Recycling and Mineral Evolution of Multi-Industrial Solid Waste in Green and Low-Carbon Cement: A Review

Abstract

The accelerated industrialization in China has precipitated a dramatic surge in solid waste generation, causing severe land resource depletion and posing substantial environmental contamination risks. Simultaneously, the cement industry has become characterized by the intensive consumption of natural resources and high carbon emissions. This review aims to investigate the current technological advances in utilizing industrial solid waste for cement production, with a focus on promoting resource recycling, phase transformations during hydration, and environmental management. The feasibility of incorporating coal-based solid waste, metallurgical slags, tailings, industrial byproduct gypsum, and municipal solid waste incineration into active mixed material for cement is discussed. This waste is utilized by replacing conventional raw materials or serving as active mixed material due to their content of oxygenated salt minerals and oxide minerals. The results indicate that the formation of hydration products can be increased, the mechanical strength of cement can be improved, and a notable reduction in CO₂ emissions can be achieved through the appropriate selection and proportioning of mineral components in industrial solid waste. Further research is recommended to explore the synergistic effects of multi-waste combinations and to develop economically efficient pretreatment methods, with an emphasis on balancing the strength, durability, and environmental performance of cement. This study provides practical insights into the environmentally friendly and efficient recycling of industrial solid waste and supports the realization of carbon peak and carbon neutrality goals.

1. Introduction

A substantial amount of industrial solid waste has been generated with the rapid industrial development in China. Industrial solid waste refers to solid materials produced during industrial activities, including various slags, dusts, and other residues discharged into the environment. These waste are primarily composed of minerals such as quartz, calcite, and hematite. According to statistics, the total volume of industrial solid waste generated in China reached 4.37 billion tons in 2023, with an accumulated stock exceeding 60 billion tons, occupying over 2 million hectares of land. The overall utilization rate of general industrial solid waste stood at 60.05% [1]. Traditionally, disposal methods such as landfilling, stockpiling, and incineration have been adopted, which suffer from low recovery efficiency of valuable elements and significant land consumption. Poor management of toxic elements allows them to spread via surface runoff, groundwater seepage, airborne dust, and atmospheric deposition, contaminating nearby water bodies, vegetation, and soil. Over time, these pollutants accumulate in the food chain, ultimately posing a threat to human health. The “Zero-Waste City” initiative prioritizes the reduction at source, efficient resource utilization, and safe disposal of bulk industrial solid waste [2]. In this context, advancing the high-volume and high-value resource utilization of industrial solid waste through technological innovation has become a critical pathway to address the tension between environmental constraints and sustainable industrial development.

As the world’s largest producer and consumer of cement, China considers cement one of the most essential basic construction materials, playing a strategically vital role in national economic growth and social development. However, the cement industry has long been prioritized in environmental monitoring due to its characteristics of high emissions, high pollution, and high energy consumption during production. China’s cement output reached approximately 1.8 billion tons, and CO₂ emissions surpassed 1.2 billion tons in 2024. This accounts for over 20% of total industrial carbon emissions nationwide and more than 70% of the emissions from the building materials sector [3]. The Ministry of Industry and Information Technology, together with other government agencies, jointly issued the Implementation Plan for Carbon Peaking in the Building Materials Industry. The plan explicitly identifies the decarbonization transformation of the cement sector as a critical component in achieving the carbon peak and carbon neutrality goals across the entire building materials industry and even within China’s broader industrial landscape [4]. Therefore, seeking alternative raw materials that are abundant, clean, efficient, and economically viable becomes essential, along with advancing technological innovation in cement production and deeply embedding the concept of ecological civilization throughout the entire process of industrial development to achieve a green transition.

Industrial solid waste, while meeting the aforementioned requirements, is also rich in reactive components such as silicon, aluminum, and calcium, which are highly compatible with the mineral phases of cement. Effective cementitious phases can theoretically be produced by appropriately adjusting the proportions of raw materials based on the chemical composition of the solid waste and subjecting them to calcination. Numerous successful experiments have demonstrated that cements prepared from industrial solid waste such as fly ash, red mud, and coal gangue can meet relevant performance requirements and have already found widespread application. The synergistic use of these waste with cement helps reduce the over-reliance on natural resources in traditional cement production while enabling value-added recycling of solid waste, creating new opportunities for the sustainable development of industries like steelmaking and metallurgy. At present, some progress has been made in studies on the application of coal-based solid waste [5,6,7,8,9], metallurgical slag [10,11,12,13,14], tailings [15,16], industrial byproduct gypsum [17,18], and municipal solid waste incineration [19,20] in cement production. There has been no review on the preparation of cement from various industrial solid waste, particularly concerning the durability and environmental performance of cement products derived from such waste, as well as summarized insights and future development directions.

A comprehensive review of recent research progress on the utilization of industrial solid waste in cement production is provided in this work. It begins with an overview of the classification and current emission status of industrial solid waste, followed by a detailed analysis and comparative evaluation of recent studies on typical types of industrial solid waste used for cement manufacturing, including coal-based solid waste, metallurgical slag, tailings, industrial byproduct gypsum, and municipal solid waste incineration. The raw material composition and its impact on properties such as mechanical strength, hydration kinetics, microstructure, and setting time are focused on in the review. Additionally, the environmental performance of cement products derived from these waste materials is discussed. Finally, the work presents an outlook on future research directions and development trends. It aims to provide valuable references for the comprehensive utilization of industrial solid waste and the sustainable development of cement production.

