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Article

Compressive Strength and Microstructure of Multi-Solid Waste Concrete Incorporated with Iron Tailings–Steel Slag–Desulfurization Ash

by
Chuanhua Zhao
1,*,
Yannian Zhang
2,
Jianbin Zhao
3,
Hui Zhang
1 and
Hao Chen
3
1
School of Intelligent Construction, Jilin University of Architecture and Technology, Changchun 130114, China
2
School of Transportation Engineering, Dalian Jiaotong University, Dalian 116028, China
3
School of Civil Engineering, Shenyang Jianzhu University, Shenyang 110168, China
*
Author to whom correspondence should be addressed.
Buildings 2026, 16(7), 1382; https://doi.org/10.3390/buildings16071382
Submission received: 6 January 2026 / Revised: 19 March 2026 / Accepted: 26 March 2026 / Published: 1 April 2026
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Versions Notes

Abstract

Iron tailings, steel slag (SS), and desulfurization ash (DA) are industrial solid wastes with high annual output and large stockpiles. To enhance their utilization rate in concrete and fully utilize the synergistic effect of iron tailings powder (ITP), SS, and DA, a multi-solid-waste ISD (ITP-SS-DA) concrete was prepared. In this study, ITP, SS, and DA were used as composite mineral admixtures to replace 30% of the cement, and iron tailings sand (ITS) and iron tailings waste rock (ITR) were used as aggregates. The effects of water/binder ratio (w/b), ITP fineness, and mineral admixture proportion on the compressive strength of ISD concrete were investigated. The influence of ITP fineness on the microstructure was analyzed based on mercury intrusion porosimetry (MIP) and backscattered electron (BSE) tests. The results show that the w/b has a significant effect on the early-age compressive strength, but its effect diminishes at mid-to-late ages. ISD composite mineral admixtures with properly ball-milled ITP enhance compressive strength, refine the pore structure, and increase the compactness of the interfacial transition zone (ITZ). Appropriately increasing the proportion of SS and adjusting the ratio of ITP to DA can promote the synergistic effect of mineral admixtures, thus enhancing compressive strength. Compared with cement concrete, ISD concrete exhibits slightly lower compressive strength but still meets the design requirements and presents a significantly superior microstructure when the w/b, ITP fineness, and admixture proportion are suitable.

