Design and Research on the Preparation of Pervious Concrete Using Carbonized Steel Slag as a Full Component

Abstract

To address the environmental pressures and resource waste caused by massive stockpiling of steel slag, this study developed a carbonated steel slag pervious concrete binder using 40% steel slag powder as the primary cementitious component combined with CaO and MgO. The mechanical performance evolution was investigated, while XRD, SEM, and TG-DTG microcharacterization techniques were employed to reveal the carbonation mechanism and strength formation principles. The results demonstrate that when CaO and MgO contents reached 5% and 15%, respectively, the 28d compressive strength of mortar increased by 134.49% compared to the reference group. Microstructural analysis confirmed that CaO reacted to form CaCO₃ crystals, while MgO enhanced strength by regulating CaCO₃ crystal morphology to optimize product structure. Using steel slag as an aggregate, carbonated steel slag pervious concrete was prepared to investigate the influence mechanisms of B/A ratio and W/B ratio on compressive strength, permeability coefficient, and carbonation effects. The post-carbonation strength increase was adopted to evaluate carbonation efficiency. Increasing B/A ratio enhanced paste filling in aggregate voids, raising 28d compressive strength to 24.76 MPa, but thickened paste coating layers reduced permeability coefficient to 0.33 mm/s while impeding CO₂ diffusion, decreasing carbonation strength growth rate by 22.76%. Initial W/B ratio elevation improved workability to increase strength to 23.76 MPa, whereas excessive water caused paste sedimentation and strength reduction. As W/B ratio rose, permeability coefficient decreased by 65.6%, while carbonation strength growth rate increased. The carbonated steel slag pervious concrete contained approximately 82% steel slag, demonstrating high resource utilization efficiency of steel slag and significant potential for carbon emission reduction.

1. Introduction

In China, the primary treatment method for steel slag involves open-air stockpiling [1], which consumes substantial land resources. Therefore, its utilization has become an urgent issue to address. Pervious concrete has gained significant attention in urban construction due to its positive effects on mitigating urban heat island effects and drainage pollution control. The key research focus on pervious concrete currently lies in resolving the conflicting relationship between mechanical properties and permeability performance [2]. Given steel slag’s inherent advantages of high strength and excellent abrasion resistance [3], its application in pervious concrete preparation can significantly enhance mechanical performance while maintaining equivalent permeability. Lang et al. [4] demonstrated improved mechanical and permeability properties in steel slag pervious concrete compared with natural aggregates by using steel slag with 4.75–9.5 mm particle size. Wang et al. [5] achieved a 35% increase in compressive strength and 50% improvement in splitting tensile strength by replacing crushed stone with steel slag in pervious concrete preparation.

The cementitious components in steel slag exhibit mineral phase similarities with cement, making partial substitution of cement with steel slag powder a feasible approach [6]. Zhao Xuguang et al. [7] demonstrated that partial cement replacement with steel slag effectively reduces shrinkage in slag powder–cement composite mortars. Moreover, f-CaO and f-MgO in steel slag react with CO₂ to form CaCO₃ and MgCO₃, which densify the matrix through pore-filling effects. Carbonation treatment of steel slag-based cementitious materials therefore enhances specimen compactness and strength. Zhang Feng et al. [8,9] developed a binder system using steel slag powder as the primary cementitious material with 20% MgO and CaO additions. The analysis after carbonization shows that MgO facilitates the formation of calcium magnesium carbonate (Ca_xMg_1−xCO₃), concurrently improving mechanical properties and accelerating carbonation kinetics. The inherent pore structure of pervious concrete, particularly its interconnected porosity, provides optimal pathways for CO₂ diffusion. Employing steel slag as an aggregate in carbonation-cured pervious concrete effectively neutralizes unstable components (e.g., f-CaO and f-MgO), resolving durability issues like cracking and expansion caused by poor volume stability in construction applications [10,11,12]. Cao Weida et al. [13] verified that carbonation curing significantly enhances frost resistance and sulfate attack resistance in steel slag bricks, with freeze–thaw cycle resistance improved by 43.7% and sulfate erosion resistance increased by 31.2%.