2. Methodology

Sources of evidence included original research articles and review articles. Priority was given to publications from the last five years (approximately 2020–2025), with consideration for seminal works up to 10 years old to ensure a robust overview of the current panorama while also identifying research gaps and avoiding duplications. The primary information source for scientific literature was the Web of Science platform. The inclusion criteria were the following terms: (“fly ash” or “coal gangue” or “coal gasification furnace slag” or “red mud” or “steel slag” or “electrolytic manganese slag” or “iron tailings” or “phosphorus tailings” or “copper tailings” or “phosphogypsum” or “titanium gypsum” or “municipal solid waste incineration bottom ash” or “municipal solid waste incineration fly ash” and “cement”). These were searched for in the main sections of articles and combined using the Boolean operator “AND”. Each reference was systematically full-text-screened based on whether it focused on industrial solid waste, application in cement, or phase transformations during hydration. Additionally, a supplementary literature search focused on the overview of industrial solid waste and environmental performance evaluation.

3. Overview of Industrial Solid Waste

3.1. Classification of Industrial Solid Waste

Industrial solid waste can be broadly classified into two categories: general industrial solid waste and hazardous industrial solid waste. General industrial solid waste includes materials such as steel slag, blast furnace slag, fly ash, coal cinder, non-ferrous metal slag, sulfuric acid slag, desulfurization ash, waste gypsum, carbide slag, and saline mud. This category can be further divided into reactive industrial waste (e.g., blast furnace slag) and inert or low-reactivity waste (e.g., coal gangue and tailings) [5]. Hazardous industrial solid waste refers to solid materials that exhibit one or more hazardous characteristics, such as toxicity, corrosiveness, flammability, reactivity, or infectivity—examples include aluminum dross and municipal solid waste incineration fly ash [21]. Typically, general industrial solid waste is used for cement production.

3.2. Production of Industrial Solid Waste in China

In 2023, the total generation of general industrial solid waste in China reached 4.27 billion tons, with 2.58 billion tons being comprehensively utilized and 830 million tons properly disposed of. During the same period, the generation of hazardous industrial solid waste amounted to 105.465 million tons, with 105.029 million tons undergoing utilization or disposal. The top five industries in terms of general industrial solid waste generation were (1) electricity and heat production and supply, (2) ferrous metal smelting and rolling processing, (3) non-ferrous metal mining and processing, (4) coal mining and washing, and (5) ferrous metal mining. Together, these five industries accounted for a combined output of 3.30 billion tons of general industrial solid waste [22]. The emission volumes of various types of industrial solid waste in 2023 are illustrated in Figure 1.

4. Application of Coal-Based Solid Waste in Cement

Coal-based solid waste is a by-product generated during coal mining, washing, and pyrolysis processes. Numerous studies have demonstrated that fly ash, coal gangue, and coal gasification slag have been extensively utilized in cement production [23].

4.1. Application of Fly Ash in Cement

Fly ash is transported through flue gases and collected using electrostatic precipitators and filter bags during the combustion of pulverized coal in the boiler [24,25]. The chemical composition of fly ash is typically characterized by high contents of SiO₂, Al₂O₃, and CaO, while its principal mineral phases include anhydrite and calcite. The contents of SiO₂ and Al₂O₃ in fly ash are similar to those in clay, making it a viable substitute for clay in the calcination of cement clinker [26]. The residual carbon content in fly ash can also serve as an auxiliary fuel during calcination, thereby reducing overall energy consumption. Fly ash-based cement exhibits several advantages compared to clay-based cement, including reduced cracking tendency, lower drying shrinkage, lower heat of hydration, and enhanced sulfate resistance [27]. As the active mixed material in cementitious systems, fly ash reacts with the Ca(OH)₂ generated during cement hydration to form hydration products such as Ca₅Si₆O₁₆(OH)·4H₂O gel and ettringite [28]. These products fill the pore structure of the matrix, promoting strength development and improving impermeability. As shown in Figure 2, incorporating more than 20% fly ash effectively reduces the risk of plastic shrinkage-induced cracking in cement mortar (hereinafter referred to as mortar) [29]. At water-to-binder ratios of 0.4, 0.5, and 0.6, the optimal Class F fly ash replacement levels are 32.5%, 35%, and 37.5%, respectively [30]. In terms of synergistic utilization, Geetika Mishra et al. [26] found that the optimal replacement rates for biochar and high-calcium fly ash (Class C) in cement are 5% and 10%, respectively. Accelerated carbonation of the mortar, as shown in Figure 3, significantly increased the elastic modulus at both 7 and 28 days and enabled the sequestration of up to 50% of ambient CO₂ into the cement matrix. Therefore, aluminosilicate clay minerals can be replaced by fly ash in cement clinker production, with both product performance and energy efficiency being enhanced. Additionally, fly ash can be utilized as the active mixed material, resulting in improved strength, reduced cracking, and substantial carbon reduction benefits being delivered. Yet, questions remain regarding the long-term durability of these products under field exposure conditions, such as sulfate attack, freeze–thaw cycles, and carbonation.

4.2. Application of Coal Gangue in Cement

The main chemical components of coal gangue are SiO₂ and Al₂O₃, while its predominant mineral phases include kaolinite, quartz, and illite. Coal gangue differs from fly ash in both origin and physical form; it typically occurs as blocky or densely granular material, while fly ash appears as a grayish-white powder [32]. The primary chemical composition of activated coal gangue is similar to that of Portland cement, enabling its direct use as a raw material for cement manufacturing. Moreover, it possesses intrinsic pozzolanic activity, which allows the production of high-performance composite cement when used as the active mixed material [33]. Shao et al. [34] incorporated coal gangue into limestone calcined clay cement (LC³). They observed a notable increase in hydration products, a significant reduction in porosity, and a refinement of the pore structure. Figure 4 illustrates the relationship between differential intrusion volume and corresponding pore size. Further investigations by Li et al. [35] indicated that a calcination temperature of 600 °C is optimal, as it provides a broader range for kaolinite content in coal gangue. Such findings are case-specific, and comparative studies examining different gangue sources or LC³ formulations are still limited. Therefore, coal gangue can be directly calcined for cement production, with the benefit of conserving natural resources. In addition, it serves as the active mixed material that enhances the performance of composite cement.

