Characteristics of Steel Slag and Properties of High-Temperature Reconstructed Steel Slag

Abstract

The chemical composition, mineral composition, and mineral distribution characteristics of steel slag were characterized through petrographic analysis, X-ray diffraction (XRD), and particle size analysis. Limestone, silica, and silicomanganese slag were blended with converter steel slag to fabricate a reconstructed steel slag. Through burden calculation, the chemical composition ratio of this reconstructed steel slag approximated the silicate phase region. The high-temperature reconstruction process outside the furnace was simulated through reheating. The composition, structure, and cementitious characteristics of the reconstructed steel slag were investigated through X-ray diffraction (XRD), FactSage software (FactSage version 7.0 (GTT-Technologies, Aachen, Germany, 2015))analysis, scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) analysis, setting time determination, compressive strength measurement, and thermodynamic computation. The findings indicated that the primary mineral compositions of the reconstructed steel slag were predominantly silicates, such as Ca₃Al₂O₆, Ca₂SiO₄, Ca₂MgSi₂O₇, Ca₂Al(AlSiO₇), Ca₂(SiO₄), and FeAlMgO₄. In comparison with the original steel slag, these compositions underwent substantial alterations. The α′-C₂S phase appears at 1100 K and gradually transforms into α-C₂S at 1650 K. The liquid phase begins to precipitate at approximately 1550 K. Spinel exists in the temperature range from 1300 to 1700 K, and Ca₃MgSi₂O₈ melts into the liquid phase at 1400 K. As the temperature increases to 1600 K, the minerals C₂AF, Ca₂Fe₂O₅, and Ca₂Al₂O₅ gradually melt into the liquid phase. Melilite melts into the liquid phase at 1700 K. It was observed that the initial and final setting times of the reconstructed steel slag exhibited reductions of 7 and 43 min, respectively, in comparison to those of the original steel slag. In comparison with steel slag, the compressive strength of the reconstructed steel slag exhibited an increase of 0.6 MPa at the 3-day strength stage, 1.6 MPa at the 7-day strength stage, and 3.4 MPa at the 28-day strength stage. The reduction in setting time and the enhancement in compressive strength verified the improved cementitious activity of the reconstructed steel slag. Thermodynamic calculations of the principal reactions of the reconstructed steel slag at elevated temperatures verified that the primary reaction at 1748 K is thermodynamically favorable.

1. Introduction

Steel slag, a solid waste primarily composed of silicate and ferrite, is generated during the separation of molten steel from impurities in steel-making furnaces [1,2]. In 2024, China’s crude steel production reached approximately 1.005 billion tons, resulting in the generation of over 120 million tons of steel slag. Moreover, the cumulative stockpile of steel slag tailings exceeded 1.8 billion tons [3,4]. The accumulation of steel slag not only occupies precious land resources but also pollutes the environment [5,6]. The generation of converter steel slag bears resemblance to the calcination process for cement clinker production, as the slagging material employed in steelmaking is analogous to the primary raw material utilized in cement clinker manufacturing [7,8,9]. The primary mineral constituents of steel slag are as follows: 3CaO·SiO₂, 2CaO·SiO₂, RO(Bivalent metallic oxide solid solution), CaO·MgO·SiO₂, and 3CaO·MgO·2SiO₂ [10,11]. Consequently, steel slag is also referred to as over-burned Portland cement clinker [12]. Its chemical composition, mineral phase composition, hydration process, and hydration products exhibit similarities to those of cement clinker, rendering it applicable as an admixture for cement and concrete [13,14,15]. Nevertheless, the utilization of steel slag encounters challenges including variable composition [16,17], low cementitious reactivity, and suboptimal volume stability [18,19], thereby making it a less-favorable substitute for Portland cement clinker [20,21,22]. The primary reasons lie in the fact that the elevated steel-making temperature (ranging from 1550 °C to 1650 °C) surpasses the maximum calcination temperature (1450 °C) of silicate cement clinker [23,24]. This excessive burning leads to the formation of well-developed and coarse tricalcium silicate (C₃S) grains, which impedes their activation via grinding in the subsequent stage [25]. The hydration rate of tricalcium silicate (C₃S) in steel slag is significantly lower compared to that in qualified cement clinker [26,27,28]. Furthermore, all minerals within steel slag exhibit well-developed structures, are in an optimal crystallization state, possess large crystal sizes, and display low activity [29,30,31]. In the absence of quenching, tricalcium silicate (C₃S) undergoes partial decomposition into dicalcium silicate (C₂S) and secondary free calcium oxide (CaO), thereby diminishing its hydraulic reactivity [32]. Slow cooling leads to the partial conversion of β-C₂S into γ-C₂S, which exhibits nearly no hydraulicity [32]. Phosphorus in the molten steel is first oxidized to P₂O₅, which preferentially reacts with CaO and SiO₂ to form 7CaO·P₂O₅·2SiO₂ with no gelling activity [33].

