How to Cite

Stephen, A. O., Liu, C., & Xin, G. (2026). Coal Gangue as a Sustainable Construction Material: A Global Review of Mechanical Properties, Microstructural Behavior, and Performance Challenges. International Journal of Research, 13(1), 188–212. https://doi.org/10.26643/ijr/2026/1

Aremu Oluwatobi Stephen¹, Chao Liu², Guo Xin³

¹School of Civil Engineering, Xi’an University of Architecture and Technology, Xi^’an, 710055, China. aremutoby@yahoo.com ,(https://orcid.org/0009-0005-6295-4138)

²School of Civil Engineering, Xi’an University of Architecture and Technology, Xi^’an, 710055, China. Corresponding author:  chaoliu@xauat.edu.cn, (https://orcid.org/0000-0002-2521-924X )

³School of Civil Engineering, Xi’an University of Architecture and Technology, Xi^’an, 710055, China. xinguo927@163.com ,0009-0000-7841-368X, (https://orcid.org/0000-0002-7149-5413)

Abstract

   Coal gangue (CG), a substantial by-product of coal mining, has recently emerged as a promising sustainable material for concrete production. This review synthesizes 44 experimental and life-cycle studies published between 2012 and 2024 to elucidate the mechanical, microstructural, durability, and environmental performance of coal gangue concrete (CGC). At aggregate replacement levels below 30%, compressive strengths of approximately 40 MPa are generally maintained, whereas higher substitution ratios tend to diminish performance due to increased porosity and weaker interfacial transition zones (ITZs). When employed as a supplementary cementitious material (SCM), calcined gangue enhances long-term strength and ITZ bonding through pozzolanic activity. Durability outcomes are varied: resistance to freeze–thaw cycles, sulfate, and chloride attack remains acceptable at moderate replacement levels but declines under carbonation exposure. Life-cycle assessments (LCAs) indicate potential CO₂ emission reductions of 20–35%, contingent on calcination energy demand and replacement ratios. However, widespread adoption is hindered by non-standardized testing protocols, limited field validation outside China, and insufficient integration of microstructural and durability data. To address these challenges, this review proposes a four-layer evaluation framework (mechanical, microstructural, durability, and environmental), benchmark mix classifications for CGC, and a roadmap promoting field-scale validation and AI-driven optimization. Codifying the use of coal gangue within design standards and green certification systems could transform it from a mining liability into a recognized sustainable construction resource.

Keywords: Coal Gangue; Supplementary Cementitious Materials; Microstructure; Durability; Life-Cycle Assessment; Sustainable concrete.

Highlights

– Global review of 44 studies (2012–2024) on coal gangue concrete.

– ≤30% gangue substitution maintains ~40 MPa compressive strength.

– Calcined gangue improves ITZ bonding and later-age strength.

– Mixed durability; carbonation remains the main weakness.

– A four-layer framework and roadmap are proposed for codification.

1. Introduction

  Concrete remains the most widely consumed construction material globally, yet its production is responsible for nearly 8% of global carbon dioxide emissions. The increasing concern regarding climate change and resource depletion has consequently intensified research into alternative binders and aggregates derived from industrial by-products. Among these materials, coal gangue (CG), the solid residue produced during coal mining and beneficiation, presents significant potential for sustainable utilization. Annually, more than 600 million tonnes of gangue are generated worldwide [16,18], and improper disposal results in land degradation, spontaneous combustion, and water pollution. Due to its high silica and alumina content, gangue exhibits latent pozzolanic activity that can be activated through thermal or chemical treatment, enabling its application as coarse or fine aggregate, supplementary cementitious material (SCM), or filler in concrete systems [18,26,31].

  Over the past decade, numerous studies in China, India, Australia, and, more recently, Africa have demonstrated that properly processed coal-gangue concrete (CGC) can achieve satisfactory strength and durability while reducing environmental impact [11,17,19]. Despite these advancements, industrial adoption remains limited by several factors: (i) variability in mineral composition and porosity, (ii) inconsistent mix-design and testing protocols, and (iii) a shortage of field-scale validation. Existing reviews have primarily focused on single aspects such as pozzolanic reactivity or regional studies, leaving a gap in holistic understanding that integrates mechanical, microstructural, durability, and environmental dimensions [1], [2] .

