Trends and Applications of Green Binder Materials for Cemented Paste Backfill Mining in China

Abstract

The backfill binder material is the key to the cost and performance of cemented paste backfill. This study aims to understand the current situation of metal ore backfill binders, identify industry challenges, inspire research ideas, and explore development directions. Current research investigates trends and developments of backfill binders through literature review, experience summary, field research, statistical analysis, and other methods. Firstly, the main backfill binder types are summarized, including cement, metallurgical slag, thermal slag, chemical slag, and tailings binders. Secondly, the research progress regarding reactivity activation, hydration mechanism, harmful ion solidification, energy conservation, and carbon reduction is summarized. Thirdly, three industrial applications of new backfill binders are introduced and summarized. Cement is still the most common, followed by slag powder binder. The cases of steel slag binder and semi-hydrated phosphogypsum backfill have shown significant effects. Solid waste-based backfill binder materials are gradually replacing cement, which is a trend. Finally, further research is discussed, including hydration modeling and simulation, material properties under extreme environments, hardening process control, and technical standards for backfill binders. This work provides a reference and basis for promoting green and efficient paste backfill and sustainable industry development.

1. Introduction

The cemented paste backfill (CPB) technology creatively utilizes mining solid waste (waste tailings, rock, and water) to prepare a paste-like slurry backfilling for the excavated underground voids, eliminating tailings ponds and goaf; forming a high-recovery and low-dilution mining method; and providing a safe, green, and efficient solution for the industry [1]. CPB has become a prevalent practice for underground metal mining around the world.

When preparing the paste, a certain proportion of binder materials, which can account for 5 wt.% to 20 wt.%, needs to be added to meet the requirements of strength and safety. Binder materials are the main cost of CPB, accounting for 60% to 80% of the total [2]. For example, a mine with an annual production capacity of 5 million tons requires approximately 200,000 to 300,000 tons of binder material, with a cost exceeding CNY 60 million [3]. CPB binder has become a limiting factor for the economic feasibility of mining base metals such as iron in China. Tailings are becoming finer and finer owing to the advancement of mineral processing technology. As a result, the bonding of ultra-fine tailings requires more cement to achieve the design strength [4]. Some mines that adopt the downward cut-and-fill mining method require high strength at a long-term scale and high early strength within 1–2 days in order to recover adjacent stopes and improve the efficiency of the mining–filling cycle [5]. The mining industry has increasingly stringent requirements for the cost and performance of CPB binders, where traditional Portland cement can no longer meet the demand. On the other hand, cement is one of the largest carbon-emitting industries, where approximately 0.9 tons of CO₂ is released in 1 ton of cement production. China produced over 50% of the world’s total cement consumption, estimated at over 3 billion tons of CO₂ annually [6]. It is urgent to develop green cement materials to reduce greenhouse gases. Furthermore, millions of tons of solid waste are generated annually, occupying extensive lands, polluting water and solids, and threatening human health [7]. Solid industrial waste valorization in developing sustainable building materials exhibits encouraging prospects [8]. The research on developing green, low-carbon, low-cost, and efficient new backfill binder materials using bulk solid waste has become a hot topic in the current industry [9].

The development of backfill binder material began in 1969 when Australia first adopted cement for backfilling bottom pillars at the Mount Isa copper mine [10]. In order to reduce the binder costs, lead-zinc copper slag was used to partially replace cement [11]. In the early days of Canada, fly ash was used as a cement admixture in nonferrous and gold mines to reduce costs [12]. Motivated by occasional shortages and high costs of Portland cement, Glukhovsky first investigated the utilization of local raw materials, including soils and industrial wastes, as solid precursors for producing sodium silicate binders, which then were called "geopolymer" binder materials [13]. In recent decades, researchers have developed various new CPB binders using solid wastes such as slag, fly ash, and red mud, significantly reducing costs and promoting the development of CPB [14,15,16,17].

To help researchers and mining enterprises better understand the current situation of backfill binder materials in metal mines, this work reviews the current research and applications of green backfill binder material through a literature review, field research, statistical analysis, and case studies. The main content is divided into four parts. Firstly, CPB binder types are summarized and classified. Secondly, the research progress of CPB binders is reviewed. Thirdly, application cases of three binder types are introduced. Finally, the trends and further work in the research and application of CPB binders are discussed. The work aims to provide a reference and basis for promoting green and efficient paste fill and sustainable development.

2. Types of Backfill Binder Materials

The development of binder material has evolved from the earliest form of cement to various new materials. The main idea is to fully utilize multiple bulk solid wastes, which usually have potential hydraulic properties, also known as pozzolanic properties, and then prepare binders through various activation methods. According to the different originalities of pozzolanic solid wastes, CPB binders can be divided into the following types.