4.3. Application of Coal Gasification Furnace Slag in Cement

The chemical composition of coal gasification slag is primarily composed of SiO₂, Al₂O₃, CaO, and Fe₂O₃, while its main mineral components include amorphous aluminosilicate glass and residual carbon [36]. A small amount of residual carbon in coal gasification furnace slag can reduce fuel consumption during the co-burning process for cement clinker production, while also supplementing the mineral composition of the cement. Coal gasification furnace slag calcined at 400 °C exhibits the highest reactivity. As shown in Figure 5, the compressive strength of mortar increases with curing time, and the 28-day strength is 4.75% higher than that of ordinary Portland cement. The micropore structure is illustrated in Figure 6, where the total porosity is reduced by 26.76% [37]. Coal gasification furnace slag acts as a nucleation site and accelerates the setting process in cement systems when applied at low dosages (10%). However, as the dosage exceeds 30%, the unreacted slag particles primarily exist in an agglomerated state within the cement matrix, resulting in reduced hydration products, a looser structure, and lower compressive strength [38]. The strength activity indices of coarse and fine coal gasification furnace slag reach 100.9% and 82.7%, respectively, after decarbonization by calcination at 600 °C, indicating their potential as active additives in cementitious materials. Figure 7 presents the microstructural evolution of cement, coarse slag, and fine slag samples at different hydration times [39]. Therefore, coal gasification furnace slag can serve both as the raw material for cement clinker production and as the active additive. It is noteworthy that the particle structure of coal gasification furnace slag often exhibits a carbon-coated ash morphology, and residual carbon can severely hinder the hydration process. Excessive incorporation of this material into cement results in diminished performance.

Overall, coal-based solid waste is indicated by studies to possess pozzolanic or cementitious properties, with mechanical strength and durability being improved by secondary hydration reactions. The incorporation of auxiliary materials such as silica fume or slag may further enhance the performance and applicability of these cementitious systems.

5. Application of Metallurgical Slag in Cement

Metallurgical slags refer to various solid waste generated during metallurgical industrial processes. Numerous studies have explored the use of red mud, steel slag, and electrolytic manganese slag in cement production.

5.1. Application of Red Mud in Cement

Red mud is primarily composed of CaO, Fe₂O₃, Al₂O₃, and SiO₂, with its major mineral phases typically including hematite, aragonite, calcite, and gibbsite [40]. As shown in

Table 1. The application of red mud in cement.

Proportioning of Raw Materials	Density, g/cm3	Specific Surface Area, cm2/kg	Setting Time, min		Compressive Strength, MPa
			Initial	Final	3 d	28 d
Red mud–Cement = 20:80 [41]	—	—	—	—	21.10	35.50
Red mud and metakaolin composite–Cement = 50:50 [42]	3.12	355	99	158	27.00	50.30
Red mud–Slag cement = 8:92 [43]	2.42	—	—	—	22.90	56.30
Red mud–Alkali-activated slag cement = 40:60 [44]	—	—	49	62	60.00	89.93
Pretreated red mud–Slag cement = 50:50 [45]	—	—	—	—	43.39	68.79

, red mud can be used both as the raw material for producing cement clinker and as the active mixed material in Portland cement. It is typically blended with other substances such as gypsum, blast furnace slag, and coal gangue to activate its cementitious potential due to its relatively low hydraulic reactivity. The optimal dosage largely depends on the alkali content in the red mud.

As a result, cement produced with red mud exhibits mechanical properties comparable to those of ordinary Portland cement. In addition, it provides enhanced performance characteristics, such as higher early strength, moderate setting time, and improved durability.

5.2. Application of Steel Slag in Cement

The main chemical components of steel slag include CaO, SiO₂, and Fe₂O₃, while the dominant mineral phases consist of free CaO, tricalcium silicate, and dicalcium silicate [46]. Cement clinker can be partially replaced by steel slag due to its CaO content being comparable to that of limestone. As shown in Figure 8, it has been reported that higher Fe₂O₃ content lowers the allowable replacement level, with an upper limit of 14.30 wt% [47], as excessive Fe in the slag can react with f-CaO to form unstable calcium ferrites, negatively affecting strength development [48]. Moreover, trace amounts of alkalis present in steel slag can lower the eutectic temperature and promote melting during clinker production [49], contributing to a reduction in CO₂ emissions from fuel combustion. Active mineral phases, such as C₃S, β-C₂S, f-CaO, and f-MgO, endow steel slag with potential as a supplementary cementitious material. However, high replacement levels adversely affect properties such as relative strength [50] and early hydration behavior [51]. These drawbacks can be mitigated through synergistic blending strategies. Tian et al. [52] reported that replacing cement with 5% steel slag powder and 5% limestone powder extended the initial and final setting times by 39% and 25%, respectively, while reducing mortar flow by only 0.7%. The compressive strength loss was minimal, recorded at 7% and 5% after 28 and 180 days, respectively, as shown in Figure 9. In terms of functional enhancement, Wang et al. [53] developed a steel slag-based high-iron calcium sulfoaluminate cementitious material. The Fe₂O₃ introduced by steel slag facilitated the transformation from C₄A₃$\stackrel{\mathrm{-}}{\mathrm{S}}$-o to C₄A₃$\stackrel{\mathrm{-}}{\mathrm{S}}$-c. As depicted in Figure 10, this system maintained ettringite formation levels comparable to those at 20 °C even under low temperatures (0–5 °C), while also reducing the proportion of deleterious pores, allowing hydration reactions to continue effectively under sub-zero conditions. Therefore, limestone can be substituted by steel slag in cement production, with carbon emissions from fuel combustion being reduced. Additionally, stability is significantly improved when utilized as the supplementary cementitious material in synergistic systems, and its potential for large-scale utilization is enhanced.