  This review addresses that gap by synthesizing 44 publications from 2012 to 2024. It provides trend-based quantitative consolidation of mechanical data, integrates microstructural and durability evidence, and evaluates life-cycle environmental implications. Beyond synthesis, the paper introduces a four-layer evaluation framework covering mechanical, microstructural, durability, and environmental metrics, establishes benchmark CGC mix classes, and proposes a research roadmap toward international codification. The study ultimately positions coal gangue as a viable secondary resource that can support circular-economy objectives and green-construction standards.

2. Review Methodology

2.1 Literature-search strategy

  A systematic narrative approach was adopted instead of a formal meta-analysis due to the heterogeneity of the available data. The search strategy integrated electronic databases, including Scopus, Web of Science, Google Scholar, and CNKI, to retrieve publications from 2012 to 2024 using the keywords “coal gangue,” “concrete,” “aggregate,” “supplementary cementitious material,” and “durability.” The initial query yielded 1,024 records, with an additional 76 articles identified through cross-referencing and grey literature.

  Following the removal of duplicates, 950 unique records were screened by title and abstract. Of these, 750 were excluded as they were unrelated to coal-gangue-based concrete or lacked mechanical or microstructural results. Two hundred full texts were reviewed in detail, and 44 met all inclusion criteria. The selection process adhered to a PRISMA-style protocol (Figure 1), adapted from the PRISMA guidelines, to ensure transparency and reproducibility. Searches were conducted in Scopus, Web of Science, CNKI, and Google Scholar for publications from 1 January 2012 to 31 December 2024 (final search 10 January 2025). An example Scopus query is: TITLE-ABS-KEY((“coal gangue” OR “coal-gangue” OR “gangue”) AND (concrete OR mortar OR “supplementary cementitious material”)). Results were deduplicated using EndNote X9, followed by manual screening of titles and abstracts. Full-text screening was conducted using the inclusion criteria listed in Section 2.2. Data extraction fields (author, year, country, gangue form, replacement ratio, curing condition, 28-day compressive strength, durability metrics, microstructural methods) are provided in Supplementary Table S1. Detailed search strings, screening steps, and exclusion reasons are provided in Supplementary Table S2.

2.2 Inclusion and exclusion criteria

  Studies were included based on the following criteria: 1. They examined the utilization of coal gangue as an aggregate, supplementary cementitious material (SCM), or filler in concrete or mortar; 2. They provided quantitative data concerning mechanical, durability, or microstructural performance; 3. They were composed in English and published in peer-reviewed journals or reputable conference proceedings. Exclusion criteria encompassed: (i) Studies concentrating exclusively on gangue geopolymers without cement systems; (2) Studies lacking adequate experimental detail (e.g., absence of mix ratios or test methods); (3) Duplicated sources or those not subjected to peer review.

2.3 Data extraction and synthesis

  From each study, key variables were extracted: gangue form (raw, calcined, ash, ceramsite), replacement level, curing condition, mechanical results, microstructural characterisation, and durability indicators. Reported 28-day compressive-strength ranges were converted to mid-points to allow pooled comparison. Because variance data were rarely provided, numerical results were synthesised as trend-based averages rather than statistical effect sizes. This descriptive integration captures consistent performance tendencies while acknowledging methodological diversity.

2.4 Quantitative Data Synthesis and Transparency

  Reported mechanical-strength values were harmonised to 28-day compressive strength for comparability. When a study presented a range of strengths (e.g., 35–45 MPa), the midpoint (40 MPa) was recorded. For single-value reports, the stated result was used directly. Variance data (standard deviations, confidence intervals) were seldom provided across the reviewed literature; therefore, meta-analysis was not statistically feasible. Instead, descriptive synthesis and trend-based averaging were applied. Outliers—defined as values > 2× the interquartile range —  were inspected manually and retained when consistent with the reported mixture design or test conditions. Of the 44 included studies, 29 reported single values while 15 presented ranges; the latter were converted to midpoints for comparative synthesis. All extracted numeric values and corresponding metadata are provided in Supplementary Table S1, and calculations were performed in Microsoft Excel 2021 for traceability  {Citation}   . 

2.5 Quality assessment

  Methodological quality was graded as high, moderate, or low using four criteria:

(i) clarity of mix-design reporting;

(ii) specification of gangue-processing method (raw, calcined, ash, or ceramsite);

(iii) use of recognized test standards (ASTM, GB/T, EN); and

(iv) completeness of mechanical and durability datasets.

This process improved the reliability of cross-study interpretation and provided the foundation for the comparative analyses presented in later sections.

Figure 1. PRISMA-style literature-selection process for coal-gangue concrete review.

(Flow: 1,024 records → 950 screened → 200 full-text → 44 included.)

