Study on the Compressive Strength and Reaction Mechanism of Alkali-Activated Geopolymer Materials Using Coal Gangue and Ground Granulated Blast Furnace Slag

By reutilizing industrial byproducts, inorganic cementitious alkali-activated materials (AAMs) contribute to reduced energy consumption and carbon dioxide (CO2) emissions. In this study, coal gangue (CG) blended with ground granulated blast furnace slag (GGBFS) was used to prepare AAMs. The research focused on analyzing the effects of the GGBFS content and alkali activator (i.e., Na2O mass ratio and alkali modulus [SiO2/Na2O]) on the mechanical properties and microstructures of the AAMs. Through a series of spectroscopic and microscopic tests, the results showed that the GGBFS content had a significant influence on AAM compressive strength and paste fluidity; the optimal replacement of CG by GGBFS was 40–50%, and the optimal Na2O mass ratio and alkali modulus were 7% and 1.3, respectively. AAMs with a 50% GGBFS content exhibited a compact microstructure with a 28 d compressive strength of 54.59 MPa. Increasing the Na2O mass ratio from 6% to 8% promoted the hardening process and facilitated the formation of AAM gels; however, a 9% Na2O mass ratio inhibited the condensation of SiO4 and AlO4 ions, which decreased the compressive strength. Increasing the alkali modulus facilitated geopolymerization, which increased the compressive strength. Microscopic analysis showed that pore size and volume increased due to lower Na2O concentrations or alkali modulus. The results provide an experimental and theoretical basis for the large-scale utilization of AAMs in construction.


Introduction
Ordinary Portland cement (OPC) is an essential component of concrete.Generally, the production of 1 ton of OPC generates 0.8-1.0tons of carbon dioxide (CO 2 ), and as the global population increases, the demand for OPC increases.The cement industry is estimated to account for 7-8% of global CO 2 emissions, which is challenging for human societies [1][2][3].It is predicted that the production of OPC will reach 5 billion tons in 2030, and the projected emission of CO 2 could be as high as 4.7 billion tons [4].The global consensus on CO 2 reduction and emissions has promoted research on low-carbon cementitious materials as alternatives to OPC [5,6].

Materials
The CG used in this study was sourced from Zhengzhou, Henan Province, China.Before use, it was subjected to calcination at 700 • C for 3 h.The GGBFS was procured from Nanyang, Henan Province, China.
Table 1 shows the chemical compositions of the CG and GGBFS, which were determined via X-ray fluorescence.The CG was rich in SiO 2 and Al 2 O 3 , constituting 86.12 wt% of the total CG composition.The XRD patterns (Figure 1a) showed that quartz, kaolinite, and muscovite were the main minerals in the raw CG.Following the calcination process, the reflections of kaolinite disappeared, and the calcined CG exhibited a broad diffraction band centered at approximately 23.7 • (2θ) (Figure 1b), suggesting the transformation of kaolinite into an amorphous phase, while quartz was preserved in the calcined CG due to its thermal stability.The GGBFS primarily contained CaO, SiO 2 , and Al 2 O 3 , which totaled 86.11 wt% of the whole composition.The XRD pattern of GGBFS (Figure 1c) featured a broad diffraction band centered at approximately 31.5 • (2θ), indicating the GGBFS was predominantly amorphous.Calcite and dolomite were identified in the GGBFS.

Materials
The CG used in this study was sourced from Zhengzhou, Henan Province Before use, it was subjected to calcination at 700 °C for 3 h.The GGBFS was procu Nanyang, Henan Province, China.
Table 1 shows the chemical compositions of the CG and GGBFS, whi determined via X-ray fluorescence.The CG was rich in SiO2 and Al2O3, constituti wt% of the total CG composition.The XRD patterns (Figure 1a) showed tha kaolinite, and muscovite were the main minerals in the raw CG.Following the ca process, the reflections of kaolinite disappeared, and the calcined CG exhibited diffraction band centered at approximately 23.7° (2θ) (Figure 1b), sugges transformation of kaolinite into an amorphous phase, while quartz was preserve calcined CG due to its thermal stability.The GGBFS primarily contained CaO, S Al2O3, which totaled 86.11 wt% of the whole composition.The XRD pattern of (Figure 1c) featured a broad diffraction band centered at approximately 31 indicating the GGBFS was predominantly amorphous.Calcite and dolomi identified in the GGBFS.The materials used as alkali activators were composed of sodium silicate (SiO2 27.3 wt%, Na2O 8.5 wt%, and H2O 64.2 wt%) and sodium hydroxide particle Figure 2 shows the SEM images and particle size distributions of the calcined CG and GGBFS.The CG possesses a loose surface morphology and porous structure (Figure 2a), and the GGBFS exhibits a smooth surface morphology and angular shape (Figure 2b).The median particle size (d 50 ) of the CG and GGBFS are 8.59 and 10.29 µm, respectively.