5.3. Application of Electrolytic Manganese Slag in Cement

The mineral composition of electrolytic manganese slag mainly comprises quartz, muscovite, pyrite, and rhodochrosite. Its major chemical constituents are SiO₂, Al₂O₃, and Fe₂O₃ [54]. The primary oxides present in electrolytic manganese slag are similar to those in cement, enabling its use as the raw material for cement clinker production when co-processed with clay, limestone, and aluminosilicate corrective materials. Electrolytic manganese slag also holds potential as the active mixed material due to its latent pozzolanic activity. Additionally, its inherent sulfate content can activate low-reactivity mineral admixture. Desulfurized electrolytic manganese slag, by removing excessive gypsum, overcomes prior dosage limitations. As shown in Figure 11, the optimal replacement level is determined to be 10% desulfurized electrolytic manganese slag combined with 90% cement [55]. The primary obstacle to its cementitious application is the presence of NH⁴⁺-N. Lan et al. [56] achieved over 95% immobilization of both NH⁴⁺-N and Mn²⁺ by incorporating MgO and CaHPO₄·2H₂O. Wang et al. [57] effectively controlled toxic components through a combined strategy of cement solidification and direct electro-curing. Yet, further validation is required for these approaches regarding engineering feasibility, cost, and long-term stability under field conditions. In the field of composite binders, He et al. [58] developed a ternary system consisting of active electrolytic manganese slag, slag, and cement. As illustrated in Figure 12, the blended mortar exhibited favorable flowability, compressive strength, and refined microstructure when the electrolytic manganese slag content remained below 15%. The system’s dual alkali–sulfate activation mechanism significantly enhanced the reactivity of both slag and electrolytic manganese slag, thereby promoting the generation of hydration products. Furthermore, unhydrated electrolytic manganese slag effectively filled the pore structure of the mortar matrix due to its fine particle size, contributing to improved compressive strength. Thus, electrolytic manganese slag can be utilized in cement either as a substitute for raw materials or as the active mixed material. It is essential, however, to pre-remove NH⁴⁺-N and to evaluate the long-term stability of electrolytic manganese slag-based materials.

In summary, metallurgical slags exhibit considerable potential to enhance cement performance. Nevertheless, the high alkalinity of red mud, Fe₂O₃ content restrictions in steel slag, along with excess ammonium nitrogen and sulfate in electrolytic manganese slag currently limit their large-scale utilization. Therefore, the development of cost-effective and efficient pretreatment technologies is crucial for the comprehensive utilization of metallurgical slags.

6. Application of Tailings in Cement

Tailings refer to the solid waste generated from metal or non-metal ores after valuable concentrates have been extracted through beneficiation processes. Iron tailings, phosphorus tailings, and copper tailings have already been utilized in cement production, contributing to the advancement of comprehensive tailings utilization.

6.1. Application of Iron Tailings in Cement

The chemical composition of iron tailings is characterized by high contents of SiO₂, Al₂O₃, and Fe₂O₃, while the main mineral phases are quartz, gibbsite, and hematite [59]. Iron tailings, with a chemical composition similar to that of clay, can serve as a silica-alumina source in cement raw mixes. Additionally, their Fe₂O₃ content makes them suitable as an iron corrective material, compensating for the low iron content typically present in clay. According to Liu et al. [60], a synergistic effect exists between ultrafine iron tailings powder and slag powder in supersulfated cement systems. As shown in Figure 13, the mortar exhibits compressive strength increases of 20.4% at 3 days and 44.1% at 28 days when 40% of the cement is replaced with iron tailings powder. Microstructural analysis revealed that the incorporation of iron tailings increases the amount of hydration products in the supersulfated cement matrix. In addition, it modifies the ratio between Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O and CaO·Al₂O₃·SiO₂·H₂O, which contributes to a denser hardened structure. In contrast, Zhang et al. [61] further demonstrated that substituting 28% LC³ with iron tailings yields a compressive strength of up to 42 MPa at 28 days. Additionally, a life cycle assessment revealed significant environmental benefits, including reductions of 43.6% in global warming potential, 37.2% in energy consumption, and 35.5% in ozone depletion potential. Therefore, clay can be effectively replaced by iron tailings as a source of Si, Al, and Fe in cement clinker production. They also serve as high-strength supplementary cementitious materials with enhanced environmental performance when combined with multi-waste synergy technologies.

6.2. Application of Phosphorus Tailings in Cement

The main mineral components of phosphorus tailings are dolomite, apatite, and a little quartz. Its main chemical components are CaO, MgO, and P₂O₅ [62]. Phosphorus tailings can be directly incorporated into cement raw meal as a partial replacement for limestone. The phosphorus content can also function as a mineralizer when appropriately controlled. As shown in Figure 14, replacing 30% of cement with untreated phosphorus tailings results in approximately a 25.0% reduction in flexural strength across all tested curing ages. Correspondingly, compressive strength decreases by 34.7%, 38.9%, and 40.7% at 3, 28, and 90 days, respectively [63]. In contrast, the findings of Liu et al. [62] demonstrated that the cement system exhibited enhanced hydration activity and improved microstructure, which is typically associated with better strength development, when the phosphorus tailings were finely ground, as illustrated in Figure 15. This contrast suggests that mechanical performance is highly sensitive to particle fineness and processing conditions. Therefore, the appropriate addition of phosphorus tailings can positively influence cement clinker mineral formation and cement strength development. Careful monitoring and adjustment of sulfur, magnesium, and other impurity levels are essential to ensure the stability of cement mineral composition and performance.