Building upon this foundation, this study achieved full-component utilization of steel slag to prepare carbonated steel slag pervious concrete. First, steel slag powder was employed as a high-volume cement substitute to develop high-strength cementitious materials for pervious concrete. The carbonation efficiency of the binder was optimized through synergistic incorporation of CaO and MgO. XRD, TG, and SEM microstructural characterization techniques were utilized to analyze the phase composition and microstructural characteristics of the carbonated steel slag binder, elucidating the carbonation mechanism of steel slag-based materials. Second, natural aggregates were entirely replaced with steel slag to fabricate full-component steel slag pervious concrete, followed by carbonation treatment. The effects of gradation, W/B ratio, and B/A ratio on the mechanical properties, permeability, and carbonation efficiency of carbonated steel slag pervious concrete were systematically investigated as follows: B/A ratio correlates with paste coating thickness. Increasing B/A ratio thickened the paste layer, hindering CO₂ penetration and reducing carbonation efficiency, thereby weakening the strength enhancement from carbonation. W/B ratio governs internal porosity of the paste. Elevated W/B ratio increased inter-paste porosity, facilitating CO₂ infiltration and enhancing carbonation efficiency, which significantly strengthened mechanical performance. Both factors influence CO₂ permeation efficiency and carbonation effects, yet the impacts of paste layer thickness and porosity on carbonation remain underexplored in current research. Finally, an optimized formulation for carbonated steel slag pervious concrete was established, providing technical support for full-component utilization of steel slag in pervious concrete production. The proposed mix design enables the preparation of 1 ton of carbonated steel slag pervious concrete to reduce cement consumption by 0.068 tons and natural aggregate usage by 0.765 tons, collectively mitigating approximately 0.1 ton of CO₂ emissions, demonstrating substantial environmental benefits.

2. Materials and Methods

2.1. Raw Materials

The raw materials used in this study included steel slag, cement, CaO, MgO, and mineral powder, with their chemical compositions listed in

Table 1. Chemical constituents and content of raw materials (wt %).

Chemical Composition	MnO	MgO	Al2O3	SiO2	SO3	K2O	CaO	Fe2O3	LOI
Steel slag	4.19	9.24	4.22	18.57	0.43	0.27	36.77	19.85	0.06
Cement	1.33	2.57	6.65	21.96	2.46	0.40	57.92	3.34	3.42
Lime	0.05	13.39	0.13	1.63	0.04	0.06	82.19	0.11	2.15
Reactive MgO	0.06	88.52	0.42	2.76	0.61	0.03	3.18	0.26	3.62
Mineral powder	0.25	7.81	13.60	29.73	1.94	0.47	41.23	0.33	1.07

. The cement was P.O 42.5 cement produced by a company in Shanxi, featuring a specific surface area of 337 m²/kg, initial and final setting times of 173 min and 248 min, 3d and 28d compressive strengths of 28.9 MPa and 51.6 MPa, and 3d and 28d flexural strengths of 5.7 MPa and 9.3 MPa, respectively. Steel slag was sourced from a company in Shanxi, with its basic properties and particle size distribution presented in

Table 2. Performance of steel slag.

Loose Packing Voidage (%)	Apparent Density (g·cm−3)	Crush Value (%)	f-CaO Content (%)
38.8	3.10	16.5	3.4

. The steel slag was sieved into different particle size ranges (0–2.36 mm, 2.36–4.75 mm, 4.75–9.5 mm, and 9.5–16 mm) using a vibration sieve shaker. Particles below 2.36 mm were further ground to a specific surface area of 380 m²/kg using a ball mill. The incorporated CaO was derived from quicklime produced by a company in Gaoxian County, with a measured CaO content of 82.19%, digestion time of 12 min, and digestion temperature of 84 °C. MgO was sourced from a Jiangsu-based company, exhibiting an MgO content of 88.52%. The mineral powder was supplied by a Wuhan-based company.

2.2. Mix Proportion Design

This study developed a carbonated steel slag-based pervious concrete cementitious system by partially replacing cement with steel slag powder and incorporating CaO, MgO, and mineral powder. The synergistic effects of CaO-MgO binary component dosages on the mechanical properties of carbonated steel slag mortar were specifically investigated. CaO participates in carbonation reactions, where the formed CaCO₃ plays a significant role in enhancing specimen strength. MgO not only improves carbonation efficiency [14] but also modifies the crystal morphology of CaCO₃, enabling denser stacking of carbonate products to enhance mechanical strength [9,15]. The mix proportions are detailed in

Table 3. Mix ratio design of steel slag carbonized mortar test.

