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Article

The Influence of Coal Gangue on the Mechanical Properties of Ground-Granulated Blast Furnace Slag-Based Geopolymers

by
Xiaoping Wang
1,2,
Feng Liu
3,
Weizhi Chen
1,
Kaifeng Xing
2,
Kexian Zhuo
4 and
Lijuan Li
1,*
1
School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou 510006, China
2
School of Architecture and Engineering, Huangshan University, Huangshan 245041, China
3
School of Future Transportation, Guangzhou Maritime University, Guangzhou 510725, China
4
Bureau of Public Works of Pingshan District, Shenzhen 518118, China
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(15), 2695; https://doi.org/10.3390/buildings15152695
Submission received: 3 July 2025 / Revised: 28 July 2025 / Accepted: 29 July 2025 / Published: 30 July 2025
(This article belongs to the Special Issue Next-Gen Cementitious Composites for Sustainable Construction)
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Abstract

The reuse of coal gangue (CG) and ground-granulated blast furnace slag (GGBFS) to synthesize geopolymers presents a sustainable strategy for industrial waste recycling. This study investigates the influences of various GGBFS/CG mixtures on the mechanical behavior and microstructure of the synthesized geopolymers. Results show that the geopolymer matrix is composed of calcium aluminosilicate (C-(A)-S-H) and sodium aluminosilicate (N-A-S-H) hydrates, which is essential for enhancing the compressive strength of the specimens. With 100% GGBFS, the geopolymer matrix sets in 17 min, reaching a compressive strength of 107.55 MPa after 28 days. As the CG content increases, both compressive strength and compactness decrease gradually, while the setting time prolongs. When the GGBFS/CG mass ratio is 1:1, the specimens’ setting time increases by 64.7% (from 17 to 28 min). The corresponding compressive strengths at 3 days, 7 days, and 28 days are recorded to be 46.73 MPa, 53.25 MPa, and 54.59 MPa, respectively. Specimens with 100% CG exhibit a prolonged setting time (122 min), but the compressive strength is just 21.80 MPa. Microscopic analysis reveals that specimens with 50% CG have smaller average pore diameters (22.84 nm) and a compact microstructure. These findings indicate that the GGBFS content significantly influences geopolymer performance, highlighting the effective utilization of GGBFS/CG wastes.

1. Introduction

Ordinary Portland cement (OPC) is a crucial material used in cement production with annual output surpassing 4 billion tons. However, its manufacturing process faces challenges such as high energy use and significant emissions of carbon dioxide (CO2) [1]. Projections indicate that the production of 1 ton of OPC results in 0.8 to 1.0 tons of CO2, and it also consumes a significant amount of non-renewable energy [2,3]. By 2030, global OPC accumulation is projected to reach 5 billion tons, which poses a significant global warming and energy challenge for society [4,5]. Moreover, as limestone serves as the raw material for manufacturing OPC, prolonged mining operations are likely to cause a critical depletion of limestone reserves over the forthcoming 25 to 50 years [6]. Therefore, to mitigate the cement industry’s carbon footprint, the adoption of low-carbon cementitious materials as alternatives to OPC has gained attention [7,8,9].
Geopolymers are inorganic materials formed by [AlO4] and [SiO4] tetrahedra, which are interconnected by shared oxygen atoms [10]. It exhibits excellent mechanical properties and sustainable characteristics, such as high early strength [11], thermal stability [12], and resistance to corrosion [13,14] with lower energy consumption and CO2 emissions [15,16]. In recent decades, geopolymers have attracted substantial attention due to their remarkable performance [17,18]. Environmental impact assessments confirmed that geopolymer systems incorporating industrial wastes achieved 44–64% lower CO2 emissions than OPC, highlighting their potential in carbon mitigation [19]. Geopolymers are synthesized from precursors (e.g., metakaolin [20], fly ash [21], and slag [22]) and alkali activators (e.g., sodium silicate, sodium hydroxide). The synthesis of geopolymers can effectively reduce the industrial by-products. The properties of geopolymer mechanics depend on hydrated calcium silicoaluminate (C-(A)-S-H) and hydrated sodium silicoaluminate (N-A-S-H), and these properties change according to the amount of calcium-rich precursors present, resulting in different mechanical properties.
Ground granulated blast furnace slag (GGBFS) is a by-product formed during the iron smelting operation in blast furnaces; the chemical components are composed of calcium aluminosilicate and silica. The high calcium oxide (CaO) content in GGBFS promotes the rapid hardening of geopolymers and accelerates compressive strength development. This industrial by-product undergoes the reaction with alkaline activator at ambient and high temperatures, resulting in the formation of a geopolymer matrix. The matrix of the geopolymer primarily consists of C-A-S-H hydrates [23], which play an essential role in enhancing mechanical properties, which offer advantages such as high compressive strength, optimal pore structure, and low cost. The obtained geopolymer-based GGBFS is commonly utilized in civil engineering. Liu et al. [24] prepared bathroom components using slag/fly ash-based geopolymer; Deb et al. [25] created geopolymers with a slag/fly ash ratio of 2:8, achieving a compressive strength peak of 51.0 MPa; Bernal [26] investigated metakaolin/slag-based geopolymers with a 5:2 slag/fly ash ratio, which resulted in compressive strengths of 45.1 MPa. Additionally, Pazhani and Venkatesan [27] demonstrated that the inclusion of 10% rice husk ash into slag-based geopolymers yielded compressive strengths of 72.3 MPa. Collectively, these studies show that GGBFS-based geopolymers require free calcium precursors, and the rapid setting caused by C-(A)-S-H gel formation hinders their suitability for construction applications.
Coal gangue (CG), a by-product produced during the operations of coal mining and washing, constitutes 10–20% of the overall output [28]. Worldwide, the accumulation of CG waste has reached critical levels, and it has become a major solid waste in several countries, such as Australia, South Africa, India, and China [29,30]. CG is a crystalline material. Some of the literature indicates that CG is primarily composed of oxides like Al2O3, MgO, and SiO2 as well as minerals like quartz, kaolinite, feldspar, and muscovite. It has been shown to be a potential precursor (rich aluminosilicate) for preparing geopolymers [31]. Wang et al. [32] identified that 700 °C is an optimal temperature for CG calcination to form metakaolin, while Li et al. [31] showed that the particle size of 200 mesh is beneficial for achieving higher compressive strength, and CG-based geopolymers forming N-(A)-S-H gel are beneficial for reducing autogenous shrinkage. Zhang et al. [33] investigated how prolonging the duration of mechanical activation (ranging from 1 to 20 h) can enhance the reactivity of CG and improve the characteristics of geopolymers. Meanwhile, Zhao et al. [34] researched the mechanical activation of CG in a stirred media mill over a period of 30 to 120 min, finding that this process promotes better performance in cement applications, including higher compressive strength and lower porosity. Balczár et al. [35] noted in their research on kaolin-based geopolymer mortar that after 240 min of mechanical activation, the maximum compressive strength slightly decreases, which is potentially caused by agglomeration occurring with 60% amorphous content. These studies have laid the groundwork for the broader use of CG.
This research explores the combination of GGBFS with CG, in which alkali activators such as sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) solutions are utilized. It analyzes how the GGBFS/CG ratio influences the mechanical properties and microstructure of geopolymers. To evaluate the mineral composition and mechanical properties, the geopolymers were analyzed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, images from scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis. Additionally, the pore characteristics of the geopolymers were analyzed by mercury intrusion porosimetry (MIP). This research enhances the understanding of GGBFS/CG-based geopolymers, and it is an effective utilization for GGBFS/CG wastes.