6.3. Application of Copper Tailings in Cement

The chemical composition of copper tailings is mainly composed of SiO₂, Al₂O₃, and Fe₂O₃, while the dominant mineral phases include andradite and dolomite [64]. Sandstone and clay can be replaced by copper tailings for the production of Portland cement. Heavy metals contained in the tailings can replace Ca²⁺ in Ca₂SiO₄ and Ca₃SiO₅, thereby increasing the reactivity of Ca₂SiO₄ and promoting its hydration, which ultimately enhances the long-term hydration strength of cement clinker [65]. Pei et al. [66] prepared a low-calcium Portland cement clinker using copper tailings as the siliceous raw material. The results showed that CaSiO₃ was the dominant phase when the Ca/Si ratio was less than or equal to 1.0, whereas a composite Ca₂SiO₄-Ca₃SiO₅ system formed when the ratio ranged between 1.0 and 1.4. Moreover, the Ca₃SiO₅ content increased with the rising calcination temperature. As illustrated in Figure 16, the compressive strength of the carbonated sample exceeded 100 MPa at 1200 °C and a Ca/Si ratio of 1.4. Cheng et al. [67] used copper tailings at 20.31 wt% as cement raw meal and 15 wt% as the active mixed material to produce Portland cement that met the Chinese national standard for P·O42.5-grade cement. An annual economic benefit of USD 293.00 million and a reduction of 44,000 tons of CO₂ emissions per year can be achieved. Both studies diverge in terms of the optimal incorporation ratio and performance indicators. Pei et al. focused on high-temperature strength development in synthetic clinker systems, whereas Cheng et al. validated industrial applicability at moderate replacement levels. Therefore, copper tailings can serve multiple functions in cement production, such as acting as a mineralizer or iron raw material, significantly contributing to reduced calcination energy consumption and enhanced late-stage strength. The utilization of tailings as the supplementary cementitious material enables higher incorporation levels, contributing to both environmental sustainability and economic efficiency. In addition, further efforts should be directed toward exploring the synergistic utilization of copper tailings with other industrial solid waste in cementitious systems.

In summary, the silicon, aluminum, and iron oxides present in tailings provide the essential chemical foundation for cement production. Tailings can meet current cement standards, regardless of whether they are used as raw materials for cement clinkers or as active mixed material. They can be transformed into high-quality supplementary cementitious materials with a high substitution rate (≥50%) through synergistic utilization strategies.

7. Application of Industrial Byproduct Gypsum in Cement

Industrial byproduct gypsum refers to the secondary products or waste residues containing calcium sulfate as the main component, generated during various chemical reactions in industrial processes. Among them, phosphogypsum and titanium gypsum show the greatest potential for large-scale utilization in cementitious materials.

7.1. Application of Phosphogypsum in Cement

The main components of phosphogypsum are calcium sulfate dihydrate (CaSO₄·2H₂O) and SiO₂, which together account for more than 90% of its total composition [68]. As summarized in

Table 2. Reported phosphogypsum incorporation rates in various types of cement.

Proportioning of Raw Materials	Compressive Strength MPa
	3 d	28 d
2.5% of washed and filtered phosphogypsum in ordinary Portland cement [69]	20.30	53.60
15% of calcined phosphogypsum in supersulfated cement [70]	87.60	99.30
72% of phosphogypsum in calcium sulfoaluminate cement [71]	86.70	112.60
10% of thermally modified phosphogypsum in calcium sulfoaluminate cement [72]	46.50	88.40
20% of phosphogypsum in calcium sulfoaluminate cement [73]	28.10	41.59
73% of phosphogypsum in cement co-produced with sulfuric acid [74]	13.00	28.95
4% of phosphogypsum in eco-cement [75]	—	45.80

, phosphogypsum has already been used as the raw material in various types of cement, including ordinary Portland cement [69], supersulfated cement [70], calcium sulfoaluminate cement [71,72,73], cement co-produced with sulfuric acid [74], and eco-cement [75]. In terms of synergistic utilization, Wei et al. prepared high-sulfur cementitious materials by synergistically utilizing phosphogypsum, red mud, and blast furnace slag. The results showed that the soluble phosphorus in phosphogypsum was completely stabilized when the red mud content reached 20%, significantly alleviating the setting delay and thereby improving the 3-day compressive strength [76]. Wang et al. found that the mortar specimens reached a 14-day compressive strength of 19.0 MPa and a flexural strength of 4.3 MPa under a water–binder ratio of 0.33, when mixed with 46.9% phosphogypsum, 26.5% red mud, 4.1% fly ash, 12.2% quicklime, and 10.2% cement [77]. Furthermore, as shown in Figure 17, the feasibility of producing iron-rich cement clinker using steel slag, iron slag, and phosphogypsum has been demonstrated through kiln trials at the industrial scale [78].

Phosphogypsum can serve as a versatile raw material for multifunctional cement production. In comparison, the use of phosphogypsum in calcium sulfoaluminate cement production presents broader application prospects. It can be incorporated as a component of cement raw meal, as a post-blending gypsum source, or as a calcium raw material, allowing for significantly increased addition rates.