Sample	Cement/%	Steel Slag/%	Mineral Powder/%	MgO/%	CaO/%
Z0-20	25	40	15	0	20
Z5-15			15	5	15
Z10-10			15	10	10
Z15-5			15	15	5
Z20-0			15	20	0
Z0-15			20	0	15
Z5-10			20	5	10
Z10-5			20	10	5
Z15-0			20	15	0
Z0-10			25	0	10
Z5-5			25	5	5
Z10-0			25	10	0
Z0-5			30	0	5
Z5-0			30	5	0
Z0-0			35	0	0

, comprising 15 formulations labeled as Z-x-y (where x represents the percentage of MgO and y denotes the percentage of CaO). Group Z0-0 served as the control group. A triangular isenthalpic diagram was plotted to visually analyze the relationship between the dosages of CaO, MgO, as well as mineral powder and the mortar strength evolution. XRD and TG analyses were conducted on samples Z15-5, Z15-0, Z0-15, Z0-0, and non-carbonated Z15-5. SEM analysis was performed on samples Z15-5, Z0-0, and non-carbonated Z15-5 to elucidate the carbonation mechanism and strength development of the cementitious system through microstructural characterization.

Based on the optimized binder formulation, natural aggregates were entirely replaced with steel slag to prepare carbonated pervious concrete with varying gradations (mix proportions detailed in

Table 4. Different gradations of pervious concrete.

Sample	Particle Size
S1	2.36–4.75 mm
S2	4.75–9.5 mm
S3	9.5–16 mm
S4	2.36–4.75 mm:9.5–16 mm = 4:6
S5	2.36–4.75 mm:4.75–9.5 mm = 4:6
S6	4.75–9.5 mm:9.5–16 mm = 5:5

), investigating the relationship between gradation adjustments and the mechanical/permeability performance of carbonated pervious concrete. Additionally, mixtures with different B/A ratios and W/B ratios were designed (mix proportions in

Table 5. Pervious concrete mix ratio under different conditions.

Sample	B/A Ratio	W/B Ratio
G1	0.15	0.35
G2	0.20	0.35
G3	0.25	0.35
G4	0.30	0.35
G5	0.35	0.35
G6	0.30	0.30
G7	0.30	0.40

) to study their influence patterns on mechanical properties, permeability, and carbonation efficiency: Groups G1–G5 (B/A ratios: 0.15, 0.20, 0.25, 0.30, 0.35): Increasing B/A ratio thickened the paste coating layer, enhancing compressive strength but reducing permeability coefficient as paste occupied internal voids. The thickened paste layer also impeded CO₂ penetration, diminishing carbonation efficiency. Groups G4, G6, G7 (W/B ratios: 0.30, 0.35, 0.40): Elevated W/B ratio initially improved binder fluidity, enabling better aggregate coating and enhanced interfacial bonding. However, excessive water (W/B = 0.40) diluted the binder, causing binder runoff from aggregates and reducing compressive strength. Consequently, compressive strength exhibited an initial increase followed by a decrease, while permeability coefficient declined continuously. Higher W/B ratios increased internal paste porosity, facilitating CO₂ penetration and improving carbonation efficiency. These findings systematically clarify the dual effects of B/A and W/B ratios on the mechanical–permeability–carbonation coupling behavior of carbonated steel slag pervious concrete.

2.3. Specimen Preparation

Mortar Preparation: Carbonated steel slag mortar was prepared to characterize the mechanical properties of the cementitious materials in carbonated steel slag pervious concrete. Raw materials were weighed and placed into a mixing pot, followed by low-speed mixing for 1 min with gradual addition of water and standard sand. Subsequently, high-speed mixing was conducted for another 1 min. The mixture was then poured into 40 mm × 40 mm × 40 mm molds and compacted via vibration on a shaking table. After demolding post-24 h curing, the specimens were dried at room temperature for 24 h and subjected to carbonation in a chamber under controlled conditions: 99.9% CO₂ concentration, 0.1 MPa pressure, 2 h carbonation duration, and 60 ± 5% humidity. Compressive strength was measured at curing ages of 1d, 3d, 7d, and 28d.

Paste Preparation: The preparation process for the paste used in microscopic testing was similar to that of the mortar, with the exception that no standard sand was added. After curing for 28d, the paste specimens were crushed into small pieces and ground into powder. The small pieces were used for SEM analysis, while the powdered samples were subjected to XRD and TG testing.