2. Experimental Methods

2.1. Materials

GGBFS was procured from Nanyang Shuoyu Mineral Products Co., Ltd., Nanyang, China, while raw CG was acquired from Hongqi Coal Industry Co., Ltd., Zhengzhou, China. The chemical compositions of both materials were conducted via X-ray fluorescence, which are shown in Table 1. The GGBFS mainly contained SiO2, Al2O3, and CaO, with a total content (mass fraction, the same below) of 86.11%. The CG mainly contained SiO2 and Al2O3, with a total content of 86.12%, and CaO content accounted for 1.73%. The raw CG should be calcined in a muffle furnace (~700 °C); the heating rate was 10 °C/min.
The XRD patterns of raw materials (CG, calcined CG, and GGBFS) are shown in Figure 1. There was a broad diffraction band centered at ~31.5° (2θ) in GGBFS (Figure 1c), indicating that GGBFS had an amorphous phase with a high reactivity; some crystals, such as calcite and dolomite, were contained in GGBFS. The raw CG was signified with quartz, kaolinite, and muscovite (Figure 1a). Following the calcination at 700 °C, the diffraction peaks corresponding to kaolinite were disappearing, and the center of broad diffraction shifted ~23.7° (2θ) (Figure 1b). This indicated that the kaolinite structure had been disrupted, and the phase transformed into an amorphous phase of metakaolin. The SEM images of raw materials (calcined CG and GGBFS) are illustrated in Figure 2. The calcined CG had a porous structure and loose morphology (Figure 2a), while GGBFS displayed an angular shape and smooth morphology (Figure 2b).
The NaOH particles, having a purity of ≥96%, were purchased from Tianjin Bochuang Chemical Co., Ltd., Tianjin, China. To create a 10 mol/L sodium hydroxide solution, NaOH was dissolved in distilled water. Additionally, a Na2SiO3 solution was prepared with 3.3 modulus, which was purchased from Foshan Koning New Material Technology Co., Ltd., Foshan, China.

2.2. Geopolymers Preparation

Figure 3 illustrates the process involved in preparing geopolymers based on GGBFS/CG, and the detailed mixtures are provided by Table 2. Firstly, the GGBFS and CG precursors were thoroughly mixed for 2 min, and the alkali activators were composed of Na2SiO3 and NaOH solution; the mass ratio of Na2O was 7.0%, the modulus ratio (SiO2/Na2O) was 1.3, and the liquid-to-solid ratio was 0.55. Secondly, the precursors with alkaline activators and the homogeneous gel slurry were formed by stirring for 5 min, then pouring slurry into silica molds measured at 20 mm × 20 mm × 20 mm, and vibrating for 1.5 min. All samples are covered with a thin polyethylene film and cured in regulated temperature at 60 °C. Subsequently, the samples were demolded and cured in room temperature (~25 °C) for 3 days, 7 days, and 28 days before being tested. The obtained sample was labeled as “GSXCY”, where “S” represented GGBFS and “C” represented CG. The mass ratio of GGBFS to CG is indicated as “X:Y”.

2.3. Characterization Methods

2.3.1. Workability and Setting Properties

The workability of geopolymer slurry was assessed by the truncated conical device following GB/T 8077–2012 [36], and the setting times were determined by the Vicat apparatus (Wuxi Zhongke Building Materials Instrument Co., Ltd., Wuxi, China) as per GB/T 1346–2011 [37].

2.3.2. Compressive Strength Tests

The strength tests for geopolymer specimens were conducted using an STS100K machine (Kaiqiangli Testing Instrument Co., Ltd., Xiamen, China), following GB/T 17671–2020 [38]. Each group comprised three samples; the final compressive strength was determined by averaging the results from the testing process.