Li Zaibo et al. [34] found that the appropriate mix ratio of converter slag, electric arc furnace steel slag, and coal bottom ash in MSS (modified steel slag), i.e., 85:12.75:2.25, could reduce free CaO content of MSS for the pilot production line and considerably improve its cementitious properties. Li Jianxin et al. [35] used lime, electric furnace reduction slag, slag, cinder, and fly ash to reconstruct and modify steel slag, and found that calcium-regulating materials can promote the formation of A and B minerals in reconstructed steel slag.

This research intends to utilize lime, silica, and silicomanganese slag to reconstruct steel slag at high temperatures, modifying its chemical and mineral composition to stabilize its properties and improve its cementitious activity. Moreover, the enthalpy of molten steel slag discharged from converter steelmaking reaches as high as 1.670 MJ/t. Leveraging the enthalpy of molten steel slag, which amounts to 1.670 MJ/t for reconstruction, can facilitate waste-heat recovery and yield steel slag with favorable composition and performance, thereby attaining energy conservation and environmental protection. To enhance the early gelatinization of the reconstructed steel slag and the formation conditions of the steel slag during the cooling process, it is essential to add a certain amount of Al₂O₃ to regulate the melting range and viscosity. Therefore, incorporating materials with high CaO and SiO₂ content and moderate Al₂O₃ content during the high-temperature reconstruction process of steel slag is advantageous. This addition promotes the mineralization of f-CaO in steel slag, consequently reducing its content and guaranteeing the volume stability of the composite cementitious materials composed of steel slag and cement.

2. Materials and Methods

2.1. Raw Materials

2.1.1. Chemical Composition Analysis of Raw Materials

The four raw materials employed in this study were all procured from the Portland cement clinker manufacturing enterprises (Baotou, China). Limestone and silica are both natural mineral raw materials. Steel slag is the waste residue generated during the steel-making process in a converter, and silicomanganese slag is the waste residue produced during the smelting of silicon-manganese iron alloy in submerged arc furnaces.

Table 1. Chemical composition of raw materials (wt,%).

Raw Material	SiO2	Al2O3	Fe2O3	CaO	MgO	Na2O	MnO	P2O5	SO3	TiO2	Others
limestone	3.52	1.25	0.78	52.08	0.57	-	0.03	0.03	0.04	0.07	41.63
Silica	90.40	5.62	0.81	0.94	0.18	0.10	-	0.11	0.41	0.13	1.3
Steel slag	14.50	8.35	20.6	40.10	6.09	0.139	5.51	1.45	1.09	0.876	1.29
Silicomanganese slag	39.10	12.30	1.19	24.10	6.04	0.89	10.8	0.03	2.91	0.20	2.44

presents the chemical composition analysis results of the four raw materials obtained by XRF (X-ray fluorescence).

2.1.2. Analysis of the Mineral Composition of Steel Slag

The converter slag employed in the experiment is a non-magnetic powder. It is derived from the hot steel slag discharged during the steelmaking process, which undergoes a series of treatments including water-cooling, subsequent crushing, magnetic separation, screening, and grinding. The XRD analysis depicted the primary mineral composition of the steel slag in Figure 1 and

Table 2. The primary mineral composition of the steel slag.

Number	Composition
1	Ca3Al2O6
2	Ca2Fe2O5
3	α-Ca2(SiO4)
4	Ca2[Fe(Fe0.866Al0.134)O5]
5	β-Ca2(SiO4)
6	Ca3Mg(SiO4)2

.

