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Article

Investigation into the Static Mechanical Properties of Ultra-High-Performance Geopolymer Concrete Incorporating Steel Slag, Ground Granulated Blast-Furnace Slag, and Fly Ash

by
Yan-Hua Cai
1,
Tao Huang
1,
Bo-Yuan Huang
1,
Chuan-Bin Hua
1,
Qiang Huang
1,
Jing-Wen Chen
1,
Heng-Liang Liu
2,
Zi-Jie He
2,
Nai-Bi Rouzi
2,
Zhi-Hong Xie
3 and
Gai Chen
2,*
1
Zhanjiang Power Supply Bureau, Guangdong Power Grid Co., Ltd., Zhanjiang 524200, China
2
School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou 510006, China
3
Navigational Engineering Department, Guangzhou Maritime University, Guangzhou 510725, China
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(14), 2535; https://doi.org/10.3390/buildings15142535
Submission received: 16 June 2025 / Revised: 12 July 2025 / Accepted: 16 July 2025 / Published: 18 July 2025
(This article belongs to the Special Issue Next-Gen Cementitious Composites for Sustainable Construction)
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Abstract

The utilization of steel slag (SS) in construction materials represents an effective approach to improving its overall recycling efficiency. This study incorporates SS into a conventional ground granulated blast-furnace slag (GGBS)–fly ash (FA)-based binder system to develop a ternary system comprising SS, GGBS, and FA, and investigates how this system influences the static mechanical properties of ultra-high-performance geopolymer concrete (UHPGC). An axial point augmented simplex centroid design method was employed to systematically explore the influence and underlying mechanisms of different binder ratios on the workability, axial compressive strength, and flexural performance of UHPGC, and to determine the optimal compositional range. The results indicate that steel slag has a certain negative effect on the flowability of UHPGC paste; however, with an appropriate proportion of composite binder materials, the mixture can still exhibit satisfactory flowability. The compressive performance of UHPGC is primarily governed by the proportion of GGBS in the ternary binder system; an appropriate GGBS content can provide enhanced compressive strength and elastic modulus. UHPGC exhibits ductile behavior under flexural loading; however, replacing GGBS with SS significantly reduces its flexural strength and energy absorption capacity. The optimal static mechanical performance is achieved when the mass proportions of SS, GGBS, and FA are within the ranges of 9.3–13.8%, 66.2–70.7%, and 20.0–22.9%, respectively. This study provides a scientific approach for the valorization of SS through construction material applications and offers a theoretical and data-driven basis for the mix design of ultra-high-performance building materials derived from industrial solid wastes.