2.3.3. Microscopic Properties Tests

The XRD tests for precursors and synthesized samples were acquired by a Bruker D8 Advance diffractometer (Bruker Corporation, Mannheim, Germany). Data were accumulated within a 2θ range of 3–70°. The FTIR spectra of synthesized samples were employed by a Nicolet IS50 spectrometer (Thermo Fisher, Waltham, MA, USA), the spectral range was 400–4000 cm−1. The SEM was equipped with EDX system, using an S-3400N-II apparatus (Hitachi, Tokyo, Japan), the voltage and current operated at 35 kV and 10 mA, respectively. The MIP was determined by an Auto Pore IV 9510 instruments (Micromeritics, Norcross, GA, USA), and the intrusion pressures range was 0.5–33,000 psi.

3. Results

3.1. Workability and Setting Time

3.1.1. Workability

The fluidity of geopolymer was demonstrated in Table 2; it ranged from 107.6 mm to 167.7 mm with different mixtures. As the GGBFS content increased, the slurry fluidity increased. When the GGBFS content was 20%, the fluidity increased 9.76% (118.1 mm), and the fluidity of the GS1C1 (50% GGBFS content) and GS (100% GGBFS content) samples was raised by 25.74% and 55.86%, respectively, compared to GC (100% CG content) specimens. The phenomenon was related to two factors. (1) CG featured a porous structure, allowing the materials to absorb more free water. (2) As shown in Figure 2, CG had a smaller particle size and needed more free water participating in geopolymerization, resulting in less water being beneficial for the slurry fluidity. Kramar et al. [39] discovered that GGBFS-based mortar had higher fluidity than FA-based mortar (180 mm vs. 152 mm). Furthermore, GS1C1 samples exhibited a fluidity of ~135.3 mm, meeting the high-fluidity specifications (120 ± 2 mm) of cementitious materials. These indicated that the GGBFS content could effectively regulate the fluidity requirements of geopolymer slurry.

3.1.2. Setting Time

The initial and final setting times of geopolymer specimens are illustrated in Figure 4. As the mixtures of precursors altered, the geopolymerization of the specimens was affected within the different reactivity for both GGBFS and CG. In Figure 4, it is evident that the final setting times for the geopolymers were notably less than the initial setting times with most of the specimens exhibiting relatively brief setting durations. This was mainly due to the specimens being cured in a regulated environment at 60 °C, which enhanced the geopolymerization reaction.
Moreover, the initial and final setting times of the GGBFS-based geopolymer (GS) were 13 min and 17 min, respectively, and those were 99 min and 122 min for the CG-based geopolymer (GC). For the GS specimens, the rapid solidification posed a challenge for the geopolymer application in the practical structure. When the proportion of CG increased 100%, the setting times of the geopolymer gradually extended to 86 min (initial setting times) and 105 min (final setting times), respectively, which was more than the GS specimens. Moreover, as the mass ratio of GGBFS/CG was 1:4, the final setting time was 68 min, while it was 28 min for GS1C1 specimens. These setting time indicated that incorporating free calcium precursors such as CG into GGBFS-based geopolymer could be beneficial for retardation in the geopolymerization process.

3.2. Compressive Strength

The strength of geopolymer specimens after varying curing periods of 3, 7, and 28 days is illustrated in Figure 5. As the curing time increased, the strength also rose, indicating that the geopolymer reaction was ongoing. With the addition of GGBFS content, the strength of the specimens was enhanced. The compressive strength of GS at 28 days reached 107.55 MPa, which was significantly higher than that of GC specimens (21.80 MPa). Moreover, the specimens exhibited a rapid increase in strength at the 3 days, achieving approximately 65% of their strength at 28 days. This trend was due to the following factors: (1) The geopolymer reaction was rapidly going in the initial stages, and (2) the controlled temperature accelerated the formation of geopolymers.
As the mass ratio of GGBFS/CG changed from 1:4 to 4:1, the strength of the specimens at 28 days varied between 28.27 MPa and 95.88 MPa with an increase of 239.2%. Similarly, when the ratio of GGBFS/GC increased from 2:3 to 3:2, the compressive strength at 28 days increased from 47.72 MPa to 65.63 MPa with an increase of 37.5%. In addition, the compressive strengths of GS1C1 specimens (50% GGBFS content) at 3 days, 7 days, and 28 days were 46.73 MPa, 53.25 MPa, and 54.59 MPa, respectively, and it exhibited a suitable setting time. These indicated that the addition of CG into GGBFS-based geopolymer could be satisfied with the strength and applied in practice engineering structures.

3.3. Microstructural Characterization

3.3.1. XRD Analysis

The XRD patterns of the geopolymer specimens are presented in Figure 6. A broad reflection was observed in the GC specimens, which was centered around ~29.0° (2θ). With the addition of GGBFS, this center was gradually shifted toward higher values (~31.5° (2θ)), indicating that more amorphous phases were generated. As GGBFS content increased to 100%, the center of broad reflection, which corresponded to amorphous C-(A)-S-H, was shifted to ~31.5° (2θ). The diffraction peaks of quartz and muscovite were still present, indicating that quartz and muscovite minerals did not participate in the geopolymerization process [40].
When the mass ratio of GGBFS/CG was less than 2:3, the XRD patterns resembled that of GC specimens, suggesting that the geopolymerization product was primarily the amorphous N-(A)-S-H, which was dominant at this stage. As the amount of GGBFS increased, more crystalline C-(A)-S-H was generated. As the GGBFS/CG mass ratio reached 1:1, the XRD patterns were aligned with the patterns of GS specimens; both the amorphous N-(A)-S-H and crystalline C-(A)-S-H contributed to the compressive strength of geopolymer specimens.