2.1. Portland Cement

Owing to the development of the construction industry, Portland cement is one of the earliest binder materials, and it has been widely used for hundreds of years. Jinchuan Nickel is the earliest mine in China to introduce the cemented backfill mining method [18]. It mainly uses Portland cement as the binder material, combined with coarse aggregates composed of rod-milled sand and waste rocks. The slurry concentration is 79–81 wt.%, with a cement content of 310 kg/m³ to obtain the desired strength at 3, 7, and 28 days of 1.5, 2.5, and 5 MPa, respectively. The advantages of Portland cement are its stable and reliable quality and its wide range of sources. However, research shows that when ultra-fine tailings with particles below 74 μm exceed 75 wt.%, the 28-day CPB strength is only 1.6 MPa using Portland cement at a cement-to-sand mass ratio of 1:4 [19]. Portland cement can be considered unsuitable for cementing ultra-fine tailings. In addition, in some non-metallic mines such as potash mines, special cement, such as aluminum magnesium cement, should be used to bind the brine and salt [20]. Overall, Portland cement is still the most popular CPB binder, but it will gradually be replaced by newly developed green materials.

2.2. Metallurgical Slag Binders

According to the China Statistical Yearbook 2023, the general industrial solid waste production in China was 411.4 million tons, with a comprehensive utilization of 237 million tons, accounting for only 57%. General industrial waste usually includes blast furnace slag, steel slag, red mud, nonferrous metal slag, fly ash, coal slag, etc. Metallurgical solid wastes are widely used to develop backfill binder materials.

2.2.1. Iron and Steel Industry Slag

The production of pig iron and crude steel in China accounts for more than half of the world’s total [3]. Blast furnace slag (BFS) and steel slag (SS), the most common and largest metallurgical solid wastes, have an annual output of 300 million tons. Their production process is shown in Figure 1. BFS has a glass content (CaO-SiO₂-Al₂O₃ solid solution) of over 80 wt.%, which is widely used as a supplementary cementitious material (SCM) in cement and concrete. BFS binder, which is composed of about 70 wt.% BFS powder and 30 wt.% activators (clinker, gypsum, and others) has been applied successfully. Studies show that under the same conditions, the CPB strength with BFS binder is 2–3 times that of cement CPB, while the manufacturing cost is only 40%–45% that of cement [21]. BFS can be activated by alkali and sulfate to generate large quantities of hydrated calcium silicate (aluminate) and ettringite hydration products, which have a good bonding effect on fine tailings [22]. The fineness of the slag and the curing temperature are crucial to CPB performance. It is usually necessary to grind it to a specific surface area above 400 m²/kg. The finer the slag, the higher the activity. The curing temperature of the BFS binder should be above 10 °C, owing to the hydration rate, which is severely reduced in low-temperature environments. In contrast, a temperature around 30 °C can promote early strength without affecting later strength [23]. A case study in the Doyon mine (Quebec, QC, Canada) used a combination of 30 wt.% Portland cement and 70 wt.% BFS, representing 5 wt.% of the total dry tailings weight to improve its environmental behavior in the sulfide tailings CPB [24].

The BFS powder is used extensively in cement, and its price is close to that of cement in China. Many researchers have begun to use steel slag, developing binder materials to reduce costs and improve the utilization rate of solid waste. Steel slag is a by-product of steelmaking, which can be divided into basic oxygen furnace (BOF) slag, electric arc furnace (EAF) slag, refining furnace slag, etc. Most steel slag in China is BOF slag, and its main mineral composition is blite, which can be considered a "burnt clinker" that exhibits low hydrability. Therefore, it is usually necessary to combine it with other pozzolanic materials, such as slag and fly ash, to develop steel slag binders. There are several SS binder systems, including SS–cement, SS-BFS, SS-BFS-cement, SS-BFS-gypsum, SS-BFS-silica fume, and SS-gypsum-fly ash. The weight percentage of SS powders in binder materials can range from 30% to 60% [25]. The SS-BFS-gypsum system exhibits good performance, and its CPB strength is not lower than that of cement CPB under the same conditions, while its production cost is only about half that of cement [3]. Researchers from the US suggested that EAF slags should be aged for at least 6 months and BOF slags for at least 24 months in open air conditions before being used as backfill material, considering their mechanical performance, heavy metals leaching, and particle size distribution [26].

In summary, BFS- and SS-based binders are the most likely to replace cement and have begun to be promoted in many mine practices.