1. Introduction

With the rapid development of industry, sectors such as mining, chemical, steel, and coal power have generated a large amount of industrial solid waste, and its production and stockpiling increase yearly. According to the relevant literature, the total stockpile of industrial solid waste in China has reached approximately 60 billion tons, and with an annual increase of about 3 billion tons [1,2]. Among them, iron tailings, steel slag (SS), and desulfurization ash (DA) are the main industrial solid wastes. Iron tailings are produced during the beneficiation process of iron ore [3], characterized by high reserves (over 5 billion tons) and annual output (over 300 million tons), yet have a utilization rate of less than 30% [4,5]. SS is a by-product produced from the steelmaking process [6,7]. In China, more than 100 million tons of SS are generated annually, but its utilization rate remains below 30% [6,7]. As another by-product, DA is produced from flue gas desulfurization in steel plants, including dry and semi-dry desulfurization processes [8,9]. The annual output of semi-dry flue gas DA in China is approximately 20 million tons, with its cumulative stockpile reaching billions of tons [8]. Nevertheless, the utilization rate of such DA is less than 10% [9]. Given the above, owing to the low utilization rates of these three major solid wastes, the large stockpiles not only occupy massive amounts of land but also cause serious pollution to water, air, and soil if improperly disposed [2,4,5,7,8,9,10,11,12,13]. Therefore, how to efficiently utilize these bulk solid wastes, advance their resource utilization rate, and establish a green industrial model has become a core issue requiring an urgent solution.
Studies have shown that the utilization of iron tailings, SS, and DA in building materials is an effective disposal method. Since iron tailings mainly contain SiO2, Fe2O3, and Al2O3 [10,11], numerous studies have demonstrated that they can be used as aggregates or supplementary cementitious materials [3,4,5,10,11,13]. Regarding aggregates, some studies [14,15,16,17,18] have investigated the effects of iron tailings sand (ITS) content and substitution methods on concrete or cement mortar. The results indicated that, with the proper incorporation of ITS, both mechanical strength and durability met design requirements and were even improved to some extent. Similarly, Kang et al. [19] showed that concrete with iron tailings fine and coarse aggregates exhibited superior compressive strength and chloride ion resistance compared to conventional concrete. On the other hand, as a supplementary cementitious material, iron tailings powder (ITP) has been widely investigated. However, the large number of crystalline phases in ITP leads to low reactivity and restricts its dosage [3,10,11]. Therefore, existing studies have mainly focused on its dosage and activation methods, as well as their effects on the properties of mortar and concrete. Liu et al. [20] found that the pozzolanic activity of ITP was related to its specific surface area, and that ITP with a large specific surface area promoted hydration product formation, refined the pore structure, and enhanced the late-age compressive strength of cement paste. The research of Gu et al. [21] showed that mechanically activated iron ore tailings were suitable as a supplementary cementitious material, and its dosage in ultra-high-performance concrete did not exceed 20%. Cheng et al. [22] indicated that the maximum cement replacement rate of mechanically activated ITP was 30% in common concrete, and it could reach up to 40% with a proper amount of water-reducing agent. Zhang et al. [23] demonstrated that both mechanical and mechanochemical activation can effectively improve the chemical activity of ITP, as well as enhance the compressive strength and refine the pore structure of iron ore tailing-based ternary supplementary cementitious materials. In addition, Cheng’s team also investigated the effects of siliceous ITP on the properties of the interfacial transition zone (ITZ) [24] and the durability of concrete [25].
SS is mainly classified into basic oxygen furnace slag and electric arc furnace slag based on the production process, with the former accounting for a relatively high proportion, and its main components are CaO, SiO2, Al2O3, and Fe2O3 [12]. Although the hydration process of SS is similar to that of cement [26], its cementitious activity is much lower [26,27,28]. Therefore, with SS used as a supplementary cementitious material, scholars have primarily focused on the effects of SS content, fineness, and the incorporation of active admixtures or additives on the properties of mortar or concrete. Wang et al. [27] found that increasing the SS content had a negative effect on the strength and durability of concrete, while this negative effect was weaker at lower w/b. Han et al. [29] indicated that elevated temperature obviously promoted the hydration of blended cement with SS. Shi et al. [30] found that blended mortar incorporating 10% superfine SS as a cement replacement exhibited higher 28-day compressive strength than pure cement mortar, whereas the strength gradually decreased when the replacement ratio exceeded 10%. Pang et al. [31] reported that superfine SS possessed higher hydration activity but inhibited cement hydration. Moreover, due to the low activity of SS, it was often combined with highly active admixtures in mortar or concrete [5,12,32,33,34,35,36,37].
DA is mainly composed of CaO and SO3. However, owing to its complex composition and the adverse effects of f-CaO and CaSO3, which readily induce hydration expansion at room temperature, its large-scale application has been restricted [38,39]. Moreover, as reported in the literature, DA is rarely used alone as a supplementary cementitious material in cement mortar or concrete. Most studies have focused on its blending with other mineral admixtures or activators, yet the incorporation content of DA remains relatively low [9,37].
Due to the limitations of using ITP, SS, and DA as single auxiliary cementitious materials in concrete or mortar, the blending of multiple solid wastes has attracted increasing research attention. To date, scholars have conducted extensive research on binary systems such as ITP-SS [34], ITP-DA [40], and SS-DA [33,37], as well as composite systems of ITP, SS, and other higher activity admixtures (e.g., slag and fly ash) [5,32,34,35,36,41]. However, studies on ITP, SS, and DA (abbreviated as ISD) used as ternary composite mineral admixtures in mortar or concrete remain scarce. To the best of our knowledge, only Professor Zhang’s team has conducted relevant research in this field so far. The research findings [42] indicated that using ISD mineral admixtures as supplementary cementitious materials to partially replace cement was feasible. Although the effects of various dosages of ISD composite mineral admixtures on the compressive performance and microstructure of concrete have been investigated, with results showing the optimal dosage of 20% by mass [42], the influence mechanisms of other factors on ISD concrete, as well as the synergistic effects among ITP, SS, and DA admixtures, remain to be further studied.
Therefore, in this study, multi-solid-waste concretes were prepared using 30% ISD composite mineral admixtures and full iron tailings aggregates. The effects of w/b ratio, ITP fineness, and admixture proportion on the compressive strength of ISD concrete were investigated. The influence of ITP fineness on the microstructure was analyzed via MIP and BSE tests. This work aims to explore the influence of key factors on ISD concrete containing 30% composite mineral admixtures and investigate the synergistic mechanism of ITP, SS, and DA. The results can provide a theoretical basis and reference for the engineering application of ITP, SS, and DA.

2. Materials and Methods

2.1. Materials

2.1.1. Raw Materials

(1) Cement: ordinary Portland cement (P·O 42.5) was produced by Shenyang Shanshui Gongyuan Cement Co., Ltd., Shenyang, China. The main chemical components and the physical properties of the cement are shown in Table 1 and Table 2.
(2) Aggregates: ITS and iron tailings waste rock (ITR) were used as the fine aggregate and coarse aggregate, respectively, both supplied by Liaoning Yilifang Sand Industry Co., Ltd., Benxi, China. The main physical properties of the aggregates are listed in Table 3 and Table 4, while their particle gradation curves are shown in Figure 1 and Figure 2.
(3) Mineral admixtures: the high-silicon ITP was provided by Liaoning Yilifang Sand Industry Co., Ltd., and its specific surface area was 646 m2/kg. The converter SS was produced by China Baowu Iron & Steel Group Co., Ltd. (Shanghai, China) and had a specific surface area of 1022 m2/kg. The calcareous DA was supplied by Lingshou Aoda Refractory Materials Processing Factory (Shijiazhuang, China), with a specific surface area of 3335 m2/kg. The main chemical compositions of the mineral admixtures are shown in Table 5, and the XRD patterns and particle size distribution of the mineral admixtures are presented in Figure 3 and Figure 4, respectively.
(4) Water: drinking water.
(5) Water-reducing agent: the air-entraining water-reducing agent was purchased from Shenyang Shengxinyuan Building Materials Co., Ltd., Shenyang, China.