2.1.3. Phase Equilibrium Calculation of Steel Slag

The Equilib module within the FactSage software(FactSage version 7.0 (GTT-Technologies, Aachen, Germany, 2015))was employed to compute the equilibrium phase alterations of the tested steel slag in relation to temperature under an air atmosphere. This approach minimizes the Gibbs free energy of the entire system to determine the state of a specified element or compound system upon attaining chemical equilibrium under certain conditions, encompassing the concentration of each mineral. The ambient atmospheric pressure was set at 1 atmosphere, with an oxygen partial pressure of 0.21 atmosphere, and the reaction temperature spanned from 700 to 1800 Kelvin. The components utilized in the thermodynamic calculation were based on the chemical composition analysis results presented in Table 1.

As shown in the thermodynamic calculation results presented in Figure 2, the main mineral phases olivine, Ca₃MgAl₄O₁₀, C₂AF, Ca₃MgSi₂O₈, Ca₂Fe₂O₅, Ca₂Al₂O₅ exist in the lower temperature range of 700–1100 K. The olivine phase disappeared at 1100 K, and the α′-C₂S phase began to appear at approximately 1150 K, and gradually transformed into α-C₂S at 1650 K. The RO phase exists from low to high-temperature regions, and the spinel phase exists at 1450–1600 K. The liquid phase began to precipitate at 1550 K. With the increase in temperature, the mineral phase containing Fe and Al and Ca₃MgSi₂O₈ phase gradually integrated into the liquid phase, and disappeared completely at 1700 K.

2.1.4. Petrographic Examination of Steel Slag

The surface of steel slag was subjected to erosion by a 1% NH₄Cl solution, and subsequent observations were conducted under a reflective microscope. As depicted in Figure 3, the A-mineral exhibits a blue coloration, the B-mineral presents a brownish-yellow hue, and the RO phase appears white.

2.1.5. Particle Size Analysis of Raw Materials

Particle size analysis of the raw materials was performed, the red curve depicts the cumulative frequency distribution, whereas the black curve represents the fitted curve, specifically the geometric mean μ_g(q₀). Figure 4 shows the particle size analysis results of steel slag, D₁₀ = 3.36 μ m, D₅₀ = 32.52 μm, D₉₀ = 57.35 μm. Figure 5 shows the particle size analysis results of limestone, D₁₀ = 1.54 μm, D₅₀ = 7.47 μm, D₉₀ = 45.73 μm. Figure 6 shows the particle size analysis results of silica, D₁₀ = 2.29 μm, D₅₀ = 16.47 μm, D₉₀ = 56.83 μm. Figure 7 shows the particle size analysis results of silicomanganese slag, D₁₀ = 3.96 μm, D₅₀ = 26.73 μm, D₉₀ = 71.27 μm.

2.2. Experimental Methods

An X-ray diffractometer (Rigaku, Tokyo, Japan, SMARTLAB (9)) is utilized, featuring a test scanning speed ranging from 8°/min to 10°/min, a scanning range spanning from 10° to 90°, a maximum high-frequency generator power of 9 kW, a maximum rated current of 200 mA, a rated tube voltage of 45 kV for the diffractometer, and Cu serving as the target material.

An X-ray fluorescence spectrometer (PANalytical B.V., Almelo, The Netherlands, AxiosmAX) is applicable for the analysis of all elements between B(5) and U(92), excluding N(7), with an analytical range from ppm to 100%.

A scanning electron microscope (Carl Zeiss, Oberkochen, Germany, ZEISS EVO 18), with an accelerating voltage of 0.2–30 kV, a magnification of 5–1,000,000 times, and a probe current of 0.5 pA–5 μA, is used.

A reflective microscope (Keyence, Osaka, Japan, VHX-5000) with 54 million pixels is also employed.

Further, a laser particle size analyzer (Jinan Winner Particle Instrument Stock Co., Ltd., Jinan, China, Winner2008) is employed with the following conditions: measurement range, 0.01–2000 mm; power of He–Ne laser, >2 mW; wavelength, 632.8 nm; ultrasonic machine power, 40 W; frequency, 40 kHz; and focal length, 50 mm.

2.3. High-Temperature Reconstruction Test of Steel Slag

The chemical composition of steel slag is distinguished by a relatively lower content of CaO and SiO_2, along with a relatively higher content of Fe₂O₃ and Al₂O₃. The mineral composition has low C₃S and C₂S content. To enhance the cementitious activity of steel slag, it is imperative to regulate the chemical and mineral composition of steel slag. In this research, limestone, silica, and silicomanganese slag were incorporated to alter the proportion of CaO–SiO₂–Al₂O₃ components from the steel slag region to the Portland cement clinker region (Figure 8). The proportion of raw materials is presented in

Table 3. Experimental mix proportion (wt,%).

