The Utilization of Slag, Steel Slag, and Desulfurization Gypsum as Binder Systems in UHPC with Iron Tailings and Steel Fibers—A Review

Abstract

Ultra-high-performance concrete (UHPC) is known for its outstanding strength and durability but is often limited by the high cost of traditional materials, like cement, fine aggregates, and silica fume. This review examines the use of industrial by-products—specifically, iron tailings, steel slag, and desulfurization gypsum—as sustainable alternatives in UHPC mix design. These materials serve as supplementary cementitious components and fine aggregates, helping reduce environmental impacts and production costs. This study highlights the synergistic hydration mechanisms between Portland cement and waste-based materials, leading to improved microstructure and long-term strength. The role of steel fibers in enhancing crack resistance is also discussed. Challenges related to workability, cost, and lack of standardization are addressed, along with opportunities for innovative mix designs, low-carbon binders, and 3D printing. Overall, this paper underscores the potential of industrial by-products to advance sustainable, high-performance UHPC solutions.

1. Introduction

Concrete is a widely used construction material with substantial annual usage in infrastructure and real estate projects. Ultra-high-performance concrete (UHPC) is a novel building material based on high-reactivity powder concrete, offering superior mechanical properties, including a compressive strength exceeding 120 MPa and a flexural strength surpassing 20 MPa [1,2]. UHPC utilizes highly reactive binding materials, increases the dosage of these materials, reduces the water–cement ratio, optimizes particle size distribution using only fine aggregates, and incorporates steel fibers to enhance mechanical performance, resulting in exceptional strength and durability [3,4,5,6]. Currently, the materials used in preparing UHPC primarily consist of cement, mineral admixtures, and quartz sand, which can be expensive and challenging to promote for widespread use.

Steel is an important industrial product; social development and construction rely heavily on it. China is the largest steel producer with 49.5% from the world production [7], leading to substantial solid waste generation. China produces more 100 million tons of steel slag annually (2016) [7], but less than 40% of it is effectively utilized. Moreover, tailings generated during iron ore mining and beneficiation processes are increasing at over 1 billion tons per year [7]. Solid waste occupies land, threatens the surrounding environment, and can even harm human health [8,9]. Steel slag has a short history of being utilized as an industrial by-product in China. Reporting on the recovery of steel slag in China only began in the 1970s [10]. The utilization of steel slag in China has undergone three stages, which are illustrated in Figure 1. With escalating environmental issues, China has developed a green circular economy to improve resource efficiency [11,12]. It has also set the goal of ‘carbon neutrality’. ‘Made in China 2025’ also proposes green development and pursuing a sustainable path of ecological civilization [13,14]. By 2025, a resource recycling industry will be established, and a resource recycling system covering the entire society will be completed. By then, resource utilization efficiency will be substantially improved, and the replacement ratio of renewable and primary resources will increase further [15].

Using solid waste materials as a replacement to prepare UHPC could reduce production costs and increase the comprehensive utilization of solid waste. Many scholars have researched this direction, and related technologies have been successfully applied in practical production, with emerging industries gaining momentum [16,17].

This paper presents a novel approach by focusing on the incorporation of industrial by-products, such as steel slag, iron tailings, and desulfurization gypsum, into UHPC. These waste-derived materials offer a sustainable alternative to conventional raw materials, promoting the principles of a circular economy by recycling waste from other industries. The key novelty of this work lies in its exploration of how these by-products improve not only the environmental footprint of UHPC production but also its mechanical properties, making it a more cost-effective and eco-friendly material for sustainable construction.

In this study, we analyze the impact of these industrial by-products on the performance of UHPC, highlighting their potential to enhance both the sustainability and durability of the material. By addressing these gaps, this paper offers new insights into the potential of industrial waste materials for improving the overall sustainability of UHPC in construction.

This paper presents state-of-the-art studies on solid waste-based UHPC. First, the development of UHPC is reviewed. Then, the use of solid waste binders and fine aggregates in preparing UHPC is reviewed. Additionally, the incorporation of various fibers into UHPC is reviewed. Finally, an economic analysis of solid waste UHPC is provided to evaluate its benefits.

2. The Development of UHPC

2.1. The Performance of UHPC

UHPC technology has undergone several decades of development and is gradually maturing. It has improved in mechanical performance and durability, showcasing excellent development potential.

Initially, Bache [18] optimized particle gradation to achieve the densest-packed state, producing concrete with compressive strengths ranging from 150 to 200 MPa. In 1994, Larrard and Sedran [19] used quartz sand with a maximum particle size of 0.4 mm as aggregates. They formulated concrete specimens with 28-day compressive strengths reaching 164.9 MPa, introducing “ultra-high-performance concrete”. Pandya [20] studied the effects of binder and aggregate fineness on the compressive strength and flowability of non-proprietary UHPC but did not mention the specific 200 MPa compressive strength target. Figure 2 shows the trends in UHPC research publications from the 1990s to the present, illustrating a significant increase in the number of publications in recent years.

Currently, there is no unified standard for UHPC. The academic community in France defines it as a fiber-reinforced material with non-brittleness and a compressive strength above 150 Mp [21]. In Japan, it is a composite material composed of fine aggregates, cement, and volcanic ash. Under fiber-reinforced conditions, it should have a compressive strength of at least 150 MPa [22].

The excellent performance of UHPC goes beyond just its strength. With the addition of steel fibers, UHPC shows good resistance to deformation and high toughness. It is dense, has low permeability, and can even heal small cracks on its own, making it much more durable than regular materials. UHPC is strong, durable, and tough, with a long service life, resistance to damage, and the ability to block water from passing through [23].

2.2. The Application of UHPC

Ultra-high-performance concrete (UHPC) has a wide range of applications, from infrastructure to artistic constructions, thanks to its strength and durability. However, these applications often come with a high environmental cost, primarily due to the use of Portland cement and other virgin raw materials. Recent studies have shown that the use of industrial by-products, such as steel slag and iron tailings, not only reduces the carbon footprint but also enhances the durability and mechanical properties of UHPC.

By replacing conventional raw materials with these waste materials, we can reduce environmental impacts while maintaining or even improving the performance of UHPC. For instance, steel slag can improve the compressive strength of UHPC, while iron tailings help to reduce the need for natural sand, further contributing to sustainable practices in construction.

UHPC has numerous applications in regions such as Europe, the United States, Japan, and South Korea, where it is widely promoted. Industry standards related to UHPC have already been established in these countries and territories [24]. Although China began researching UHPC relatively late, it has achieved significant milestones. For instance, experimental studies on UHPC beams were conducted during the construction of the Qiancao Railway Luannbo Canal Bridge. Figure 3 illustrates various applications of ultra-high-performance concrete (UHPC) in real-world infrastructure projects, including bridges, dams, parking garages, and water treatment plants. These images demonstrate how UHPC, particularly when made with sustainable materials, such as steel slag and iron tailings, contributes to the durability and longevity of critical infrastructure.