1. Introduction

Ultra-high-performance concrete (UHPC) represents one of the most significant advancements in the field of civil engineering materials. Owing to its outstanding mechanical properties and durability, UHPC has emerged as a key material driving the development of modern construction toward lightweight and long-service-life structural systems [1,2,3,4,5]. Currently, UHPC has been successfully applied in various engineering domains, including long-span bridges, supertall buildings, protective structures for nuclear power plants, and the strengthening of existing infrastructure [6,7,8,9,10]. However, the production of UHPC typically involves a high dosage of cement, often exceeding 800 kg/m3 [11,12]. The cement industry is a well-known carbon-intensive sector, with the production of one ton of cement clinker emitting approximately 0.87 tons of CO2 into the atmosphere, in addition to consuming substantial amounts of non-renewable fossil energy [13,14,15]. From the perspective of sustainable development in the construction industry, the large-scale application of UHPC in practical engineering projects faces significant challenges [16].
To address certain limitations of UHPC, the development of ultra-high-performance geopolymer concrete (UHPGC) using low-cost industrial by-products such as fly ash (FA), ground granulated blast-furnace slag (GGBS), and red mud as alternatives to cement has emerged as a promising and environmentally friendly solution [17,18,19,20]. These industrial wastes are rich in reactive aluminosilicate compounds, which, under the activation of alkaline solutions, undergo dissolution, ionic migration and rearrangement, followed by polycondensation reactions to form stable three-dimensional polymeric gel networks [2,21,22,23,24]. Although the production of alkaline activators introduces some environmental burden, geopolymer binders still exhibit superior carbon emission control per unit compared to cement-based systems, primarily due to the elimination of high-temperature calcination and the effective utilization of industrial solid wastes [25]. Relevant studies have shown that the life-cycle carbon emission intensity of UHPGC is significantly lower than that of UHPC, demonstrating a transformative potential in carbon footprint reduction [26,27]. Furthermore, UHPGC exhibits superior corrosion resistance and fire resistance compared to conventional UHPC [28,29]. It also possesses desirable properties such as rapid setting and high early strength, making it particularly suitable for structural rehabilitation and strengthening applications [30,31]. On the basis of these advantages, further investigation into the influence mechanisms of geopolymer binder compositions on UHPGC performance is essential to provide a theoretical foundation for the synergistic optimization of material performance and carbon reduction [32,33,34].
Steel slag (SS) is a by-product generated during the steelmaking process, primarily formed through complex physicochemical reactions in the refining stage, and its discharge typically accounts for approximately 15% of crude steel output [35,36]. The physicochemical characteristics of SS are closely associated with the metallurgical route and raw material composition [37,38]. Although SS produced under different process parameters and post-treatment regimes—such as basic oxygen furnace slag, open-hearth slag, and electric arc furnace slag—shares common features in major oxide composition, the proportion of individual oxides can vary significantly depending on the process [39,40,41,42]. According to recent statistics, the annual SS generation in China has exceeded 150 million tons, while the comprehensive utilization rate remains relatively low at only 20–30%, far below that of European countries [43,44,45]. Conventional disposal methods, such as open-air stockpiling and simple landfilling, not only occupy vast industrial land over time and increase land-use pressure and management costs but may also cause environmental hazards due to the leaching of heavy metals (e.g., Cr, Pb) into soil and groundwater through rainwater infiltration [46,47,48,49]. Fortunately, SS exhibits latent hydraulic activity and can be used to partially replace cementitious materials, enabling large-scale applications in concrete admixtures, subgrade stabilization, and the development of novel eco-friendly construction materials [50,51,52]. This offers a crucial breakthrough for the resource utilization of SS. Xu et al. [53] and Liu et al. [54] investigated the effects of using steel slag as aggregate on the mechanical properties of asphalt concrete. Their results indicated that the porous surface characteristics of steel slag can effectively enhance the adhesion between asphalt and aggregate, thereby improving the load-bearing capacity of asphalt concrete. Yan et al. [55] reported that a synergistic hydration effect between SS and silica fume could produce UHPGC with a compressive strength as high as 120 MPa. Shi et al. [56] found that the incorporation of SS significantly prolonged the setting time of UHPGC, and the optimal static mechanical performance (compressive strength of 157.1 MPa and flexural strength of 16.3 MPa) was achieved when the mass ratio of SS to GGBS was 3:7. Xu et al. [57] developed a UHPGC system using GGBS, SS, and silica fume, and observed that increasing the SS-to-GGBS replacement ratio improved the workability of UHPGC, whereas both the compressive and flexural strengths decreased to varying extents. The interactions among multiple materials used in combination may contribute positively to the enhancement in concrete properties [58,59]. Filazi et al. [58] demonstrated that the interactions among cement, slag, and hemp fibers effectively improved the mechanical performance and water absorption characteristics of concrete. However, existing research primarily focuses on the influence of SS as a single supplementary material replacing highly reactive binders in UHPGC, with emphasis on microstructural mechanisms and macroscopic properties. Studies on the synergistic effects of ternary binder systems comprising SS, GGBS, and FA remain limited, leaving a research gap in this promising area.
To enhance the comprehensive utilization of SS and promote its application in construction materials, this study centers on a ternary binder system composed of SS, GGBS, and FA. An axial point augmented simplex centroid design method was employed to systematically vary the gradient ratios of the binder components. Based on alkali activation technology, an SS–GGBS–FA-based UHPGC was prepared. The synergistic effects of the ternary binder system on the workability, compressive strength, and flexural performance of UHPGC were thoroughly investigated. The optimal mix proportion range of the SS–GGBS–FA composite binder was identified to satisfy different static mechanical performance requirements of UHPGC.

2. Experimental Program

2.1. Raw Materials

The binders used for preparing the UHPGC primarily consisted of steel slag (SS), Grade 105 ground granulated blast-furnace slag (GGBS), and Class F fly ash (FA). To improve the flowability of the UHPGC, silica fume (SF) was additionally incorporated as a supplementary cementitious material [55,60]. SS was sourced from Yuanheng Water Purification Materials Co., Ltd. (Gongyi, China), while both GGBS and FA were supplied by Longze Water Purification Materials Co., Ltd. (Gongyi, China). SF was provided by Elkem International Trading (Shanghai) Co., Ltd. (Shanghai, China). The chemical compositions of the binder materials are listed in Table 1. Two types of fine aggregates with different particle size distributions—medium sand (MS) and fine sand (FS)—were used to minimize the porosity of the UHPGC [12]. These were supplied by Xiangyuan Co., Ltd. (Jiangmen, China). The particle size distributions of the binder and fine aggregates are shown in Figure 1.
An alkaline activator with a modulus of 1.5 was prepared by mixing a pre-formulated 14 mol/L sodium hydroxide solution (NH) and a sodium silicate solution (NS, modulus = 2.25, containing 29.99 wt% SiO2, 13.75 wt% Na2O, and 56.26 wt% H2O), the latter of which was supplied by Yourui Refractories Co., Ltd. (Jiaxing, China). The activator and water were added simultaneously during the mixing process using a planetary mixer. Barium chloride (BaCl2) was used as a set retarder due to its ability to react with sodium silicate, forming a barium silicate precipitate that deposits on the binder particle surfaces, producing an insoluble and dense layer that imparts a retarding effect [22,61]. Solid NaOH and BaCl2, with purities of 96% and 99%, respectively, were obtained from Xilong Scientific Co., Ltd. (Shantou, China). The steel fibers used in this study were straight steel fibers supplied by Shanchi Building Materials Co., Ltd. (Xingyang, China). These fibers were selected to ensure adequate bonding performance while avoiding the potential negative effects of deformed fibers on workability and mechanical property consistency. The physical properties of the fibers are summarized in Table 2.