AAMs Preparation
The preparation process is illustrated in Figure 3, and the specific mixing CG/GGBFS-based AAMs is detailed in Table 2.The AAMs were prepared by NaOH and Na2SiO3 to distilled water to formulate the alkali activators, and the pr (calcined CG and GGBFS) were mixed for 2 min in a planetary mixer.The alkali ac were then blended with the precursors for 5 min to create a uniform slurry pa paste was subsequently poured into silica molds and vibrated on an electric v table for 90 s to eliminate the remaining bubbles.A thin polyethylene film was ap cover all samples.Following this, the samples were cured in a controlled environ 60 °C for 24 h, unmolded, and left at ambient temperature (~25 °C) at 3, 7, and 28 conducting the test.The materials used as alkali activators were composed of sodium silicate solution (SiO 2 27.3 wt%, Na 2 O 8.5 wt%, and H 2 O 64.2 wt%) and sodium hydroxide particles (purity ≥96%).Distilled water was used for adjusting alkalinity.All chemical agents were obtained from laboratory and commercial vendors.

AAMs Preparation
The preparation process is illustrated in Figure 3, and the specific mixing ratio of CG/GGBFS-based AAMs is detailed in Table 2.The AAMs were prepared by adding NaOH and Na 2 SiO 3 to distilled water to formulate the alkali activators, and the precursors (calcined CG and GGBFS) were mixed for 2 min in a planetary mixer.The alkali activators were then blended with the precursors for 5 min to create a uniform slurry paste.The paste was subsequently poured into silica molds and vibrated on an electric vibration table for 90 s to eliminate the remaining bubbles.A thin polyethylene film was applied to cover all samples.Following this, the samples were cured in a controlled environment at 60 • C for 24 h, unmolded, and left at ambient temperature (~25 • C) at 3, 7, and 28 d before conducting the test.The prepared AAMs were labeled SXCY-W-Z, where S and C represent the raw precursors of GGBFS and CG; X and Y represent the mass ratios of GGBFS and CG; W indicates the Na2O content (mass ratio of Na2O); and Z indicates the alkali modulus (SiO2/Na2O) of the alkali activator.The liquid/solid ratio was 0.55.For example, S1C1-7-1.3signified the specimens were fabricated with a mass ratio of GGBFS/CG of 1:1, and the Na2O content and alkali modulus were 7 wt% and 1.3, respectively.The prepared AAMs were labeled S X C Y -W-Z, where S and C represent the raw precursors of GGBFS and CG; X and Y represent the mass ratios of GGBFS and CG; W indicates the Na 2 O content (mass ratio of Na 2 O); and Z indicates the alkali modulus (SiO 2 /Na 2 O) of the alkali activator.The liquid/solid ratio was 0.55.For example, S 1 C 1 -7-1.3signified the specimens were fabricated with a mass ratio of GGBFS/CG of 1:1, and the Na 2 O content and alkali modulus were 7 wt% and 1.3, respectively.

Physical Properties Tests
The fluidity of AAM paste was determined using a shortened conical apparatus (Cangzhou, China).The setting times of AAM paste were determined using a Vicat apparatus (Shanghai, China).The compressive strength of AAM samples was determined using an STS100K testing machine (Xiamen, China).Three samples were tested, and the average results were considered the final compressive strength.

Microscopic Properties Tests
XRD patterns of the samples (raw materials and AAMs) were recorded on a D8 Advance diffractometer (Bruker, Mannheim, Germany) using Cu Kα radiation.The diffractometer was operated at 40 kV and 30 mA, and the scanning range was 3-70 • (2θ) with a scanning speed of 3 • /min.FTIR spectroscopy of AAM samples was recorded on a Nicolet IS 50 spectrometer (Thermo Fisher, Waltham, MA, USA).The spectra ranged from 400 to 4000 cm −1 with a resolution of 4 cm −1 and 64 scans.SEM images and EDX results of the samples (raw materials and AAMs) were obtained using an S-3400N-II instrument (Hitachi, Tokyo, Japan).The acceleration voltage was 35 kV, and the current was 10 mA.The MIP of AAM samples was measured using an AutoPore IV 9510 porosimeter (Micromeritics, Norcross, GA, USA).The intrusion pressures ranged from 0.5 to 33,000 psi with a contact angle of 130 • and a surface tension of 485 dynes/cm.