2.1.2. Raw Material Preparation

(1) Drying treatment
The coarse aggregate, fine aggregate, and mineral admixtures were dried in an oven at 105 °C for 24 h to achieve a moisture content below 1%.
(2) Ball milling of ITP
The dried ITP samples were ball-milled for 1.5 h, 2.0 h, and 2.5 h, respectively. As the ball milling time increased, the specific surface area of the ITP first increased and then decreased. Specifically, the specific surface areas of ITP milled for 1.5 h and 2.0 h were 1290.3 m2/kg and 1589.3 m2/kg, respectively. However, due to the agglomeration of small-sized particles, the specific surface area of ITP milled for 2.5 h decreased to 1311.9 m2/kg instead.
The particle size distribution and cumulative distribution of ITP with different ball milling times are shown in Figure 5. The experimental results indicate that as the ball milling time increases, the number of micro-sized ITP particles increases. However, the relationship between the fineness and specific surface area of ITP is non-linear.
Figure 6 displays the SEM images of ITP at different ball milling times. It is evident that the particle size of ITP decreases after ball milling, but the surface remains rough and irregular in shape.

2.2. Mix Proportion of Concrete

The cement concrete (C-1) and ISD concrete specimens were designed based on the C40 concrete strength grade, as required in the Specification for Mix Proportion Design of Ordinary Concrete (Chinese standard, JGJ 55-2011) [43] and Technical Code for Application of Mineral Admixtures (Chinese standard, GB/T 51003-2014) [44], as well as the previous studies [42]. The mix proportions of the concretes are shown in Table 6. In this study, the design slump of concrete ranged from 150 mm to 200 mm.
For ISD concretes, three types of concrete samples (w/b group, fineness group, and admixture proportion group) were designed, in which 30% of Portland cement was replaced by ISD composite mineral admixtures, and ITS and ITR were used as aggregates. Among these, the ball milling time of ITP was 2.0 h for both the w/b group and the admixture proportion group. As for the fineness group, the ball milling time of ITP was 0 h (ISD-D-1), 1.5 h (ISD-D-2), 2.0 h (ISD-D-3), and 2.5 h (ISD-D-4).

2.3. Specimens Preparation

The raw materials were weighed according to the mixing proportions in Table 6 and then added to the mixer in three batches for blending. Initially, the fine and coarse aggregates were poured into the mixer for 60 s; subsequently, the cement and mineral admixtures were poured into the mixer for another 60 s; finally, the water and water-reducing agent were added into the mixer to continue stirring for 120 s. Then, the fresh concrete was poured into 100 mm × 100 mm × 100 mm moulds and vibrated for 30 s on a vibrating table. After 24 h of curing (20 ± 1 °C, relative humidity ≥ 95%), the specimens were demoulded and transferred to a standard curing room (20 ± 2 °C, relative humidity over 95%). They were cured for 7 d, 14 d, and 28 d, respectively. In addition, six specimens were prepared for each mix proportion in the fineness group, while three were prepared for the other mix proportions.

2.4. Test Methods

2.4.1. Compressive Strength Test

The compressive strength of specimens cured for 7 d, 14 d, and 28 d was tested in accordance with the National Standard of the People’s Republic of China, “Standard for test methods of concrete physical and mechanical properties” (GB/T50081-2019) [45]. In the test, the WANCE universal testing machine (Shenzhen, China)was used to measure the compressive strength. For each mix proportion, three specimens were tested, and the compressive strength value was determined according to the following principle: the compressive strength of each proportion is taken as the arithmetic mean of the results from the three specimens; if the maximum or minimum value exceeds 15% of the median value, the median value is adopted as the compressive strength value. If both the extreme values exceed 15% of the median value, the test results for the group are invalid. Moreover, due to the non-standard size of the specimens, the compressive strength of each group needs to be multiplied by a conversion factor of 0.95.

2.4.2. The MIP Test

The MIP test was performed at a testing centre in Dalian, China. After 28 days of standard curing, 15 mm-thick slices were cut from the concrete specimens using a cutting machine, with the cutting plane parallel to and 15 mm from the surface. Subsequently, core samples were extracted from these slices using an electric drill equipped with a hollow drill bit (inner diameter: 8–14 mm). The selected samples were ensured to be free of coarse aggregates and surface cracks. The core samples were then immersed in absolute ethanol for 7 days to stop further hydration, after which they were dried in a vacuum oven at 50 ± 2 °C for 3 days. Finally, the pore structure was measured using an AutoPore IV 9510 automatic mercury porosimeter (Micromeritics Instrument Corporation, Norcross, GA, USA), with a maximum mercury intrusion pressure of 414 MPa. Next, the parameters of the pore structure were calculated and analyzed.

2.4.3. The BSE Test

The BSE tests of the interfacial transition zone (ITZ) in concrete were also conducted by the testing centre in Dalian. After standard curing of the concrete for 28 days, the sheet specimens with a thickness of approximately 3–5 mm were cut from it using a cutting machine. The first cut was made parallel to and 15 mm away from the surface. Afterward, the core samples with a diameter of 8–14 mm and a thickness of about 3–5 mm were extracted from sheet samples using a drill with a hollow drill bit, and they comprised paste and aggregates. Subsequently, the selected core samples were kept in absolute ethanol for 7 days to stop hydration and thereafter dried in a vacuum oven for 3 days at 50 ± 2 °C. Before testing, the dried specimens underwent natural epoxy resin curing for 24 h, followed by grinding, polishing, acoustic cleaning, and drying at 50 ± 2 °C for 24 h. In the test, the morphology of the ITZ was examined at 500× magnification using a ZEISS GeminiSEM 300 scanning electron microscope (Carl Zeiss Microscopy GmbH, Germany) in BSE mode, and grayscale images with a resolution of 1024 × 768 pixels were acquired for analysis. After extracting the boundary, the ITZ was separated into 10 consecutive strips ranging from the aggregate surface to 50 μm, each with a width of 5 μm. Subsequently, the overflow-point method was used to determine the upper threshold of pores [24]. Then, the porosity and unhydrated particles [46] were analysed and quantified using ImageJ (version 1.52p) processing software.