3.3.2. FTIR Analysis

The FTIR patterns of geopolymer specimens are presented in Figure 7. All specimens exhibited absorption bands at approximately 1650 cm−1 and 1446 cm−1, which corresponded to the bending vibration (O-H bands) [41] and the asymmetric tensile vibration (C-O bands) [42]. The absorption bands located around 697 cm−1 and 480 cm−1 represented the internal extension (Si-O bonds) [43] and the stretching vibration (Si-O-T bonds) [44,45]. Moreover, the bands at approximately 801 cm−1 were linked to the bending vibrations (Si-O bonds) [46].
In Figure 7, there were broad bands in the range of 800–1200 cm−1, which were associated with the formation of amorphous products [14,47]. Compared to the FTIR spectra, the center of the peak wavenumbers for GC specimens was higher than that of GS specimens. The difference was mainly related to the Si, Al, and Ca content, which influenced the degree of geopolymerization. More Si atoms in geopolymerization resulted in higher wavenumbers, while an increase of Ca atoms led to the wavenumbers shifting to a lower region [48]. Therefore, with the addition of GGBFS, more Ca atoms caused the lower wavenumbers for the center of broad bands.

3.3.3. SEM/EDX Results

The SEM images of GC and GS specimens are displayed in Figure 8a–d, while the EDX analysis is illustrated in Figure 8e,f. The GC specimens exhibited a loose morphology with numerous large pores that resulted from trapping air. Some isolated particles, such as raw CG, were observed within the geopolymer specimens. As shown in Figure 8e, the GC matrix primarily consisted of O, Na, Si, and Al, exhibiting a Ca/Si ratio of 0.03, suggesting that the geopolymer was predominantly composed of N-A-S-H gels. In contrast, Figure 8c,d revealed that GS specimens possessed a compact microstructure, and the geopolymer gels were firmly interconnected. Figure 8f indicated that the GS matrix was composed of O, Na, Si, Al, and Ca, with a Ca/Si ratio of 1.27, suggesting that the geopolymer was primarily C-(N)-A-S-H gel. This discrepancy suggests that more Ca atoms were participating in geopolymerization for the GS specimens, which led to the increased compressive strength illustrated in Figure 5.
The SEM images of GS2C3, GS1C1, and GS3C2 specimens are displayed in Figure 9a–f, while the EDX analysis results are illustrated in Figure 9g. As the GGBFS content increased, the geopolymer matrix transitioned from a porous morphology to a uniform, dense microstructure. Some smaller pores existed in GS2C3 specimens, resulting in a lower compressive strength for the specimens. When the GGBFS/GC mass ratio exceeded 50% or 60%, both the specimens (GS1C1 and GS3C2) exhibited a more compact morphology due to the greater formation of geopolymers generated. According to Figure 9g, all geopolymer specimens were composed of O, Na, Si, Al, and Ca, and with a higher proportion of GGBFS (40%, 50%, and 60%), the number of Ca atoms increased (ranging from 6.0% to 8.6%), and more C-(N)-A-S-H gels were generated during the geopolymerization process as well.

3.3.4. MIP Results

The distribution of pore sizes for GC, GS1C1, and GS specimens is displayed in Figure 10, while the pore parameters are detailed in Table 3. The results were analyzed by MIP. The pore diameters ranged from 10 to 200 nm, which correspond to the medium and large capillary pores; the more larger capillary pores there were, the poorer the mechanical properties [49].
With 100% CG content, the GC specimens exhibited approximately 35.70 nm pore diameters and 31.74% porosity. When the GGBFS/CG mass ratio was modified to 1:1, both the pore diameters and porosity were reduced. The pore diameters of the GS1C1 specimens were approximately 22.84 nm, and the porosity was 18.38%. The decreases were 36.0% and 42.1%, respectively, compared to GC specimens. Figure 10b shows that the cumulative intrusion of GS1C1 specimens was 0.105 mL/g, while it was 0.208 mL/g for GC specimens. When the GGBFS content reached 100%, the pore diameters and porosity of GS specimens further decreased to 17.06 nm and 3.36%, respectively, and the cumulative intrusion was 0.017 mL/g. These results suggest that the higher reactivity of GGBFS incorporated in the geopolymerization process leads to the generation of more C-(N)-A-S-H gels, which were beneficial for a compact microstructure that aligned with the compressive strength findings detailed in Section 3.2.

4. Discussion

The study revealed that the content of Ca atoms significantly influenced the compressive strength and microstructure of geopolymer specimens, which were dependent on the ratio of GGBFS/CG. As the GGBFS content increased, there was a higher generation of hydrated calcium silicoaluminate (C-(A)-S-H) gels that were leading to a three-dimensional network formation, and the compressive strength was enhanced [50]. However, the compressive strength of the GC (100% CG content) specimens at 28 days was 21.80 MPa, while it was 54.59 MPa for the GS1C1 (50% GGBFS content) specimens. The strength of the GS1C1 specimens was comparable to that of OPC for grade 52.5, and it exhibited a suitable setting time. When the GGBFS content increased from 50% to 100%, the compressive strength increased to 52.96 MPa, which was an increase of 97.0%. These indicate that the compressive strength of GGBFS/CG-based geopolymers was prominently related to the GGBFS incorporation ratio.
Additionally, the rapid reactivity of GGBFS contributed to the shortening of geopolymer slurry setting times. Numerous studies investigated that the utilization of retarding admixtures effectively extends the setting times during the geopolymerization. Wang et al. [51] identified the zinc nitrate and sodium gluconate mixtures that were effectively retarding the GGBFS-based geopolymers. Cong et al. [52] adjusted the properties of fly ash (FA)-based geopolymers via borate addition. Both Kalina [53] and Gong [54] explored how sodium phosphate affects the hydration process of geopolymers made from red mud and slag. Moreover, Brough et al. [55] noted that the setting time could be extended from 4 to 20 h when the geopolymers were incorporated with 0.5 wt% malic acid. This paper provided a foundation for further exploration of whether controlling and optimizing the setting time of geopolymers based on GGBFS throughout the reaction process is achievable.