2.2.2. Nonferrous Metal Metallurgical Slag

Copper, nickel, lead, zinc, and other nonferrous metallurgical slags, shown in Figure 2, are also important components of general industrial solid waste, with an annual yield of over 30 million tons. Compared with BFS, the content of calcium and aluminum oxides in nonferrous metallurgical slag is lower, and the content of iron oxides is higher, resulting in lower reactivity, higher specific area, sharp shape, and harmful ions content, which makes them challenging to utilize [27]. Many researchers have studied the utilization of nonferrous metallurgical slag in cement based on the experience of BFS. Zhu et al. [28] addressed the problem of low activity caused by high iron content and low calcium content in copper slag by using high-temperature reconstruction, mechanical activation, chemical excitation, and reduced quenching methods. The prepared binder CPB strength was still lower than that of cement. Liu et al. [29] developed a lead-zinc smelting slag-based ecological mine backfill by mechanical and alkali activation methods. The CPB strength met the requirements for high-strength grade self-weight backfill, the solidification ratio for each heavy metal exceeded 70%, and the cost of CPB was reduced. Wang et al. [30] produced a cemented backfill material with secondary smelting water-granulated nickel slag, composite activator (gypsum, carbide slag, Na₂SO_4, and clinker), and iron ore tailings. The 28-day flexural and compressive strength of CPB material reached 1.99 MPa and 3.38 MPa, which meet the requirements of mine backfill. Behera et al. [31] utilized lead-zinc mill tailings and lead-zinc smelter fuming furnace slag as paste backfilling for an underground metalliferous mine in India and found that the use of crushed fuming furnace slag as a cement replacement showed encouraging results of strength development, and the paste backfilling cost per ton was reduced significantly. Overall, due to properties of low reactivity, limited distribution, and hard grinding, utilization of nonferrous metallurgical slag in backfill binder materials is limited in individual cases. The properties of this type of slag determine that it is more suitable for use as backfill aggregate.

2.3. Thermal Slag

Thermal slag refers to residues generated after incineration in thermal power plants, incinerators, etc., mainly including fly ash and circulating fluidized bed (CFB) slag.

2.3.1. Fly Ash

Fly ash is a substance discharged from the flue of a power plant after coal combustion and collected by a dust collector. Its composition is similar to high-alumina clay, mainly in a glassy state, with SiO₂ and Al₂O₃ contents accounting for more than 80%. Under alkaline conditions, FA can undergo hydration reactions and produce gel-like products. According to China’s national standard GB/T 1596 [32], FA can be classified into three grades based on fineness, water demand ratio, loss on ignition, moisture content, and sulfur trioxide content. FA grade I has been widely used as SCM in cement and concrete. In mine backfill, FA grades II and III are usually used to partially replace Portland cement, which can improve the flowability of the slurry and the later strength of the CPB but reduces the early strength [18]. FA is more used in coal mines because of its ability to improve the strength, impermeability, and chemical resistance of the backfill body [33]. In metal mines, Behera et al. [34] investigated the efficacy of FA as a partial replacement (25 wt.%) for Portland cement for paste backfill application, finding that UCS development is more sensitive toward FA replacement. Xiao et al. [35] proposed a new backfill binder made with 40 wt.% of low-quality Class F FA to totally replace Portland cement. The result showed that the new FA binder could meet the strength requirement of three different mines regarding different subsequent filling and cut-and-fill mining methods, and the binder costs were reduced by more than 30%. In Australia, backfilling with FA as a partial cement replacement and plasticizer enabled continuous longwall mining through old stub headings; however, proper drainage and prevention of erosion should be achieved to maximize its stabilization [36]. Due to the prevalence of FA in thermal power plants, utilizing FA to partially replace or prepare new binder materials has inspiring prospects.

2.3.2. Circulating Fluidized Bed Slag

The CFB slag is the combustion residue brown ash from the circulating fluidized bed, consisting of complex substances of fuel ash, unburned carbon particles, and other impurities. The main mineral composition includes quartz, calcite, hard gypsum, lime, and hematite. Zhang et al. [37] prepared clinker-free cemented backfill materials containing CFB slag (20–40 wt.%). The material exhibited excellent filling performance and promoted the formation of a greater polymerization degree of the C-A-S-H phase. Yang et al. [38] pointed out that CFB contains a large amount of calcium sulfite and free calcium oxide, which have risks of volumetric expansibility and instability. It can be combined with BFS slag to prepare binder materials, minimizing expansion. Liu et al. [39] reviewed the rheology, mechanics, microstructure, and durability of CFB binders and pointed out that adding nanomaterials or modifying CFB slag may become a large-scale utilization method. The utilization of CFB in backfill binders is still limited; further research should be conducted, since more than 280 million tons of CFB slag are produced every year in China [40]. Longo et al. [41] presented some case studies from South Africa and the US, demonstrating the recipes and process design that can be implemented to solve coal combustion residues by measuring the paint filter test, hydraulic conductivity, strength as defined by Unconfined Compressive Strength (UCS) tests, and acid generating potential.