Raw Material	Proportion
Steel slag	51.37%
limestone	38.86%
Silica	8.7%
Silicomanganese slag	1.08%

. The content of active minerals, such as A-mineral and B-mineral, in steel slag is augmented by elevating the content of CaO and SiO₂ in steel slag and the molar ratio of calcium to silicon (C/S).

The apparatus utilized in the experiment is depicted in Figure 9, primarily encompassing the heating furnace body, argon protection device, and controller. During the calcination process, the heating rate from 0 °C to 900 °C was set at 10 °C/min, and from 900 °C to the target temperature, it was adjusted to 5 °C/min. The calcination temperature was maintained at 1450 °C, with a holding duration of 20 min. Subsequently, the reconstructed steel slag was cooled via a rapid air-cooling approach to facilitate the formation of the glass phase.

2.4. Calculation of Gibbs Free Energy of Chemical Reaction in the Process of Steel Slag Reconstruction

According to Kirchhoff’s formula:

$${∆}_r{H}_m^{\theta }(T)={∆H}_0+\int {∆}_r{C}_{p,m}^{\theta }\left(B\right)dT$$

(1)

Because:

$${∆}_r{C}_{p,m}^{\theta }=∆a+∆bT+∆c^{\prime }{T}^{-2}$$

(2)

Among, $∆a=\sum _B{\vartheta }_B{a}_B$, $∆b=\sum _B{\vartheta }_B{b}_B$, $∆c^{\prime }=\sum _B{\vartheta }_B{c}_B$

Substituting Equation (2) into Equation (1), we get:

$${∆}_r{H}_m^{\theta }(T)={∆H}_0+∆aT+{\displaystyle \frac{1}{2}}∆b{T}^2-∆c^{\prime }{\displaystyle \frac{1}{T}}$$

(3)

According to the Gibbs–Helmholtz equation:

$${\left[{\displaystyle \frac{\mathit{\partial }\left(G/T\right)}{\mathit{\partial }T}}\right]}_p=-{\displaystyle \frac{H}{T^2}}$$

(4)

For the chemical reaction $0=\sum _B{\vartheta }_{B}B$ under standard conditions, the Gibbs–Helmholtz equation is expressed as:

$${\displaystyle \frac{d\left[{\vartheta }_B{G}_m^{\theta }\left(B\right)/T\right]}{dT}}=-{\displaystyle \frac{{\vartheta }_B{H}_m^{\theta }\left(B\right)}{T^2}}$$

(5)

The algebraic sum of Equation (5) with respect to B is obtained:

$${\displaystyle \frac{d\left({∆}_r{G}_m^{\theta }/T\right)}{dT}}=-{\displaystyle \frac{{∆}_r{H}_m^{\theta }}{T^2}}$$

(6)

Perform an indefinite integral on Equation (6):

$$-{\displaystyle \frac{{∆}_r{G}_m^{\theta }}{T}}=\int {\displaystyle \frac{{∆}_r{H}_m^{\theta }}{T^2}}dT+I$$

(7)

Substituting Equation (3) into Equation (7):

$${∆}_r{G}_m^{\theta }={{∆}_{r}H}_0-∆aTlnT-{\displaystyle \frac{1}{2}}∆b{T}^2-{\displaystyle \frac{1}{2}}∆c^{\prime }{T}^{-1}-IT$$

(8)

For each chemical reaction, at standard conditions (298 K), ${∆}_r{G}_m^{\theta },{{∆}_{r}H}_0$ in Equation (8) can be calculated from thermodynamic data by consulting the thermodynamic data booklet. Further calculations for $∆a$, $∆b$, and $∆c^{\prime }$ at standard conditions led to I.

Further, the thermodynamic data manual was consulted to obtain the basic thermodynamic data for the reactants and products, and $∆a$,$∆b$, and $∆c^{\prime }$ at the corresponding reaction temperature were calculated. According to Equation (8), the ${∆}_r{G}_m^{\theta }$ of chemical reactions at the corresponding reaction temperatures were determined, indicating whether the chemical reaction can be carried out at a certain temperature.