2.3. The Preparation Processes of UHPC

The design principle of UHPC is based on the theory of maximum packing density, aiming to achieve maximum density or minimum porosity and an optimal particle size distribution. UHPC accomplishes this by optimizing the gradation of materials, allowing fine particles to fill the voids and cracks within the structure, thereby enhancing the overall compactness of the structure.

UHPC differs significantly from ordinary concrete in terms of curing methods. UHPC’s binder materials incorporate mineral admixtures to optimize the internal pore structure. The aggregates used consist of fine aggregates with optimized gradation, and the particle size range of these fine aggregates is typically limited. During preparation, a specific amount of steel fibers is added to improve the toughness and strength of the specimens. Pressurized heat curing is frequently employed to accelerate the hydration reaction and enhance the microstructure [25,26,27].

Recent research has focused on enhancing UHPC’s sustainability and cost-effectiveness through various activation strategies and alternative materials. Reactive powder concrete (RPC), a type of UHPC, shows an improved microstructure and mechanical properties compared to high-performance concrete [28]. Physical and chemical activation methods, including nanomaterials and multi-cementitious systems, have been investigated to boost UHPC performance [29]. Studies indicate that supplementary cementitious materials can partially replace cement and silica fumes in UHPC production, reducing environmental impacts and costs [30]. Rice husk ash has emerged as a promising alternative to silica fume, while fly ash, ground granulated blast furnace slag, and metakaolin can be combined with silica fume [31]. These advancements facilitate the development of more sustainable and economical UHPC formulations.

However, UHPC has its drawbacks. It is relatively expensive and more challenging to design. While a low water–cement ratio enhances strength, it can complicate construction and lead to self-shrinkage problems. Although China has developed UHPC-related standards, there is still a lack of construction experience, and the technology’s application is not yet fully mature.

2.4. Contribution of UHPC to Sustainable Development Goals (SDGs)

As sustainability becomes an increasingly important aspect of modern construction, ultra-high-performance concrete (UHPC), particularly when made with industrial by-products, such as steel slag and iron tailings, offers significant potential in addressing several key Sustainable Development Goals (SDGs). The use of UHPC in construction can help achieve the following SDGs:

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SDG 9—Industry, Innovation, and Infrastructure

UHPC’s remarkable strength and durability enhance the resilience and longevity of infrastructure projects, reducing the need for frequent repairs and replacements. By utilizing UHPC, especially in bridges, buildings, and roads, we can create long-lasting infrastructure that supports sustainable economic growth [32].

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SDG 12—Responsible Consumption and Production

UHPC promotes the use of industrial by-products, such as steel slag and iron tailings, which are often considered waste materials. By incorporating these sustainable materials into concrete production, UHPC reduces the reliance on virgin raw materials and helps lower the environmental footprint of construction processes. This aligns with the goal of promoting responsible production and consumption [33].

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SDG 13—Climate Action

The production of conventional concrete is responsible for significant CO2 emissions, primarily due to the use of Portland cement. However, by integrating industrial waste materials like steel slag and iron tailings into UHPC, we reduce the need for cement and help mitigate climate change. Furthermore, the enhanced durability of UHPC contributes to longer-lasting structures, reducing the environmental impact over the lifecycle of buildings and infrastructure [34].

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SDG 11—Sustainable Cities and Communities

The use of UHPC in urban infrastructure can support the development of more sustainable cities. Its application in construction reduces the need for frequent repairs and maintenance, contributing to more resilient urban environments. Moreover, UHPC’s ability to incorporate recycled materials aligns with the principles of a circular economy, a key feature of sustainable urban development [35].

2.5. Industrial By-Products in UHPC: Uses and Benefits

The integration of industrial by-products into ultra-high-performance concrete (UHPC) offers numerous environmental and performance advantages. This section explores the different types of industrial by-products that are increasingly being used in UHPC formulations, focusing on their use as fine aggregates, supplementary cementitious materials (SCMs), and other potential applications.

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Steel Slag as a Fine Aggregate

Steel slag, a by-product of steel production, has been increasingly used as a fine aggregate in UHPC. Its incorporation not only reduces the demand for natural aggregates but also improves the mechanical properties of UHPC, such as compressive strength and durability. Steel slag is rich in calcium silicate and aluminate, which contribute to the formation of a denser microstructure in the concrete matrix [36,37].

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Iron Tailings as a Fine Aggregate

Iron tailings, a by-product of iron ore mining, are another industrial waste material that can be utilized as a fine aggregate in UHPC. Iron tailings help in reducing the environmental impact of both the mining industry and concrete production. Their particle size distribution and chemical composition allow for better particle packing in UHPC, improving its strength and workability [38,39].

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Desulfurization Gypsum as Supplementary Cementitious Material (SCM)

Desulfurization gypsum, produced during the flue gas desulfurization process in coal-fired power plants, is an effective supplementary cementitious material in UHPC. Its high calcium sulfate content enhances the hydration reaction, improving the early-age strength and durability of the concrete. The use of desulfurization gypsum also helps mitigate the environmental impact of gypsum mining [38,40], as is shown in Figure 4.

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Other Potential Industrial By-Products

Apart from the major by-products discussed, other materials, like fly ash, blast furnace slag, and rice husk ash, can also be considered as SCMs in UHPC. These by-products provide additional pozzolanic effects that enhance the material’s durability and long-term strength while lowering the overall cost and environmental impact of concrete production [41,42].

2.6. Main Components of UHPC with Industrial By-Product Binders

UHPC is a dense, high-strength cementitious material composed of selected components optimized for maximum mechanical properties and durability. In the context of sustainable UHPC, especially when incorporating industrial by-products, the role and behavior of each component are crucial.

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Binder Materials

Traditional UHPC uses large amounts of Portland cement and silica fume. In this study, the binder system consists primarily of slag, steel slag, and desulfurization gypsum, forming a ternary clinker-free matrix. These materials contribute to the formation of hydration products, such as C–S–H gels and ettringite, providing strength and durability through a synergistic hydration mechanism [43]. Figure 5 shows the effects of BOFS and GBFS ratios on UHPC strength.

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Aggregates

Unlike conventional concrete, UHPC excludes coarse aggregates to enhance homogeneity and reduce weak points in the matrix. Only fine aggregates are used, such as quartz sand and iron tailings, the latter serving as an industrial by-product with good particle size distribution and compatibility. The use of iron tailings also improves packing density and microstructure densification [38,44].

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Admixtures

UHPC requires a very low water-to-binder ratio (typically < 0.25). To ensure workability and dispersion of particles, high-range water reducers or polycarboxylate-based superplasticizers are essential. These admixtures reduce viscosity and improve flowability without increasing water content [45,46].

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Steel Fibers

While not a by-product, steel fibers are critical for improving tensile strength, ductility, and crack resistance—especially in UHPC systems with modified hydration behavior due to non-cementitious binders. In this work, three types of steel fibers (straight, hooked-end, and corrugated) are used [47].