2.4. Chemical Industry Slag

Chemical waste refers to solid waste generated during the production processes of the chemical industry. The pollution caused by chemical waste is widespread, making it difficult to control. The typical chemical slag in the mine backfill field is mainly phosphogypsum and red mud.

2.4.1. Phosphogypsum

Phosphogypsum (PG) is produced in tens of millions of tons annually from the production of phosphate fertilizers, phosphoric acid, and other phosphate chemicals. Most PG is stacked on the surface with extremely low utilization rates. Using PG as a raw material for filling underground voids has become an important disposal approach. The main component of PG is calcium sulfate dihydrate, which contains incompletely decomposed phosphate ore, residual phosphoric acid, fluoride, acid-insoluble substances, organic matter, etc. Min et al. [42] studied the strength performance of PC backfill through four binder types and found that the hydration process of CSA and CAC exhibited markedly fast hydration and UCS improvement. Nizevičienė et al. [43] investigated the neutralization process of the acid impurities of PG through mechanical activation together with the neutralizing zeolite additive. The hazardous phosphate impurities can be absorbed with higher compression strength. Wang et al. [44] proposed a hemihydrate PG backfill mixture consisting of 60 wt.% PG, 2.3 wt.% of quicklime, 5.7 wt.% of tailings, and 32 wt.% of water. Though the mechanical strength can meet requirements, interaction effects between temperature and pH on 28-day strength degradation were observed. The application of PG as backfill material is promising and of great significance in solving the problem of PG disposal in China. Hemihydrate PG can be prepared into binder materials for common CPB methods within the first 137 h of aging; thereafter, it can be used only as an inactive aggregate [45]. The application of PG still faces problems such as a fast reaction rate, easy pipe blockage, and emitting irritating odors after solidification and decay.

2.4.2. Red Mud

Red mud is an industrial waste generated during alumina production from bauxite, including sintering red mud and Bayer red mud (BRM). The BRM is the most common, producing 1.2–1.5 tons for every 1 ton of alumina. Wang et al. [46] investigated the durability, microstructure, permeability, and environmental properties of CPB with BRM, FA, and cement; the CPB strength reached 1.25 MPa and 2.37 MPa after curing for 3-day and 28-day, respectively, and the heavy metal concentration and pH value of the leachate were within the standard range. BRM is a strongly alkaline substance, which is prone to risks such as frosting, corrosion, and groundwater pollution. Strong alkalinity is the key factor restricting the utilization of BRM. However, the alkalinity can be used to promote the hydration of pozzolanic materials such as fly ash and slag. Li et al. [47] used BRM as an alternative activator to sodium hydroxide with BFS as the precursor; the result showed that CPB exhibited significantly better mechanical properties, with a 57%–94% increase in uniaxial compressive strength at 28 days compared with CPB made with Portland cement. Suchita et al. [48] pointed out that neutralization/treatment of red mud using different techniques such as using mineral acids, acidic waste (pickling liquor waste), coal dust, superphosphate, and gypsum as amenders; CO₂; or sintering with silicate material and seawater is the only alternative to make the bauxite residue environmentally benign. Further studies should focus on exploring the economic viability of these processes for better waste management and disposal of red mud.

2.5. Ultra-Fine Tailings

Tailings usually contain a large number of inert substances such as SiO₂ and are used as aggregates in backfill. However, converting these crystalline, non-reactive phases into more amorphous, reactive phases can activate the tailings’ cementitious activity, which can then be used for cementitious material. Research showed that mechanical, thermal, chemical, and coupled activation technologies were usually used in tailings processing [49]. Grinding into ultra-fine particles is essential for activated tailings, which can increase the specific surface area, improve the surface energy and activity, increase the degree of participation in hydration reactions, promote the formation of a large number of acicular ettringite and C-S-H gel, improve the pore structure of hydration products, and lead to denser microstructure [50]. At present, mailing gold and iron tailings are used as mineral admixtures in cement and concrete with an incorporation rate of less than 10 wt.%.

For the above five main types of backfill binders, the active Ca-Si-Al chemical compositions, which are taken from the literature and field investigation, are shown in Figure 3. The diagram can provide a basis for activity evaluation, modification methods, and utilization of potential backfill binder material. Key properties and some examples of the green mine backfill binders are listed in

Table 1. Key properties and examples of current green mine backfill binder types.