5. Conclusions

This study investigated the influences of various GGBFS/CG mixtures on the mechanical and microstructural characteristics of the synthesized geopolymers. A series of experimental tests resulted in the following conclusions.
As the proportion of GGBFS increased in raw precursors, it significantly influenced the mechanical properties and microstructural features of the geopolymer. GGBFS-based geopolymers exhibited rapid setting behavior, whereas precursors incorporating CG prolonged the setting process. When the GGBFS/CG mass ratios increased from 1:4 to 4:1, the compressive strength at 28 days ranged from 28.27 MPa to 95.88 MPa. Similarly, as the GGBFS/CG mass ratios varied from 2:3 to 3:2, the strength increased from 47.72 MPa to 65.63 MPa for the specimens. At a GGBFS/CG mass ratio of 1:1, the specimens achieved a compressive strength of 54.59 MPa with a setting time of 28 min, representing a 64.7% increase.
Microstructural analysis revealed that when the CG content reached 100%, the microstructure of the obtained specimens exhibited a loose morphology with numerous pores, resulting in poor compressive strength (21.8 MPa). Increasing the GGBFS content in raw precursors allowed more calcium (Ca) atoms to participate in geopolymerization, promoting the generation of amorphous calcium aluminosilicate hydrate (C-A-S-H) gel and calcium silicate hydrate (C-S-H) crystals. This process induced a denser microstructure and higher compressive strength.
Although this research demonstrates that better mechanical and microstructural characteristics were obtained, supporting the practical application of GGBFS/CG geopolymers in engineering structures, a comprehensive investigation into the long-term durability of geopolymers based on GGBFS/CG is still necessary. Future studies should incorporate durability evaluations, including tests for water absorption, corrosion resistance, and freeze–thaw resistance, to more effectively assess their characteristics under actual engineering conditions and enhance their reliable use in sustainable construction.

Author Contributions

Conceptualization, X.W. and F.L.; data curation, X.W. and W.C.; formal analysis, X.W.; funding acquisition, F.L., L.L., X.W. and K.X.; investigation, X.W. and W.C.; methodology, X.W. and K.X.; resources, F.L. and L.L.; project administration, F.L. and L.L.; software X.W.; supervision, F.L.; writing—original draft preparation, X.W.; writing—review and editing, X.W., K.X. and K.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant Nos. 12072080 and 12032009), and the Applied Research Foundation of Anhui province (Grant Nos. hxkt2024260, hxkt2024344 and hxkt2025138).

Data Availability Statement

The original data will be made available upon requirement.