  All numerical data (S1–S4) were extracted from peer-reviewed studies with cross-verification of units and parameters. Outliers were checked and normalized by the equivalent binder replacement ratio.

3. Overview of Coal Gangue as a Construction Material

3.1 Origin and classification

  Coal gangue is a solid waste generated during coal mining and beneficiation processes. It typically constitutes 15–20% of the raw coal extracted, containing clay minerals, quartz, feldspar, pyrite, and residual carbonaceous matter. When disposed of untreated, it contributes to land subsidence, spontaneous combustion, and surface-water pollution. Gangue can be broadly divided into:

• Primary gangue, interbedded with coal seams during extraction; and

• Secondary gangue, produced during coal washing and processing [39].

Further classification may be based on mineralogy, thermal behaviour, and physical texture, as shown in Table 1.

Table 1. Classification of coal gangue by origin, mineralogy, and behaviour

Type / Criterion	Basis of classification	Typical characteristics
Primary gangue	Inter-bedded with coal seams	Hard, dense shale-like material
Secondary gangue	By-product of washing/processing	Slurry tailings or waste heaps
Mineralogical	XRD/petrographic phases	Quartz, kaolinite, feldspar
Thermal behaviour	Reactivity after calcination	Formation of amorphous aluminosilicates (600–900 °C) Physical texture   Colour, porosity, and shape     Grey–black, flaky, porous
Physical texture	Colour, porosity, and shape	Grey–black, flaky, porous

3.2 Global distribution and availability

  Global production of coal gangue exceeds 600 million tonnes per year, with China accounting for over 70% of this volume. Other major producers include India, South Africa, and Australia. Despite this abundance, utilisation rates remain below 30% in most regions. Figure 2 illustrates the approximate distribution of known gangue reserves and highlights data scarcity across Africa and South America.

Figure 2. Estimated global distribution of coal-gangue reserves and research activity density distributed as China → 65%, India → 15%, Europe → 10%, Africa → 5%, Others → 5%

3.3 Chemical and mineral composition

  Typical oxide composition derived from XRF/XRD analyses includes SiO₂ (45–65%), Al₂O₃ (15–35%), and minor oxides such as Fe₂O₃, CaO, MgO, and K₂O [14,28]. These constituents are comparable to those of Class F fly ash, suggesting potential pozzolanic reactivity. However, impurities such as unburnt carbon, sulfides, and expansive clays can adversely affect cement hydration and dimensional stability. Pre-treatment through calcination (600–800 °C) or chemical activation can therefore enhance performance.

3.4 Forms of application in concrete

  Coal gangue can serve in several roles within cementitious systems:

1. Coarse or fine aggregate, replacing natural stone or sand at 10–50%;

2. Supplementary cementitious material (SCM), after calcination and grinding;

3. Filler or lightweight aggregate, as in ceramsite production.

Appropriate processing, crushing, grading, calcination, and blending—enables acceptable workability and strength comparable to conventional concrete at low substitution levels [17,18].

4. Mechanical Properties of Coal-Gangue Concrete (CGC)

4.1 Compressive strength

  Compressive strength remains the most reported indicator of CGC performance. Across 44 reviewed studies, low-to-moderate aggregate replacement (≤30%) preserves 28-day compressive strength at approximately 38–44 MPa, while high substitution (>50%) leads to a significant reduction due to increased porosity and weak ITZ bonding [10,23]. When used as a calcined SCM (≈10–15%), coal gangue can slightly increase later-age strength by enhancing hydration reactions [28].

Table 2. Summary of 28-day compressive strength at varying gangue replacement levels(n=44).

Mix type	Gangue role/replacement (%)	Strength range (MPa)	Mean (MPa)	Relative to control
Control concrete	0	40–45	42.5	—
Aggregate replacement	20	38–44	41.0	Comparable
Aggregate replacement	50	30–36	33.0	Decreased
Calcined SCM	10	42–48	45.0	Improved

Figure 3. Variation of mean 28-day compressive strength with coal-gangue replacement ratio.

(Shows consistent performance up to ~30% replacement; drops beyond 50%.)

4.2 Tensile and flexural strength

  Splitting-tensile and flexural strength values are more sensitive to microcracking at the ITZ. Reductions of 10–30% are common when untreated gangue aggregates are used. Improved bonding and reduced cracking can be achieved with superplasticisers, silica fume, or pre-soaked aggregates [12,21]. Enhanced ITZ densification correlates with increased flexural resilience.