3. Results and Discussion

3.1. Chemical Composition and Mineral Composition of Reconstructed Steel Slag

XRF was used to examine the chemical composition of the air-cooled reconstructed steel slag, and the results are shown in

Table 4. Chemical composition of reconstructed steel slag.

Name	SiO2	Al2O3	Fe2O3	CaO	MgO	Na2O	MnO	P2O5	SO3	TiO2	Others
Modification steel slag	19.8	8.2	12.3	48.4	3.74	0.18	2.51	1.21	0.31	0.96	2.39

. The mineral composition of the reconstructed steel slag was analyzed using XRD (Figure 10). The main ones are Ca₃A_l2O₆ (COD ID: 9014359) cubic crystal system and space group Pm-3m(221), Ca₂SiO₄ (COD ID: 1535815) monoclinic crystal system, space group P121/n1(14), Ca₂MgSi₂O₇ (COD ID: 2106179) tetragonal crystal system, space group P-421m (113), Ca₂Al(AlSiO₇) (COD ID: 1000048) tetragonal crystal system, space group P-421m (113), Ca₂(SiO₄) (COD ID: 1546025) orthorhombic crystal system, spatial group Pbnm (62), FeAlMgO₄ (COD ID: 2310729) cubic crystal system, space group Fd-3m (227). The above crystal structure was plotted using Diamond software(Diamond version 3.2k (Crystal Impact GbR, Bonn, Germany, 2014)) as shown in Figure 11.

3.2. Phase Equilibrium Calculation of Reconstructed Steel Slag

The Equilib module within the FactSage software was employed to compute the temperature dependency of the equilibrium phase in reconstituted steel slag within an air atmosphere. Through the minimization of the Gibbs free energy of the entire system, the chemical equilibrium state of a specified element or compound system under particular conditions, including the concentration of each mineral, was ascertained. The ambient atmospheric pressure was set at 1 atm, with an oxygen partial pressure of 0.21 atm, and the reaction temperature spanned from 700 to 1800 K. The components utilized in the thermodynamic calculation were based on the chemical composition analysis results presented in Table 3.

From the phase equilibrium calculation results shown in Figure 12, the main mineral phases C₂AF, Ca₂Fe₂O₅, olivine, Melilite, Ca₃MgSi₂O₈, Ca₃MgAl₄O₁₀, Ca₂Al₂O₅ exist in the lower temperature range of 700–1100 K. The α′-C₂S phase appears at 1100 K and gradually transforms into α-C₂S at 1650 K. The liquid phase begins to precipitate at approximately 1550 K. Spinel exists in the temperature range from 1300 to 1700 K, and Ca₃MgSi₂O₈ melts into the liquid phase at 1400 K. As the temperature increases to 1600 K, the minerals C₂AF, Ca₂Fe₂O₅, and Ca₂Al₂O₅ gradually melt into the liquid phase. Melilite melts into the liquid phase at 1700 K.

3.3. Micro-Morphology and Elemental Analysis of Reconstructed Steel Slag

The morphology and microzone elemental composition of the reconstructed steel slag were analyzed using scanning electron microscopy combined with energy spectrum analysis. Figure 13 shows the morphology and the occurrence area of O, Al, Si, Ca, Fe and Mg. The energy spectrum analysis results are shown in Figure 14. According to the molar ratio, microzone (a) is mainly silicate (C₃S or C₂S), microzone (b) contains a small amount of calcium magnesium ferrite and a large amount of unreacted MgO, microzone (c) is mainly silicate, with small amounts of Fe and Al salts, and microzone (d), which is lighter color in the image and shows obvious grain boundaries, is a solid solution of the silicate phase with small amounts of Fe and Al salts.

3.4. Setting Time

The setting time was ascertained by grinding the reconstructed steel slag and gypsum dihydrate to a specific surface area of 350 ± 10 m²/kg, with the residue on the 80 mm sieve being less than 4%, for the purpose of preparing P·I type silicate cement. In accordance with GB/T 1346-2011 [36] “Test Methods for Water Requirement of Normal Consistency, Setting Time and Soundness of the Portland Cement”, the test results are presented in Figure 15. The test was conducted with a mixture of 30% steel slag T1 (or reconstructed steel slag T2) and 70% Portland cement. In comparison with the original steel slag, the initial setting time of the reconstructed steel slag was shortened by 7 min, and the final setting time was shortened by 43 min.