Acknowledgments

The authors gratefully thank all technical personnel from the Structural Laboratory of Guangdong University of Technology for their assistance during the experiment.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Lin, J.; Liu, R.; Liu, Y.; Zhuo, K.; Chen, Z.; Guo, Y. High-strength and high-toughness alkali-activated composite materials: Optimizing mechanical properties through synergistic utilization of steel slag, ground granulated blast furnace slag, and fly ash. Constr. Build. Mater. 2024, 422, 135811. [Google Scholar] [CrossRef]
  2. Chen, G.; Zheng, D.; Chen, Y.; Lin, J.; Wei, J.; Guo, Y.; Chen, Z.; Lan, X. Development of high performance geopolymer concrete with waste rubber and recycle steel fiber: A study on compressive behavior, carbon emissions and economical performance. Constr. Build. Mater. 2023, 393, 131988. [Google Scholar] [CrossRef]
  3. Aly, A.; El-Feky, M.; Kohail, M.; Nasr, E. Performance of geopolymer concrete containing recycled rubber. Constr. Build. Mater. 2019, 207, 136–144. [Google Scholar] [CrossRef]
  4. Liu, Q.; Hu, Z.; Wang, X.; Zhao, H.; Qian, K.; Li, L.; Meng, Z. Numerical study on cracking and its effect on chloride transport in concrete subjected to external load. Constr. Build. Mater. 2022, 325, 126797. [Google Scholar] [CrossRef]
  5. Tan, Y.; He, Y.; Cui, X.; Liu, L. Design and performance optimization of alkali-activated waste coal bottom ash/slag porous concrete. Constr. Build. Mater. 2022, 359, 129413. [Google Scholar] [CrossRef]
  6. Bahmani, H.; Mostofinejad, D. A review of engineering properties of ultra-high-performance geopolymer concrete. Dev. Built. Environ. 2023, 14, 100126. [Google Scholar] [CrossRef]
  7. Xi, F.; Davis, S.J.; Ciais, P.; Crawford-Brown, D.; Guan, D.; Pade, C.; Shi, T.; Syddall, M.; Lv, J.; Ji, L.; et al. Substantial global carbon uptake by cement carbonation. Nat. Geosci. 2016, 9, 880–883. [Google Scholar] [CrossRef]
  8. Tran, T.; Kim, Y.; Kang, G.; Dinh, B.; Do, T. Feasibility of reusing marine dredged clay stabilized by a combination of by-products in coastal road construction. Transp. Res. Rec. 2019, 2673, 519–528. [Google Scholar] [CrossRef]
  9. Guo, Y.; Cai, Y.; Xie, Z.; Xiao, S.; Zhuo, K.; Cai, P.; Lin, J. Experimental investigation of GFRP bar bonding in geopolymer concrete using hinged beam tests. Eng. Struct. 2025, 322, 119036. [Google Scholar] [CrossRef]
  10. Pobłocki, K.; Pawlak, M.; Drzeżdżon, J.; Gawdzik, B.; Jacewicz, D. Clean production of geopolymers as an opportunity for sustainable development of the construction industry. Sci. Total Environ. 2024, 928, 172579. [Google Scholar] [CrossRef]
  11. Zurinskas, D.; Vaiciukyniene, D.; Stelmokaitis, G.; Dorosevas, V. Clayey soil strength improvement by using alkali activated slag reinforcing. Minerals 2020, 10, 1076. [Google Scholar] [CrossRef]
  12. Zhang, H.; Kodur, V.; Qi, S.; Cao, L.; Wu, B. Development of metakaolin–fly ash based geopolymers for fire resistance applications. Constr. Build. Mater. 2014, 55, 38–45. [Google Scholar] [CrossRef]
  13. Kong, L.; Zhao, W.; Xuan, D.; Wang, X.; Liu, Y. Application potential of alkali-activated concrete for antimicrobial induced corrosion: A review. Constr. Build. Mater. 2022, 317, 126169. [Google Scholar] [CrossRef]
  14. Aiken, T.A.; Gu, L.; Kwasny, J.; Huseien, G.F.; McPolin, D.; Sha, W. Acid resistance of alkali-activated binders: A review of performance, mechanisms of deterioration and testing procedures. Constr. Build. Mater. 2022, 342, 128057. [Google Scholar] [CrossRef]
  15. Puertas, F.; Fernández-Jiménez, A. Mineralogical and microstructural characterisation of alkali-activated fly ash/slag pastes. Cem. Concr. Compos. 2003, 25, 287–292. [Google Scholar] [CrossRef]
  16. Luukkonen, T.; Abdollahnejad, Z.; Yliniemi, J.; Kinnunen, P.; Illikainen, M. One-part alkali-activated materials: A review. Cem. Concr. Res. 2018, 103, 21–34. [Google Scholar] [CrossRef]
  17. Farooq, M.; Bhutta, A.; Banthia, N. Tensile Performance of Eco-Friendly Ductile Geopolymer Composites (EDGC) Incorporating Different Micro-Fibers. Cem. Concr. Compos. 2019, 103, 183–192. [Google Scholar] [CrossRef]
  18. Zhang, B.; Feng, Y.; Xie, J.; Dai, J.; Chen, W.; Xue, Z.; Li, L.; Li, Y.; Li, J. Effects of Pretreated Recycled Powder Substitution on Mechanical Properties and Microstructures of Alkali-Activated Cement. Constr. Build. Mater. 2023, 406, 133360. [Google Scholar] [CrossRef]
  19. Cloete, S.; Giuffrida, A.; Romano, M.C.; Zaabout, A. The Swing Adsorption Reactor Cluster for Post-Combustion CO2 Capture from Cement Plants. J. Clean. Prod. 2019, 223, 692–703. [Google Scholar] [CrossRef]
  20. Chen, L.; Wang, Z.; Wang, Y.; Feng, J. Preparation and properties of alkali activated metakaolin-based geopolymer. Materials 2016, 9, 767. [Google Scholar] [CrossRef]
  21. Li, X.; Zhao, Y.; Hu, Y.; Wang, G.; Xia, M.; Luo, B.; Luo, Z. Influence of multiple factors on the workability and early strength development of alkali-activated fly ash and slag-based geopolymer-stabilized soil. Materials 2022, 15, 2682. [Google Scholar] [CrossRef]
  22. Zhang, Q.; Ji, T.; Yang, Z.; Wang, C.; Wu, H. Influence of different activators on microstructure and strength of alkali-activated nickel slag cementitious materials. Constr. Build. Mater. 2020, 235, 117449. [Google Scholar] [CrossRef]
  23. Ghorbani, S.; Stefanini, L.; Sun, Y.; Walkley, B.; Provis, J.; De Schutter, D.; Matthys, S. Characterisation of alkali-activated stainless steel slag and blast-furnace slag cements. Cem. Concr. Compos. 2023, 143, 105230. [Google Scholar] [CrossRef]