3. Results and Discussion

3.1. Compressive Strength

3.1.1. The Effect of w/b on the Compressive Strength of ISD Concrete

The compressive strengths of the w/b group ISD concrete and C-1 concrete are shown in Figure 7. The results indicate that the compressive strength of ISD concrete decreases with increasing w/b at each test age. This is mainly due to the fact that under the synergistic effect of the water-reducing agent, the low-activity admixtures dilute the concentration of the blend slurry, increasing the actual w/b and hydration space of the system, resulting in more free water [6,47]. Moreover, as the w/b increases, the number of capillary pores and connected pores caused by free water migration and bleeding also increases accordingly. This leads to a decrease in compactness and thus a reduction in compressive strength [48,49].
In Figure 7, it can also be clearly observed that an increasing w/b has a marked impact on the 7-day compressive strength, while its effect on the 28-day strength is less pronounced. In particular, compared with ISD-W-1, the 7-day compressive strength of ISD-W-2 (w/b = 0.44) and ISD-W-3 (w/b = 0.46) decreases by 17.88% and 17.58%, respectively, but the reductions in compressive strength decrease to 5.85% and 9.37% at 28 days. This is primarily because at the early hydration stage, mineral admixtures mainly provide a physical filling effect [11,23,31]; however, as the w/b increases, the filling effect becomes inadequate to counteract the adverse impact of the increased internal porosity. In contrast, during the mid- to late-stages, the hydration of SS and the pozzolanic reaction of a small amount of ITP generate more C-S-H and C-A-H gels to fill the pores, thus improving the pore structure and enhancing the compressive strength [28,50].
Furthermore, at the same w/b (0.44), the compressive strength of the ISD-W-2 specimens is approximately 20% lower than that of the C-1 specimens at all ages. In contrast, the ISD-W-1 specimens (w/b = 0.42) display a much smaller strength reduction. The trend of the w/b effect is consistent with the research of Wang et al. [28] and Han et al. [3]. The test results demonstrate that when the dosage of ISD mineral admixtures is high, appropriately decreasing the w/b can mitigate the adverse influences of their low activity and the dilution effect on concrete strength [3,28,33]. This can increase the pore ion concentration, promote cement hydration, and fully exert the micro-aggregate and pozzolanic effects of the admixtures [28,33,51,52].

3.1.2. The Effect of ITP Fineness on the Compressive Strength of ISD Concrete

Figure 8 presents the compressive strength of fineness group ISD concrete specimens and C-1 concrete at different ages. The results indicate that the ISD-D-2 specimen achieves the highest compressive strength (32.2 MPa at 7 d and 44.3 MPa at 28 d) and exhibits the least strength reduction compared to the C-1 specimen. This demonstrates that the ITP milled for 1.5 h (D50: 21.2 μm), together with SS (D50: 6.54 μm), DA (D50: 7.64 μm), and cement, form an optimal particle size distribution, leading to a positive synergistic effect. During the early hydration process, ITP, SS, and DA provide an effective physical filling effect, meanwhile the finely milled ITP supplies more nucleation sites for cement to promote hydration, thus enhancing the early compressive strength. In the middle and later stages, the hydration of SS plays a dominant role, and the activity of a portion of micron-sized ITP is stimulated [20,22,27,53]; consequently, more active silica–alumina components in the ISD mineral admixture react with calcium hydroxide (CH) to generate calcium silicate hydrates (C-S-H) and calcium aluminate hydrates (C-A-H) gels, which refine the pore structure, thereby enhancing the 28-day compressive strength [20,27,33,34].
However, as the ball milling time of ITP increases, the compressive strength for ISD-D-3 (ITP ball-milled for 2.0 h) and ISD-D-4 (ITP ball-milled for 2.5 h) specimens decreases instead. Of these, the ISD-D-3 specimen (specific surface area: 1589.3 m2/kg) exhibits the lowest 28-day compressive strength (40.2 MPa). This is attributed to the excessive fineness of the ITP, which diminishes particle uniformity [53], degrades the particle size distribution of the admixtures, and thereby weakens their synergistic effects. Specifically, the proportion of particles below 10 μm is only 3.37% for the un-milled ITP, while this proportion increases to 38.09% in those milled for 2.0 h. This means that in ISD-D-3, a substantial amount of low-reactivity micron-sized ITP fills the spaces between cement particles, displacing the more reactive SS and DA. It is evident that the physical filling effect and limited pozzolanic activity of the ITP are inferior to the hydration reactivity of SS in terms of strength contribution, thereby the ISD-D-3 specimen with the highest specific surface area exhibits the lowest compressive strength. In addition, this also demonstrates that the compressive strength of ISD concrete can be enhanced through the synergistic effect among the mineral admixtures, when the fineness of ITP is proper and the quantity of small-sized SS and DA between cement particles is higher than that of ITP. Furthermore, the 28-day compressive strength of the ISD-D-1 specimen (un-milled for ITP, specific surface area: 646 m2/kg, D50: 144 μm) decreases by only 2.7% compared to the ISD-D-2 specimen, while it increases by 7.2% compared to the ISD-D-3 specimen, further supporting this viewpoint.
Furthermore, it is noteworthy that the fineness of ITP has no obvious effect on the 14-day compressive strength. The primary reason is that between 7 and 14 days, cement hydration remains the dominant process, while the contribution of secondary hydration from ISD mineral admixtures is limited [53,54]. Thus, the compressive strengths of the concrete are similar.
According to studies [20,53,54], when the grinding time is within a certain range, the specific surface area of ITP increases as the grinding time increases, and the activity index improves accordingly. However, for the ISD concrete, the results demonstrate no positive correlation between ITP fineness and compressive strength. Instead, the compressive strength of ISD concrete containing appropriately ball-milled ITP can be improved by the synergistic effects of ITP, SS, and DA.