Binder Types	Examples	Key Features	References
Cement and blend cement	Ordinary Portland cement	Not applicable for ultra-fine tailings	[19]
	Slag, FA, etc., blend cement	SCM replacement ratio 20–80 wt.%	[51]
Metallurgical slag developed binders	Alkali + BFS + gypsum	70–90 wt.% BFS, excellent binding performance	[21,52,53]
	SS + BFS + gypsum	30–60 wt.% SS, good binding performance, low-cost	[3,54]
Nonferrous metallurgical slag binders	Cement partially replaced by copper, nickel, lead, zinc slag	Low reactivity of slag, slag replace ratio < 30 wt.%, heavy metal iron leaching risk	[29,30]
Thermal slag developed binders	FA + BFS + gypsum; FA + SS + gypsum	30–60 wt.% FA, good binding performance, low cost	[17,35,55]
Chemical industry slag-based binders	PG + BFS + alkali	Up to 70 wt.% of PG can be used, good binding performance, low cost	[45,56,57]
	BRM + BFS + gypsum	Good performance, low cost, alkaline leaching risk	[47,58]
Tailings-based binders	Tailings + alkali + BFS	Tailings ratio < 30 wt.%, limited tailing types	[34,59]

.

3. Research Development of Backfill Binder Materials

The previous section summarized five different types of backfill binder materials. Solid waste-based binders mostly require activation, have similar hydration mechanisms, and have advantages in performance, economy, and the environmental perspective. These properties are hotspots of current research.

3.1. Research on Slag Activation Methods

The primary raw materials for new backfill binders are bulk solid wastes with pozzolanic activity. Under normal circumstances, these materials do not undergo hydration reactions, except alkali and other activation methods are introduced.

The quality or reactivity of slag is critical for its utilization. From Ashfaque’s study, there is a wide range of standardized and non-standardized methods used to evaluate reactivity, which can be divided into direct methods measuring physical properties or indirect methods measuring chemical properties [60]. Among them, the strength activity index, which is the compressive strength of concrete/mortar containing additives compared with reference concrete/mortar samples at different curing ages, is used in Chinese national standard GB/T 18046-2017 evaluating BFS’s reactivity [61]. Various pre-treatment techniques and methods can be employed to enhance the reactivity, such as grinding, calcination, and modified catalysis. Grinding to finer particles can damage the structure of the glass body, causing cracks and distortions in the crystals, reducing crystallinity, and increasing the specific surface area of reaction, and thus, enhancing activity. Calcination can achieve the decomposition and recombination of minerals, obtaining more useful active components. Modified catalysis optimizes the functional groups on the particle surface by adding modifiers, achieving improved surface activity, solubility, and other properties.

The potential reactivity of slag needs to be activated to obtain hydraulic properties. Typical activation methods include mechanical, thermal, chemical, and combined techniques. Xie et al. [62] investigated the mechanical grinding activation of magnesium slag for backfill and showed that the optimal grinding time should consider energy consumption and technical benefits, and the corresponding specific surface area and 45 µm sieve residue are 503 m²/kg and 3.83%, respectively. A study showed that the multi-phased cement clinker prepared at 1400 °C with the raw material containing 16.86% steel slag has comparable grindability and soundness to the normal Portland cement clinker [63]. Higher curing temperature can lead to a decrease in arsenic released from the slag–CPB, faster hydration rate and pozzolanic reaction, and lower volume of connective pores [64]. Alkali and sulfate, such as sodium hydroxide, calcium sulfate, and sodium sulfate, are commonly seen in chemical activation slag with excellent performance [3,47,49]. Industrial salt and alkali activators include desulfurization gypsum, fluorogypsum, water glass, clinker, carbide slag, steel slag, red mud, mirabilite, etc.

Figure 4 summarizes the current activation methods for CPB binders. Due to the differences in physicochemical properties, diverse slag types, and different application scenarios, it is difficult to determine the optimal activation scheme in practice, and combined activation experiments need to be conducted.

3.2. Research on Hydration Mechanism

The study of the hydration mechanism of binder materials focuses on the feasibility of the hydration reaction, hydration product analysis, and reaction rate (hydration kinetics). The laws of thermodynamics and the principle of minimum Gibbs free energy are the theoretical bases. Although different types of raw solid waste are used, and the hydration mechanism varies for different binders, the hydration process has an outstanding common feature, which is the formation process of hydrated calcium silicate (aluminate) and ettringite (AFt), as shown in Equation (1):

$$\{\begin{array}{c}x{\mathrm{Ca}}^{2+}+\mathrm{O}{\mathrm{H}}^{-}+\mathrm{Si}{\mathrm{O}}_2\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}-\mathrm{S}-\mathrm{H}+{\mathrm{H}}^{+}\\ {\mathrm{Ca}}^{2+}+\mathrm{O}{\mathrm{H}}^{-}+{\mathrm{Al}}_2{\mathrm{O}}_3\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}-\mathrm{A}-\mathrm{H}+{\mathrm{H}}^{+}\\ \mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3+2\mathrm{CaO}+3\mathrm{CaS}{\mathrm{O}}_4·2{\mathrm{H}}_2\mathrm{O}+30{\mathrm{H}}_2\mathrm{O}\to \mathrm{AFt}\end{array}$$