2.2. Mix Proportions and Specimen Preparation

According to previous studies [1,22,57,62], the binder system of UHPGC matrices typically features a GGBS mass proportion exceeding 50%, while the proportions of SS and FA are generally below 20% and 30%, respectively. Therefore, in this study, the mass proportions of SS, GGBS, and FA in the SS–GGBS–FA ternary binder system were set within the ranges of 0–30%, 50–80%, and 20–50%, respectively, to comprehensively investigate the influence of each component on the performance of the UHPGC. The dosages of the SF and the retarder were fixed at 5% and 1% by mass of the total binder, respectively, while the steel fiber content was maintained at 2% by volume of UHPGC.
In addition, considering the limitation of the traditional simplex centroid design in capturing axial information [1,63], this study adopted an axial point augmented simplex centroid design to optimize the composition of the solid binder components in the SS–GGBS–FA ternary system of the UHPGC matrix. The distribution of the component design and the corresponding mix proportions of the UHPGC are shown in Figure 2 and Table 3, respectively. A total of ten mix combinations were developed. The performance responses of these ten binder combinations under different proportions are expressed as a function Y ( x 1 , x 2 , x 3 ) , as defined in Equation (1).
Y x 1 , x 2 , x 3 = β 1 x 1 + β 2 x 2 + β 3 x 3 + β 12 x 1 x 2 + β 13 x 1 x 3 + β 23 x 2 x 3 + β 123 x 1 x 2 x 3
Here, x 1 ( i = 1 , 2 , 3 ) represents the dosage of each raw material, while β i ,   β i j , and β i j w   ( i , j , w = 1 , 2 , 3 ) denote the coefficients of the main and interaction effects of the binder components—namely, SS, GGBS, and FA. For example, β 12 refers to the interaction coefficient between SS and GGBS.
A 30 L planetary mortar mixer was used for the preparation of the UHPGC, and the detailed mixing procedure is illustrated in Figure 3. Firstly, the binder materials were loaded into the mixer and dry-mixed for 3 min. Then, the fine aggregates and set retarder were added and mixed for an additional 3 min to ensure uniform distribution of all dry powders. Subsequently, the pre-prepared water and alkaline activator solution were slowly poured into the mixer and blended for 3 min. Finally, with the mixer running, steel fibers were gradually added and mixed for another 3 min. After mixing, the fresh UHPGC paste was cast into molds—pre-coated with a release agent—placed on a vibrating table to eliminate entrapped air. All molds were then covered with plastic sheets to prevent moisture loss and surface cracking during the initial setting phase. After 24 h of curing under ambient laboratory conditions, the specimens were demolded and transferred to a standard curing chamber at 20 ± 2 °C and relative humidity not less than 95% for 28 days before testing.

2.3. Test Setup and Loading

2.3.1. Flowability Test

The flowability of fresh UHPGC paste was evaluated in accordance with ASTM C1437-20 [64], using a standard flow table apparatus for mortar. Prior to testing, the conical mold was pre-wetted to achieve a saturated surface condition, ensuring it would not absorb additional water from the fresh UHPGC and thus preserving the accuracy of the test results. The fresh paste was then filled into the mold, and any excess material spilled on the flow table was removed. Subsequently, the mold was lifted vertically and slowly, and the flow table was actuated for 25 drops. The final spread diameter of the paste was measured using a steel square ruler.

2.3.2. Axial Compression Performance Test

The uniaxial compressive behavior of the UHPGC was evaluated in accordance with ASTM C469/C469M-22 [65], using cylindrical specimens with a diameter of 50 mm and a height of 100 mm. The test setup is illustrated in Figure 4a,b. A universal testing machine was employed to conduct the compression test under a loading rate of 0.2 mm/min [66]. To monitor the compressive deformation of the UHPGC, one strain gauge was affixed at the mid-height along the axial direction of each specimen, and two linear variable differential transformers (LVDTs) were positioned accordingly. The calculation formula for the elastic modulus is as follows:
E = S 2 − S 1 / ε 2 − 0.000050 ,
where S2 is the stress corresponding to 40% of the ultimate load; S1 is the stress corresponding to an axial strain of 0.000050; and ε2 is the longitudinal strain produced by stress S2.

2.3.3. Third-Point Flexural Performance Test

The flexural behavior of the UHPGC was assessed following the ASTM C78/C78M-22 [67] standard using prismatic specimens with dimensions of 40 mm (width) × 40 mm (height) × 120 mm (length). The experimental setup is shown in Figure 5a,b. A universal testing machine was employed to perform the three-point bending test with a loading rate of 0.1 mm/min and a span length of 120 mm. To record the mid-span deflection of the specimen, an LVDT was mounted beneath the loading head. The flexural strength and cracking strength of the UHPGC were determined using the conventional three-point bending strength formula [11]. According to relevant studies [68], the cracking point of UHPGC is identified as the point at which the load–deflection curve deviates from linearity. Furthermore, the flexural toughness of the UHPGC was quantified in accordance with ASTM C1609/C1609M-24 [69] by calculating the area under the load–deflection curve corresponding to mid-span deflections of L/600 and L/150, respectively.