3.1.3. The Effect of Mineral Admixture Proportion on the Compressive Strength of ISD Concrete

Figure 9 gives the compressive strength of ISD concrete specimens with different mineral admixture proportions. It is evident that the compressive strengths of ISD-P-2 and ISD-P-8 are higher at 28 days, reaching 44.3 MPa and 44.2 MPa, respectively. In contrast, the compressive strengths of ISD-P-1 (30% ITP alone), ISD-P-3 (ITP:DA = 15%:15%), and ISD-P-4 (ITP:SS:DA = 15%:7.5%:7.5%) are lower at all ages, of which the 28 d values are merely 35.6, 34.7, and 35.3 MPa, respectively. Furthermore, for the ISD-P-4 to ISD-P-7 specimens, the mass proportion of ITP decreases in arithmetic steps of 4.5%, while those of SS and DA increase in steps of 2.25%. As a result, the compressive strength at all ages shows a gradual increasing trend as the ITP content decreases. Additionally, as can be seen from Figure 9, in ISD ternary admixture concrete with an ITP proportion not exceeding 15%, all specimens except ISD-P-5 (28 d strength: 39.2 MPa) meet the C40 compressive strength requirement. And it is clear that the higher the SS proportion, the greater the strength improvement.
In Figure 9, the main reasons for the lower compressive strength of the ISD-P-1 can be summarized as follows. First, the reduction in cement content (with a substitution rate of 30% ITP) leads to a corresponding decrease in its hydration products (C-S-H and Aft) [22,55], thereby weakening the strength foundation of the cementitious system. Second, ITP has lower reactivity. According to research by Zhang et al. [42], the activity index of blend mortar specimens with 30% ITP alone (the same composition and fineness as in this study) was only 65.36%. Their experiments indicated that low-activity ITP primarily acted as a micro-aggregate filler, and only a small amount of active SiO2 and Al2O3 reacted with the cement hydration product CH to form additional C-S-H and C-A-H. Consequently, ITP plays a limited role in the later-stage compressive strength of concrete. Third, the w/b of concrete is relatively high. As discussed in Section 3.1.1, when concrete incorporates substantial amounts of low-reactivity mineral admixtures, a relatively high w/b can adversely affect the microstructure after free water evaporation [3]. In conclusion, the compressive strength decreases due to insufficient hydration products and the deterioration of the pore structure.
In the binary admixtures concrete (Figure 9), ISD-P-2 (with 15% ITP and 15% SS) exhibits a significant increase in compressive strength. This is mainly because SS contains more C2S and less C3S, as well as its large specific surface area of 1022 m2/kg, resulting in relatively higher hydration activity [12]. During hydration, gel substances such as C-S-H generated from SS hydration effectively fill the pores and enhance the density of concrete [34]. Simultaneously, the alkaline environment provided by its hydration product CH promotes cement hydration and enhances the pozzolanic effect of ITP. Thus, through the synergistic effect of ITP and SS, the strength of ISD-P-2 is significantly improved. In comparison, the ISD-P-3 (with 15% ITP and 15% DA) shows a slight reduction relative to ISD-P-1 (30% ITP alone). The main reasons are as follows: during hydration, the high CaO content (93.85%) in DA leads to the dissolution of a large amount of Ca2, which reacts with OH− to form low-strength CH crystals (some CH may also be produced by the hydration of f-CaO); meanwhile, high-silica ITP contains less active SiO2 and Al2O3 [10,11], so the gel substances generated by consuming CH through the pozzolanic effect are relatively minor; furthermore, the high concentration of Ca2+ in DA probably inhibits cement hydration [56]. Consequently, the compressive strength of the ISD-P-3 samples is reduced due to the aforementioned factors.
In the ternary admixtures ISD concrete (Figure 9), the ISD-P-4 specimen shows the lowest compressive strength. The primary reasons are as follows: first, the activity of ITP is relatively low [11,42], but the dosage is relatively high; second, the alkaline environment (provided by 7.5% DA and cement) has limited effectiveness in stimulating SS activity; and third, the insufficient content of active silica–alumina components in ITP [23,24], which weakens pozzolanic reactions and thus reduces compressive strength. In addition, it can be clearly observed that as the ITP dosage decreases (by equal increments of 4.5%) and the SS and DA dosages correspondingly increase (by equal increments of 2.25%), the compressive strength of specimens ISD-P-4 to ISD-P-7 shows an upward trend. This further indicates that reducing the dosage of lower activity ITP helps mitigate its diluting effect on cement slurry; simultaneously, the abundant CaO in SS and DA enhances the alkalinity of the cementitious system, thereby stimulating SS activity and promoting cement hydration [33]; more importantly, as the SS dosage increases, the more active silica–alumina components in SS are dissolve and release, which react with CH to form C-S-H or C-A-H gel, and enhancing the mechanical properties of the ISD concrete. Furthermore, it can also be observed that the dosage of ITP is below 10% for the ISD-P-6 to ISD-P-9, and regardless of the ratio adjustment between SS and DA, the compressive strengths of all specimens still meet the concrete requirements of C40. Among them, the ISD-P-8 specimen with the maximum SS dosage (ITP:SS:DA = 6%:16%:8%) achieves the highest compressive strength at 28 days. This further demonstrates that for the ternary ISD admixture concrete, when the ratio of SS is appropriately increased and the ratios of ITP and DA are correspondingly decreased, the compressive strength of multi-waste concrete can be effectively enhanced through the synergistic effect of micro-aggregate filling and secondary hydration reactions [5]. Lastly, it is also worth noting that specimens ISD-P-7 and ISD-P-9, with a higher dosage of DA, exhibit slightly lower strength compared to ISD-P-8. This may be attributed to the higher calcium ion concentration from DA, which inhibits the hydration of both SS and cement.
The above results indicate that when ITP, SS, and DA are proportioned properly, the micro-aggregate filling effect of ITP, the weak cementitious properties of SS, and the alkali-activated effect of DA can be fully utilized. The admixtures exhibit synergistic effects through complementary advantages, thereby promoting the hydration of cementitious materials, improving microstructure and enhancing concrete strength. However, due to the limitations of experimental data, the hydration mechanism of ISD concrete and the synergistic effects among mineral admixtures need to be further investigated.