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Microstructural Characteristics

At the microscopic level, the dense UHPC matrix exhibits low porosity, refined capillary pores, and high amounts of C–S–H and AFt phases. The synergistic hydration among slag, steel slag, and gypsum produces a robust internal structure that contributes to both early and long-term strength [48].

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Applications

UHPC produced with industrial by-products has been successfully used in artificial reef elements, precast building components, and marine structures, where its low permeability, corrosion resistance, and sustainability offer clear advantages [49].

3. The Application of Mine Tailings as the Aggregate of UHPC

Government regulations on natural sand mining are becoming stricter as the availability of natural sand decreases and costs rise each year. Finding a more economical alternative material for fine aggregates in place of natural sand effectively reduces costs [50,51]. Mine tailings, which consist mainly of components like sand, can be utilized as fine aggregates. Table 1 presents the impact of iron ore tailings (IOT) on the mechanical properties of cement-based materials.

Iron mine tailings are waste products generated during mineral processing. Due to differences in the properties of the original ore and the beneficiation process, iron tailings have complex compositions that may vary between sources. However, the main components of iron tailings are generally similar, typically containing SiO₂, Al₂O₃, Fe₂O₃, and small amounts of CaO and MgO [52].

Tailings represent a significant type of solid waste, and their storage can lead to numerous problems and hazards. Consequently, extensive research and the application of methods for effectively managing tailings have been conducted both domestically and internationally. Various methods exist for the comprehensive treatment of iron tailings, including reprocessing, manufacturing construction materials, and creating composite materials [53].

Many scholars have researched the application of iron tailings in concrete for several years, and some have successfully implemented it in production. Iron tailings, derived from the crushing and grinding iron ore, have fine particle sizes, making them suitable for use as fine aggregates. They can also be blended with manufactured sand to optimize the fine aggregate gradation. The waste rock produced after crushing iron ore generally has larger particle sizes and is suitable for use as coarse aggregates [54,55].

Research indicates that using iron tailings as fine aggregates for concrete production impacts its workability and mechanical properties, but within reasonable limits. Relevant standards have been established in China [56,57].

Research has shown promising results in using iron tailings mixed with manufactured sand as a substitute for natural river sand in concrete production. Studies indicate that concrete containing iron tailings can achieve comparable or even superior mechanical properties to conventional concrete [58,59]. The optimal ratio of iron tailings to manufactured sand is 6:4, providing good flowability and strength. Regarding durability, concrete with iron tailings demonstrates similar freeze-thaw resistance to river sand concrete. Interestingly, drying shrinkage in concrete made with iron tailings is slightly lower than in conventional concrete for both C30 and C60 grades [60]. However, more recent research has reported higher shrinkage in iron tailing sand concrete than in river sand concrete, necessitating shrinkage mitigation strategies, such as super absorbent polymers and controlled permeable formwork liners [61].

Nur Ain Hamiruddin et al. [62] studied the influence of different sand gradations on UHPC. They found that concrete specimens with a sand gradation between 600 and 1180 μm exhibited excellent performance, showing a uniform increase in compressive strength at both 7 days and 28 days.

Zhao et al. [63] used iron tailing sand to replace natural aggregates in UHPC and investigated its impact on mechanical properties and pore structure. They found that completely substituting natural aggregates with tailing sand was not favorable for the workability and compressive strength of UHPC. Furthermore, as the content of tailing sand increased, porosity rose, and the bond between tailing sand and the matrix in the transition zone weakened.

Iron ore tailings, as a partial replacement for fine aggregates in concrete production, can influence workability and mechanical properties, but only within reasonable limits. Every year, millions of tons of iron ore tailings are produced during iron ore processing. They are typically disposed of in landfills, quarries, rivers, and seas, among other locations, leading to environmental issues. Assessing whether iron ore tailings can substitute fine aggregates in UHPC significantly increases the proportion of solid waste in UHPC [64].

Table 1. IOT impact on the mechanical properties of cement-based material.

	Study	Key Findings	Remarks
Mechanical Properties	Lv et al. (2022) [65]	Up to 25% IOT enhances strength: compressive, +12%; tensile, +18%; flexural, +13%.	Optimal at 25% IOT. Excessive amounts reduce effectiveness.
	Shettimaet al. (2016) [66]	Strength is improved with 25% IOT: compressive, +12.90%; tensile +20.50%.	Finer particles enhance performance. Too much IOT can affect workability.
	Liu et al. (2023) [67]	Optimal strength at 25% IOT: compressive, +7.4%–10.1%; tensile, +5.30 to 9.90%; flexural, +9.1 to 12.6%.	Adjust workability for best results. Optimal at 25% IOT.
Fiber Reinforced Concrete	Y. Li et al. (2024) [68]	IOT with carbon fiber enhances strength: +17% with 30% IOT and 0.6% fiber.	Synergistic effect. Up to 30% IOT recommended.
	C. Wang et al. (2024) [69]	Significant gains with IOT fine aggregates and fibers: +42% strength with 50% IOT; +93% with 0.5% PVA fibers.	Fibers counteract brittleness. Improves overall performance.
	Zhao et al. (2023) [70]	Fiber addition mitigates IOT’s negative effects; improved strength with 1.5% fiber.	IOT decreases performance. Fiber addition is essential for maintaining mechanical properties.
Ultra-High-Performance Concrete	H. Heraiz et al. (2024) [38]	IOT with 2% of steel fiber enhances the compressive strength by 19%.	Effective in high-performance concrete.
	Zhanget al. (2020) [71]	Compressive strength is improved with 40% IOT by +14.3%.	Effective in high-performance concrete.
	Carrasco et al. (2017) [72]	Evaluated Young’s modulus indicates potential for high-performance mixes with IOT.	Promising for high-performance applications.
	Shi et al. (2024) [73]	Replacement with 100% IOT reduces properties compared to quartz sand.	Inferior performance as a complete substitute. Limited effectiveness in ultra-high-performance concrete.

Table 1. IOT impact on the mechanical properties of cement-based material.