3. Results and Discussion

3.1. Flowability

Figure 6 illustrates the flowability of the fresh UHPGC paste across different mix groups. The flow diameters of all fresh UHPGC pastes exceeded 250 mm, thereby meeting the requirements specified in the European Guidelines for Self-Compacting Concrete [70]. This satisfactory flowability can be attributed to the dispersing effect of NS present in the alkaline activator on geopolymer. In addition, the incorporation of SF as a supplementary cementitious material provided a lubricating effect, which helped reduce particle agglomeration and inter-particle friction or collision within the fresh matrix, thereby enhancing the flowability of the UHPGC [60,71,72].
To further clarify the influence of different binder proportions on the flowability of the fresh UHPGC paste, a contour plot of the flow diameter was constructed, as shown in Figure 7. Based on the experimental data, a quantitative relationship between flowability and the ternary composite binder system (SS–GGBS–FA) was established, as presented in Equation (3), with a coefficient of determination of R2 = 0.972:
Y f l = − 965 x 1 + 406 x 2 + 707 x 3 + 2312 x 1 x 2 + 3704 x 1 x 3 − 1149 x 2 x 3 − 8275 x 1 x 2 x 3
According to the regression equation, the univariate coefficient β1 for SS is negative, indicating that an increase in the SS content exerts a certain adverse effect on the flowability of the paste. This negative influence may be attributed to the following: (1) the hydration of free CaO in SS, which consumes available mixing water; and (2) the loose and porous structure of SS, which absorbs water and reduces the amount of free water available in the mix [63]. In contrast, both GGBS and FA exhibit positive effects on the flowability of the UHPGC paste. This is mainly due to the spherical morphology of FA, which induces the so-called “ball-bearing effect,” facilitating the movement of solid particles within the paste [73,74]. Previous studies have reported that the high reactivity of GGBS could lead to early consumption of Ca species and subsequently reduce paste flowability [75,76]. However, a comparison between experimental groups with constant GGBS content—such as RUN 1 vs. RUN 7 and RUN 4 vs. RUN 9—and those with constant FA content—such as RUN 2 vs. RUN 7 and RUN 3 vs. RUN 8—shows that the influence of varying SS content on flowability is not consistent. This suggests that additional mechanisms may be involved in the composite binder system. As shown in Figure 1, SS, GGBS, and FA used in this study exhibit significantly different particle size distributions, which together satisfy the “overall wide distribution with locally narrow ranges” criterion required for dense particle packing. Therefore, the observed inconsistencies in flowability trends may result from the formation of a densely packed particle matrix at certain proportions, which reduces the demand for free water and thereby enhances flowability. This also highlights the potential advantage of composite binder systems.

3.2. Compressive Behaviors

3.2.1. Failure Mode

Figure 8 illustrates the typical failure modes of the UHPGC specimens under uniaxial compression for different mix groups. Unlike conventional concrete, which tends to fail in a brittle and fragmented manner, UHPGC retains a relatively high degree of integrity after reaching peak stress. Multiple secondary cracks develop around the primary crack, which is mainly attributed to the crack-bridging effect of steel fibers. These fibers help suppress crack propagation and redistribute localized stress to surrounding uncracked areas, thereby mitigating stress concentration and contributing to a failure mode with noticeable ductile characteristics [77]. Moreover, as shown in Figure 8, variations in the proportions of components within the ternary binder system lead to corresponding changes in UHPGC failure modes, which can be categorized as vertical splitting failure, diagonal shear failure, or mixed-mode failure with intersecting cracks. For UHPGC with a denser matrix and fewer internal voids—such as mix S0G80F20 (with high GGBS content and more complete alkali activation)—energy tends to accumulate internally under load without gradual dissipation, resulting in a highly brittle vertical splitting failure upon fracture [78]. In contrast, specimens containing SS exhibit a principal crack oriented at an angle to the specimen’s axis, indicative of a typical shear failure mode. This behavior can be attributed to the low-calcium binder system, which limits the formation of C-A-S-H gel and leads to a more porous matrix structure with weaker fiber–matrix interfacial bonding [79].