4.3 Stress–strain characteristics

  Coal-gangue concrete generally exhibits a lower elastic modulus (10–25% lower than conventional concrete) and a broader post-peak deformation zone, indicating improved ductility and energy-absorption capacity [23,40]. Such behaviour is beneficial in composite systems such as concrete-filled steel tubes (CFSTs), where confinement offsets intrinsic brittleness.

5. Durability and Environmental Resistance

5.1 Overview

  Durability represents a crucial determinant of long-term viability for coal-gangue concrete (CGC). Performance depends on gangue treatment, pore refinement, and aggregate–paste interaction. Although compressive strength can remain satisfactory, environmental resistance varies considerably with replacement level and curing regime [10,29].

5.2 Freeze–thaw and wet–dry cycles

  Most studies indicate that CGC incorporating ≤30% treated gangue maintains adequate freeze–thaw resistance over 150–300 cycles, with relative dynamic modulus losses below 15% [38]. The internal porosity of gangue aggregates enables partial stress relief during freezing, whereas excessive substitution (>40%) increases microcrack propagation and scaling. Similar patterns appear in wet–dry tests, where calcined gangue mixes show improved dimensional stability relative to untreated material.

5.3 Sulfate and chloride attack

  Resistance to sulfate attack improves slightly with calcined gangue additions because of reduced calcium hydroxide content and the formation of secondary C-A-S-H phases. Strength retention after 180 days of Na₂SO₄ exposure commonly exceeds 80% for moderate substitution ratios. Conversely, chloride-ion diffusion coefficients increase marginally due to open-pore connectivity when coarse gangue aggregates dominate the mix [20]. Incorporation of supplementary SCMs such as fly ash or silica fume can offset this effect.

5.4 Carbonation and acid resistance

  Carbonation remains the weakest durability parameter of CGC. The higher porosity of untreated gangue promotes CO₂ ingress and CaCO₃ formation along the ITZ, leading to strength reductions of 10–25% after accelerated tests [13]. Partial substitution with calcined gangue or the use of surface sealants mitigates but does not eliminate this vulnerability. Acid exposure (H₂SO₄ or HCl) produces comparable deterioration trends, particularly in mixes containing pyritic gangue.

  Carbonation depth increased with higher gangue replacement ratios, confirming that carbonation is a key durability concern. Carbonation-related durability parameters are summarised in Supplementary Table S4.

5.5 Coupled deterioration mechanisms

  Few studies explore the combined effects of carbonation–chloride or freeze–thaw–sulfate cycles. Limited evidence suggests synergistic deterioration, where microcracking from thermal cycling accelerates ion penetration. Figure 5 illustrates the overall ranking of durability indices compiled from representative data.

Figure 5. Radar chart of relative durability indices of coal-gangue concretes (freeze–thaw, sulfate, chloride, carbonation, acid).

5.6 Environmental and leaching behaviour

  Toxic-element leaching tests (TCLP, GB/T 5086) reveal that heavy-metal concentrations mainly Fe, Mn, and trace Pb—remain well below regulatory thresholds when gangue is encapsulated within the cement matrix [26]. Life-cycle assessments indicate potential CO₂-emission reductions of 20–35% relative to conventional concrete, contingent on local calcination energy sources. However, sustainability benefits diminish if gangue requires long-distance transport or high-temperature activation. Supplementary Table S3 – Assumptions and boundary conditions extracted from five representative life-cycle assessment studies (2012–2024) underpinning the 20–35 % CO₂-reduction range discussed in Sections 5.6 and 7.1.

Table 3. Summary of the durability performance of coal-gangue concrete

Durability factor	Typical test duration	Optimum gangue substitution (%)	Relative performance vs control	Governing mechanism
Freeze–thaw	150–300 cycles	≤30% (calcined)	Comparable	Pore-structure buffering
Sulfate attack	180 days	≤25%	Slightly improved	Reduced CH, C-A-S-H formation
Chloride penetration	90 days	≤20% + fly ash	Moderate increase	Porous ITZ, open pores
Carbonation	28 days CO₂	≤15% (calcined)	Weaker	Porosity, CaCO₃ in ITZ
Acid resistance	60 days	≤10%	Decreased	Pyrite oxidation
Leaching safety	—	—	Acceptable	Metal immobilisation

5.7 Summary of durability trends

  Durability of CGC is thus application-specific. Properly treated gangue performs satisfactorily in environments governed by physical rather than chemical degradation. Nonetheless, carbonation and acid resistance remain research priorities before large-scale adoption.