Fluidity
The fluidity of the freshly mixed AAM pastes is shown in Table 2.The paste fluidity results changed from 107.6 to 167.7 mm as the mixture varied and was notably affected by the GGBFS content.For example, compared to C-7-1.3, the fluidity of pastes with GGBFS contents of 20%, 40%, 50%, 60%, 80%, and 100% increased by 9.76%, 20.17%, 25.74%, 32.71%, 43.49%, and 55.86%, respectively.This was attributed to the following: (1) CG has a more porous structure that can absorb free water; and (2) The smaller particle size of CG, inducing a larger specific surface area compared to GGBFS (Figure 2), increases the available water for the AAM paste, resulting in increased fluidity when CG is substituted with GGBFS.Kramar et al. [39] found that the fluidity of slag-based mortar was higher than that of FA (180 vs. 152 mm).However, the paste fluidity of the S 1 C 1 -W-1.3samples was not altered (around 135.3 ± 3 mm).With changes in Na 2 O content from 6% to 9%, the fluidity increased from 133.2 to 138.2 mm.Similar changes were observed for the S 1 C 1 -7-Z samples; the fluidity changed from 127.3 to 139.5 mm as the SiO 2 /Na 2 O modulus increased from 0.9 to 1.5, respectively.This indicates that the Na 2 O content and modulus of the alkali activator were insensitive to the AAM paste fluidity.

Setting Time
Figure 4 illustrates the setting times of the AAM pastes.The setting time was closely related to the geopolymerization rate, which was influenced by either the precursor materials or alkali activators.The final setting times of the AAM pastes appeared to be shortened, indicating that curing the samples in the controlled environment (60 fluidity increased from 133.2 to 138.2 mm.Similar changes were observed for the S1C1-7-Z samples; the fluidity changed from 127.3 to 139.5 mm as the SiO2/Na2O modulus increased from 0.9 to 1.5, respectively.This indicates that the Na2O content and modulus of the alkali activator were insensitive to the AAM paste fluidity.

Setting Time
Figure 4 illustrates the setting times of the AAM pastes.The setting time was closely related to the geopolymerization rate, which was influenced by either the precursor materials or alkali activators.The final setting times of the AAM pastes appeared to be shortened, indicating that curing the samples in the controlled environment (60 °C) accelerated the geopolymerization process.The setting times of the AAM pastes decreased as the GGBFS content increased, impacting both the initial and final setting times.Compared to the C-7.0-1.3 sample, the final setting times of samples with 100% GGBFS content (S-7.0-1.3) were significantly shortened, with an 86.1% reduction.This can be attributed to the higher reactivity of GGBFS; the reaction of the AAM paste increased when CG was replaced with GGBFS.
With increasing Na2O content, the setting time was slightly reduced (from 30 to 22 min), indicating that higher alkalinity (Na2O concentration) promoted the dissolution of Al and Si ions, thereby accelerating the geopolymerization process.However, with a higher alkali modulus (SiO2/Na2O), both the initial and final setting times of the AAM pastes were shortened.For example, the final setting time of S1C1-7.0-0.9 was 35 min, while that of S1C1-7.0-1.5 was shortened to 26 min when the alkali modulus increased to 1.5.Studies have reported that the higher alkali modulus in the reaction is conducive to geopolymerization, resulting in more AAM pastes and shorter paste hardening times [40].

Compressive Strength
Figure 5 illustrates the compressive strengths of the AAM specimens at various curing ages (i.e., 3 d, 7 d, and 28 d).The strength of the specimens exhibited a gradual increase throughout the curing period, indicating a continuous ongoing process of geopolymerization.Notably, there was a rapid increase in compressive strength during the early stages, especially by day 3.The compressive strength by day 3 was about 75% of that of the 28 d specimens, and by day 7, the compressive strength of the specimens was about 85% of that of the 28 d specimens.This was attributed to two factors: (1) more The setting times of the AAM pastes decreased as the GGBFS content increased, impacting both the initial and final setting times.Compared to the C-7.0-1.3 sample, the final setting times of samples with 100% GGBFS content (S-7.0-1.3) were significantly shortened, with an 86.1% reduction.This can be attributed to the higher reactivity of GGBFS; the reaction of the AAM paste increased when CG was replaced with GGBFS.
With increasing Na 2 O content, the setting time was slightly reduced (from 30 to 22 min), indicating that higher alkalinity (Na 2 O concentration) promoted the dissolution of Al and Si ions, thereby accelerating the geopolymerization process.However, with a higher alkali modulus (SiO 2 /Na 2 O), both the initial and final setting times of the AAM pastes were shortened.For example, the final setting time of S 1 C 1 -7.0-0.9 was 35 min, while that of S 1 C 1 -7.0-1.5 was shortened to 26 min when the alkali modulus increased to 1.5.Studies have reported that the higher alkali modulus in the reaction is conducive to geopolymerization, resulting in more AAM pastes and shorter paste hardening times [40].