3.2.2. Stress–Strain Curves and Compressive Parameters

Figure 9 presents the stress–strain curves of the UHPGC specimens from different mix groups under uniaxial compression. Based on these curves, the compressive strength and elastic modulus of all the UHPGC mixes were determined, and the corresponding test results are summarized in Table 4. Overall, the UHPGC designed in this study exhibited high compressive strength and elastic modulus. Among them, mix S0G50F50 recorded the lowest compressive strength at 81.38 MPa, which still meets the performance criteria for high-strength concrete. In comparison, the compressive strengths of S15G50F35 and S30G50F20 are 10.3% and 15.6% higher than that of S0G50F50, respectively, indicating that the incorporation of SS enhances the mechanical properties and highlights the advantages of the blended binder system.
To further investigate the influence of the SS–GGBS–FA ternary binder system on the compressive behavior of UHPGC, contour plots of the compressive strength and elastic modulus were generated, as shown in Figure 10. A quantitative correlation between the compressive strength and the ternary binder composition was established, as expressed in Equation (4), with an R2 value of 0.958:
Y c = 189 x 1 + 96.7 x 2 − 149 x 3 − 189 x 1 x 2 + 252 x 1 x 3 + 427 x 2 x 3 − 330 x 1 x 2 x 3
As shown in Equation (4), the influence coefficients of SS and GGBS ( β 1 and β 2 ) are both positive, indicating their beneficial effect on the compressive strength of UHPGC. In particular, when the GGBS content in the ternary binder system is relatively low (<60%), increasing the proportion of SS leads to a noticeable enhancement in compressive strength. This suggests that the incorporation of SS may improve the macroscopic mechanical properties of UHPGC through certain mechanisms. Similarly, a comparison between mixes with constant GGBS content—such as RUN 1 vs. RUN 7 and RUN 4 vs. RUN 9—and those with constant FA content—such as RUN 2 vs. RUN 7 and RUN 3 vs. RUN 8—reveals that replacing part of the FA with SS while keeping the GGBS content unchanged can enhance the compressive strength. In contrast, when the FA content is fixed and SS is used to replace GGBS, a reduction in compressive strength is observed. One possible explanation is that SS typically has a smaller particle size, which helps disperse agglomerates in the fresh UHPGC mixture, increases the contact surface area between the binder and the alkaline activator, promotes the dissolution of reactive species, and contributes to void filling and pore structure refinement [80,81]. Another possible explanation is that the CaO content in SS is significantly higher than that in FA and is comparable to that of GGBS, thereby introducing more soluble Ca2+ into the ternary binder system, facilitating the formation of C-A-S-H gels that act as nucleation sites. This accelerates the polymerization reaction and generates more reaction products, thereby enhancing matrix densification [82,83]. The interaction coefficients β 13 and β 23 , representing the synergistic effects of SS–FA and GGBS–FA, are also positive, suggesting that these combinations are favorable for improving the compressive strength of UHPGC. This also indicates that SS, with its high calcium content, has a certain potential to partially substitute GGBS. In contrast, the interaction coefficient β 12 between SS and GGBS is negative, which may be attributed to a deficiency in silica and alumina caused by their combination, leading to strength degradation.
Based on the experimental results, a quantitative correlation between the elastic modulus and the ternary composite binder system was also established, as shown in Equation (5), with an R2 value of 0.928:
Y E = − 414 x 1 + 16.8 x 2 + 38.9 x 3 + 925 x 1 x 2 + 1313 x 1 x 3 − 51.0 x 2 x 3 − 2954 x 1 x 2 x 3
It is noteworthy that although the UHPGC exhibited the highest compressive strength when the proportion of GGBS in the ternary binder system reached its maximum, its elastic modulus did not achieve the optimum value. We believe that the elastic modulus does not necessarily increase in direct proportion to the compressive strength. One possible explanation for this discrepancy is matrix shrinkage. During early-age shrinkage or drying, internal microcracks may develop within the material, which can reduce the stiffness while having a limited impact on the compressive strength [84]. Previous studies have shown that the incorporation of steel slag can mitigate autogenous shrinkage in alkali-activated pastes [85]. Therefore, the S0G80F20 mixture, which does not contain steel slag, may have experienced greater shrinkage-induced internal damage compared to the mixes containing slag. This could explain why S0G80F20, despite exhibiting the highest compressive strength, showed a relatively lower elastic modulus. As indicated by the parameters in Equation (5), the combinations of SS–GGBS and SS–FA had the most significant positive effects on the elastic modulus. This suggests that an appropriate Ca/Si ratio in the binder system is beneficial for ensuring a high level of elastic modulus in UHPGC.

3.3. Flexural Behaviors

3.3.1. Failure Mode

Figure 11 shows the typical failure patterns of the UHPGC from different groups after the three-point bending tests. As observed in the figure, all failed UHPGC specimens exhibited a certain degree of ductility, with an irregularly developed primary crack forming in the flexural–shear zone at mid-span. This behavior can be attributed to the significant chemical bonding and mechanical friction between the steel fibers and the matrix, which effectively suppressed the initiation of microcracks in the UHPGC and hindered the propagation and coalescence of existing cracks [60].