3.2. Pore Structure

The pore size distribution, cumulative pore volume, and proportions of pore volume for the fineness group ISD concrete and C-1 concrete at 28 days are shown in Figure 10. As can be seen from Figure 10a, compared to specimen C-1, the pore size distribution curves of the fineness group ISD concrete show a leftward shift, with a decrease in the most probable pore size and an increase in the peak value. Specifically, the most probable pore size of the C-1 specimen is 0.077 µm, whereas the values for the fineness group ISD concrete (ISD-D-2: 0.055 µm; ISD-D-1 and ISD-D-3: both 0.050 µm) are significantly smaller. This reveals that adding ITP ball-milled to a certain fineness in ISD concrete can markedly refine the pore structure and reduce the impact of interconnected pores through micro-aggregate filling and pozzolanic effect, thereby enhancing the compactness of concrete [20,23,34,57].
As shown in Figure 10b,c, ISD-D-2 has the largest cumulative pore volume (0.08941 mL/g), slightly higher than that of ISD-D-1 (0.08419 mL/g), but both exhibit excellent pore size distributions. Specifically, samples ISD-D-2 and ISD-D-1 show higher combined proportions of harmless pores (pore size < 20 nm) and less harmful pores (pore size 20–50 nm) [58], at 50.87% and 48.99%, respectively. Moreover, ISD-D-2 has the highest proportion of harmless pores (31.73%), and the lowest proportion of more harmful pores (pore size > 200 nm) [58], which is a mere 14.02%. In contrast, ISD-D-3 exhibits the smallest cumulative pore volume (0.07507 mL/g), which is close to that of the C-1 specimen (0.07736 mL/g); however, both specimens display a significant increase in the combined proportions of harmful pores (pore size 50–200 nm) [58] and more harmful pores, reaching 57.72% and 57.64%, respectively. Additionally, the proportion of more harmful pores in ISD-D-3 is the largest, reaching 30.39%.
Therefore, the above results indicate that the effect trend of ITP fineness on pore structure is consistent with its influence on compressive strength, as described in Section 3.1.2. Overall analysis shows that the ISD-D-2 specimen exhibits the optimal pore structure, which further demonstrates that ball-milled ITP for 1.5 h achieves an optimum particle size distribution with SS and DA. This promotes the synergistic effects of physical filling and secondary hydration, thereby refining the pore structure and enhancing the compressive strength. In contrast, the ISD-D-3 specimen (ITP ball-milled for 2.0 h) exhibits the highest proportion of pores above 50 nm. This indicates that a greater quantity of micron-sized ITP particles fills the spaces between cement hydration products with increasing ball milling time, primarily acting as physical fillers [11]. However, in terms of refining the pore structure, physical filling is less effective than the chemical filling offered by the C-S-H and C-A-H gels [34]. Consequently, the number of harmful and more harmful pores increases, leading to a deterioration in pore structure and a subsequent decrease in compressive strength. Moreover, the ISD-D-1 specimen (un-milled for ITP) exhibits significantly superior pore structure and compressive strength compared to the ISD-D-3 specimen, which once again fully substantiates the above viewpoint.
In conclusion, the pore structure test demonstrates that the fineness of ITP within the mineral admixture does not positively correlate with its impact on the pore structure of ISD concrete. For this reason, incorporating moderately ball-milled ITP can optimize pore structure and enhance concrete strength.