Preparation of Carbonated Steel Slag Pervious Concrete: Pre-weighed cement, steel slag powder, mineral powder, magnesium oxide, quicklime, and steel slag aggregates were added to a mixing bowl and dry-mixed. Water was then poured into the mixture and further mixed. The prepared mixture was placed into 100 mm × 100 mm × 100 mm cubic molds, demolded after 24 h of static curing, and dried at room temperature for 24 h. The specimens were carbonated in a carbonation chamber under CO₂ concentration of 99.9%, pressure of 0.1 MPa, duration of 2 h, and 60 ± 5% humidity. After curing for 7d and 28d, the specimens underwent compressive strength testing and permeability coefficient measurement.

2.4. Performance Testing

According to the “Test method of cement mortar strength (ISO method)” (GB/T 17671-2021 [16]), the compressive strengths of carbonated steel slag mortar at 1d, 3d, 7d, and 28d were tested. Following the “Standard for test methods of concrete physical and mechanical properties” (GB/T 50081-2019 [17]), the 7d and 28d compressive strengths of pervious concrete blocks were tested, with three samples per group.

According to the “Technical specification for pervious cement concrete pavements” (CJJ/T 135-2009 [18]), the permeability coefficient was tested using the constant head method. By measuring the overflow water volume Q within time t, the permeability coefficient K was calculated using Equation (1).

$$K={\displaystyle \frac{QL}{AHt}}$$

(1)

where K is the permeability coefficient (mm/s); L represents the specimen thickness (mm); H denotes the water head difference (mm); Q corresponds to the overflow water volume within time t (mL); A is the cross-sectional area of the specimen (mm²).

The paste was prepared and ground into powder. The mineral compositions and product quantities of the steel slag pervious concrete before and after carbonation curing were determined using an Empyrean X-ray diffractometer manufactured by PANalytical B.V. (Almelo, Netherlands) and an STA 449 F3 Jupiter synchronous thermal analyzer produced by NETZSCH (Selb, Germany). (temperature range: RT~1450 °C, heating rate: 0.1–50 K/min). The microscopic morphology of the samples was observed using a QUANTA FEG 450 field emission environmental scanning electron microscope manufactured by FEI Company (Hillsboro, OR, USA).

3. Results

3.1. Design of the Carbonated Steel Slag Cementitious System

3.1.1. Effect of Carbonated Steel Slag Cementitious System Composition on Strength

The relationships between the admixture quantities of mineral powder, CaO, MgO, and compressive strength are plotted in the triangular isenthalpic diagram shown in Figure 1. This method can effectively identify the maximum gradient direction of the dependent variable (compressive strength) following variations in the three factors (mineral powder, CaO, MgO), enabling precise localization of extreme value ranges for the dependent variable and providing scientific basis for mix proportion optimization in complex cementitious systems.

Figure 1 reveals the regulation mechanism of MgO and CaO admixture quantities on the mechanical properties of carbonated steel slag-based cementitious materials. When the MgO admixture is fixed at 0%, the mortar strength first increases and then decreases as the CaO admixture rises from 0% to 20%, reaching its peak at 5% CaO content with a 28d compressive strength of 23.56 MPa (standard deviation: 20.12 MPa). Compared with the reference group (Z-0-0), the strengths at 1d, 3d, 7d, and 28d curing ages are enhanced by 94.3%, 75.4%, 97.8%, and 103.6%, respectively. At lower CaO admixture levels, the carbonation reaction generates calcite crystals (CaCO₃), which can fill pores and serve as crystallization nuclei to effectively accelerate the growth of surrounding C-S-H gels and promote hydration [19]. However, excessive CaO admixture leads to significant compressive strength reduction, likely due to the higher water demand during specimen formation in high-CaO mixtures, resulting in overdried specimens that insufficiently supply water for carbonation reactions, thereby inhibiting the carbonation process [20].

When the CaO admixture is 0%, the MgO admixture exhibits a parabolic strength variation characteristic within the 0–20% range, with the optimal dosage occurring at 15%. At this 15% MgO admixture, the 28d compressive strength reached 20.9 MPa (standard deviation: 17.32 MPa). The 1d, 3d, 7d, and 28d strengths are increased by 86.5%, 74.3%, 98.9%, and 80.6%, respectively, compared to the reference group. At lower MgO admixture levels, Mg²⁺ can be incorporated into CaCO₃ formed by the reaction of CO₂ with CaO, generating Ca_xMg_1−xCO₃, which densifies the mortar [9], thereby enhancing overall strength. However, with increasing MgO admixture, excess MgO generates Mg(OH)₂ that resists carbonation, forming weakly bonded zones that reduce compressive strength [14].