3.3.2. Load–Deflection Curves and Flexural Parameters

Figure 12 presents the load–deflection curves of the UHPGC from different groups obtained through three-point bending tests. All the UHPGC specimens exhibited a generally consistent trend in their load–deflection behavior. In the elastic stage, the homogeneous material composed of fibers and matrix jointly resisted the applied load until the tensile stress at the bottom exceeded the matrix strength, leading to the formation of the first crack. However, the UHPGC did not fail immediately upon crack initiation; instead, it continued to carry load due to the bridging effect of the steel fibers. As the load increased, microcracks within the UHPGC further developed and propagated, and the bottom tensile stress approached the critical threshold of the fiber bridging capacity, reaching the peak load. At this stage, the UHPGC exhibited a distinct deflection-hardening behavior [86,87]. Subsequently, as the steel fibers were progressively pulled out from the cracks, their bridging effect weakened rapidly, leading to the formation of the main crack and transition into the softening phase. Notably, the UHPGC demonstrated pronounced post-peak ductility, which is beneficial for enhancing structural safety and disaster resistance.
Table 5 summarizes the flexural performance parameters of the UHPGC from different groups. The cracking strength, flexural strength, and toughness at L/150 of the UHPGC ranged from 4.50 to 8.39 MPa, 5.07 to 9.51 MPa, and 1131 to 2317 N·mm, respectively. Furthermore, as shown in Table 5, within the ternary cementitious system, when the proportion of FA remained constant, increasing the SS content while decreasing the GGBS content (comparing S0G80F20, S15G65F20, and S30G50F20) resulted in a gradual decrease in the UHPGC’s cracking strength, flexural strength, and toughness at L/150, with maximum reductions of 20.6%, 25.6%, and 27.4%, respectively. This indicates that replacing GGBS with SS as a cementitious material adversely affects the flexural performance of UHPGC. The underlying reason is that SS contains more unhydrated γ-C2S and RO phases (solid solutions of CaO, MgO, FeO, and MnO) compared to GGBS, exhibiting limited reactivity and requiring further activation treatment [88]. Zhao et al. [89] reported that SS and GGBS have a synergistic hydration effect, with the bending strength of geopolymer mortar reaching a peak when the mass ratio of SS to GGBS in the cementitious materials was 2:3. The explanation for this is that the SS powder used by Zhao et al. had a smaller median particle size (18.16 μm) and higher CaO content (46.28 wt%), thereby fully utilizing the reactivity of SS.
To further investigate the influence of the SS-GGBS-FA ternary cementitious system on the flexural performance of UHPGC, contour maps of the cracking deflection, cracking strength, peak deflection, flexural strength, toughness at L/600, and toughness at L/150 were plotted, as shown in Figure 13. From Figure 13a,c, it is clearly observed that when the SS content remains constant in the ternary system, increasing the GGBS content while decreasing the FA content causes the cracking deflection and peak deflection of the UHPGC to first increase and then decrease. The appropriate introduction of GGBS is beneficial for enhancing the flexural deformation capacity of UHPGC. Moreover, Figure 13b,d–f show similar contour patterns for the strength and toughness parameters of the UHPGC’s flexural performance.
Based on the experimental results, quantitative correlations between the flexural strength and flexural toughness (at L/150) with the ternary cementitious system were established, as expressed in Equations (6) and (7), with R2 values of 0.823 and 0.797, respectively, as follows:
Y F = − 113 x 1 + 7.09 x 2 + 10.2 x 3 + 26 x 1 x 2 + 537 x 1 x 3 + 3.60 x 2 x 3 − 1226 x 1 x 2 x 3
Y T = 10 3 × ( − 22.6 x 1 + 2.35 x 2 + 4.75 x 3 + 51.6 x 1 x 2 + 115 x 1 x 3 − 4.91 x 2 x 3 − 263 x 1 x 2 x 3 )
Combining Figure 12 and Equations (6) and (7), it can be seen that lower proportions of SS in the ternary cementitious system correspond to regions on the contour map where the UHPGC exhibits higher strength and energy absorption capacity. Taking S0G50F50 as an example, it shows the highest strength and toughness among all UHPGC groups. This is likely because S0G50F50 features stronger chemical bonding at the fiber–matrix interface and a more uniform fiber distribution, enabling steel fibers to more effectively transfer stress during crack propagation while delaying fiber pullout [90].

3.4. Design of Ternary Cementitious System UHPGC

As previously discussed, the compressive and flexural performances of the UHPGC vary significantly under different proportions of the SS–GGBS–FA ternary binder system. To design UHPGC with both favorable compressive and flexural properties, this study applied the augmented centroid design method with axial points to optimize the mix proportions of the ternary binder system. Based on the previous literature [91,92,93], contour maps of four performance indicators—compressive strength, elastic modulus, flexural strength, and flexural toughness at L/150—were plotted. A multi-objective superimposition analysis method was adopted to integrate the boundary conditions of these performance parameters within a unified coordinate system, thereby achieving the coordinated design of UHPGC’s compressive and flexural performance. According to the material characteristics obtained in this study, a compressive strength > 100 MPa, elastic modulus > 18 GPa, flexural strength > 8 MPa, and flexural toughness at L/150 > 1800 N·mm were selected as the threshold values for the overlapping contour analysis. The resulting optimized mix design range is shown in Figure 13. The gray shaded area represents the optimal region of the SS–GGBS–FA ternary binder system that satisfies the comprehensive performance criteria.
As shown in Figure 14, when the binder system comprises 9.3–13.8% SS, 66.2–70.7% GGBS, and 20.0–22.9% FA by mass, the coordinated optimization of the UHPGC’s compressive and flexural properties can be achieved. This optimized range enables a balanced improvement in the key performance indicators of the material and fulfills the integrated performance requirements of UHPGC for engineering applications.

4. Conclusions

This study employed an augmented simplex centroid design method to formulate 10 groups of UHPGC based on an SS–GGBS–FA ternary binder system, investigating the influence of the binder proportions on the workability, uniaxial compressive behavior, and flexural performance of UHPGC. An optimal range of mix proportions achieving balanced comprehensive performance was identified. The main conclusions are as follows:
  • All groups of freshly mixed UHPGC pastes exhibited good workability, with flow diameters exceeding 250 mm. An appropriate proportioning of the ternary composite binder system can mitigate the negative impact of steel slag on the flowability of UHPGC paste.
  • The 10 UHPGC mixtures designed by the augmented simplex centroid method maintained good integrity during uniaxial compressive testing. Variation in the proportions of steel slag, GGBS, and fly ash in the ternary binder system altered the failure modes of the UHPGC. High-calcium systems tend to exhibit vertical splitting failure, whereas low-calcium systems are more prone to diagonal shear failure. Increasing the GGBS content within the ternary binder system helped maintain high levels of compressive strength and elastic modulus.
  • In the three-point bending tests, UHPGC specimens with different ternary binder ratios consistently exhibited a certain degree of ductility after failure. Replacing GGBS with steel slag as a binder component led to a reduction in the flexural performance of the UHPGC.
  • Utilizing the augmented simplex centroid design method and considering multiple mechanical performance indicators, an optimal mix proportion range was established. When the proportions of steel slag, GGBS, and fly ash were controlled within 9.3–13.8%, 66.2–70.7%, and 20.0–22.9%, respectively, the UHPGC met the key performance criteria of a uniaxial compressive strength > 100 MPa, elastic modulus > 18 GPa, flexural strength > 8 MPa, and toughness at L/150 > 1800 N·mm simultaneously.
Although this study focused on the static mechanical properties of UHPGC, its long-term durability (including shrinkage behavior, carbonation resistance, and performance at elevated temperatures) is crucial for practical applications. Therefore, the authors plan to further investigate the long-term durability of UHPGC incorporating the SS–GGBS–FA ternary binder system in future studies.