(1)

The hydration process of CPB binder materials can be divided into five stages. The first stage is the hydrolysis of the alkaline activator, forming an alkaline solution environment. The second stage is the equilibrium of precipitation dissolution of $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ generated by ${\mathrm{OH}}^{-}$ and ${\mathrm{Ca}}^{2+}$. In this stage, pozzolanic materials such as slag and fly ash are dissolved by ${\mathrm{OH}}^{-}$ to form ${\mathrm{H}}_2{\mathrm{SiO}}_4^{2-}$ and ${\mathrm{H}}_2{\mathrm{AlO}}_3^{-}$. In the third stage, ${\mathrm{Ca}}^{2+}$ reacts with ${\mathrm{H}}_2{\mathrm{SiO}}_4^{2-}$ and ${\mathrm{H}}_2{\mathrm{AlO}}_3^{-}$ to produce a quantity of gel products such as hydrated calcium silicate and hydrated calcium aluminates. The fourth stage involves the reaction of ${\mathrm{SiO}}_4^{2-}$ with ${\mathrm{H}}_2{\mathrm{AlO}}_3^{-}$ and ${\mathrm{Ca}}^{2+}$ to form microcrystalline AFm and AFt. The fifth stage is the gradual slag deposition and gel growth of hydration products on the surface of the particles, creating a complex structure of flocs. For example, the hydration process of the SS–BFS–gypsum binder system, modified from Hao’s study [65], is shown in Figure 5. Helinski et al. [66] proposed a model showing that the pore-water pressure change depends on the amount of volume change associated with the cement hydration, the incremental stiffness change in the soil, and the porosity of the material. The rate of hydration and volumes of water consumed during hydration were unique for each cement–tailings combination regardless of the mix proportions.

The hydration heat of the metallurgical slag binders can be divided into the rapid reaction period, induction period, acceleration period, deceleration period, and slow hydration period. However, compared with Portland cement, the hydration heat release is significantly reduced, and the duration of the induction and acceleration periods is shortened considerably [67]. Low hydration heat is beneficial for reducing the risk of cracking or strength reduction caused by thermal stress in a massive filling body. The hydration kinetics model can be derived from the hydration heat monitoring curve. Then, a specific model for such materials can be established to describe the degree of hydration, hydration products, and other characteristics. Numerous hydration kinetics have been built from the literature, e.g., Byfors, Knudsen, Hansen and Pedersen, Basma et al., Nakamura et al., Cervera et al., Schindler and Folliard, and Bentz [68].

3.3. Research on Harmful Ion Solidification

Metallurgical slag and tailings may contain harmful metal ions such as lead, arsenic, chromium, and cadmium, which are prone to migration. The issue of harmful ion leaching should be carefully examined before utilization. A study by Zhu et al. [69] showed that C–S–H exhibits various disordered layered structures at different Ca/Si ratios, exhibiting strong ion adsorption performance. The principle of heavy metal ions solidification in C–S–H gel can be divided into four ways: adsorption, ion substitution, insoluble matter generation, and encapsulation [69]. For example, in the arsenic solidification system of slag cement backfill, the calcium ions generated during the hydration process will react with the free arsenate or hydrogen arsenate ions in the pore fluid to achieve rapid arsenic fixation, that is, calcium–arsenic combination [70]. Zhang et al. [71] found that the metallurgical slag binder material has a higher lead solidification efficiency than Portland cement, mainly because of the adsorption of ${\mathrm{Pb}}^{2+}$ on the hydrated C–S–H gel and the synergistic effect of ${\mathrm{Pb}}^{2+}$ entering into the AFt and salt structure in the same phase. A suitable CPB mixture design to control the release of all heavy metals in CPB is of practical importance for groundwater quality [72]. The CPB prepared with metallurgical slag binders has smaller porosity, smaller pore diameter, and a large amount of gel-like hydration products, improving strength performance and facilitating the solidification of harmful ions [3].

3.4. Research on Energy Conservation and Emission Reduction

The cement industry, which produces 2.38 billion tons annually, contributes to more than 10% of the country’s total carbon dioxide emissions. Each unit ton of clinker production emits 0.85–0.90 tons of carbon dioxide. Solid waste-based backfill binders are prepared by grinding industrial solid waste such as slag, steel slag, gypsum, and fly ash. This not only avoids environmental damage caused by the excavation of clay minerals required for clinker but also significantly reduces energy consumption and carbon emissions by requiring grinding without sintering. Traditional cement binders can be responsible for up to 70% of the greenhouse gas emissions in the mine backfill process [73]. Research has shown that the environmental impact of cementitious materials throughout their entire lifecycle can be divided into stages: raw material production, raw material transportation, cementitious material preparation, cementitious material transportation, and the construction stage. The carbon emissions of solid waste-based binders can be reduced by 18.62% to 38.35% compared with traditional Portland cement materials [74]. Liu et al. proposed the concept of carbon dioxide backfill, defined the calculation boundaries for carbon footprint and carbon consumption in CO₂ backfill, and calculated that the carbon consumption of 1 ton of CO₂ filling material throughout the entire lifecycle can reach 200–550 kg [75]. Therefore, solid waste-based backfill not only has advantages in the comprehensive utilization of solid waste but also in energy conservation and emissions reduction, forming zero carbon and negative carbon backfill.