3.1.2. Microstructural Analysis of the Carbonated Steel Slag Cementitious System

(1) Phase Analysis of the Carbonated Steel Slag Cementitious System

The XRD patterns of the carbonated steel slag cementitious system are shown in Figure 2. After carbonation treatment, the peak intensities of calcium-containing phases in the specimens decrease, while new diffraction peaks corresponding to CaCO₃ and Ca_xMg_1−xCO₃ emerge. Due to the overlapping peak positions of these two phases in the XRD patterns, their differentiation becomes challenging [21]. A comparison between the Z15-5 group and groups with single CaO or MgO admixtures reveals similar reaction products among all three. However, the Z15-5 group exhibits stronger calcite peaks, indicating a synergistic interaction between CaO and MgO under carbonation conditions. Although the Z0-0 group still shows calcite diffraction peaks after carbonation, their intensities remain comparatively weak. Comparative analysis of the Z15-5 group before and after carbonation demonstrates distinct phase evolution: uncarbonated specimens display characteristic peaks of Ca(OH)₂ and C₃S, whereas these peaks virtually disappear in carbonated samples and replaced by prominent calcite peaks. This confirms the participation of both Ca(OH)₂ and C₃S in carbonation reactions, with calcite being the dominant carbonation product.

(2) Quantitative Analysis of Carbonation Products in the Carbonated Steel Slag Cementitious System

Figure 3 and

Table 6. Quality loss rate of different components.

Sample	C-S-H Gel Dehydration/%	Decomposition of Ca(OH)2/%	Decomposition of CaCO3/%
Uncarbonized Z15-5	5.18	3.22	5.52
Carbonized Z15-5	4.13	2.09	14.50
Z15-0	5.47	3.22	10.56
Z0-5	5.00	2.42	12.23

show the TG-DTG curves of carbonated steel slag paste specimens. The samples exhibit mass changes in three temperature ranges. In the temperature range of 50–300 °C, C-S-H gel undergoes dehydration; at 300–450 °C, Ca(OH)₂ decomposes under high temperatures; and at 650–800 °C, CaCO₃ and Ca_xMg_1−xCO₃ undergo thermal decomposition. As shown in Figure 3a, Z15-5 generated more CaCO₃ than Z15-0, with CaCO₃ contents of 14.5% and 10.56% for Z15-5 and Z15-0, respectively. The addition of 5% CaO increased CaCO₃ content by 37.3% after carbonation. Analysis of Figure 3b reveals that the CaCO₃ contents before and after adding 15% MgO were 12.23% and 14.5%, respectively. Although Z15-5 and Z0-5 had the same CaO dosage, the CaCO₃ generation rate in the Z15-5 group increased by 18.6%, indicating that MgO enhances carbonation efficiency. Combined with XRD and strength test results, MgO alters CaCO₃ crystal morphology to form Ca_xMg_1−xCO₃, enabling denser packing of carbonate products and improved mechanical strength. Figure 3c displays the TG-DTG curves of the Z15-5 group before and after carbonation. The pre-carbonation sample contained minor CaCO₃, while the post-carbonation sample retained unreacted Ca(OH)₂. The pre-carbonation sample contained 3.22% Ca(OH)₂ and 5.52% CaCO₃, whereas post-carbonation levels shifted to 2.09% Ca(OH)₂ and 14.5% CaCO₃. This confirms the conversion of Ca(OH)₂ to CaCO₃ during carbonation, consistent with XRD analysis results.

(3) Microstructural Analysis of Carbonated Steel Slag Cementitious Materials

SEM analysis was performed on paste specimens of Z15-5 and Z0-0 groups before and after carbonation, with results shown in Figure 4. As observed in Figure 4b, the carbonated specimen exhibits smaller voids and a denser structure, with numerous regular cubic calcite crystals (approximately 1–5 μm in size) embedded within abundant C-S-H gel. The generated CaCO₃ during carbonation serves as nucleation sites, facilitating C-S-H gel growth and promoting hydration reactions. Magnification reveals that extensive calcite crystals fill the specimen’s pores, significantly enhancing its strength. A comparison between Figure 4a,b demonstrates that the pre-carbonation specimen retains substantial layered Ca(OH)₂ crystals, confirming the consumption of Ca(OH)₂ during carbonation. Enlarging Figure 4b reveals acicular CaCO₃, indicating that exposure of the specimen to atmospheric CO₂ generates a small amount of CaCO₃.