	Study	Key Findings	Remarks
Mechanical Properties	Lv et al. (2022) [65]	Up to 25% IOT enhances strength: compressive, +12%; tensile, +18%; flexural, +13%.	Optimal at 25% IOT. Excessive amounts reduce effectiveness.
	Shettimaet al. (2016) [66]	Strength is improved with 25% IOT: compressive, +12.90%; tensile +20.50%.	Finer particles enhance performance. Too much IOT can affect workability.
	Liu et al. (2023) [67]	Optimal strength at 25% IOT: compressive, +7.4%–10.1%; tensile, +5.30 to 9.90%; flexural, +9.1 to 12.6%.	Adjust workability for best results. Optimal at 25% IOT.
Fiber Reinforced Concrete	Y. Li et al. (2024) [68]	IOT with carbon fiber enhances strength: +17% with 30% IOT and 0.6% fiber.	Synergistic effect. Up to 30% IOT recommended.
	C. Wang et al. (2024) [69]	Significant gains with IOT fine aggregates and fibers: +42% strength with 50% IOT; +93% with 0.5% PVA fibers.	Fibers counteract brittleness. Improves overall performance.
	Zhao et al. (2023) [70]	Fiber addition mitigates IOT’s negative effects; improved strength with 1.5% fiber.	IOT decreases performance. Fiber addition is essential for maintaining mechanical properties.
Ultra-High-Performance Concrete	H. Heraiz et al. (2024) [38]	IOT with 2% of steel fiber enhances the compressive strength by 19%.	Effective in high-performance concrete.
	Zhanget al. (2020) [71]	Compressive strength is improved with 40% IOT by +14.3%.	Effective in high-performance concrete.
	Carrasco et al. (2017) [72]	Evaluated Young’s modulus indicates potential for high-performance mixes with IOT.	Promising for high-performance applications.
	Shi et al. (2024) [73]	Replacement with 100% IOT reduces properties compared to quartz sand.	Inferior performance as a complete substitute. Limited effectiveness in ultra-high-performance concrete.

4. Innovative Solid Waste Binders in UHPC: Performance and Mechanisms

This section explores the use of novel solid waste materials, such as GBFS, BOFS, and desulfurization gypsum, as binders and iron tailings as fine aggregates in ultra-high-performance concrete (UHPC). These materials are by-products of industrial processes and offer sustainable alternatives to conventional raw materials. By highlighting their unique roles, this section focuses on their potential to enhance the mechanical performance and sustainability of UHPC, moving beyond traditional Supplementary Cementitious Materials (SCMs), like fly ash and slag, and towards waste-derived resources that contribute to circular economy practices.

4.1. Supplementary Cementitious Materials Used in Ultra-High-Performance Concrete

Most research on UHPC incorporating mineral admixtures utilizes a small amount of cement replacement. Limited research exists on using high volumes of mineral admixtures to prepare UHPC. The presence of unreacted cement particles and the high heat of hydration caused by using a large amount of cement in ultra-high-performance concrete, which results in high self-shrinkage, have led to investigations into the possibility of using industrial solid waste materials as partial replacements for cement and other highly active components. Various industrial solid waste materials, such as tailings [74] and metallurgical by-products [75,76], have been studied for their applications in UHPC.

Edwin et al. [77] used a vacuum high-speed mixing method to replace 5%, 10%, 15%, and 20% of cement with copper slag in reactive powder concrete (RPC). Their study found that under vacuum conditions, the workability of RPC decreased compared to atmospheric conditions; however, with an increase in copper slag content, the workability gradually improved. RPC achieved the highest compressive strength of 158 MPa when 10% copper slag replaced cement under vacuum mixing conditions. Yazici et al. [78] utilized 20%, 40%, and 60% blast furnace slag to replace cement in RPC. After steam curing, RPC containing a high proportion of GGBFS achieved compressive strengths exceeding 250 MPa and reduced the demand for a high-efficiency water reducer. Furthermore, Li et al. [79] incorporated 10%, 20%, and 30% steel slag powder to substitute cement in UHPC. Their study found that steel slag powder initially affected the strength of UHPC but had a minor impact on long-term compressive strength. Additionally, the self-shrinkage strain of UHPC decreased as the steel slag content increased.

4.2. Current State of Solid Waste Binders (Slag–Steel Slag–Desulfurization Gypsum) and Iron Tailings as Fine Aggregates

Significant research has been conducted on the slag–steel slag–desulfurization gypsum binder system, which has now reached a mature stage and has been successfully applied in practical engineering. Based on previous studies by our research group [42,80], UHPC prepared using steel slag, slag, and gypsum as binder materials has demonstrated excellent performance. In these systems, iron tailings are incorporated as fine aggregates, contributing to improved particle packing while maintaining overall UHPC quality. This approach enables the development of high-performance concrete with a high proportion of industrial solid waste, reducing production costs and environmental impacts. Wand [81] reported that the synergistic interaction among steel slag, slag, and desulfurization gypsum allows these materials to effectively replace cement in binder systems. This ternary combination has yielded promising results. For instance, Li et al. [82] noted that while a high steel slag content (>20%) can reduce mortar strength, a mix containing only 1% cement, with iron tailings as aggregates, achieved a compressive strength of 61 MPa, making it suitable for applications such as artificial reef concrete at a significantly lower cost. Liu et al. [83] further investigated the hydration mechanism of this binder system through heat evolution and hydration kinetics analysis. Their study confirmed that finely ground slag, steel slag, and gypsum, when mixed in optimized proportions, could produce concrete with compressive strengths up to 71 MPa at 28 days and 47 MPa at earlier curing ages. The performance enhancement is attributed to the synergistic hydration of the three components: steel slag provides an alkaline environment and reactive species; gypsum supplies SO₄²⁻ ions, promoting ettringite (AFt) formation; and slag dissolves in a high-pH medium to release additional reactive silica and alumina. These reactions result in the generation of C-S-H gels and AFt phases, significantly improving the strength and durability of the UHPC matrix.

4.3. Microstructural Development in UHPC

The microstructure of UHPC is crucial in determining its mechanical performance and durability. The combination of Portland cement with supplementary cementitious materials (SCMs), slag, steel slag, and desulfurization gypsum (DG), significantly affects the microstructural evolution of UHPC. The hydration of Portland cement generates calcium silicate hydrate (C-S-H) gel, calcium hydroxide (CH), and ettringite, which form the primary binding phases. When SCMs are added, they react with CH to produce more C-S-H gel through pozzolanic reactions, leading to a denser and more refined microstructure [84].

A key characteristic of UHPC is its low water-to-binder (W/B) ratio, which helps create a dense, homogeneous microstructure with minimal capillary pores. The incorporation of supplementary cementitious materials (SCMs) further refines the pore structure by filling the spaces between cement particles and aggregates. This refinement decreases porosity and enhances the mechanical properties of UHPC, including compressive strength, flexural strength, and resistance to chemical attacks. Additionally, the use of SCMs improves the interfacial transition zone (ITZ) between the paste and aggregates, which is typically the weakest part of conventional concrete [85].

Ettringite formation is an important part of UHPC’s microstructure development. Ettringite crystals fill the pores and microcracks in the cement paste, helping to increase early strength and reduce permeability. The presence of sulfate ions, typically provided by desulfurization gypsum (DG), is essential for ettringite formation. The combined effect of Portland cement and SCMs ensures a steady supply of calcium, aluminum, and sulfate ions, which supports the growth of ettringite and further improves the microstructure [86].