6. Microstructural Behaviour

6.1 SEM and microcrack morphology

  Scanning electron microscopy (SEM) studies reveal that untreated gangue aggregates exhibit weak bonding and open microcracks at the ITZ, often filled with secondary ettringite or CaCO₃ crystals (Figure 6a). After calcination, the gangue surface becomes rougher and more reactive, forming a denser C-S-H gel matrix at the interface (Figure 6b) [21,31].

⸻

Figure 6. Representative SEM micrographs showing (a) untreated-gangue ITZ with porous structure and (b) calcined-gangue ITZ with dense hydration products.

6.2 XRD and hydration products

  X-ray diffraction (XRD) patterns confirm the transformation of kaolinite into amorphous metakaolin during calcination at 700–800 °C, thereby enhancing pozzolanic potential. The presence of new phases such as mullite, quartz, and gehlenite correlates with improved compressive strength and durability. Quantitative phase analysis indicates that amorphous content increases from approximately 25% (raw) to 55% (calcined), promoting secondary hydration reactions [28].

6.3 ITZ characterisation

  Back-scattered electron imaging and nano-indentation measurements reveal that the ITZ in calcined-gangue concretes has higher micro-hardness and lower porosity than that of control samples. The thickness of the ITZ reduces from roughly 40 µm to 25 µm, and Ca/Si ratios decline due to additional alumina supplied by the gangue. This microstructural densification directly explains improved mechanical stability at moderate replacement levels.

6.4 Porosity and pore-size distribution

  Mercury-intrusion porosimetry (MIP) and BET tests show that total porosity decreases slightly (2–5%) after calcined-gangue incorporation, accompanied by a shift toward finer pores (< 50 nm). Such refinement limits moisture ingress and enhances freeze–thaw resistance, corroborating macroscopic results. Untreated gangue, by contrast, produces a broader pore spectrum and higher connectivity, which explains its weaker durability.

6.5 Microstructure–performance correlation

  Integrated analysis of SEM, XRD, and MIP data confirms a direct correlation between microstructural densification and macroscopic strength retention. Figure 7 summarises this linkage, highlighting the role of calcination in refining the ITZ and reducing permeability pathways.

Figure 7. Schematic correlation between coal-gangue treatment, ITZ densification, and macro-mechanical performance.

6.6 Summary

  Microstructural evidence confirms that the primary mechanism of performance enhancement in coal-gangue concrete is the transformation of kaolinite into reactive aluminosilicate phases during calcination. These reactions strengthen the ITZ, reduce pore connectivity, and underpin the favourable strength and durability trends identified earlier.

7. Integrated Synthesis and Global Comparison

7.1 Global performance synthesis

  Consolidating the 44 reviewed studies reveals consistent trends linking mechanical, durability, and microstructural parameters.  When treated, gangue is used as a coarse or fine aggregate, mechanical properties remain stable up to approximately 30% substitution, with mean compressive strength values around 40 MPa.  Above this threshold, performance declines due to increased porosity and weakened ITZ cohesion.  When ground and calcined as a supplementary cementitious material, gangue improves both compressive and tensile strength by 5–10% at later ages [39,31].

  Durability follows a similar pattern:  moderate replacement retains acceptable freeze–thaw and sulfate resistance, while carbonation remains the dominant weakness.  Life-cycle analyses indicate potential CO₂-emission savings of 20–35%, strongly dependent on calcination energy and transportation logistics.  Together, these data position calcined gangue as a credible, lower-carbon SCM and untreated gangue as a partial aggregate for non-structural or secondary applications.

  The reviewed LCA studies reported CO₂ reductions ranging from 20% to 35%, depending on the energy source and transport distance. Details of life-cycle assessment assumptions are provided in Supplementary Table S3.

7.2 Regional distribution of research

  Research activity remains highly concentrated in East Asia, which accounts for roughly 65% of published studies.  Europe and Australia contribute 20%, while Africa and South America together represent less than 5%.  Figure 8 illustrates this distribution and identifies key performance themes by region.

Figure 8. Geographical distribution of coal-gangue-concrete research (2012–2024) and dominant performance topics by region.

  Regional disparities correspond closely to coal-production volumes and policy support for waste valorisation.  China’s governmental funding and abundant gangue stockpiles have driven large-scale pilot projects and field demonstrations.  In contrast, African investigations remain largely laboratory-scale due to limited calcination infrastructure and inconsistent supply chains [11].