A comparison between Figure 4c,d reveals that the Z0-0 group specimens before carbonation exhibit characteristics identical to the Z15-5 group, containing abundant hexagonal plate-like calcium hydroxide crystals. After carbonation, although some calcium hydroxide crystals in the Z0-0 group disappear, residual crystals remain with surface gel coverage, showing compact inter-crystalline bonding and the emergence of needle-like CaCO₃ structures. This indicates partial carbonation occurred, yet the effect remained incomplete due to the absence of synergistic interaction between MgO and CaO. Unlike the Z15-5 group, the Z0-0 group lacked appropriate additions of MgO and CaO, hindering sufficient carbonation progress. Consequently, limited CaCO₃ formation failed to achieve structural densification and significant strength enhancement as observed in the Z15-5 group. XRD and TG analyses confirm that in the Z15-5 group, incorporated CaO can be carbonated into CaCO₃ crystals, while MgO integrates into CaCO₃ to modify crystal morphology, forming Ca_xMg_1−xCO₃. This promotes tighter packing of carbonate products, thereby improving the overall specimen strength.

3.2. Research on Carbonated Steel Slag Pervious Concrete

3.2.1. Effect of Gradation on Permeability and Mechanical Properties

The influence of different aggregate gradations on the performance of carbonated steel slag pervious concrete is illustrated in Figure 5. For single-graded aggregates (S1–S3), the compressive strength of carbonated steel slag pervious concrete decreases with increasing steel slag aggregate particle size, while the permeability coefficient increases accordingly. This phenomenon arises because the use of larger aggregates compromises the uniformity of pervious concrete specimens and reduces the number of contact points, thereby diminishing mechanical performance. As aggregate particle size increases, the width and thickness of cementitious bonding zones between adjacent aggregates decrease, resulting in expanded continuous pores that enhance water permeability.

For binary gradations (S4–S6), the 28d compressive strength exceeds that of single gradations, consistent with previous studies by Gan Bingqing et al. [22]. This is attributed to the incorporation of additional aggregate sizes in binary gradations, which increases the number of contact points and interfacial contact area between coarse aggregates. Furthermore, fine aggregates fill the voids between most coarse aggregates, effectively enhancing the strength of pervious concrete. Among the tested ratios, the 2.36–4.75 mm:4.75–9.5 mm = 4:6 blend exhibited the highest strength. This superiority stems from its smaller aggregate size, which provides a larger specific surface area compared to the other two blends. When the packing porosity and B/A ratio are identical across all three blends, the 2.36–4.75 mm:4.75–9.5 mm = 4:6 blend achieves the thinnest binder layer thickness. During carbonation, this reduced layer thickness facilitates CO₂ penetration, enabling more effective participation of steel slag in carbonation reactions, thereby improving specimen strength.

3.2.2. Effects of B/A Ratio and W/B Ratio on Permeability and Mechanical Properties

The effects of different B/A ratios and W/B ratios on the compressive strength and permeability coefficient of carbonated steel slag pervious concrete are shown in Figure 6. As observed in Figure 6a, the compressive strength of steel slag pervious concrete specimens increases with rising B/A ratio. When the B/A ratio increases from 0.15 to 0.35 during 28d curing, the compressive strength improves by 61.97%, reaching a maximum of 24.76 MPa with a standard deviation of 22.61 MPa. This enhancement occurs because higher B/A ratios increase the thickness of cementitious coatings around steel slag aggregates, strengthening interfacial bonding and improving mechanical properties. Simultaneously, the thickened coatings reduce inter-aggregate pores, decreasing the effective porosity and causing the permeability coefficient to drop from 2.23 mm/s to 0.33 mm/s, which is consistent with findings reported by Torres et al. [23]. Figure 6b demonstrates that the compressive strength initially increases then decreases with an ascending W/B ratio, peaking at 23.76 MPa (standard deviation: 20.93 MPa) when the W/B ratio reaches 0.35. Conversely, the permeability coefficient shows a continuous decline throughout the W/B ratio increase. Mechanistically, moderate W/B ratio elevation improves cementitious material fluidity, enhancing aggregate coating completeness and interfacial adhesion strength. Excessive W/B ratios cause over-dilution of cementitious materials, leading to material runoff from aggregates and weakened structural integrity. Furthermore, continuous W/B ratio increases progressively reduce pore connectivity in carbonated steel slag pervious concrete, systematically lowering its permeability coefficient.