Compressive Strength
Figure 5 illustrates the compressive strengths of the AAM specimens at various curing ages (i.e., 3 d, 7 d, and 28 d).The strength of the specimens exhibited a gradual increase throughout the curing period, indicating a continuous ongoing process of geopolymerization.Notably, there was a rapid increase in compressive strength during the early stages, especially by day 3.The compressive strength by day 3 was about 75% of that of the 28 d specimens, and by day 7, the compressive strength of the specimens was about 85% of that of the 28 d specimens.This was attributed to two factors: (1) more soluble silicates participated in the reactions during the initial stages of AAM curing, and (2) an elevation in curing temperature accelerated the geopolymerization rate.Furthermore, with an increase in GGBFS content, the compressive strength of the AAM showed a significant improvement.The 28 d compressive strength of S-7.0-1.3 (100% GGBFS content) was 393.34% higher than that of C-7.0-1.3 (100% CG content).When the specimens had a CG/GGBFS ratio of 1:1, the compressive strength of S1C1-7.0-1.3 reached 54.59 MPa, equivalent to that of grade 52.5 OPC.
Moreover, an increase in the Na2O mass ratio from 6% to 8% initially improved compressive strength, followed by a decline in strength as the ratio increased to 9% (S1C1-9.0-1.3).Compared to S1C1-6.0-1.3, the compressive strengths of S1C1-7.0-1.3 and S1C1-8.0-1.3 increased by 8.44% and 7.21%, respectively, indicating that a higher Na2O mass ratio was beneficial for generating more paste gels and achieving higher compressive strength through enhanced dissolution of GGBFS and CG particles [41].Increasing the Na2O mass ratio to 9% had a notable impact on the compressive strength of the AAMs.This was for two reasons: (1) inadequate alkalinity at low Na2O concentrations hindered polymerization; and (2) higher Na2O concentrations led to more precursors rapidly reacting with alkali activators, resulting in geopolymer gels forming on precursor surfaces and limiting their participation in polymerization.
With a rise in the alkali modulus, the compressive strength of the AAM specimens was only slightly affected.As the alkali modulus changed from 0.9 to 1.3, the compressive strength increased by 11.52%.However, the S1C1-7.0-1.5 specimens, with a modulus of 1.5, showed a lower compressive strength (49.85 MPa) than that of the S1C1-7.0-1.

XRD Analysis
Figure 6 illustrates the XRD patterns of the AAM specimens.There is a broad reflection within the 18°-40° (2θ), with a prominent peak at ~29° (2θ).Compared to the C-7.0-1.3 specimens, the patterns of the AAM specimens shifted toward a higher angle of ~30° (2θ) due to the inclusion of GGBFS.This indicates the presence of amorphous phases, which primarily consist of sodium aluminate silicate hydrate (N-A-S-H) and calcium aluminate silicate hydrate (C-A-S-H) [42][43][44].He et al. [44] identified that the N-A-S-H gel Furthermore, with an increase in GGBFS content, the compressive strength of the AAM showed a significant improvement.The 28 d compressive strength of S-7.0-1.3 (100% GGBFS content) was 393.34% higher than that of C-7.0-1.3 (100% CG content).When the specimens had a CG/GGBFS ratio of 1:1, the compressive strength of S 1 C 1 -7.0-1.3 reached 54.59 MPa, equivalent to that of grade 52.5 OPC.
Moreover, an increase in the Na 2 O mass ratio from 6% to 8% initially improved compressive strength, followed by a decline in strength as the ratio increased to 9% (S 1 C 1 -9.0-1.3).Compared to S 1 C 1 -6.0-1.3, the compressive strengths of S 1 C 1 -7.0-1.3 and S 1 C 1 -8.0-1.3 increased by 8.44% and 7.21%, respectively, indicating that a higher Na 2 O mass ratio was beneficial for generating more paste gels and achieving higher compressive strength through enhanced dissolution of GGBFS and CG particles [41].Increasing the Na 2 O mass ratio to 9% had a notable impact on the compressive strength of the AAMs.This was for two reasons: (1) inadequate alkalinity at low Na 2 O concentrations hindered polymerization; and (2) higher Na 2 O concentrations led to more precursors rapidly reacting with alkali activators, resulting in geopolymer gels forming on precursor surfaces and limiting their participation in polymerization.
With a rise in the alkali modulus, the compressive strength of the AAM specimens was only slightly affected.As the alkali modulus changed from 0.9 to 1.3, the compressive strength increased by 11.52%.However, the S 1 C 1 -7.0-1.5 specimens, with a modulus of 1.5, showed a lower compressive strength (49.85 MPa) than that of the S 1 C 1 -7.0-1.