Author Contributions

Conceptualization, Y.-H.C. and G.C.; methodology, Y.-H.C., T.H. and G.C.; software, T.H., C.-B.H. and N.-B.R.; validation, Y.-H.C. and Q.H.; formal analysis, T.H., J.-W.C., H.-L.L. and Z.-J.H.; investigation, T.H. and C.-B.H.; resources, Y.-H.C.; data curation, G.C.; writing—original draft preparation, Y.-H.C., T.H. and B.-Y.H.; writing—review and editing, Y.-H.C. and G.C.; visualization, Q.H.; supervision, G.C.; project administration, Z.-H.X. and G.C.; funding acquisition, Z.-H.X. and G.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Zhanjiang Power Supply Bureau, Guangdong Power Grid Co., Ltd., China Southern Power Grid Co., Ltd., grant number 0308002025030103JJ00001.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to confidentiality issues.

Acknowledgments

The authors gratefully acknowledge the financial support provided by Zhanjiang Power Supply Bureau, Guangdong Power Grid Co., Ltd., China Southern Power Grid Co., Ltd.

Conflicts of Interest

Authors Yan-Hua Cai, Tao Huang, Bo-Yuan Huang, Chuan-Bin Hua, Qiang Huang and Jing-Wen Chen were employed by the company Zhanjiang Power Supply Bureau, Guangdong Power Grid Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The sponsors had no role in the design, execution, interpretation, or writing of the study.

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Figure 1. Particle size distribution curves of binder materials and aggregates.
Figure 1. Particle size distribution curves of binder materials and aggregates.
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Figure 2. UHPGC binder material composition distribution.
Figure 2. UHPGC binder material composition distribution.
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Figure 3. The preparation and curing process of specimens.
Figure 3. The preparation and curing process of specimens.
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Figure 4. Test setup of axial compressive test. (a) Three-dimensional diagram of test setup; (b) loading device.
Figure 4. Test setup of axial compressive test. (a) Three-dimensional diagram of test setup; (b) loading device.
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Figure 5. Test setup of third-point flexural test. (a) Three-dimensional diagram of test setup; (b) loading device.
Figure 5. Test setup of third-point flexural test. (a) Three-dimensional diagram of test setup; (b) loading device.
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Figure 6. The flowability of freshly mixed UHPGC paste.
Figure 6. The flowability of freshly mixed UHPGC paste.
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Figure 7. The contour plot of flowability for freshly mixed UHPGC paste.
Figure 7. The contour plot of flowability for freshly mixed UHPGC paste.
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Figure 8. The typical axial compression failure modes of UHPGC under different mix proportions.
Figure 8. The typical axial compression failure modes of UHPGC under different mix proportions.
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Figure 9. The axial compressive stress–strain curves of UHPGC with different mix proportions.
Figure 9. The axial compressive stress–strain curves of UHPGC with different mix proportions.
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Figure 10. The contour plot of the axial compression performance parameters. (a) Compressive strength, (b) elastic modulus.
Figure 10. The contour plot of the axial compression performance parameters. (a) Compressive strength, (b) elastic modulus.
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Figure 11. The typical three-point flexural failure modes of UHPGC under different mix proportions.
Figure 11. The typical three-point flexural failure modes of UHPGC under different mix proportions.
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Figure 12. The three-point flexural load–deflection curves of UHPGC with different mix proportions.
Figure 12. The three-point flexural load–deflection curves of UHPGC with different mix proportions.
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Figure 13. The contour plot of the three-point flexural performance parameters. (a) Crack deflection, (b) crack strength, (c) peak deflection, (d) flexural strength, (e) toughness at L/600, (f) toughness at L/150.
Figure 13. The contour plot of the three-point flexural performance parameters. (a) Crack deflection, (b) crack strength, (c) peak deflection, (d) flexural strength, (e) toughness at L/600, (f) toughness at L/150.
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Figure 14. Optimal ternary cementitious system UHPGC.
Figure 14. Optimal ternary cementitious system UHPGC.
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Table 1. Chemical composition of binder materials (wt%).
Table 1. Chemical composition of binder materials (wt%).
MaterialsSiO2Al2O3Fe2O3CaOK2OSO3MgOOther
SS16.828.1929.5133.520.100.774.097.00
GGBS34.5017.701.0334.0001.646.015.12
FA53.9731.154.164.012.0401.013.67
SF95.450.300.111.201.140.390.660.75
Table 2. Properties of steel fiber.
Table 2. Properties of steel fiber.
ShapeLength (mm)Diameter (mm)Tensile Strength (MPa)Aspect Ratio (L/d)
Straight130.2260065
Table 3. Mix proportions (kg/m3).
Table 3. Mix proportions (kg/m3).
RunMix IDsSSGGBSFASFAggregateActivatorWaterBaCl2Steel Fiber
FSMSNHNS
1S0G50F50 104304304527263468361789157
2S0G65F350559301
3S0G80F200688172
4S5G55F4043473344
5S5G70F2543602215
6S10G60F3086516258
7S15G50F35129430301
8S15G65F20129559172
9S20G55F25172473215
10S30G50F20258430172
1. The mix ID “S0G50F50” denotes that the mass proportions of SS, GGBS, and FA in the SS–GGBS–FA ternary binder system are 0, 50%, and 50%, respectively. Other mix IDs follow the same naming convention.
Table 4. The axial compression performance parameters of UHPGC with different mix proportions.
Table 4. The axial compression performance parameters of UHPGC with different mix proportions.
RunMix IDsCompressive Strength (MPa)Elastic Modulus (GPa)
1S0G50F5081.38 (0.77)15.50 (1.60)
2S0G65F35109.54 (1.99)13.44 (1.20)
3S0G80F20116.76 (1.11)13.16 (0.44)
4S5G55F4089.67 (1.24)11.04 (0.15)
5S5G70F25104.54 (5.30)14.68 (0.61)
6S10G60F30102.41 (2.48)12.77 (0.38)
7S15G50F3589.75 (3.27)11.81 (1.24)
8S15G65F2099.77 (2.01)21.63 (1.43)
9S20G55F2596.11 (2.14)17.52 (2.37)
10S30G50F2094.05 (1.90)15.48 (4.02)
Note: Values in parentheses represent the standard deviations of the test results from three specimens.
Table 5. The three-point flexural performance parameters of UHPGC with different mix proportions.
Table 5. The three-point flexural performance parameters of UHPGC with different mix proportions.
RunMix IDsCrack Deflection (mm)Crack Strength (MPa)Peak Deflection (mm)Flexural Strength (MPa)Toughness at L/600 (N⋅mm)Toughness at L/150 (N⋅mm)
1S0G50F500.167 (0.047)8.39 (0.720)0.293 (0.138)9.51 (1.17)446 (77)2317 (82)
2S0G65F350.195 (0.107)7.95 (0.70)0.317 (0.161)8.67 (0.20)351 (118)1997 (53)
3S0G80F200.107 (0.034)7.18 (0.75)0.281 (0.085)7.99 (0.43)390 (78)1972 (149)
4S5G55F400.190 (0.031)7.24 (0.41)0.340 (0.068)7.59 (0.11)291 (30)1787 (28)
5S5G70F250.239 (0.073)8.08 (0.32)0.359 (0.031)8.46 (0.30)298 (132)1964 (115)
6S10G60F300.082 (0.034)5.77 (0.49)0.304 (0.205)6.53 (0.14)377 (11)1575 (148)
7S15G50F350.153 (0.074)6.59 (0.29)0.303 (0.140)6.76 (0.32)326 (33)1672 (83)
8S15G65F200.271 (0.044)7.77 (0.45)0.393 (0.120)7.92 (0.37)292 (42)1823 (76)
9S20G55F250.102 (0.039)4.50 (0.42)0.230 (0.118)5.07 (0.14)246 (40)1131 (87)
10S30G50F200.123 (0.037)6.66 (0.48)0.193 (0.067)7.07 (0.13)399 (43)1682 (55)
Note: Values in parentheses represent the standard deviations of the test results from three specimens.
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MDPI and ACS Style