7.2. Application of Titanium Gypsum in Cement

The main chemical components of titanium gypsum are SO₃, CaO, and Fe₂O₃. Its primary mineral phases are gypsum dihydrate and hemihydrate gypsum [79]. In the red mud–titanium gypsum–blast furnace slag binder system, both compressive and flexural strengths increase with the addition of 6%–26% titanium gypsum, as shown in Figure 18 [80]. As illustrated in Figure 19, titanium gypsum effectively shortens the setting time of fly ash-carbide slag composite binders, improves compressive strength, optimizes pore structure, and enhances the overall performance of the material when used as an additive [81]. These results highlight the promising role of titanium gypsum in composite binders. However, studies on titanium gypsum are relatively scarce. Efficient impurity removal methods should be developed in future research to overcome the potential negative impacts on cement setting behavior, mechanical strength evolution, and long-term durability.

In summary, industrial byproduct gypsums have been widely applied in cement systems. Their utilization is expanding from single-component cements toward multi-source composite binder systems. The incorporation of modifying agents is currently being primarily investigated for performance enhancement. Further investigations are required to elucidate the hydration mechanisms and to understand the effects of impurities on cementitious behavior.

8. Application of Municipal Solid Waste Incineration in Cement

Incinerating one ton of municipal solid waste typically generates 200–250 kg of bottom ash and 10–30 kg of fly ash in municipal solid waste incineration power plants [82]. Numerous studies have explored the incorporation of these ashes into cement production to alleviate environmental pressures.

8.1. Application of Municipal Solid Waste Incineration Bottom Ash in Cement

The chemical composition of bottom ash contains CaO, SiO₂, Al₂O₃, and Fe₂O₃. The main mineral components are quartz and calcite [83,84]. As the active mixed material, bottom ash benefits from high-temperature pre-curing, which accelerates early hydration and improves compressive strength. As shown in Figure 20, this treatment promotes the transformation of ettringite into CaSO₄·2H₂O, and the conversion of hydration products into loose, porous, flocculent, and reticular Ca₅Si₆O₁₆(OH)·4H₂O gels. However, the uneven accumulation of hydration products in the early stage hinders strength development at later stages. In contrast, as shown in Figure 21, curing at lower temperatures (20 °C) results in the formation of more crystalline, needle-like ettringite and a reticulated Ca₅Si₆O₁₆(OH)·4H₂O gel with the denser, more homogeneous matrix [85]. A trade-off between early strength gains and long-term performance is indicated by these contrasting findings, depending on the curing temperature. Natural sand in mortar can also be partially replaced by bottom ash. As illustrated in Figure 22, early compressive strength increases with higher substitution levels, mainly due to the pozzolanic reactivity of pretreated bottom ash. It reacts with Ca(OH)₂ in hydration products to form compounds such as 3CaO·Al₂O₃·CaCl₂·10H₂O and CaO·Al₂O₃·SiO₂·H₂O [86]. In terms of supplementary cementitious materials, Cheng et al. [87] investigated the effect of bottom ash on the hydration and carbonation behavior of reactive magnesia cement. As shown in Figure 23, the results suggest an optimal replacement rate of 5%, which promotes early hydration and compressive strength development while minimizing the risk of microcracking and durability degradation. Excessive bottom ash leads to the formation of layered double hydroxides, which inhibit further hydration. Therefore, bottom ash serves multiple beneficial roles in cementitious systems: as the active mixed material, its hydration kinetics and strength development can be improved by controlling pre-curing temperature; as a sand replacement, it significantly enhances early strength; and as the supplementary cementitious material in reactive magnesia cement, low replacement levels yield optimal performance. Furthermore, the influence of bottom ash impurities (e.g., chlorides, heavy metals) on setting behavior and microstructure evolution remains insufficiently addressed.

8.2. Application of Municipal Solid Waste Incineration Fly Ash in Cement

Fly ash contains high content of SiO₂, CaO, and Al₂O₃ [88]. The chemical composition of fly ash closely matches that of cement raw materials, making it a potential substitute for cement clinker [89]. However, its presence tends to inhibit the formation of hydration products such as 3CaO-SiO₂ and 3CaO-Al₂O₃, thereby delaying early strength development [90]. In calcium sulfoaluminate cement systems, fly ash—along with flue gas desulfurization gypsum and aluminum ash—can serve as a major raw material. Notably, fly ash contributes to the formation of Ca₄Al₆O₁₂(SO)₄ by providing a high content of essential elements, with replacement levels reaching up to 35%. As shown in Figure 24, over 90% of the chloride salts can be removed from fly ash via two-stage water washing, and the leaching concentrations of heavy metals remain well below national regulatory limits. Compared with Portland cement containing fly ash, the calcium sulfoaluminate cement system requires less limestone and a lower sintering temperature (1200 °C) [91]. Wang et al. developed a quaternary cementitious material composed of carbide slag, red mud, fly ash, and municipal solid waste incineration fly ash. The main hydration products included CaSiO₃·nH₂O gel, calcium chloroaluminate hydrate, and ettringite, which filled the pore spaces of the matrix and thus enhanced the strength of the cementitious material [92]. The most environmentally sustainable approach to fly ash utilization is the production of eco-friendly cement. Hu et al. [93] developed a low-carbon cement composed of CO₂-activated incineration fly ash (CIFA), calcined clay, and Portland cement. As shown in Figure 25, CIFA retarded the hydration of Ca₃SiO₅ and calcium aluminates, with compressive strength reaching 18.0 MPa at 3 days and 47.6 MPa at 56 days. Moreover, toxic metals such as Cd and Pb were immobilized as insoluble, inert compounds. Therefore, the stabilization of pollutants can be achieved by utilizing the high-temperature environment of the cement kiln and the mineral structure of cement clinker to immobilize heavy metals, along with the pretreatment of fly ash to reduce chloride interference. Applications include direct substitution of raw materials and the development of multicomponent blended systems, which offer significant environmental and performance benefits.