XRD Analysis
Figure 6 illustrates the XRD patterns of the AAM specimens.There is a broad reflection within the 18 • -40 • (2θ), with a prominent peak at ~29 • (2θ).Compared to the C-7.0-1.3 specimens, the patterns of the AAM specimens shifted toward a higher angle of ~30 • (2θ) due to the inclusion of GGBFS.This indicates the presence of amorphous phases, which primarily consist of sodium aluminate silicate hydrate (N-A-S-H) and calcium aluminate silicate hydrate (C-A-S-H) [42][43][44].He et al. [44] identified that the N-A-S-H gel was the product of low-calcium materials, and the C-A-S-H gel was predominant in rich-calcium precursors.Additionally, minerals like quartz and muscovite were detected in the specimens, suggesting that CG was insufficient for the geopolymerization reaction [45].
due to the reaction between the GGBFS and alkali activators [46,47].However, increasing the Na2O mass ratio from 6% to 8% did not significantly impact mineral composition (Figure 6b).Compared to S1C1-6.0-1.3, the C-A-S-H diffraction peak of S1C1-8.0-1.3 was enhanced, indicating that more geopolymerization gels generated at higher alkalinity, resulting in the higher compressive strength of S1C1-8.0-1.3.

FTIR Spectroscopy
Figure 7 displays the FTIR spectra of the AAM specimens between 400 and 1800 cm −1 , while Table 3 provides the vibrational modes of FTIR band assignments for the selected samples.In Figure 7a, a broad band between 800 and 1200 cm −1 was observed and shifted, indicating the presence and alteration of amorphous products [48,49].With an increase in GGBFS content, the spectra of the specimens shifted towards lower wavenumbers.For example, compared to C-7.0-1.3, the center of the S1C1-7.0-1.3 and S-7.0-1.3 broad bands moved from 1027 to 1007 and 971 cm −1 , respectively.Research [50] indicated that the N-A-S-H gel is primarily associated with 1030 cm −1 , while the C-A-S-H gel is mainly focused on 940 cm −1 .This suggests that the components of the AAM specimens were transformed from N-A-S-H to C-A-S-H as the GGBFS content increased.The intensity of the C-A-S-H crystalline phase increased with higher GGBFS content due to the reaction between the GGBFS and alkali activators [46,47].However, increasing the Na 2 O mass ratio from 6% to 8% did not significantly impact mineral composition (Figure 6b).Compared to S 1 C 1 -6.0-1.3, the C-A-S-H diffraction peak of S 1 C 1 -8.0-1.3 was enhanced, indicating that more geopolymerization gels generated at higher alkalinity, resulting in the higher compressive strength of S 1 C 1 -8.0-1.3.
Varying the SiO 2 /Na 2 O modulus of the alkali solution had minimal effect on geopolymers' compound synthesis.Compared to S 1 C 1 -7.0-0.9, the XRD patterns of S 1 C 1 -7.0-1.3 and S 1 C 1 -7.0-1.5 showed a lower intensity of the quartz diffraction peak, suggesting the greater consumption of quartz crystals in S 1 C 1 -7.0-1.3 and S 1 C 1 -7.0-1.5.This confirms that a higher SiO 2 /Na 2 O modulus promoted geopolymerization.

FTIR Spectroscopy
Figure 7 displays the FTIR spectra of the AAM specimens between 400 and 1800 cm −1 , while Table 3 provides the vibrational modes of FTIR band assignments for the selected samples.In Figure 7a, a broad band between 800 and 1200 cm −1 was observed and shifted, indicating the presence and alteration of amorphous products [48,49].With an increase in GGBFS content, the spectra of the specimens shifted towards lower wavenumbers.For example, compared to C-7.0-1.3, the center of the S 1 C 1 -7.0-1.3 and S-7.0-1.3 broad bands moved from 1027 to 1007 and 971 cm −1 , respectively.Research [50] indicated that the N-A-S-H gel is primarily associated with 1030 cm −1 , while the C-A-S-H gel is mainly focused on 940 cm −1 .This suggests that the components of the AAM specimens were transformed from N-A-S-H to C-A-S-H as the GGBFS content increased.In Figure 7b, as the Na2O mass ratio increased, the characteristic bands decreased from 1016 cm −1 (S1C1-6.0-1.3) to 1005 cm −1 (S1C1-9.0-1.3).Additionally, with an increase in SiO2/Na2O modulus from 0.9 to 1.5, the center of the broad band decreases from 1015 cm −1 (S1C1-7.0-0.9) to 1006 cm −1 (S1C1-7.0-1.5).These minimal changes in FTIR peaks indicated limited impacts on the composition of the AAM specimens within the activated system.8a-g.In Figure 8a, the C-7.0-1.3 specimens possess a loose microstructure that coexists with isolated particles in the matrix.As the GGBFS content increased, the microstructure of the S1C1-7.0-1.3 and S-7.0-1.3 specimens became more compact (Figure 8c,e), and there were fewer smaller pores in the stone body.This suggested that more AAM gels (C-(N)-A-S-H) were produced as the GGBFS content increased, leading to enhanced compressive strength.The high-magnification SEM images (Figure 8d,f) showed microcracks in the bodies of the S1C1-7.0-1.3 and S-7.0-1.3 specimens.Research [61][62][63] revealed that the shrinkage value of the AAMs increased with the addition of GGBFS, contributing to the formation of more microcracks.The EDX analysis (Figure 8g, spot 1) revealed that the C-7.0-1.3 specimens were primarily composed of N-A-S-H, while the S1C1-7.0-1.3 and S-7.0-1.3 specimens (Figure 8g, spots 2 and 3) contained C-(N)-A-S-H, which aligns with the elemental analysis.Additionally, the analyzed AAM samples exhibited varying Ca/Si ratios.For example, the Ca/Si ratio of C-7.0-1.3 was 0.03, considerably lower than that of S1C1-7.0-1.3 (0.59) and S-7.0-1.3 (1.27).This result suggests that the formation of more C-A-S-H gel was generated with increasing GGBFS content.Temuujin et al. [31] demonstrated that the more C-A-S-H gel generated, In Figure 7b, as the Na 2 O mass ratio increased, the characteristic bands decreased from 1016 cm −1 (S 1 C 1 -6.0-1.3) to 1005 cm −1 (S 1 C 1 -9.0-1.3).Additionally, with an increase in SiO 2 /Na 2 O modulus from 0.9 to 1.5, the center of the broad band decreases from 1015 cm −1 (S 1 C 1 -7.0-0.9) to 1006 cm −1 (S 1 C 1 -7.0-1.5).These minimal changes in FTIR peaks indicated limited impacts on the composition of the AAM specimens within the activated system.