3.3. Interface Transition Zone

Figure 11 displays the microstructure (500× magnification) of the ITZ for fineness group ISD concrete specimens and C-1 specimens at 28 days. It can be clearly observed from Figure 11a that the ITZ of specimen C-1 is relatively loose, exhibiting more large pores and partially interconnected cracks. In comparison, as shown in Figure 11b–d, the fineness group ISD concrete specimens present a pronounced improvement in the ITZ. Specifically, the ITZ of samples ISD-D-2 and ISD-D-3 is relatively denser, with fewer microcracks, smaller pore sizes, and a more uniform pore distribution. This indicates that the composite mineral admixtures of ITP (ball-milled to a certain fineness), SS, and DA can refine the ITZ and effectively inhibit the propagation of microcracks.
Figure 12a presents the porosity of the ITZ for the fineness group ISD concrete and C-1 specimen at 28 days. It is clear that the fineness group ISD concrete specimens exhibit lower porosity in the ITZ than the C-1 specimen. This can be primarily attributed to three factors: first, the “wall effect” [59] promotes the enrichment of micron-sized ITP, SS, and DA in the ITZ, thereby reducing the growth space of CH crystals, and diminishing their size; second, the mineral admixture particles provide nucleation sites for CH crystals [51,52], which make the orientation of CH crystals complex [24]; third, the gel products such as C-S-H and C-A-H, which are derived from the self-hydration of SS and the pozzolanic reaction between active SiO2 and Al2O3 in ITP with CH, effectively fill the pores [20,24,27]. The synergistic effect of the above aspects markedly optimizes the pore structure in the ITZ, thereby reducing porosity. Moreover, Figure 12a also shows that the porosity values of the ISD-D-1 and ISD-D-2 specimens are relatively close, while the ISD-D-3 specimen has the lowest porosity. This illustrates that an increase in the micron-sized ITP can provide a better physical filling effect and reduce the porosity [3,20]. Furthermore, it can be observed that within 20 μm from the aggregate surface, the porosity of each specimen increases as the distance from the aggregate decreases. This is primarily due to the “wall effect” and the “microbleeding effect” [59,60].
Figure 12b presents the distribution of unhydrated particles in the ITZ for the fineness group ISD concrete and the C-1 concrete at 28 days. It is clearly observed that the ISD-D-2 specimen (ITP ball-milled for 1.5 h) has the smallest number of unhydrated particles, and its degree of hydration is similar to that of the C-1 specimen. The result agrees with Section 3.1.2 and Section 3.2. This further demonstrates that moderately ball-milled ITP can form an optimal particle size distribution with SS and DA and promote synergistic effects between the composite mineral admixtures. Therefore, the microstructure is improved and macroscopic strength is enhanced for the ISD concrete.
In contrast, the ISD-D-3 specimen (ITP ball-milled for 2.0 h) has the largest number of unhydrated particles and the lowest porosity. The results indicate that more micron-sized ITP accumulates in the ITZ with the increase of ITP fineness, and it primarily provides physical filling and refines the micropores. However, due to the low reactivity of ITP, the unhydrated particles increase in the ITZ with increasing ITP fineness, and this leads to a decrease in the compressive strength.

4. Conclusions

In this study, multi-solid-waste concrete incorporating 30% ISD composite mineral admixtures was prepared. Based on the results of compressive strength and microstructure, the following conclusions can be drawn:
  • The increase in the w/b has a significant impact on the early-age compressive strength of ISD concrete. However, its effect on the compressive strength at mid-to-late ages gradually diminishes due to the combined action of physical filling and secondary hydration of the ISD mineral admixtures. Appropriately lowering the w/b can enhance the compactness of ISD concrete and thus improve its compressive strength;
  • The moderately ball-milled ITP enhances its cementitious activity and optimizes the particle size distribution of ISD admixtures, thereby refining the pore structure and improving the compressive strength through filler, pozzolanic, and nucleation effects;
  • The ISD mineral admixtures containing appropriately ball-milled ITP can consume CH through the pozzolanic reaction to form more C-S-H and C-A-H gels, thereby reducing the quantity of unhydrated particles in the ITZ. The physical filling and secondary hydration of these multi-solid waste mineral admixtures effectively decrease the porosity and restrict the development of microcracks in the ITZ of concrete, thus enhancing the density of the ITZ;
  • The test results indicate that appropriate proportions of ITP, SS, and DA can facilitate the synergistic effects of physical filling, alkali activation, and secondary hydration among the composite mineral admixtures, thereby improving the compressive strength of the concrete. Moreover, moderately increasing the proportion of SS and adjusting the ratio of ITP to DA can significantly promote the synergistic effects of these mineral admixtures;
  • When the w/b, ITP fineness, and admixture proportion are suitable, the compressive strength of ISD concrete is slightly lower than that of cement concrete but still meets the design strength requirements, while its pore structure and ITZ are significantly superior to those of cement concrete;
  • Although this study has verified the feasibility of applying ITP, SS, and DA in concrete, the hydration mechanisms and the synergistic effects among cementitious materials have not been fully elucidated. Furthermore, the mechanical properties and durability of ISD concrete need to be further investigated.

Author Contributions

C.Z.: Conceptualization, methodology, formal analysis, writing—review and editing, writing—original draft, software, visualization, funding acquisition, validation. Y.Z.: Data curation, funding acquisition, methodology, conceptualization, supervision, validation, visualization. J.Z.: Investigation, project administration. H.Z.: Validation, formal analysis. H.C.: Software, validation. All authors have read and agreed to the published version of the manuscript.

Funding

Key Project of National Natural Science Foundation of China (Grant No.52234004), Liaoning Province Key Research and Development Projects (Grant No.2024JH2/102400016), and Jilin Province Research Project of Education Department (Grant No. JJKH20231364KJ).