  24. Liu, J.; Wu, C.; Li, J.; Liu, Z.; Xu, S.; Liu, K.; Su, Y.; Fang, J.; Chen, G. Projectile impact resistance of fibre-reinforced geopolymer-based ultra-high performance concrete (G-UHPC). Constr. Build. Mater. 2021, 290, 123189. [Google Scholar] [CrossRef]
  25. Deb, P.; Nath, P.; Sarker, P. The effects of ground granulated blast-furnace slag blending with fly ash and activator content on the workability and strength properties of geopolymer concrete cured at ambient temperature. Mater. Des. 2014, 62, 32–39. [Google Scholar] [CrossRef]
  26. Buchwald, A.; Tatarin, R.; Stephan, D. Reaction progress of alkaline-activated metakaolin-ground granulated blast furnace slag blends. J. Mater. Sci. 2009, 44, 5609–5617. [Google Scholar] [CrossRef]
  27. Venkatesan, R.; Pazhani, K. Strength and durability properties of geopolymer concrete made with Ground Granulated Blast Furnace Slag and Black Rice Husk Ash. KSCE J. Civ. Eng. 2016, 20, 2384–2391. [Google Scholar] [CrossRef]
  28. Geng, J.; Zhou, M.; Li, Y.; Chen, Y.; Han, Y.; Wan, S.; Zhou, X.; Hou, H. Comparison of red mud and coal gangue blended geopolymers synthesized through thermal activation and mechanical grinding preactivation. Constr. Build. Mater. 2017, 153, 185–192. [Google Scholar] [CrossRef]
  29. Jablonska, B.; Kityk, A.; Busch, M.; Huber, P. The structural and surface properties of natural and modified coal gangue. J. Environ. Manag. 2017, 190, 80–90. [Google Scholar] [CrossRef]
  30. Afrakoti, M.T.P.; Choobbasti, A.J.; Ghadakpour, M.; Kutanaei, S.S. Investigation of the effect of the coal wastes on the mechanical properties of the cement-treated sandy soil. Constr. Build. Mater. 2020, 239, 117848. [Google Scholar] [CrossRef]
  31. Li, Z.; Gao, Y.; Zhang, J.; Zhang, C.; Chen, J.; Liu, C. Effect of particle size and thermal activation on the coal gangue based geopolymer. Mater. Chem. Phys. 2021, 267, 124657. [Google Scholar] [CrossRef]
  32. Wang, X.; Liu, F.; Pan, Z.; Chen, W.; Muhammad, F.; Zhang, B.; Li, L. Geopolymerization of Coal gangue via alkali-activation: Dependence of mechanical properties on alkali activators. Buildings 2024, 14, 787. [Google Scholar] [CrossRef]
  33. Zhang, W.; Dong, C.; Huang, P.; Sun, Q.; Li, M.; Chai, J. Experimental Study on the Characteristics of Activated Coal Gangue and Coal Gangue-Based Geopolymer. Energies 2020, 13, 2504. [Google Scholar] [CrossRef]
  34. Zhao, Y.; Qiu, J.; Ma, Z.; Sun, X. Eco-friendly treatment of coal gangue for its utilization as supplementary cementitious materials. J. Clean. Prod. 2021, 285, 124834. [Google Scholar] [CrossRef]
  35. Balczár, I.; Korim, T.; Kovács, A.; Makó, É. Mechanochemical and Thermal Activation of Kaolin for Manufacturing Geopolymer Mortars—Comparative Study. Ceram. Int. 2016, 42, 15367–15375. [Google Scholar] [CrossRef]
  36. GB/T 8077-2012; National Standard of the People’s Republic of China: Test Methods for Uniformity of Concrete Admixtures. Standardization Administration of China: Beijing, China, 2012. (In Chinese)
  37. GB/T 1346-2011; National Standard of the People’s Republic of China: Test Methods for Water Requirement of Normal Consistency, Setting Time and Soundness of Portland Cement. Standardization Administration of China: Beijing, China, 2011. (In Chinese)
  38. GB/T 17671-2020; National Standard of the People’s Republic of China: Test Method for Strength of Hydraulic Cement Mortar (ISO Method). Standardization Administration of China: Beijing, China, 2020. (In Chinese)
  39. Kramar, S.; Šajna, A.; Ducman, V. Assessment of alkali activated mortars based on different precursors with regard to their suitability for concrete repair. Constr. Build. Mater. 2016, 124, 937–944. [Google Scholar] [CrossRef]
  40. Van Jaarsveld, J.; van Deventer, J. Effect of the alkali metal activator on the properties of fly ash-based geopolymers. Ind. Eng. Chem. Res. 1999, 38, 3932–3941. [Google Scholar] [CrossRef]
  41. Fernández-Jiménez, A.; Palomo, A. Mid-infrared spectroscopic studies of alkali-activated fly ash structure. Microporous Mesoporous Mater. 2005, 86, 207–214. [Google Scholar] [CrossRef]
  42. Li, N.; Farzadnia, N.; Shi, C. Microstructural changes in alkali-activated slag mortars induced by accelerated carbonation. Cem. Concr. Res. 2017, 100, 214–226. [Google Scholar] [CrossRef]
  43. Ryu, G.; Lee, Y.; Koh, K.; Chung, Y. The mechanical properties of fly ash-based geopolymer concrete with alkaline activators. Constr. Build. Mater. 2013, 47, 409–418. [Google Scholar] [CrossRef]
  44. Sun, Z.; Vollpracht, A. One year geopolymerisation of sodium silicate activated fly ash and metakaolin geopolymers. Cem. Concr. Compos. 2019, 95, 98–110. [Google Scholar] [CrossRef]
  45. Koshy, N.; Dondrob, K.; Hu, L.; Wen, Q.; Meegoda, J.N. Synthesis and characterization of geopolymers derived from coal gangue, fly ash and red mud. Constr. Build. Mater. 2019, 206, 287–296. [Google Scholar] [CrossRef]
  46. Yu, Q.; Li, S.; Li, H.; Chai, X.; Bi, X.; Liu, J.; Ohnuki, T. Synthesis and characterization of Mn-slag based geopolymer for immobilization of Co. J. Clean. Prod. 2019, 234, 97–104. [Google Scholar] [CrossRef]
  47. Yaseri, S.; Masoomi Verki, V.; Mahdikhani, M. Utilization of high volume cement kiln dust and rice husk ash in the production of sustainable geopolymer. J. Clean. Prod. 2019, 230, 592–602. [Google Scholar] [CrossRef]
  48. Zhang, M.; Zhao, M.; Zhang, G.; El-Korchi, T.; Tao, M. A multiscale investigation of reaction kinetics, phase formation, and mechanical properties of metakaolin geopolymers. Cem. Concr. Compos. 2017, 78, 21–32. [Google Scholar] [CrossRef]