SEM/EDX Results
Figures 8-10 illustrate the SEM/EDX findings of the AAM samples.The microstructures of the specimens with various GGBFS contents (i.e., 0%, 50%, and 100%) are examined in Figure 8a-g.In Figure 8a, the C-7.0-1.3 specimens possess a loose microstructure that coexists with isolated particles in the matrix.As the GGBFS content increased, the microstructure of the S 1 C 1 -7.0-1.3 and S-7.0-1.3 specimens became more compact (Figure 8c,e), and there were fewer smaller pores in the stone body.This suggested that more AAM gels (C-(N)-A-S-H) were produced as the GGBFS content increased, leading to enhanced compressive strength.The high-magnification SEM images (Figure 8d,f) showed microcracks in the bodies of the S 1 C 1 -7.0-1.3 and S-7.0-1.3 specimens.Research [61][62][63] revealed that the shrinkage value of the AAMs increased with the addition of GGBFS, contributing to the formation of more microcracks.The EDX analysis (Figure 8g, spot 1) revealed that the C-7.0-1.3 specimens were primarily composed of N-A-S-H, while the S 1 C 1 -7.0-1.3 and S-7.0-1.3 specimens (Figure 8g, spots 2 and 3) contained C-(N)-A-S-H, which aligns with the elemental analysis.Additionally, the analyzed AAM samples exhibited varying Ca/Si ratios.For example, the Ca/Si ratio of C-7.0-1.3 was 0.03, considerably lower than that of S 1 C 1 -7.0-1.3 (0.59) and S-7.0-1.3 (1.27).This result suggests that the formation of more C-A-S-H gel was generated with increasing GGBFS content.Temuujin et al. [31] demonstrated that the more C-A-S-H gel generated, the higher the compressive strength achieved for FA-based AAMs.As the Na 2 O mass ratio increased from 6% to 8%, there was no apparent difference observed between the S 1 C 1 -0.6-1.3 and S 1 C 1 -0.8-1.3 specimens, with some pores being evident in the matrix of their compact microstructure (Figure 9a,c).The high-magnification SEM images showed an increase in AAM gels generated with greater Na 2 O content; however, some microcracks occurred in the stone body (Figure 9b,d).The EDX results, as depicted in Figure 9e (spots 1, and 2), demonstrated that the Ca/Si ratios were similar between the S 1 C 1 -0.6-1.As the Na2O mass ratio increased from 6% to 8%, there was no apparent difference observed between the S1C1-0.6-1.3 and S1C1-0.8-1.3 specimens, with some pores being evident in the matrix of their compact microstructure (Figure 9a,c).The highmagnification SEM images showed an increase in AAM gels generated with greater Na2O content; however, some microcracks occurred in the stone body (Figure 9b,d).The EDX results, as depicted in Figure 9e (spots 1, and 2), demonstrated that the Ca/Si ratios were similar between the S1C1-0.6-1.3 and S1C1-0.8-1.3 (0.26 vs. 0.27); the (Ca + Na)/(Si + Al) ratios of the S1C1-0.6-1.3 and S1C1-0.8-1.3 were 0.45 and 0.43, respectively.Furthermore, the lower Ca/Si ratio in S1C1-0.6-1.3 resulted in a decrease in compressive strength.
Figure 10 illustrates the SEM images and EDX findings of the S1C1-7.0-0.9 and S1C1-7.0-1.5 specimens, with both exhibiting compact morphology (Figure 10a,c).In addition, the high-magnification SEM images showed that less AAM gel was generated in the S1C1-7.0-0.9 specimens due to the incomplete reaction of the low soluble silicate content.The EDX results, as depicted in Figure 10e (spots 1 and 2), demonstrate that the Ca/Si ratios of the S1C1-7.0-0.9 and S1C1-7.0-1.5 (0.26 vs. 0.25) specimens were similar, and the (Ca + Figure 10 illustrates the SEM images and EDX findings of the S 1 C 1 -7.0-0.9 and S 1 C 1 -7.0-1.5 specimens, with both exhibiting compact morphology (Figure 10a,c).In addition, the high-magnification SEM images showed that less AAM gel was generated in the S 1 C 1 -7.0-0.9 specimens due to the incomplete reaction of the low soluble silicate content.The EDX results, as depicted in Figure 10e (spots 1 and 2), demonstrate that the Ca/Si ratios of the S 1 C 1 -7.0-0.9 and S 1 C 1 -7.0-1.5 (0.26 vs. 0.25) specimens were similar, and the (Ca + Na)/(Si + Al) ratios of the S 1 C 1 -7.0-0.9 and S 1 C 1 -7.0-1.5 were 0.37 and 0.36, respectively, suggesting that more Si atoms were involved in the S 1 C 1 -7.0-1.5 reaction.