3.5. Compressive Strength

For the determination of compressive strength, the reconstructed steel slag and dihydrate gypsum were pulverized to a specific surface area of 350 ± 10 m²/kg, with a residue on an 80 mm sieve of <less than 4%, to produce P·I Portland cement. The compressive strength was measured in accordance with the GB/T 17671-2021 [37] Test Method of Cement Mortar Strength (ISO method), and the test results are presented in Figure 16. The test was conducted by blending 30% steel slag T1 (or reconstructed steel slag T2) with 70% Portland cement. In comparison with the steel slag, the reconstructed steel slag exhibited an increment in compressive strength of 0.6 MPa at 3 days, 1.6 MPa at 7 days, and 3.4 MPa at 28 days.

3.6. Calculation of Gibbs Free Energy of Chemical Reaction in Reconstructed Steel Slag

Main chemical reactions:

C + S = CS

(9)

2C + S = β-C₂S

(10)

3C + S = C₃S

(11)

β-C₂S + C = C₃S

(12)

3C + α-A = C₃A

(13)

2C + CA = C₃A

(14)

2C + F = C₂F

(15)

The thermodynamic parameters of the main minerals are shown in

Table 5. Thermodynamic parameters of main minerals.

Name	${∆}_{\mathit{f}}{\mathit{H}}_{\mathit{m}}^{\mathit{\theta }}$(298 K) (kJ/mol)	${∆}_{\mathit{f}}{\mathit{G}}_{\mathit{m}}^{\mathit{\theta }}$(298 K) (kJ/mol)	${\mathit{S}}^{\mathit{\theta }}$(298 K) (J/K·mol)	Cp = a + b × 10−3T + ${\mathit{c}}^{\mathit{\prime }}$× 105T−2 (J/K·mol)
				a	b	${\mathbf{c}}^{\mathbf{\prime }}$
CaO	−635.089	−603.509	38.074	49.622	4.519	−6.945
SiO2	−908.346	−854.509	43.396	46.898	31.501	−10.092
α-Al2O3	−1675.692	−1582.271	50.936	103.851	26.267	−29.091
Fe2O3	−824.248	−742.294	87.404	98.282	77.822	−14.853
CaO·SiO2	−1634.940	−1549.656	81.919	111.462	15.062	−27.280
β-2CaO·SiO2	−2304.823	−2190.269	127.742	145.896	40.752	−26.192
3CaO·SiO2	−2929.202	−2783.898	168.599	208.572	36.066	−42.468
CaO·Al2O3	−2326.304	−2208.820	114.223	153.093	22.301	−35.480
3CaO·Al2O3	−3587.801	−3411.786	205.899	250.873	31.338	−49.371
2CaO·Fe2O3	−2139.279	−2001.686	188.782	183.301	86.860	−28.744

, and the calculation results are shown in

Table 6. The calculation results.

Name	${∆}_{\mathit{r}}{\mathit{G}}_{\mathit{m}}^{\mathit{\theta }}$	${∆}_{\mathit{r}}{\mathit{G}}_{\mathit{m}}^{\mathit{\theta }}$(kJ/mol)
		298 K	1400 K	1500 K	1600 K	1748 K
1	${∆}_r{G}_m^{\theta }=-91505-∆aTlnT-{\displaystyle \frac{1}{2}}∆b{T}^2-{\displaystyle \frac{1}{2}}∆c^{\prime }{T}^{-1}+75.84T$	−91.638	−115.93	−117.74	−119.40	−121.63
2	${∆}_r{G}_m^{\theta }=-126299-∆aTlnT-{\displaystyle \frac{1}{2}}∆b{T}^2-{\displaystyle \frac{1}{2}}∆c^{\prime }{T}^{-1}-10.81T$	−128.742	−139.07	−139.98	−140.89	−142.25
3	${∆}_r{G}_m^{\theta }=-115589-∆aTlnT-{\displaystyle \frac{1}{2}}∆b{T}^2-{\displaystyle \frac{1}{2}}∆c^{\prime }{T}^{-1}+54.14T$	−118.86	−160.47	−164.38	−168.29	−174.04
4	${∆}_r{G}_m^{\theta }=10710-∆aTlnT-{\displaystyle \frac{1}{2}}∆b{T}^2-{\displaystyle \frac{1}{2}}∆c^{\prime }{T}^{-1}+64.96T$	9.88	−21.38	−24.38	−27.37	−31.78
5	${∆}_r{G}_m^{\theta }=-6842-∆aTlnT-{\displaystyle \frac{1}{2}}∆b{T}^2-{\displaystyle \frac{1}{2}}∆c^{\prime }{T}^{-1}-52.22T$	−18.988	−52.95	−55.42	−57.78	−61.11
6	${∆}_r{G}_m^{\theta }=8681-∆aTlnT-{\displaystyle \frac{1}{2}}∆b{T}^2-{\displaystyle \frac{1}{2}}∆c^{\prime }{T}^{-1}-23.87T$	4.052	−9.89	−11.06	−12.23	−13.94
7	${∆}_r{G}_m^{\theta }=-44853-∆aTlnT-{\displaystyle \frac{1}{2}}∆b{T}^2-{\displaystyle \frac{1}{2}}∆c^{\prime }{T}^{-1}-106.28T$	−52.374	−193.50	−204.12	−214.73	−230.44