Advanced characterization methods, such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP), are commonly used to investigate the microstructural development of UHPC. These techniques help visualize the evolution of hydration products, pore structure, and the interfacial transition zone (ITZ) over time. For example, SEM images show the dense network of C-S-H gel and ettringite in UHPC, while MIP data reveal how adding SCMs reduces pore size and volume. These findings are essential for optimizing the mix design and curing conditions to achieve the desired performance [87]. The images in Figure 6 reveal differences in microstructures, with the solid waste binder system demonstrating a denser network of hydration products, such as C-S-H and CH, compared to the more irregular microstructure observed in conventional concrete.

4.3.1. Formation of C-S-H Gel and Ettringite in Hybrid Binder Systems

The formation of calcium silicate hydrate (C-S-H) gel and ettringite is critical to the microstructural development in hybrid binder systems used in UHPC. These hydration products are primarily responsible for binding particles and developing mechanical properties. In hybrid systems, which combine Portland cement with supplementary cementitious materials (SCMs), such as ground granulated blast furnace slag (GBFS), basic oxygen furnace slag (BOFS), and desulfurization gypsum (DG), the formation of C-S-H gel and ettringite is influenced by the chemical composition and reactivity of the materials [89].

C-S-H gel is the primary binding phase in cementitious systems, formed through the hydration of tricalcium silicate (C₃S) and dicalcium silicate (C₂S) in Portland cement. In hybrid systems, SCMs, such as GBFS and BOFS, react with calcium hydroxide (CH), a by-product of Portland cement hydration, to generate additional C-S-H gel through pozzolanic reactions. This secondary C-S-H gel contributes to the densification of the microstructure and enhances the long-term strength and durability of UHPC. The low water-to-binder (W/B) ratio in UHPC further promotes the formation of a dense and homogeneous C-S-H gel network [90].

Ettringite, a hydrous calcium aluminum sulfate mineral (C₆AS₃H₃₂) is another key hydration product in hybrid binder systems. It forms during the early stages of hydration through the reaction of tricalcium aluminate (C₃A) with calcium sulfate (CaSO4), which is often supplied by desulfurization gypsum (DG). Ettringite crystals fill the pores and microcracks in the cement paste, contributing to early-age strength and reducing permeability. In hybrid systems, the availability of aluminum and sulfate ions from SCMs, such as BOFS, and DG enhances ettringite formation, further refining the microstructure [91].

The synergy between C-S-H gel and ettringite formation is crucial for optimizing the performance of hybrid binder systems. While C-S-H gel provides primary binding and long-term strength, ettringite contributes to early-age strength and pore-filling. The balance between these two phases depends on factors such as the chemical composition of the materials, the W/B ratio, and the curing conditions. For example, a higher sulfate content from DG can promote ettringite formation, while a finer particle size of SCMs can enhance the pozzolanic reaction and C-S-H gel formation [92]. Figure 7 illustrates the bonding configuration of silicon atoms in calcium silicate hydrate (C-S-H) gel, which plays a central role in determining the microstructural integrity and strength of UHPC.

4.3.2. Pore Structure Refinement and Durability Enhancement

The pore structure of UHPC is a critical factor influencing its durability and long-term performance. In hybrid binder systems, which combine Portland cement with supplementary cementitious materials (SCMs), such as ground granulated blast furnace slag (GBFS), basic oxygen furnace slag (BOFS), and desulfurization gypsum (DG), the refinement of the pore structure is achieved through the synergistic effects of hydration products and a low water-to-binder (W/B) ratio. The dense and homogeneous microstructure of UHPC, characterized by minimal capillary pores, significantly enhances its resistance to permeability, chemical attacks, and environmental degradation [93].

One of the primary mechanisms of pore structure refinement in UHPC is the formation of calcium silicate hydrate (C-S-H) gel. The hydration of Portland cement and the pozzolanic reaction of SCMs create a dense network of C-S-H gel that fills the interstitial spaces between cement particles and aggregates. This reduces overall porosity and refines the pore size distribution, leading to a more impermeable and durable material. The low W/B ratio in UHPC further promotes the formation of a dense C-S-H gel network, minimizing large capillary pores [94].

Ettringite, another key hydration product in hybrid binder systems, also contributes to the refinement of pore structure. Ettringite crystals form during the early stages of hydration, filling the pores and microcracks in the cement paste. This process not only enhances early-age strength but also reduces the permeability of the concrete. The availability of sulfate ions from desulfurization gypsum (DG) and aluminum ions from SCMs, such as BOFS, ensures continuous ettringite formation, further refining the pore structure and improving durability [95].

4.3.3. Hydration Mechanisms and Early-Age Strength Development

The hydration of Portland cement is a complex chemical process that plays a pivotal role in the early-age strength development and microstructure formation of UHPC. When Portland cement meets water, it undergoes a series of exothermic reactions, leading to the formation of primary hydration products, such as calcium silicate hydrate (C-S-H) gel, calcium hydroxide (CH), and ettringite (AFt). These phases bind the particles and develop the concrete’s mechanical properties [96].

In UHPC, the hydration process is significantly influenced by the low water-to-binder (W/B) ratio and the presence of supplementary cementitious materials (SCMs), such as ground granulated blast furnace slag (GBFS) and basic oxygen furnace slag (BOFS). The low W/B ratio results in a dense microstructure with minimal capillary pores, enhancing the strength and durability of the concrete. The inclusion of SCMs further refines the pore structure. It contributes to the formation of additional C-S-H gel through pozzolanic reactions, which occur when SCMs react with calcium hydroxide (CH) produced during cement hydration [97].

The rapid formation of C-S-H gel and ettringite primarily governs the early-age strength development of UHPC. C-S-H gel is the main binding phase, providing concrete with compressive and tensile strength. Ettringite, on the other hand, enhances early-age strength by filling interstitial spaces between particles and reducing porosity. However, ettringite formation is highly dependent on the availability of sulfate ions, which can be supplied by materials such as desulfurization gypsum (DG) [85].

The particle size distribution and specific surface area of the cementitious materials also influence the hydration kinetics of Portland cement in UHPC. Finer particles react more quickly, leading to faster strength development. However, using ultrafine materials can increase the risk of early-age cracking due to autogenous shrinkage, a critical consideration in UHPC formulations [41]. Figure 8 presents the mechanism by which ettringite seeds accelerate early-strength development in mortar, supporting their role in refining microstructures and improving performance during initial curing stages.

4.3.4. Synergistic Effects of Portland Cement and Supplementary Cementitious Materials

Combining Portland cement with supplementary cementitious materials (SCMs) in UHPC creates a synergistic effect that enhances mechanical properties and durability. SCMs, such as ground granulated blast furnace slag (GBFS), basic oxygen furnace slag (BOFS), and fly ash, react with the calcium hydroxide (CH) produced during Portland cement hydration to form additional calcium silicate hydrate (C-S-H) gel. This secondary reaction, known as the pozzolanic reaction, refines the microstructure and reduces porosity, thereby improving strength and durability [97].