In summary, fly ash is rich in reactive silica and alumina phases that can promote cement hydration, while bottom ash typically contains lower levels of heavy metals, ensuring better environmental safety. Their combined use is more conducive to large-scale utilization, provided that material compatibility is optimized and long-term environmental safety is comprehensively evaluated.

9. Environmental Performance Evaluation of Cement Prepared from Industrial Solid Waste

Heavy metal ions present in industrial solid waste are a major source of environmental pollution. Therefore, the environmental performance of cement produced using such waste must be carefully evaluated. Existing studies assessing the environmental impact of industrial solid waste-based cement have identified the high concentrations of heavy metals as the primary obstacle to its large-scale application. These toxic elements are typically immobilized through incorporation into cement hydration products. The dominant hydration phases include ettringite and Ca₅Si₆O₁₆(OH)·4H₂O gels. In the case of ettringite, cation exchange reactions occur wherein heavy metal ions are substituted into the crystal lattice, forming a chemically stable structure. Meanwhile, Ca₅Si₆O₁₆(OH)·4H₂O gels exhibit high specific surface area and ion exchange capacity, enabling the immobilization of heavy metals through mechanisms such as adsorption and ion substitution. These processes lead to the formation of stable compounds. Consequently, cement produced from industrial solid waste can meet environmental regulatory standards after hydration-induced stabilization of heavy metals [5].

The total environmental impact value of producing 1 ton of cement entirely from industrial solid waste is –0.0045 pt. The main environmental benefits arise from raw material acquisition (75.8%) and mixing/pre-treatment (24.0%), while clinker calcination (71.3%) and grinding (13.7%) are the primary sources of environmental damage. In comparison with conventional cement production, the use of carbon-containing materials is reduced through raw material substitution in this process, thereby decreasing energy and resource consumption as well as pollutant emissions. Carbon emissions are reduced by 60%–80%, making the process environmentally beneficial overall [94].

The performance differences between single-waste and multi-waste cementitious systems are shown in

Table 3. Performance differences between single-waste and multi-waste cementitious systems.

Multiple Solid Waste Systems	Performance Difference Compared with Single Solid Waste
5% biochar + 10% fly ash + cement [26]	Flexural strength and elastic modulus improve significantly with the addition of biochar.
25% steel slag powder + 15% limestone powder + cement [52]	The activity index is higher.
15% electrolytic manganese slag + 15% slag + cement [58]	The compressive strength increased by 83% compared with the use of electrolytic manganese slag alone.
40% slag powder + 40% iron tailings powder + cement [60]	The incorporation of iron tailings resulted in a 20.4% and 44.1% increase in compressive strength at 3 and 28 days, respectively.
20% red mud + phosphogypsum + blast furnace slag + cement [76]	The soluble phosphorus is transformed into inert material and is completely stabilized when red mud dosing reaches 20%. The slow setting is significantly improved, resulting in an increase in 3-day compressive strength.
46.9% phosphogypsum + 26.5% red mud + 4.1% fly ash + 12.2% quicklime + 10.2% cement [77]	Under the alkaline stimulation of materials such as red mud and quicklime, silicon ions and aluminum ions in industrial solid waste such as fly ash are activated to exhibit high activity, which can react with OH− and Ca2+ in the system to form Ca5Si6O16(OH)·4H2O and CaSiO3·nH2O gels. Some active aluminumions can react with OH−, SO42− and Ca2+ to form 3CaO·Al2O3·3CaSO4·32H2O.
12.5% titanium gypsum + fly ash + calcium carbide residue [81]	The setting time of fly ash–carbide slag composite binders is effectively shortened, compressive strength is improved, pore structure is optimized, and the overall performance of the material is enhanced when titanium gypsum is used as an additive.

. A summary of the utilization of industrial solid wastes in cementitious systems is presented in

Table 4. Summary of the utilization of industrial solid waste in cementitious systems.