7.3 Comparative trends with other waste materials

  Compared with other mineral by-products—fly ash, slag, and rice-husk ash—coal gangue displays lower intrinsic reactivity but higher abundance and lower cost.  Its performance improves significantly after calcination, narrowing the gap with traditional SCMs. Compared with other aluminosilicate SCMs such as fly ash and metakaolin [22,39], coal gangue exhibits lower amorphous content and slower pozzolanic reactivity; however, its high alumina–silica ratio after calcination enhances long-term C–A–S–H and N–A–S–H gel formation, contributing to improved durability in blended concretes. Recent advances in alternative SCMs (e.g., calcined clays and gangue hybrids) [40] further highlight the potential of gangue-based binders in carbon-neutral construction. Figure 9 and Table 4 summarise relative property indices derived from typical datasets.

Figure 9. Normalised performance indices of coal gangue and other common supplementary cementitious materials (fly ash, slag, silica fume, rice-husk ash).

Table 4. Comparative summary of SCM performance indices

Property category	Coal gangue (calcined)	Fly ash	Slag	Silica fume	Rice-husk ash
Pozzolanic activity	Moderate	Moderate–high	High	Very high	High
Compressive strength (28 days)	95–105% of control	100–110%	110–120%	115–130%	105–115%
Carbonation resistance	Low–moderate	Moderate	High	High	Moderate
Cost and availability	Very high availability	High	Moderate	Low	Moderate
CO₂-reduction potential	20–35%	20–40%	30–50%	15–25%	25–40%

Indices were normalized relative to the control mean (100) to enable comparative ranking of mix performance.

7.4 Field applications and pilot projects

  Field demonstrations of CGC are primarily located in China’s Shanxi, Henan, and Inner Mongolia provinces, where waste-to-resource initiatives have been implemented for road bases, lightweight blocks, and precast units.  Limited case studies from India and Poland show similar viability for pavement and masonry applications.  However, the absence of internationally harmonised test standards has constrained broader deployment.

7.5 Policy and Industrial Pathway

  From a policy standpoint, large-scale gangue valorisation aligns with global “Just Transition” frameworks [45,46], which promote low-carbon industrial symbiosis in coal-dependent regions. Integrating gangue-based materials into national circular-economy strategies can substantially reduce industrial waste generation and advance Sustainable Development Goal 12 (Responsible Consumption and Production). Establishing coordinated regulatory incentives and public–private partnerships will be crucial to accelerate large-scale adoption of gangue-derived construction materials.

7.6 Summary of integrated trends

Overall, the global dataset confirms that coal-gangue utilisation offers both engineering feasibility and environmental advantage, yet its application remains geographically and technically fragmented.  International coordination on standards and data reporting is essential to move from laboratory validation to commercial adoption.

8. Standardisation and Research Gaps

8.1 Lack of codified testing standards

  Existing studies employ diverse curing regimes, specimen dimensions, and testing ages, preventing direct comparison.  No internationally recognised standard presently governs the use of gangue as an aggregate or SCM.  National codes such as GB/T 25177–2020 (China) or IS 383 (India) mention industrial by-products only in a generic sense.  Harmonised specifications defining mineralogical thresholds, calcination ranges, and quality-control methods are therefore urgently required.

8.2 Inconsistent characterisation protocols

  Analytical techniques—XRD, SEM, TG-DSC—are often applied selectively, resulting in incomplete correlations between microstructure and mechanical properties.  Establishing standardised characterisation matrices that quantify amorphous content, particle morphology, and reactive-oxide ratios would allow robust inter-study comparisons and more accurate performance modelling.

8.3 Data gaps and regional imbalance

  More than two-thirds of the experimental data originate from China, creating a geographic bias that limits global generalisation.  Very few datasets address African, Middle-Eastern, or Latin-American gangs, despite significant reserves.  Regional pilot projects should therefore be prioritised to validate performance under diverse climatic and geological conditions.

8.4 Limited durability and long-term datasets

  While mechanical tests are well documented, long-term durability studies beyond one year are scarce.  Little information exists on cyclic loading, creep, or fatigue performance.  Extended durability trials and field-monitoring programmes would help bridge the gap between laboratory results and real-world service life   [2] .

8.5 Microstructure–durability integration

  Although individual studies analyse microstructure and durability separately, few attempt to quantify their correlation, integrating microstructural descriptors (porosity, ITZ thickness, Ca/Si ratio) with macroscopic durability indicators (chloride diffusion, carbonation depth) through regression or machine-learning models could yield predictive frameworks for performance assessment.

