of the development of a denser and stronger polysialate network during the alkali activation process, aided by the presence of portlandite released during the geopolymerization reaction. The extended curing time also allows for an adequate duration of polymerization to occur, which leads to an improvement in flexural strength [36,37].

Ultrasonic pulse velocity (UPV) test
UPV is a non-destructive technique employed to assess the consistency, porosity, and crack assessment in concrete, functioning as an indicator for the enduring attributes and hardened strength of materials. UPV has been shown to be an effective technique for qualitatively evaluating the outcomes of experiments aimed at improving concrete materials, including enhanced compressive strength and decreased cracking [27]. Results from UPV tests conducted on alkali activated GGBS paste specimens of varying NaOH molar concentrations cured for 28 days are presented in figure 12, with UPV values ranging from 2587 to 3968 m s −1 .  The highest UPV values were observed for mixtures with the highest NaOH molar concentrations, which is correlated to the rate of the geopolymerization reaction, where higher NaOH concentrations may lead to stronger specimens [38]. The increase in UPV with increasing curing time is primarily a by-product of geopolymerization, which results in a more rigid paste matrix and generates a denser microstructure in the binder phase.

SEM and EDS analysis of AAP
The microstructure of 8 M GGBS-based alkali-activated pastes under three different curing regimes was investigated using SEM analysis. Samples were collected from the innermost part of the 28-day specimens and examined at 4000x magnification with a scale of 10 μm. Figure 13(a) shows the thermally cured specimen developed micro cracks due to shrinkage caused by rapid moisture loss. Figure 14(a) displays the flaky formation of calcium silicate hydrate (C-S-H) gel, while figure 15(a) illustrates a compact structure with no residue, indicating complete reaction of the alumino-silicate with needle-shaped ettringite present [39].
The prompt addition of calcium oxide content via GGBS enhanced the IST and FST performance of the AAP paste, promoting the formation of a densely packed microstructure. Additionally, the use of GGBS resulted in the creation of C-S-H network with a honeycomb-like structure, resulting in increased packing efficiency. The exothermic interaction among calcium and the alkali solution played a prominent role in promoting the occurrence of subsequent C-S-H and C-A-S-H gel configurations. These gel formations were crucial  contributors to the overall enhancement of strength in the geopolymer microstructure [3,40]. At atmospheric temperature, the microstructure of specimens was superior to those cured at 70°C, as it facilitated GGBS dissolution, accelerated the geopolymeric process, and filled voids in the interfacial transition zone (ITZ) and matrix with alumino-silicate gel [41].
The prompt addition of calcium oxide content via GGBS enhanced the IST and FST performance of the AAP, promoting the formation of a densely packed microstructure. Additionally, the use of GGBS resulted in the creation of C-S-H network with a honeycomb-like structure, resulting in increased packing efficiency. The exothermic interaction among calcium and the alkali solution played a prominent role in promoting the occurrence of subsequent C-S-H and C-A-S-H gel configurations. These gel formations were crucial contributors to the overall enhancement of strength in the geopolymer microstructure [3,40]. At atmospheric temperature, the microstructure of specimens was superior to those cured at 70°C, as it facilitated GGBS dissolution, accelerated the geopolymeric process, and filled voids in the interfacial transition zone (ITZ) and matrix with alumino-silicate gel [41].