One of the key benefits of using SCMs in UHPC is their ability to reduce the environmental impact of concrete production. By partially replacing Portland cement with industrial by-products, like GBFS and BOFS, the carbon footprint of UHPC can be significantly reduced. Studies have shown that a 30%–40% replacement of Portland cement with SCMs can lower CO₂ emissions by a similar percentage while maintaining or even enhancing the mechanical properties of the concrete. This makes SCMs an essential component of sustainable UHPC formulations [99].

The synergy between Portland cement and SCMs is especially evident in the development of microstructures. The inclusion of SCMs promotes the formation of a denser and more homogeneous matrix characterized by a refined pore structure and an enhanced interfacial transition zone (ITZ) between the paste and aggregates. This results in improved mechanical properties, such as higher compressive and flexural strength; increased resistance to chemical attacks; and reduced permeability [100]. Figure 9 illustrates the early-age hydration process of cement paste containing metakaolin (MK), highlighting the formation of primary hydration products and the synergistic reactions that enhance UHPC performance.

4.4. Environmental and Economic Implications of Reducing Portland Cement Content

The production of Portland cement significantly contributes to global CO₂ emissions, accounting for approximately 8% of anthropogenic CO₂ emissions worldwide. This results primarily from the calcination of limestone and the high energy consumption needed for clinker production. Reducing the Portland cement content in ultra-high-performance concrete (UHPC) by incorporating supplementary cementitious materials (SCMs), such as ground granulated blast furnace slag (GBFS), basic oxygen furnace slag (BOFS), and fly ash, provides substantial environmental benefits. Studies have shown that replacing 30%–40% of Portland cement with SCMs can decrease CO₂ emissions by a comparable percentage, making UHPC more sustainable and aligned with global decarbonization goals [101].

Reducing the content of Portland cement can lead to economic advantages alongside environmental benefits. SCMs are often industrial by-products or waste materials and are generally less expensive than Portland cement. For example, GBFS and BOFS are by-products of the steel industry, and their utilization in UHPC lowers material costs while providing a sustainable solution for waste management. The cost savings associated with using SCMs can make UHPC more economically viable for large-scale applications, such as infrastructure projects and high-rise buildings [102].

However, reductions in Portland cement content must be carefully balanced to ensure that the mechanical properties and durability of UHPC are not compromised. While SCMs can enhance long-term strength and durability through pozzolanic reactions, their slower reaction kinetics compared to Portland cement can result in lower early-age strength. To address this challenge, researchers have explored using chemical activators, such as sodium hydroxide or calcium sulfate, to accelerate the hydration of SCMs and improve early-age performance. Additionally, optimizing the particle size distribution and fineness of SCMs can enhance their reactivity and improve overall performance [103].

The environmental and economic implications of reducing Portland cement content extend beyond the production phase. UHPC with a lower cement content typically exhibits improved durability, including higher resistance to chemical attacks, reduced permeability, and lower shrinkage. These properties can extend the service life of concrete structures, lowering maintenance costs and further enhancing the sustainability of UHPC. Life cycle assessment (LCA) studies have demonstrated that using SCMs in UHPC can significantly reduce the environmental impact over the entire lifecycle of a structure, from production to demolition [104].

4.5. Challenges and Outlook

While the use of industrial by-products, such as slag, steel slag, and desulfurization gypsum, offers significant environmental and economic benefits, several challenges and limitations must be acknowledged to ensure responsible implementations in UHPC systems.

One key issue is the variability in chemical composition and physical properties of these by-products, which can affect hydration behavior, early-age strength, and long-term durability. For example, steel slag may contain free CaO or MgO, which can lead to expansion and cracking if not properly stabilized or pretreated. Similarly, iron tailings may have low reactivity and require fine grinding to achieve a desirable packing density and bonding.

Additionally, the long-term durability of UHPC made with such materials remains a concern, especially under aggressive environmental conditions, such as chloride exposure or freeze–thaw cycles. While initial results are promising, further studies are needed to assess creep, shrinkage, and microstructural stability over an extended service life.

Ensuring material quality control, regional characterization, and real-world field testing will be essential for translating lab-scale findings into reliable structural applications.

5. Reinforcement Synergy: Role of Steel Fibers in Waste-Based UHPC

Although steel fibers are not classified as industrial by-products, their inclusion is essential in the performance enhancement of UHPC, especially when using alternative binders derived from solid waste. Numerous studies demonstrate that steel fibers significantly contribute to mechanical performance, particularly in overcoming brittleness introduced by non-traditional cementitious materials, such as steel slag, desulfurization gypsum, and iron tailings.

Recent research shows that the interaction between steel fibers and clinker-free or low-cement matrices—often containing industrial by-products—can enhance crack bridging and can control microcracking, compensating for early-age strength limitations often seen in SCM-rich systems [3,105,106]. Furthermore, the fiber–matrix interface and dispersion efficiency are especially influenced by the particle morphology and mineral composition of waste-based mixes.

Thus, this section justifies the inclusion of steel fibers in the context of enhancing the structural performance and durability of UHPC formulated with industrial waste materials. Their mechanical synergy plays a vital role in making these sustainable mixes viable for real-world applications.

5.1. Influence of Fiber Shape on Crack Bridging and Load Transfer

The shape of steel fibers in UHPC is crucial in how they contribute to crack bridging and load transfer, significantly affecting the material’s overall mechanical performance. Different fiber shapes offer varying degrees of interlocking within the concrete matrix, influencing their effectiveness in preventing crack propagation and enhancing the load-bearing capacity. Figure 10 shows the cracking patterns of different fibers (straight, hooked end, and corrugated).

5.1.1. Straight Fibers

Although easier and cheaper to produce, straight fibers are less effective at transferring loads than hooked-end fibers. These fibers can still improve crack resistance by providing some reinforcement, but they generally lack the mechanical anchorage offered by hooked-end fibers. This leads to lower pull-out resistance and, consequently, a reduced ability to prevent crack propagation. Despite their limitations, straight fibers are often used in cost-sensitive applications where high performance is not the primary requirement [107].

5.1.2. Hooked-End Fibers

Hooked-end fibers provide superior anchorage and better load transfer due to their ability to “lock” into the concrete matrix. This shape enhances the fibers’ capacity to bridge cracks and resist their propagation under stress. The increased surface area and interlocking nature of hooked-end fibers enable them to resist pull-out forces, improving their tensile and flexural strength performance. Studies have shown that hooked-end fibers are more effective than straight fibers in enhancing the post-cracking performance of concrete [108].

5.1.3. Corrugated Fibers

With their undulating shape, corrugated fibers compromise the cost and performance of straight and hooked-end fibers. Corrugated fibers’ rough surface and shape offer better interlocking with the concrete matrix, thus enhancing crack bridging and load transfer. These fibers demonstrate superior crack resistance compared to straight fibers. They are more cost-effective than hooked-end fibers, making them suitable for various UHPC applications where a balance of performance and cost is essential [109].