By adjusting the B/A ratio to modify the paste layer thickness and adjusting the W/B ratio to alter the internal porosity of the paste, the effects of carbonated steel slag pervious concrete under different B/A ratios and W/B ratios on pre/post-carbonation strength and carbonation growth rate are shown in Figure 7. As seen in Figure 7a, the 28d carbonation strength growth rates of carbonated steel slag pervious concrete under different B/A ratios were 48.09%, 40.02%, 36.3%, 33.03%, and 25.33%, respectively. It can be observed that as the B/A ratio increased, the carbonation strength growth rate gradually decreased. When the B/A ratio rose from 0.15 to 0.35, the carbonation strength growth rate decreased by 22.76%. The analysis indicates that increasing the B/A ratio enhances the coating paste quantity on steel slag aggregates, leading to thicker paste layers that reduce CO₂ penetration efficiency and consequently diminish carbonation effectiveness. Figure 7b shows that under different W/B ratios, the strength growth rates of carbonated steel slag pervious concrete after 28 days of curing were 28.7%, 33%, and 36.7%, respectively. The carbonation strength growth rate gradually increased with higher W/B ratios. This is attributed to increased W/B ratio elevates water content, which raises the internal porosity of the paste, facilitating CO₂ penetration into specimens and enhancing the carbonation strengthening effect on steel slag specimens.

4. Discussion

1. The influence of CaO and MgO dosages on the mechanical properties of carbonated steel slag cementitious materials was studied. The mechanical properties of carbonated steel slag cementitious materials initially increase and then decrease with increasing dosages of CaO and MgO, thereby exhibiting an optimal value. When the CaO content is 5% and the MgO content is 15%, the strength of the cementitious material reaches its peak value of 27.13 MPa, representing a 134.49% increase compared to the control sample without CaO and MgO addition.

2. The strengthening mechanism of CaO and MgO on carbonated steel slag cementitious materials was investigated using microscopic techniques including XRD, SEM, and TG-DTG. During carbonation, CaO generates substantial carbonation product CaCO₃, which exerts a pore-filling effect in the paste structure. Additionally, the formed CaCO₃ serves as a crystallization site, providing favorable conditions for C-S-H gel growth and promoting hydration reactions. Beyond being carbonated into MgCO₃, the incorporated MgO can combine with calcite crystals to form Ca_xMg_1−xCO₃, synergistically enhancing the mechanical properties of carbonated steel slag cementitious materials.

3. The study investigated the effects of B/A ratio and W/B ratio on the carbonation efficiency of carbonated steel slag pervious concrete. When the B/A ratio of carbonated steel slag pervious concrete specimens increases from 0.15 to 0.35, the paste layer thickens. The 28d compressive strength rises from 15.12 MPa to 24.49 MPa, resulting in a 61.97% improvement, while the permeability coefficient decreases from 2.23 mm/s to 0.33 mm/s. The thickened paste layer impedes CO₂ penetration, leading to a gradual decline in carbonation strength growth rate. With increasing W/B ratio, the compressive strength of carbonated steel slag pervious concrete initially increases and then decreases, accompanied by continuous reduction in permeability coefficient. The optimal performance occurs at a W/B ratio of 0.35, achieving a 28d compressive strength of 23.76 MPa, which represents a 23.24% enhancement compared to 19.28 MPa at a W/B ratio of 0.4. Higher W/B ratios increase pore quantity within the paste, elevating internal porosity and improving CO₂ penetration efficiency, thereby enhancing carbonation effectiveness.

4. This study achieved full-component utilization of steel slag, systematically designing steel slag applications in both cementitious materials and pervious concrete. Carbonated steel slag cementitious materials were prepared using steel slag powder for carbonated steel slag pervious concrete, while steel slag aggregates were employed with adjustments in B/A ratio, W/B ratio, and gradation to fabricate carbonated steel slag pervious concrete. However, engineering applications face challenges such as difficulties in carbonating large volumes of steel slag pervious concrete, requiring dedicated research. Furthermore, incomplete carbonation reactions may risk expansion due to f-CaO and f-MgO in steel slag aggregates. Mitigation strategies include extending carbonation duration, implementing pre-carbonation treatment of steel slag, and evaluating long-term performance through volume stability monitoring. These measures, not addressed in the current work, constitute critical directions for future research in this field.