Classification	Industrial Solid Waste	Main Chemical Composition	Typical Substitution Level	Compressive Strength (28 d)	Durability	Environmental Benefits	Potential Risks and Recommended Treatments	Reference
Aluminosilicate-rich	Fly ash	SiO2, Al2O3, and CaO	25%	41.0 MPa	Enhances resistance to chloride penetration and carbonation, and reduces water permeability and capillary sorptivity	Reduces emissions, recovers resources, sequesters carbon dioxide, and reduces cement consumption	Trace heavy metals; control dosage and monitor leaching	[28]
	Coal gangue	SiO2 and Al2O3	30%	42.3 MPa	Improves with activation	Utilizes mining waste; reduces land pressure and carbon footprint	Low activity; requires calcination or alkali activation	[34]
	Coal gasification furnace slag	SiO2, Al2O3, CaO, and Fe2O3	30%	54.0 MPa	Reduces porosity; improves impermeability	Reduces energy and cement demand	Contains unburned carbon; requires decarbonization and ball milling	[37]
	Electrolytic manganese slag	SiO2, Al2O3, and Fe2O3	10%	38.0 MPa	Acceptable after sulfate removal and activation	Utilizes Mn-rich industrial waste; cuts cement usage	Contains sulfates and Mn2+; requires detoxification and stabilization	[55]
	Iron tailings	SiO2, Al2O3, and Fe2O3	28%	42.0 MPa	Acceptable after fine grinding; dense matrix structure	Reduces raw material consumption; utilizes mining tailings	Heavy metals and sulfides; leaching test and pretreatment required	[61]
	Copper tailings	SiO2, Al2O3, and Fe2O3	15%	43.4 MPa	Reduces porosity and densifies structure	Reuses mining waste; reduces land use and CO2 emissions	Contains Cu, Fe; must stabilize to avoid rebar corrosion and leaching	[67]
Calcium-rich	Red mud	CaO, Fe2O3, Al2O3, and SiO2	20%	35.5 MPa	Low permeability; high alkalinity aids in chloride resistance	Solidify highly alkaline waste residue to reduce red mud storage and pollution	Strong alkalinity and heavy metals; must be neutralized	[41]
	Steel slag	CaO, SiO2, and Fe2O3	8%	60.3 MPa	Enhances sulfate and freeze-thaw resistance	Saves energy and generates economic benefits	High alkalinity and expansion risk; needs fine grinding and stabilization	[47]
	Municipal solid waste incineration bottom ash	CaO, SiO2, Al2O3 and Fe2O3	25%	43.0 MPa	Good frost/erosion resistance after Al removal	Reduces landfill pressure; saves cement and CO2	Al causes expansion; must remove metallic Al	[86]
Phosphorus-/Sulfur-rich	Phosphorus tailings	CaO, MgO, and P2O5	30%	31.0 MPa	Acceptable if stabilized; raw use cause expansion from sulfate salts	Enables phosphorus recovery; reduces hazardous tailing dumps	Radioactivity and soluble salts; must be stabilized and monitored	[63]
	Phosphogypsum	(CaSO4·2H2O) and SiO2	20%	41.6 MPa	Acceptable after washing or calcination	Replaces natural gypsum; utilizes P-industry by-product	Contains P, F, radioactive elements; must be treated and limited	[73]
	Titanium gypsum	SO3, CaO, and Fe2O3	20%	57.0 MPa	Reduces porosity and improve pore distribution	Reduces gypsum extraction	High impurities; requires desulfurization or calcination	[80]
Heavy-metal-/salt-rich	Municipal solid waste incineration fly ash	SiO2, CaO and Al2O3	15%	44.0 MPa	High-temp roasting improves resistance and reduces toxicity	Enables safe reuse of highly toxic incineration ash	Contains heavy metals, persistent organic pollutants; must be incinerated or stabilized	[93]

.

10. Conclusions and Prospects

The comprehensive utilization rate of industrial solid waste in China remains relatively low, highlighting the urgent need to accelerate the development of resource utilization technologies and their industrial-scale application. In recent years, extensive studies have been conducted by scholars both domestically and internationally on the use of industrial solid waste—such as coal-based solid waste, metallurgical slags, tailings, industrial byproduct gypsum, and municipal solid waste incineration—in cement production, yielding substantial research outcomes. Some of these findings have already been successfully implemented in industrial practice. This industrial solid waste is rich in silicate, aluminate, and ferrite components, which can serve as substitutes for cement clinkers and be directly incorporated into the cement system as active mineral admixtures. Their use broadens the spectrum of available raw materials and promotes the transformation of the entire industry toward environmental sustainability and low-carbon development, without compromising and even enhancing cement performance. To further advance the resource utilization of industrial solid waste and improve their overall utilization efficiency, additional efforts in optimization and system improvement are still required. The specific directions are as follows:

• Economical and efficient pretreatment methods. Pretreatment is typically required prior to the use of industrial solid waste in cement production, due to their complex composition and significant regional variability. However, current technologies such as high-temperature calcination and mechanical grinding are often hindered by high energy consumption and operational costs. Therefore, advancing precise activation and synergistic activation technologies based on the phase composition characteristics of solid waste should be the primary focus of future efforts. At the same time, it is essential to minimize secondary pollution and carbon emissions during the pretreatment process.
• Co-utilization of multiple types of industrial solid waste. The resource utilization of single-type solid waste is often constrained by limited reactive components, high pretreatment costs, and complex processing routes. In contrast, cements prepared with multiple types of industrial waste generally exhibit superior performance compared to those derived from a single waste source. It is thus recommended to fully leverage the characteristics of various reactive components and their synergistic effects. Accurate proportioning is critical for achieving high solid waste replacement levels while maintaining excellent cement properties.
• Mechanical strength and durability as fundamental evaluation criteria. Cement is frequently exposed to harsh service conditions—such as high temperatures, freezing environments, and rainwater erosion—that can significantly affect its long-term stability and structural safety. Therefore, cement produced using industrial solid waste must comply with national and regional standards. It is recommended to simulate actual loading conditions in engineering environments and integrate advanced materials characterization techniques to thoroughly investigate the evolution of the cement’s internal microstructure under prolonged mechanical stress.
• Environmental performance of cement. Although several studies have assessed the environmental performance of industrial waste-based cement, the potential risks associated with heavy metals and radioactive elements cannot be completely ruled out. Future research should adopt more advanced analytical techniques, expand the range of tested samples, and implement long-term, quantitative monitoring programs to ensure environmental safety.
• Policy and regulatory guidance. Detailed regulations should be established for the production, quality control, and engineering application of industrial waste-based cement products, based on comprehensive laboratory data and long-term monitoring from engineering demonstrations. Offering appropriate tax incentives and financial support to relevant enterprises is also essential. Furthermore, mechanisms for interdepartmental information sharing should be improved to enhance coordinated governance through multi-actor, multi-level, and cross-regional cooperation.