5.2. Effects of Fiber Volume Fraction on Workability and Strength

The volume fraction of steel fibers in UHPC significantly influences the material’s workability and mechanical strength. While increasing the fiber volume fraction generally improves the strength and durability of UHPC, it also presents challenges in terms of workability and mix handling. Therefore, optimizing the fiber content is essential to balance performance and ease of construction.

Figure 10. Pictures for cracking patterns [110].

Figure 10. Pictures for cracking patterns [110].

5.2.1. Impact on Workability

As the volume fraction of fibers increases, the workability of the mix typically decreases. This is due to the increased interaction between fibers and the concrete matrix, making it more difficult for the mixture to flow and compact properly. The presence of fibers creates a more cohesive mix, making it harder to achieve uniform distribution and increasing the likelihood of segregation. To mitigate these issues, superplasticizers are often added to improve flowability and maintain workability. However, beyond a certain volume fraction (typically above 2%), the mix may become difficult to handle without significant adjustments to the mix design [111].

5.2.2. Effect on Mechanical Strength

While the increase in fiber volume improves the mechanical properties of UHPC, such as tensile and flexural strength, the benefits are not unlimited. There is an optimal fiber content range, usually between 1%, 2%, and 3% by volume, where the mechanical properties, including crack resistance and toughness, are maximized. Beyond this range, as shown in Figure 5, the concrete may experience diminishing returns in terms of strength and may even suffer from reduced mechanical properties due to poor fiber dispersion and difficulties in mixing. The added fibers help distribute loads and reduce crack propagation, significantly enhancing the material’s resistance to cracking and improving its post-cracking behavior [3]. As illustrated in Figure 11, increasing the steel fiber content enhances both compressive and flexural strength up to an optimal range, beyond which performance may plateau or decline due to dispersion issues.

5.2.3. Trade-Offs Between Workability and Strength

The key challenge in fiber-reinforced UHPC is finding the optimal fiber volume that provides the desired mechanical performance without compromising workability. The correct balance allows for easy placement, compaction, and finishing while still achieving the enhanced strength and durability of the fibers. The fiber content should be carefully controlled to avoid the adverse effects of an excessive fiber volume on the concrete’s performance and handling characteristics.

5.3. Economic and Practical Considerations for Fiber-Reinforced UHPC

5.3.1. Cost–Benefit Analysis of Different Fiber Types

Selecting the type of fiber to use is critical in determining the overall cost-effectiveness of fiber-reinforced UHPC. The three main types of fibers used in UHPC—straight, hooked end, and corrugated—vary significantly in terms of performance, cost, and ease of integration into the concrete mix. A cost–benefit analysis helps balance these factors, allowing for selecting the most appropriate fiber type based on the specific requirements of a project, including performance criteria and budget constraints. A summary of recommended fiber types for various application scenarios and the rationale for selection is provided in

Table 2. Decision framework [110,112].

Scenario	Recommended Fiber	Rationale
Budget-constrained projects	Straight	Lowest cost and adequate for static loads.
High seismic zones	Hooked end	Superior crack resistance and worth 40% cost premium
Pumped UHPC applications	Corrugated	Balanced performance and flowability.

.

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Straight Fibers

Cost: straight fibers are typically the least expensive option. They are easier and cheaper to manufacture than other fiber types, making them a cost-effective choice for projects with tight budgets or where high performance is not the top priority [110].

Performance: While they improve concrete’s crack resistance and tensile strength, their crack-bridging and load transfer performance is less efficient compared to hooked-end fibers. They also tend to have lower pull-out resistance, which reduces their effectiveness in preventing crack propagation under stress.

Suitability: straight fibers are suitable for general applications where the primary concern is moderate strength and crack resistance without the need for advanced performance characteristics.

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Hooked-End Fibers

Cost: Hooked-end fibers are more expensive than straight fibers due to their complex manufacturing process. The hooks at the ends of the fibers provide superior anchorage, which improves their bonding with the concrete matrix [110].

Performance: These fibers offer the best load-transfer, crack-bridging, and post-cracking behavior. They are highly effective in enhancing the tensile strength, flexural strength, and overall durability of UHPC, making them ideal for high-performance applications.

Suitability: hooked-end fibers are recommended for applications that require high durability, such as bridges, tunnels, and other critical infrastructures where performance under heavy loads and exposure to harsh environments is essential.

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Corrugated Fibers

Cost: Corrugated fibers fall between straight and hooked-end fibers in terms of cost. They are more expensive than consecutive fibers but generally more affordable than hooked-end fibers [110].

Performance: Corrugated fibers provide a good balance of cost and performance. They improve crack resistance and enhance load transfer more effectively than straight fibers but not as efficiently as hooked-end fibers. The rough surface and shape of corrugated fibers offer better bonding with the concrete matrix, which enhances crack bridging and the load-transfer capacity.

Suitability: These fibers are perfect for projects that require a balance between performance and cost. They are frequently utilized in applications where high performance is necessary, but the budget cannot support the expense of hooked-end fibers.

5.3.2. Applications of Fiber-Reinforced UHPC in Civil Infrastructure

Fiber-reinforced UHPC has revolutionized civil infrastructure with its exceptional strength (150–250 MPa) and durability (chloride diffusion < 0.1 × 10⁻¹² m²/s) [113]. In bridges, UHPC allows for thinner decks (25–40 mm) that extend the service life by 3–5 times while reducing maintenance costs by 40% over 50 years [114]. The Route 29 Bridge in Virginia (2018) showcased its potential for accelerated construction, completed in just 60 days using precast UHPC elements [115].

Building construction benefits from UHPC’s high strength-to-weight ratio, allowing slender façade elements to be 50%–70% lighter than conventional cladding. Its seismic performance is particularly valuable, as beam-column joints maintain integrity at 5% drift ratios, which is a 300% better energy dissipation than standard concrete. Since 2015, over 50 high-rises in seismic zones have incorporated UHPC [116].

Transportation infrastructure uses UHPC for tunnel linings and railway sleepers. Single-pass tunnel linings can resist fire for over 2 h at 1200 °C, while UHPC sleepers last 2 to 3 times longer than timber and provide 8 to 10 dB noise reduction. Europe has installed more than 500,000 UHPC sleepers since 2015 [117].

UHPC demonstrates exceptional corrosion resistance in marine environments. While carbon steel in normal cement pastes corrodes due to chloride contamination, UHPC suppresses corrosion within days of casting [118]. Utilizing seawater and sea sand in UHPC production reduces costs and resource depletion without compromising durability [119].

UHPC is an innovative material that offers superior strength, durability, and sustainability compared to conventional concrete [120]. Despite its higher cost, UHPC’s advanced properties justify its use in critical applications and have the potential for life-cycle cost savings [121].

6. Sustainability and Environmental Impacts of UHPC Production

The incorporation of industrial by-products as supplementary cementitious materials (SCMs) in UHPC offers significant potential for reducing the carbon footprint of construction materials. By replacing 30%–50% of Portland cement with waste-derived materials like ground granulated blast furnace slag (GGBFS), silica fume, and fly ash, UHPC mixtures can achieve 40%–60% lower CO₂ emissions per cubic meter compared to conventional formulations [41]. This reduction primarily stems from avoiding the clinker production process, which accounts for approximately 90% of cement’s carbon footprint [122].