8.6 Research Gap Summary

Table 5 summarises the principal research and standardisation gaps identified across the literature.

Table 5. Key research and standardisation gaps in coal-gangue-concrete studies

Thematic area	Identified gap	Recommended action
Standards	Absence of dedicated gangue-concrete code	Develop unified test and acceptance criteria.
Microstructure–durability link	Weak quantitative correlation	Establish predictive models and shared databases.
Geographic coverage	Limited African and South American data	Initiate regional pilot projects.
Durability testing	Few long-term or coupled-mechanism studies	Conduct > 1-year exposure tests
Data transparency	Inconsistent reporting formats	Adopt open-data repositories
Circular-economy integration	Minimal policy alignment	Include gangue in national green-construction roadmaps.

  Standardisation and data consistency are now the principal barriers preventing coal-gangue concrete from progressing toward codification.  Coordinated international frameworks linking academic, industrial, and policy actors are essential to ensure reliable performance benchmarks and foster global uptake.

9. Framework Proposal and Implementation Roadmap

9.1 Four-layer evaluation framework

  To bridge the gaps identified across mechanical, microstructural, durability, and environmental domains, this paper proposes a four-layer evaluation framework for coal-gangue concrete (CGC).

  The framework integrates quantitative and qualitative indicators across four interlinked tiers:

1. Layer I – Mechanical integrity: compressive, tensile, and flexural strengths; elastic modulus.

2. Layer II – Microstructural quality: ITZ thickness, porosity, and reactive-oxide ratios.

3. Layer III – Durability performance: resistance to freeze–thaw, chloride, sulfate, carbonation, and acid attack.

4. Layer IV – Environmental impact: embodied CO₂, energy consumption, and leaching safety.

Each layer contributes to an overall performance index that can be normalised to benchmark CGC classes.

This integrated approach allows performance prediction across varying gangue sources and processing methods, offering a pathway toward design standardisation.

9.2 Benchmark classification of CGC mixes

Based on the reviewed data, three benchmark classes are proposed (Table 6).

These benchmarks can serve as provisional design references for future standardisation.

Table 6. Proposed benchmark classes for coal-gangue concrete

CGC Class	Typical gangue type	Replacement ratio	Mean 28-day strength (MPa)	Durability level	Recommended applications
Type I	Untreated aggregate	≤ 25%	35–40	Moderate	Non-structural blocks, pavements
Type II	Calcined SCM	10–15%	42–48	Good	Structural concrete, precast elements
Type III	Hybrid (aggregate + SCM)	20 + 10%	40–45	Good	Road base, CFST infill

Example: A Type II mix (20 % calcined gangue + 10 % fly ash) scores 3.5 for mechanical performance, 3.0 for durability, 4.0 for microstructure, and 3.8 for LCA efficiency, yielding an overall composite score of 3.6 (≈ Type II category)

9.3 Implementation roadmap

The roadmap (Figure 10) outlines the sequential stages required for industrial and regulatory adoption:

1. Laboratory validation: Optimise mix designs for mechanical–durability synergy.

2. Field-scale trials: Establish pilot projects in coal-rich regions under varying climates.

3. Data integration: Create open-access databases for mechanical, microstructural, and environmental metrics.

4. Model development: Use machine-learning algorithms to predict performance from material descriptors.

5. Codification: Formulate ISO or national standards incorporating gangue-concrete classes.

6. Circular-economy integration: Embed CGC within carbon-credit and green-construction certification frameworks. Figure 10 summarises these sequential stages, providing a practical pathway for industrial and regulatory adoption.

Figure 10. Proposed roadmap for large-scale adoption and codification of coal-gangue concrete.

9.4 Alignment with global sustainability targets

  Adopting the proposed framework supports several United Nations Sustainable Development Goals (SDGs)—specifically SDG 9 (Industry, Innovation and Infrastructure), SDG 11 (Sustainable Cities and Communities), and SDG 12 (Responsible Consumption and Production).

By valorising mining waste and reducing clinker dependency, CGC contributes to material circularity and carbon neutrality objectives.

10. Future Research Directions

10.1 Integration of digital and AI-based modelling

  Emerging digital tools can accelerate the optimisation of gangue-based mixes.

Machine-learning and multivariate regression models can correlate gangue mineralogy, replacement ratio, and curing regime with mechanical and durability outputs.

  Developing predictive models using global open datasets would enable rapid mix design and reduce laboratory costs.