  49. Wang, Y.; Alrefaei, Y.; Dai, J. Silico-aluminophosphate and alkali-aluminosilicate geopolymers: A comparative review. Front. Mater. 2019, 6, 106. [Google Scholar] [CrossRef]
  50. Rashad, A. Alkali-activated metakaolin: A short guide for civil Engineer—An overview. Constr. Build. Mater. 2013, 41, 751–765. [Google Scholar] [CrossRef]
  51. Wang, J.; Han, L.; Liu, Z.; Wang, D. Setting controlling of lithium slag-based geopolymer by activator and sodium tetraborate as a retarder and its effects on mortar properties. Cem. Concr. Compos. 2020, 110, 103598. [Google Scholar] [CrossRef]
  52. Cong, X.; Zhou, W.; Geng, X.; Elchalakani, M. Low field NMR relaxation as a probe to study the effect of activators and retarders on the alkali-activated GGBFS setting process. Cem. Concr. Compos. 2019, 104, 103399. [Google Scholar] [CrossRef]
  53. Kalina, L.; Bilek, V.; Novotny, R.; Moncekova, M.; Masilko, J.; Koplik, J. Effect of Na3PO4 on the hydration process of alkali-activated blast furnace slag. Materials 2016, 9, 395. [Google Scholar] [CrossRef]
  54. Gong, C.; Yang, N. Effect of phosphate on the hydration of alkali-activated red mud-slag cementitious material. Cem. Concr. Res. 2000, 30, 1013–1016. [Google Scholar] [CrossRef]
  55. Brough, A.; Holloway, M.; Sykes, J.; Atkinson, A. Sodium silicate-based alkali-activated slag mortars Part II. The retarding effect of additions of sodium chloride or malic acid. Cem. Concr. Res. 2000, 30, 1375–1379. [Google Scholar] [CrossRef]
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Figure 1. The XRD patterns of raw materials: (a) CG; (b) calcined CG, and (c) GGBFS.
Figure 1. The XRD patterns of raw materials: (a) CG; (b) calcined CG, and (c) GGBFS.
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Figure 2. The SEM images of raw materials: (a) Calcined CG; (b) GGBFS.
Figure 2. The SEM images of raw materials: (a) Calcined CG; (b) GGBFS.
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Figure 3. The preparation process of GGBFS/CG-based geopolymers.
Figure 3. The preparation process of GGBFS/CG-based geopolymers.
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Figure 4. The setting times of geopolymer slurry.
Figure 4. The setting times of geopolymer slurry.
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Figure 5. The compressive strength of geopolymer specimens.
Figure 5. The compressive strength of geopolymer specimens.
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Figure 6. The XRD patterns of geopolymer specimens.
Figure 6. The XRD patterns of geopolymer specimens.
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Figure 7. The FTIR patterns of geopolymer specimens.
Figure 7. The FTIR patterns of geopolymer specimens.
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Figure 8. The SEM images of geopolymer specimens: (a,b) GC, (c,d) Gs, and (e,f) the EDX results of highlighted spots.
Figure 8. The SEM images of geopolymer specimens: (a,b) GC, (c,d) Gs, and (e,f) the EDX results of highlighted spots.
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Figure 9. The SEM images of geopolymer: specimens (a,b) GS2C3, (c,d) GS1C1, and (e,f) GS3C2; (g) the EDX results of highlighted spots.
Figure 9. The SEM images of geopolymer: specimens (a,b) GS2C3, (c,d) GS1C1, and (e,f) GS3C2; (g) the EDX results of highlighted spots.
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Figure 10. The MIP results of geopolymer specimens: (a) incremental pore volume; (b) cumulative pore volume.
Figure 10. The MIP results of geopolymer specimens: (a) incremental pore volume; (b) cumulative pore volume.
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Table 1. The chemical compositions of GGBFS and CG (wt%).
Table 1. The chemical compositions of GGBFS and CG (wt%).
PrecursorsSiO2Al2O3Fe2O3CaONa2OMgOK2OTiO2Others
GGBFS28.0314.650.4443.430.518.570.440.992.94
CG62.8423.284.441.731.041.993.220.980.48
Table 2. The detailed mixtures of GGBFS/CG-based geopolymers.
Table 2. The detailed mixtures of GGBFS/CG-based geopolymers.
SpecimensCG (%)GGBFS (%)Na2O (%)Ms = (SiO2/Na2O)Liquid/SolidFluidity (mm)
GC10007.01.30.55107.6
GS1C48020118.1
GS2C36040129.3
GS1C15050135.3
GS3C24060142.8
GS4C12080154.4
GS0100167.7
Table 3. The pore parameters of geopolymer specimens conducted by MIP.
Table 3. The pore parameters of geopolymer specimens conducted by MIP.
SamplesTotal Pore Area (m2/g)Average Pore
Diameter (nm)		Porosity (%)
GC36.5835.7031.74
GS1C124.8922.8418.38
GS7.6417.063.36
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Wang, X.; Liu, F.; Chen, W.; Xing, K.; Zhuo, K.; Li, L. The Influence of Coal Gangue on the Mechanical Properties of Ground-Granulated Blast Furnace Slag-Based Geopolymers. Buildings 2025, 15, 2695. https://doi.org/10.3390/buildings15152695

AMA Style

Wang X, Liu F, Chen W, Xing K, Zhuo K, Li L. The Influence of Coal Gangue on the Mechanical Properties of Ground-Granulated Blast Furnace Slag-Based Geopolymers. Buildings. 2025; 15(15):2695. https://doi.org/10.3390/buildings15152695

Chicago/Turabian Style

Wang, Xiaoping, Feng Liu, Weizhi Chen, Kaifeng Xing, Kexian Zhuo, and Lijuan Li. 2025. "The Influence of Coal Gangue on the Mechanical Properties of Ground-Granulated Blast Furnace Slag-Based Geopolymers" Buildings 15, no. 15: 2695. https://doi.org/10.3390/buildings15152695

APA Style

Wang, X., Liu, F., Chen, W., Xing, K., Zhuo, K., & Li, L. (2025). The Influence of Coal Gangue on the Mechanical Properties of Ground-Granulated Blast Furnace Slag-Based Geopolymers. Buildings, 15(15), 2695. https://doi.org/10.3390/buildings15152695

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