The carbon reduction potential varies by SCM type:

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GGBFS reduces CO₂ by 0.75–0.85 kg per kg of cement replaced [123].

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Class F fly ash provides 0.65–0.75 kg CO₂ reduction per kg [124].

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Silica fume offers 0.55–0.65 kg CO₂ reduction per kg [125].

Recent studies demonstrate that optimized UHPC mixes with a 50% SCM content can achieve compressive strengths exceeding 150 MPa while reducing embodied carbon by 45%–55% [41]. Alternative aggregates, like iron ore tailings, further enhance sustainability, minimizing natural resource consumption by 30%–40%.

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Key challenges in waste utilization include the following:

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Variable chemical compositions of industrial by-products.

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Potential Impacts on early-age strength development.

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Limited availability in some regions.

Emerging solutions involve advanced characterization techniques and blending optimization to ensure consistent performance. Life cycle assessment studies confirm that UHPC with a high SCM content can reduce the global warming potential by 50%–60% over its service life [34]. Figure 12 illustrates the mass and carbon flows within the concrete cycle, highlighting the major contributors to emissions—particularly cement production—and demonstrating the potential for carbon uptake during the use and demolition phases.

7. Standardization Challenges and Future Directions

The development of UHPC has revolutionized the construction industry due to its superior strength, durability, and aesthetic potential. However, as sustainability becomes an increasingly important factor in construction practices, there are several future directions for the sustainable development of UHPC:

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Incorporation of Sustainable Materials

• Recycled Aggregates: future UHPC formulations may include a higher proportion of recycled aggregates, such as crushed concrete from demolition, to lessen the reliance on natural raw materials.
• Industrial by-products, such as fly ash, slag, silica fume, and rice husk ash, serve as alternatives to traditional cement, helping to reduce the carbon footprint of UHPC production. These materials lower emissions and enhance UHPC’s specific performance properties.
• Bio-based Additives: innovations in bio-based polymers or fibers could replace synthetic additives, providing a more sustainable and environmentally friendly method to enhance UHPC’s performance.

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Reduction of Cement Content

Cement production is a significant contributor to greenhouse gas emissions. Future research should concentrate on reducing the cement content in UHPC while maintaining or enhancing its performance. Developing low-carbon binders or supplementary cementitious materials (SCMs) would be essential in achieving this goal.

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Energy-Efficient Manufacturing Processes

UHPC manufacturing is energy-intensive. Future developments should involve optimizing the mixing, curing, and molding processes to lower energy consumption. Research into low-energy curing methods, such as ambient or low-temperature curing, could significantly reduce the environmental impact of UHPC production.

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Life Cycle Assessment and Circular Economy Integration

To evaluate environmental impacts, more comprehensive life cycle assessments (LCAs) should be incorporated into UHPC development. This includes analyzing production and end-of-life scenarios, such as recycling and reusing UHPC materials. The circular economy model, which promotes the reuse or recycling of materials, could become integral to UHPC design, encouraging its repurposing at the end of the building’s life.

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Optimized Durability and Longevity

Sustainability in UHPC means reducing its environmental impact and improving its longevity. UHPC’s high durability reduces the need for maintenance and replacement, making it an inherently sustainable material. Future research should continue to optimize the durability properties of UHPC, particularly in aggressive environments (e.g., coastal or industrial zones), to extend the service life of structures and reduce the frequency of repairs.

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Carbon Capture and Storage (CCS) Technologies

Future directions should explore integrating carbon capture and storage technologies in UHPC production to reduce the carbon footprint further. This could involve capturing CO₂ emissions from manufacturing or using carbon dioxide to cure UHPC, which has been explored in some innovative cement production technologies.

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Hybrid UHPC Systems

UHPC could be combined with other sustainable construction materials to create hybrid systems, such as UHPC-reinforced timber or UHPC mixed with geopolymer concrete. This would take advantage of each material’s strengths while minimizing the use of energy-intensive components.

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Three-dimensional Printing of UHPC

Another promising avenue is the adoption of 3D printing for construction. Three-dimensional printing using UHPC could minimize waste, reduce labor costs, and allow for more intricate designs that are impossible or inefficient with traditional methods. Research into the sustainability of 3D-printed UHPC structures, such as reduced material usage and faster construction times, would support its integration into green building practices.

Despite the growing interest and promising performance of UHPC, especially those incorporating industrial by-products, the lack of unified international standards remains a major barrier to widespread adoption. Most existing standards are either country-specific (e.g., France’s NF P18-470 [127], China’s JG/T 398-2012 [128]) or project-specific and are often based on traditional cement–silica fume systems. There is currently no consensus on performance benchmarks, testing protocols, or material qualification criteria for UHPC made with alternative binder systems, such as slag–steel and slag–gypsum combinations.

To move toward broader applications and industry acceptance, performance-based standards should be prioritized over prescriptive formulations, focusing on properties such as strength, durability, workability, and sustainability. Additionally, the development of regional material databases, standardized field performance evaluations, and collaboration between academia, industry, and regulatory bodies will be essential. Recent examples, such as China’s T/CECS 689-2020 guideline for solid-waste-based UHPC, offer a valuable reference model for integrating sustainability into future UHPC standards.

8. Conclusions

This study contributes to the growing body of knowledge on ultra-high-performance concrete (UHPC) by focusing on the utilization of industrial by-products—specifically, steel slag, iron tailings, and desulfurization gypsum—as key components in the production of UHPC. While previous research has largely focused on the general properties and performance of UHPC, this work introduces a novel approach by integrating waste materials that reduce environmental impact and enhance the material’s mechanical performance.

The findings demonstrate that using these industrial by-products in UHPC not only supports sustainable construction practices by promoting recycling but also improves the material’s durability, strength, and resistance to environmental stressors. This innovative approach contributes significantly to the field of UHPC by presenting a cost-effective and environmentally friendly alternative to traditional concrete materials.

While industrial by-products offer promising pathways for sustainable UHPC, careful consideration of their variability and long-term performance is essential to ensure durability and safety in practical applications. Future research should focus on addressing key challenges such as developing more effective activation technologies for low-reactivity waste materials, evaluating the long-term durability of UHPC in harsh environmental conditions, and establishing standardized testing protocols for alternative binder systems. Additionally, scaling up 3D printing applications using waste-based UHPC, optimizing mix designs through AI-assisted methods, and exploring the use of waste-derived fibers offer promising avenues for advancing both performance and sustainability. These efforts will be critical for ensuring that UHPC not only meets structural and environmental standards but also evolves with the needs of modern construction practices.

While industrial by-products offer promising pathways for sustainable UHPC, careful consideration of their variability and long-term performance is essential to ensure durability and safety in practical applications.