10.2 Multi-scale and coupled performance modelling

  Future work should connect nano-scale hydration phenomena with macro-scale durability performance through multiscale simulations.

  Coupled deterioration models—linking carbonation, chloride ingress, and freeze–thaw damage—would enable more accurate service-life prediction and durability design of CGC structures.

10.3 Field validation and life-cycle benchmarking

  Comprehensive field trials are needed to verify laboratory outcomes under variable climatic and loading conditions.

Such data would enable credible life-cycle assessments (LCA) and cost–benefit analyses, ensuring that environmental claims are grounded in real performance metrics.

10.4 International collaboration and data-sharing

  Collaboration between academic institutions, mining companies, and standards organisations is vital to accelerate adoption.

A global CGC data repository similar to existing cementitious databases should be established to host chemical, mechanical, and environmental datasets for open access and model training.

10.5 Geographic Bias and Applicability

  Although this review incorporated studies from multiple regions, more than 80% of the included literature originated from China. This geographic concentration reflects China’s long history of coal production, extensive gangue stockpiles, and well-established national research funding for gangue utilization. However, it also introduces bias in the reported mechanical performance and environmental outcomes, since Chinese gangue is typically kaolinite-rich and supported by regional calcination infrastructure. Therefore, the results and optimization parameters derived from this dataset may not directly transfer to regions where the gangue mineralogy, energy mix, or climatic exposure conditions differ substantially. Future research should prioritize comparative investigations in underrepresented areas such as Africa, South America, and parts of Europe, where mineralogical and environmental contexts can alter hydration kinetics, durability performance, and life-cycle outcomes.

10.6 Durability Limitations and Future Research Needs

  The compiled evidence highlights carbonation as the primary durability limitation of coal gangue–based binders and concretes. Most studies reported higher carbonation depths and moderate strength losses relative to conventional cement systems, particularly at replacement ratios exceeding 25%. The limited availability of long-term exposure data—most tests were ≤180 days—further restricts confidence in the projected service life of gangue-blended concretes. Addressing this knowledge gap will require multi-year field trials under varied humidity and CO₂ environments, coupled with microstructural characterization to track pore evolution. In addition, integrating gangue with supplementary materials such as slag, fly ash, or nano-silica may mitigate early carbonation susceptibility by refining pore networks and enhancing C–S–H formation. Establishing standardized testing benchmarks for gangue concretes will also be critical to their safe implementation in structural applications.

  Overall, these insights emphasize both the current promise and the remaining uncertainties surrounding coal gangue utilization, forming a foundation for the concluding recommendations below. Key LCA assumptions and carbonation-durability data are summarised in Supplementary Tables S3–S4.

11. Conclusion and Practical Implications

  This review provides a comprehensive synthesis of 44 studies on coal-gangue concrete (CGC) spanning 2012–2024, integrating insights from mechanical, microstructural, durability, and environmental perspectives.

Key conclusions are summarised as follows:

1. Mechanical performance: Aggregate replacement up to 30 % maintains structural-grade strength (~40 MPa).  Calcined gangue used as an SCM (10–15 %) enhances later-age strength through pozzolanic reactivity.

2. Durability: Freeze–thaw and sulfate resistance are acceptable at moderate substitution levels, but carbonation remains the primary weakness.

3. Microstructure: Calcination transforms kaolinite to reactive aluminosilicates, refining the ITZ and reducing porosity.

4. Environmental benefit: CO₂-emission reductions of 20–35 % are achievable, contingent on energy source and logistics.

5. Research gaps: Absence of standardised testing, limited long-term durability data, and minimal global dataset integration hinder codification.

6. Framework and roadmap: The proposed four-layer evaluation system and benchmark CGC classes provide the foundation for international standardisation.

Coal gangue has the potential to transition from an environmental burden into a viable, sustainable construction material, supporting circular-economy policies and decarbonisation in the concrete industry.

Acknowledgments

The authors would like to acknowledge the National Natural Science Foundation of

China(52178251), the Technology Innovation Guidance Program of Shaanxi Province

(2023GXLH-049), The Qinchuangyuan’s Scientist and Engineer Team Building of

Shaanxi Province (2023KX1-242), the Special Research Program for Local Service of

Shaanxi Province (23JC047), the Youth Innovation Team of Shaanxi

Universities(2023-2026)for financial support.Data availability

Data will be made available on request.

Conflict of Interest declaration

The authors declare that they have no affiliations with or involvement in any organization or entity with any financial interest in the subject matter or materials discussed in this manuscript.

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