. The ${∆}_r{G}_m^{\theta }$ of above reactions are all < 0 at 1748 K, confirming that these reactions can occur positively from a thermodynamic perspective.

4. Conclusions

The experimental findings indicate that the cementitious activity of the reconstructed steel slag can be enhanced by utilizing limestone, silica, and silicomanganese slag to reconstruct the converter steel slag at high temperatures in conjunction with rapid cooling. All raw materials were ground to a particle size where D₉₀ = 45–75 μm, and the proportion of the reconstructed steel slag was adjusted to be close to the silicate phase region through ingredient calculations. XRD analysis was employed to reconstruct the principal mineral composition of steel slag, which mainly consisted of silicates, such as Ca₃Al₂O_6, Ca₂SiO₄, Ca₂MgSi₂O₇, Ca₂Al(AlSiO₇), Ca₂(SiO₄), and FeAlMgO₄. This composition demonstrated substantial modifications when compared to that of the original steel slag. The Diamond software(Diamond version 3.2k (Crystal Impact GbR, Bonn, Germany, 2014)) was employed to map the crystal structure of the minerals. By utilizing the Equilib module in the FactSage software, the equilibrium phase variations with temperature in the reconstructed steel slag under an air atmosphere were computed. The main mineral phases C₂AF, Ca₂Fe₂O₅, olivine, Melilite, Ca₃MgSi₂O₈, Ca₃MgAl₄O₁₀, Ca₂Al₂O₅ exist in the lower temperature range of 700–1100 K. The α′-C₂S phase appears at 1100 K and gradually transforms into α-C₂S at 1650 K. The liquid phase begins to precipitate at approximately 1550 K. Spinel exists in the temperature range from 1300 to 1700 K, and Ca₃MgSi₂O₈ melts into the liquid phase at 1400 K. As the temperature increases to 1600 K, the minerals C₂AF, Ca₂Fe₂O₅, and Ca₂Al₂O₅ gradually melt into the liquid phase. Melilite melts into the liquid phase at 1700 K. The test was conducted with 30% steel slag and 70% Portland cement. Compared with steel slag, the initial setting time of the reconstructed steel slag was shortened by 7 min, and the final setting time was shortened by 43 min. In terms of compressive strength, the 3-day compressive strength of the reconstructed steel slag was 0.6 MPa higher than that of the steel slag, the 7-day compressive strength was 1.6 MPa higher, and the 28-day compressive strength was 3.4 MPa higher. These enhancements in setting time and compressive strength corroborate the elevated cementitious activity of the reconstructed steel slag. Thermodynamic calculations of the main reactions of the reconstructed steel slag at high temperatures suggest that the main reactions at 1748 K can occur favorably from a thermodynamic standpoint.

To tackle the problems of low cementitious activity and poor volume stability of steel slag, it is advisable to select appropriate modifying materials according to the compositional characteristics of steel slag from different iron and steel enterprises to reconstruct the chemical composition and mineral composition of steel slag during the slag discharge or treatment process. This method can broaden the application of steel slag in the concrete industry and increase the blending ratio of steel slag powder.