MIP Results
Figure 11 displays the distribution of pore size in the AAM specimens, and the pore parameters are shown in Table 4, which were analyzed by MIP.In Figure 11a, the samples exhibited a predominant pore size range of 10 to 200 nm.They were primarily medium capillary pores (10-100 nm) and large capillary pores (100-1000 nm), with large capillary pores having a negative impact on the mechanical properties of the AAMs [64].
suggesting that more Si atoms were involved in the S1C1-7.0-1.5 reaction.

MIP Results
Figure 11 displays the distribution of pore size in the AAM specimens, and the pore parameters are shown in Table 4, which were analyzed by MIP.In Figure 11a, the samples exhibited a predominant pore size range of 10 to 200 nm.They were primarily medium capillary pores (10-100 nm) and large capillary pores (100-1000 nm), with large capillary pores having a negative impact on the mechanical properties of the AAMs [64].With 100% CG, the average pore size of C-7.0-1.3 specimens was 35.70 nm, and the porosity was 31.74%.As the GGBFS content increased, the pore size and porosity of the specimens decreased.For example, the average pore sizes of S1C1-7.0-1.3 (50% GGBFS content) and S-7.0-1.3 (100% GGBFS content) were 22.84 and 17.06 nm, respectively, and the porosity decreased by 42.1% and 89.4%, respectively.Figure 11b illustrates the decline in the cumulative pore volume of the AAM specimens with added GGBFS, showing a higher cumulative intrusion for C-7.0-1.3 (0.208 mL/g) compared to that for S-7.0-1.3 (0.017 mL/g).This suggests that incorporating GGBFS boosts the reactivity of raw precursors, resulting in the generation of more AAM gels and the compact structures of the specimens.
Additionally, the S1C1-6.0-1.3 specimens exhibited a critical pore size of 31.71mm, which was greater than that of S1C1-8.0-1.3.This indicated that increasing Na2O content promotes hydrolysis of the aluminosilicate precursors and improves the compact structure of the specimens.These findings are aligned with the compressive strengths of the samples.Porosity analysis in Table 4 revealed that S1C1-6.0-1.3 had a porosity of 27.19%; therefore, the greater pore size induced the inferior compressive strength of S1C1-6.0-1.3 compared to S1C1-8.0-1.3 (Figure 5b).Fan et al. [65] emphasized the impact of pore dimensions on the mechanical characteristics of cementitious substances.With 100% CG, the average pore size of C-7.0-1.3 specimens was 35.70 nm, and the porosity was 31.74%.As the GGBFS content increased, the pore size and porosity of the specimens decreased.For example, the average pore sizes of S 1 C 1 -7.0-1.3 (50% GGBFS content) and S-7.0-1.3 (100% GGBFS content) were 22.84 and 17.06 nm, respectively, and the porosity decreased by 42.1% and 89.4%, respectively.Figure 11b illustrates the decline in the cumulative pore volume of the AAM specimens with added GGBFS, showing a higher cumulative intrusion for C-7.0-1.3 (0.208 mL/g) compared to that for S-7.0-1.3 (0.017 mL/g).This suggests that incorporating GGBFS boosts the reactivity of raw precursors, resulting in the generation of more AAM gels and the compact structures of the specimens.
Additionally, the S 1 C 1 -6.0-1.3 specimens exhibited a critical pore size of 31.71mm, which was greater than that of S 1 C 1 -8.0-1.3.This indicated that increasing Na 2 O content promotes hydrolysis of the aluminosilicate precursors and improves the compact structure of the specimens.These findings are aligned with the compressive strengths of the samples.Porosity analysis in Table 4 revealed that S 1 C 1 -6.0-1.3 had a porosity of 27.19%; therefore, the greater pore size induced the inferior compressive strength of S 1 C 1 -6.0-1.3 compared to S 1 C 1 -8.0-1.3 (Figure 5b).Fan et al. [65] emphasized the impact of pore dimensions on the mechanical characteristics of cementitious substances.
Furthermore, the porosity of the AAM specimens remained unchanged (~24.50%) as the SiO 2 /Na 2 O modulus varied, while the average pore size of S 1 C 1 -7.0-0.9 was larger (29.07 mm) than that of S 1 C 1 -7.0-1.5 (21.28 mm).This suggests that an insufficient number of Si atoms participate in the S 1 C 1 -7.0-0.9 reactions, resulting in a larger pore size (Table 4), consistent with the change in compressive strength.