4. Progress in the Application of New Backfill Binders

Owing to breakthroughs in theory and technology from a research perspective, various new backfill binder materials have been applied with good performance, low cost, and environmental friendliness. This section introduces three successful industrial cases.

4.1. Jiaogu Powder

Jiaogu powder (JGP), also known as BFS-based backfill binder material, is currently the most widely used binder material after Portland cement in China’s metal mines. It usually comprises 70 wt.% BFS powder and 30 wt.% activator (clinker and gypsum), and it has achieved significant results in commercial applications. China’s JGP originated in the Shandong province in 2004, where its first production line had an annual output of 20,000 tons for underground filling at Sanxin Company (Daye, China). Under the same cement dosage (dry contents 10 wt.%) and slurry concentration (74 wt.%), the strength of the slag-CPB was more than twice that of the cement-CPB, and the slurry flowability and workability were good [52]. After years of development and improvement, the JGP utilization has been shifted from combining the “C material” (activator) shipped from Shandong with local slag mixture to localized manufacture across the country. The raw materials are widely sourced, the grinding process is simple (industrial ball mills), the production cost is low, and the binding performance is good. According to incomplete statistics, there are currently over 50 JGP manufacturers in China. There are two utilization strategies, shown in Figure 6: (1) Finished JGP products manufactured in a grinding plant and then transported in tank cars to the backfill plant users. This method is convenient to apply, but the product price is high, and the users cannot control the quality. (2) Mine users buy the three raw materials separately and then mix them using a Raymond mill or place them in multiple powder silos in the backfill plant. This method has the lowest cost but requires accurate control of the mixing ratio to avoid activation failure accidents. If a mine has an annual usage of over 10,000 tons, it is also preferable to build a grinding plant next to the backfill plant with the benefits of quality and costs being controllable. The cost savings can enable the construction investment to be recovered within 3 years.

4.2. Steel Slag Gujie Powder

Indoor experiments have shown that the backfill binder made with BFS-SS-gypsum has good binding properties and low production costs. In 2019, steel slag Gujie powder (SSGJP) achieved its first national large-scale industrial utilization in the Zhongguan Iron Mine, Hebei province, China. The SSGJP has SS content of 40 to 45 wt.%, showing excellent binding performance for ultra-fine tailings that have –74 μm fine particles accounting for 89.2 wt.%. The backfill plant built two powder silos and independently purchased raw material powders to be mixed in the backfill controlling system, as shown in Figure 7. The BFS powder and composite powder (SS and gypsum) are, respectively, transported to a double-axis horizontal mixer by a double screw feeder and then prepared into a paste slurry with a concentration of 58–62 wt.% through a high-speed activation mixing system. Finally, the paste is transported by gravity or pump to the underground voids. The underground strength sampling shows that the 28-day strength of SSGJP-CPB with a cement-sand mass ratio of 1:8 is greater than that of the cement-CPB with a cement-sand mass ratio of 1:4. Until the end of 2024, the SSGJP has been used continuously for over 350,000 tons in the Zhongguan Iron Mine (40,000 to 50,000 tons every year). The cost of the SSGJB is reduced by more than CNY 100/ton compared with cement. The material’s performance and cost-effectiveness are significant, providing referencing examples for promoting SS binders in similar mines in China.

4.3. Phosphogypsum Backfill

Utilization of PG in mine backfill may be the most effective technology for large-scale and green disposal of PG. As early as 2005, the Kaiyang Phosphate Mine (Guiyang, China) achieved industrial utilization of PG paste fill for the first time using cement, fly ash, and PG at a mass ratio of 1:1:(4–10). The backfill system has a capacity of 40 m³/h and a CPB consolidation strength of 1.3–2 MPa. In recent years, ChanHen Chemical Co., Ltd. (Fuquan, China) developed a hemihydrate PG backfill technology (CHHPG), as shown in Figure 8. The CHHPG consists of hemihydrate PG accounting for over 95 wt.% and the rest alkaline modifiers. The paste concentration is 69 wt.%, and the strength of the CPB at all ages is greater than 3 MPa, with good fluidity, meeting the requirements of mines. ChanHen has built three backfill stations in Xiaoba, Xinqiao, and Gongjishan, forming a large-scale PG filling condition of 150,000 tons per month. The CHHPG backfill technology can solve the problem of PG and tailings storage in phosphate chemical enterprises, constructing a new circular economy model of the “integrated mining–chemical industry”.