Cai, Y.-H.; Huang, T.; Huang, B.-Y.; Hua, C.-B.; Huang, Q.; Chen, J.-W.; Liu, H.-L.; He, Z.-J.; Rouzi, N.-B.; Xie, Z.-H.; et al. Investigation into the Static Mechanical Properties of Ultra-High-Performance Geopolymer Concrete Incorporating Steel Slag, Ground Granulated Blast-Furnace Slag, and Fly Ash. Buildings 2025, 15, 2535. https://doi.org/10.3390/buildings15142535

AMA Style

Cai Y-H, Huang T, Huang B-Y, Hua C-B, Huang Q, Chen J-W, Liu H-L, He Z-J, Rouzi N-B, Xie Z-H, et al. Investigation into the Static Mechanical Properties of Ultra-High-Performance Geopolymer Concrete Incorporating Steel Slag, Ground Granulated Blast-Furnace Slag, and Fly Ash. Buildings. 2025; 15(14):2535. https://doi.org/10.3390/buildings15142535

Chicago/Turabian Style

Cai, Yan-Hua, Tao Huang, Bo-Yuan Huang, Chuan-Bin Hua, Qiang Huang, Jing-Wen Chen, Heng-Liang Liu, Zi-Jie He, Nai-Bi Rouzi, Zhi-Hong Xie, and et al. 2025. "Investigation into the Static Mechanical Properties of Ultra-High-Performance Geopolymer Concrete Incorporating Steel Slag, Ground Granulated Blast-Furnace Slag, and Fly Ash" Buildings 15, no. 14: 2535. https://doi.org/10.3390/buildings15142535

APA Style

Cai, Y.-H., Huang, T., Huang, B.-Y., Hua, C.-B., Huang, Q., Chen, J.-W., Liu, H.-L., He, Z.-J., Rouzi, N.-B., Xie, Z.-H., & Chen, G. (2025). Investigation into the Static Mechanical Properties of Ultra-High-Performance Geopolymer Concrete Incorporating Steel Slag, Ground Granulated Blast-Furnace Slag, and Fly Ash. Buildings, 15(14), 2535. https://doi.org/10.3390/buildings15142535

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