Discussion
The research determined that the compressive strength and microstructure of CG/ GGBFS-based AAMs were influenced by GGBFS and alkali activator contents.As the GGBFS content increased from 0% to 50% (or 100%), more AAM gels (C-(A)-S-H) were generated, forming a three-dimensional network and resulting in a higher compressive strength [66].For example, the compressive strength was 47.72 MPa for S 2 C 3 -7.0-1.3 (40% GGBFS content), and that of S 1 C 1 -7.0-1.3 (50% GGBFS content) was 54.59 MPa.This is favorable for the preparation of AAM concrete as an alternative to OPC materials.Additionally, as the Na 2 O mass ratio changed from 6% to 9%, the compressive strength increased by 8.44%, which was compared to that of S 1 C 1 -6.0-1.3.The variation of the SiO 2 /Na 2 O modulus from 0.9 to 1.5 only slightly affected the strength of the AAM specimens (48.3 vs. 54.59 MPa).This suggests that the compressive strength of CG/GGBFS-based AAMs is mainly dependent on the GGBFS content.A Na 2 O mass ratio of 7% and a SiO 2 /Na 2 O modulus of 1.3 were the most beneficial for generating AAM gels.
Furthermore, the setting times of the CG/GGBFS-based AAMs were faster due to the high reactivity of GGBFS.Several researchers have explored how a retarder admixture prolongs the setting times of AAM systems.For example, Wang et al. [67] found that zinc nitrate and sodium gluconate were excellent retarders for slag-based AAMs.Cong et al. [68] modified the properties of FA-based AAMs by adding borate.Both Kalina [69] and Gong [70] explored how sodium phosphate influences the hydration process of red mud and slag-based AAMs.Their findings indicated that sodium phosphate effectively prolonged the hydration and the setting time for AAMs.In related work, Brough et al. [71] reported that incorporating 0.5 wt% malic acid extended the initial setting time, which ranged from 4 to 20 h.The literature records effective retarders for controlling the setting times of AAMs, but fewer studies have discussed the effects of existing retarders on CG/GGBFS-based AAMs; this remains an investigation for future work.

Figure 2
Figure 2 shows the SEM images and particle size distributions of the calcined GGBFS.The CG possesses a loose surface morphology and porous structure (Fig and the GGBFS exhibits a smooth surface morphology and angular shape (Figure median particle size (d50) of the CG and GGBFS are 8.59 and 10.29 µm, respectiveThe materials used as alkali activators were composed of sodium silicate (SiO2 27.3 wt%, Na2O 8.5 wt%, and H2O 64.2 wt%) and sodium hydroxide particle
17,  x FOR PEER REVIEW ≥96%).Distilled water was used for adjusting alkalinity.All chemical agen obtained from laboratory and commercial vendors.

Figure 2 .
Figure 2. SEM images of CG (a) and GGBFS (b); and particle size distributions of CG (c) and G

Figure 2 .
Figure 2. SEM images of CG (a) and GGBFS (b); and particle size distributions of CG (c) and GGBFS (d).
3 specimens (54.59 MPa), a decrease of 8.68%.This finding was contrasted with previous research.Guo et al. found that the compressive strength of 28 d FA-based AAMs significantly increased (from 17.1 to 59.3 MPa) as the SiO2/Na2O modulus changed from 1.0 to 1.5 [36].
3 specimens (54.59 MPa), a decrease of 8.68%.This finding was contrasted with previous research.Guo et al. found that the compressive strength of 28 d FA-based AAMs significantly increased (from 17.1 to 59.3 MPa) as the SiO 2 /Na 2 O modulus changed from 1.0 to 1.5 [36].

Table 3 .
The vibrational modes of FTIR band assignments for the selected samples.

Table 4 .
The pore parameters for the AAM samples determined via MIP.

Table 4 .
The pore parameters for the AAM samples determined via MIP.