5. Discussion of Trends and Further Work of Backfill Binder Materials

Although significant progress has been made in the research and application of solid waste-based backfill binder materials, there are still many problems that urgently need to be solved.

5.1. Modeling and Simulation of Hydration and Hardening

Due to differences in material properties, equipment types, and production processes, the physicochemical properties of raw materials vary greatly. Substantial experiments usually need to be conducted to seek the optimal ratio, which can be time-consuming and cost-ineffective. Furthermore, in some cases, it is necessary to regulate the hydration reactions to meet the demands of early strength and slow setting. It is urgent to reveal the full hydration mechanism and to conduct hydration process modeling and simulating, and then, predict the hardening CPB properties such as hydration products, pore structure, and strength based on the physical and chemical properties of the materials and tailings.

5.2. Material Performance in Extreme Environments

With mining shifting to deep and high-altitude areas, the circumstances for backfill are becoming increasingly stringent, and the requirements for binder materials are becoming stricter in environments such as high temperature, low temperature, rich water, sulfur rich, and strong alkali. For example, research has shown that temperature significantly impacts the hydration performance of slag-based CPB. The hydration performance of slag powders is severely slowed down below 10 °C, while around 35 °C, it is beneficial for improving early strength without affecting later strength. At 50 °C, it is easy to cause internal cracking and a reduction in strength. In winter, the underground temperature of mines in Inner Mongolia, Xinjiang, and other places in China can be below zero degrees. It is worth further research and exploration on how to ensure material hydration and CPB performance, as well as to reduce the probability of freezing and pipe blockage, in such environments.

5.3. Technical Standards for Backfill Binder Materials

In the national standard GB175-2023, the strength grades of Portland cement are divided into six grades: 42.5, 42.5R, 52.5, 52.5R, 62.5, and 62.5R [76]. The strength grades of slag cement, fly ash cement, and volcanic cement are divided into 32.5, 32.5R, 42.5, 42.5R, 52.5, and 52.5R. The strength grades of composite Portland cement are divided into four levels: 42.5, 42.5R, 52.5, and 52.5R. The American Society of Testing Materials (ASTM) standard C150/C150M-24 specifies cement as types I, IA, II, IIA, III, IIIA, and V [77]. The ASTM standard C989/C989M-24 specifies slag cement for use in concrete and mortars [78]. These standards provide important guarantees for the application of construction cement. However, currently, there is no relevant grade standard or quality standard for backfill binder materials (aggregate tailings). In application, the binder material is verified based on a strength test sampled from each batch. If there are quality problems, there is no relevant evaluation standard, which may be detrimental to the industry’s development.

There are significant drawbacks to using the same grade evaluation of cement standards. In more than ten years of application experience, a contradictory situation is often found: if the backfill binder grade is tested according to the cement standards, the grade may be extremely low; however, its bonding effect (strength) on the mine tailings is better than that of 32.5 or 42.5 cement CPB. Therefore, it is necessary to explore a new method for determining the grade and quality of backfill binder materials.

6. Conclusions

(1)

New solid waste-based binders are gradually replacing traditional cement and are being applied to mine backfill through various activation methods. According to the types of solid waste raw materials, backfill binder materials can be divided into cement and blend cement, metallurgical slag, thermal slag, chemical slag, and tailings slag binders. Significant breakthroughs have been made in the research and application of binder materials.

(2)

Concerning the research on new backfill binders, a lot of work has been carried out from the aspects of reactivity enhancement, reactivity activation, hydration mechanism, harmful ion solidification, energy savings, and carbon reduction. Generally, slag binders have a better binding effect, especially for ultra-fine tailings. The primary hydration process is the generation of gel-like and ettringite hydration products. Solid waste-based backfill binders often have less harmful ion leaching and advantages in energy savings and carbon emission reduction.

(3)

In the industrial application of backfill binders, cement and blend cement are still the main types. However, blast furnace slag powder materials are rapidly becoming popular, exhibiting good performance and cost-effectiveness. Steel slag binders have achieved continuous industrial applications and have more cost advantages. Hemihydrate phosphogypsum backfill technology has achieved large-scale industrial utilization, providing a reference demonstration case for the comprehensive disposal of phosphogypsum.

(4)

There are still some urgent problems to be solved in the research and application of backfill binder materials, including hydration modeling and simulation, properties prediction, performance regulation in extreme environments, and specific technical standards, which can build the foundation for promoting green and efficient cemented paste backfill and sustainable development.