XRD evaluation of GGBS based AAP
The phase composition of the alkali-activated GGBS paste specimens was analyzed using the x-ray diffraction (XRD) technique. In figure 16, the XRD patterns of the 8 M alkali activator GGBS paste specimens with three different curing methods at 28 days are presented. The XRD profiles of the GGBS-based AAP samples treated under three different techniques revealed an amorphous structure characterized by raise in humps which signifies the strength formation phase in an alkali activated matrix. These humps facilitates the existence of calcium-based hydration products, including C-S-H, C-A-S-H/N-A-S-H, as well as other compounds such as Quartz (Q) as their end reaction products at the three different types of curing regimes [42,43]. The evidence for higher proportion of calcium in the materials resulted in rapid hardening of the paste, and the alkali-activated GGBS paste set and developed strength quickly [41,44]. The XRD configurations for the samples treated in an ambient atmosphere and in an oven exhibited very little difference form one another. The peak of Ca(OH) 2 was found at 18.048°, 34.110°, and 47.129°. The diffraction tops of Ca(OH) 2 occur after a day of hydration and are found in slurry hydration products. During hydration, the carbonization of calcium hydroxide products leads to the peaks of CaCO 3 at 29.399°. The peaks of C 3 S in cement clinker are found at 32.192°, 34.377°, and 41.297°, while Di-calcium silicate (C 2 S) has XRD peaks at 32.136°and 32.469°. The tetra calcium ferrialuminate (C 4 AF) has a low peak. Figure 17 shows the major characteristic bands of an 8 M alkali-activated GGBS paste after 28 days of curing using three different methods. The bending and stretching vibrations of the Si-O and Al-O groups are responsible for the 424.24 cm −1 peak. In case of GGBS based AAP specimens, the bending vibration bands of Si-O are observed as a peak at 436.32 cm −1 . The bending vibration of Al-O groups is detected between wave numbers 600.14 cm −1 and 607.55 cm −1 , with a maximum bond observed at 713.81 cm −1 and 718.74 cm −1 .The existence of silicate glass in GGBS during geopolymerization is indicated by the peak at the wave number of 940.05 cm −1 to 941.66 cm −1 , which represents the asymmetric lengthening vibrations of Si-O-Si and Al-O-Si bonds [22].

FTIR analysis of GGBS based AAP
The presence of carbonates in GGBS is indicated by the peaks at wave numbers 1383. 39

Water absorption test on GGBS based AAP
The permeability, mechanical performance, and durability of a material depend on its ability to absorb water from its surroundings, which is influenced by its internal pore structure. The present work intend to evaluate the water absorption characteristics of GGBS based AAP specimens which were exposed to ambient environment curing for 28 days. The findings depicted in figure 18 indicated that there was a decrease in water absorption for  the GGBS-AAP specimens when NaOH molarity varied among 1 M to 8 M. The outcomes from water absorption test indicated that the specimen with 8 M NaOH exhibited the least absorption rate at 6.33%, while the one with 1 M NaOH had the highest absorption rate at 16.03%.
An increase in the water absorption percentage significantly influenced the compressive strength of GGBS based AAP, leading to higher porosity and lower strength due to increased water retention. However, the reduction in open porosity reduced water flow into the alkali-activated component, resulting in low water absorption. Alkali-activated samples absorbed less water than similar materials, indicating greater resilience. The GGBS pastes were highly porous and permeable, but their low water absorption rate indicates a reduced risk of water damage.

Conclusions
The experimental study examined the compressive strength of alkali-activated GGBS paste cured under various alkali activation molarities, including ambient conditions, hot air oven curing, and humidity chamber curing. Microstructural analyses were conducted using SEM/EDS, XRD, and FTIR techniques. Based on the results, the following conclusions were drawn:  • The concentration of sodium hydroxide in the alkaline activator did not affect the normal viscosity of AAP.
The alkali concentration and GGBS content were the crucial factors in determining the strength of GGBSbased AAP. These characteristics significantly impacted consistency, setting, and strength properties. Higher concentrations of sodium hydroxide resulted in the maximum compressive strength of the alkali-activated slag paste.
• GGBS can be employed as a precursor material for synthesizing alkali activator binders, which can be cured under ambient conditions without requiring external heat. This is due to the fact that GGBS contains calcium and other elements that interact with alkaline solution to formulate C-S-H and other hydration products. This makes the production process more accessible and cost-effective, as it eliminates the need for specialized curing equipment and reduces energy consumption.
• At 28 days, ambient curing outperformed hot air oven curing and humidity chamber curing in terms of strength. Using GGBS to develop AAP under ambient curing conditions was suitable for site application.
• X-ray diffraction (XRD) examination revealed that the higher the calcium availability of the products, the faster the paste hardened, resulting in GGBS-based AAP with early setting and gaining strength.
• SEM and EDS evaluation confirmed the development of C-S-H, N-A-S-H, and C-A-S-H gel formations in the microstructure of the GGBS based AAP, which contributed to the improvement of its compressive strength.
• The FTIR spectra revealed the presence of Si-O and O-H bonds, carbonates, H-O-H, alkali hydroxides, and silicate glass in the GGBS-based AAP.