Data Availability Statement

All data are provided in the manuscript.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Buildings 16 01382 g001
Figure 1. Particle size distribution of ITS.
Figure 1. Particle size distribution of ITS.
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Figure 2. Particle size distribution of ITR.
Figure 2. Particle size distribution of ITR.
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Figure 3. XRD test results of mineral admixtures.
Figure 3. XRD test results of mineral admixtures.
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Figure 4. Particle size distribution of mineral admixtures.
Figure 4. Particle size distribution of mineral admixtures.
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Figure 5. Particle size distribution and cumulative distribution of ITP at different ball milling times.
Figure 5. Particle size distribution and cumulative distribution of ITP at different ball milling times.
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Figure 6. SEM images of ITP ball-milled for different times.
Figure 6. SEM images of ITP ball-milled for different times.
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Figure 7. Compressive strength of w/b group ISD concrete and C-1 concrete.
Figure 7. Compressive strength of w/b group ISD concrete and C-1 concrete.
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Figure 8. Compressive strength of fineness group ISD concrete and C-1 concrete.
Figure 8. Compressive strength of fineness group ISD concrete and C-1 concrete.
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Figure 9. Compressive strength of admixture proportioning group ISD concrete and C-1 concrete.
Figure 9. Compressive strength of admixture proportioning group ISD concrete and C-1 concrete.
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Figure 10. Pore size distribution, cumulative volume, and volume proportion of the fineness group ISD concrete and C-1 concrete at 28 days.
Figure 10. Pore size distribution, cumulative volume, and volume proportion of the fineness group ISD concrete and C-1 concrete at 28 days.
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Figure 11. BSE images of the fineness group ISD concrete and C-1 concrete at 28 days.
Figure 11. BSE images of the fineness group ISD concrete and C-1 concrete at 28 days.
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Figure 12. Porosity and unhydrated particles of ITZ in the fineness group ISD concrete and C-1 concrete at 28 days.
Figure 12. Porosity and unhydrated particles of ITZ in the fineness group ISD concrete and C-1 concrete at 28 days.
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Table 1. Main chemical components of cement %.
Table 1. Main chemical components of cement %.
ComponentsSiO2CaOMgOFe2O3Al2O3SO3Others
P.O 42.523.2656.103.962.696.892.694.41
Table 2. The physical properties of the cement.
Table 2. The physical properties of the cement.
Specific Surface Area/m2/kgSetting Time/minCompressive Strength/MPaFlexural Strength
/MPa
InitialFinal3 d28 d3 d28 d
40018526028.148.75.97.5
Table 3. Physical properties of ITS.
Table 3. Physical properties of ITS.
Apparent Density/
kg/m3		Bulk
Density/
kg/m3		Crushing
Index
(%)		Fineness
Modulus		Weight
Loss (%)		Stone Powder Content (%)Mud
Lump
Content (%)
25601620222.04.04.90
Table 4. Physical properties of ITR.
Table 4. Physical properties of ITR.
Apparent Density/
kg/m3		Bulk
Density/
kg/m3		Crushing
Index (%)		Sulfates and Sulfides
(%)		Content of Needles and Flakes (%)Mud
Content (%)		Mud
Lump
Content (%)
263016107.00.130.10
Table 5. Main chemical composition of mineral admixtures %.
Table 5. Main chemical composition of mineral admixtures %.
Mineral AdmixturesSiO2CaOMgOFe2O3Al2O3K2OSO3Na2OOthers
ITP62.267.786.3314.374.781.400.481.341.26
SS15.2042.656.0527.542.530.060.120.025.83
DA1.4793.852.000.441.140.550.190.300.06
Table 6. Mix proportion of concrete (kg/m3).
Table 6. Mix proportion of concrete (kg/m3).
NumberCementITPSSDAITSITRWater
Reducing Admixture		Waterw/b
C-142000074011104.51850.44
ISD-W-1 1760.42
ISD-W-229425505074011104.51850.44
ISD-W-3 1930.46
ISD-D-129425505074011104.51850.44
ISD-D-2
ISD-D-3
ISD-D-4
ISD-P-12941260074011104.51850.44
ISD-P-263630
ISD-P-363063
ISD-P-4633232
ISD-P-5444141
ISD-P-6255050
ISD-P-766060
ISD-P-8256734
ISD-P-9253467
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MDPI and ACS Style

Zhao, C.; Zhang, Y.; Zhao, J.; Zhang, H.; Chen, H. Compressive Strength and Microstructure of Multi-Solid Waste Concrete Incorporated with Iron Tailings–Steel Slag–Desulfurization Ash. Buildings 2026, 16, 1382. https://doi.org/10.3390/buildings16071382

AMA Style

Zhao C, Zhang Y, Zhao J, Zhang H, Chen H. Compressive Strength and Microstructure of Multi-Solid Waste Concrete Incorporated with Iron Tailings–Steel Slag–Desulfurization Ash. Buildings. 2026; 16(7):1382. https://doi.org/10.3390/buildings16071382

Chicago/Turabian Style

Zhao, Chuanhua, Yannian Zhang, Jianbin Zhao, Hui Zhang, and Hao Chen. 2026. "Compressive Strength and Microstructure of Multi-Solid Waste Concrete Incorporated with Iron Tailings–Steel Slag–Desulfurization Ash" Buildings 16, no. 7: 1382. https://doi.org/10.3390/buildings16071382

APA Style

Zhao, C., Zhang, Y., Zhao, J., Zhang, H., & Chen, H. (2026). Compressive Strength and Microstructure of Multi-Solid Waste Concrete Incorporated with Iron Tailings–Steel Slag–Desulfurization Ash. Buildings, 16(7), 1382. https://doi.org/10.3390/buildings16071382

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