Effects of Fluoride and Sulphate Mineralizers on the Properties of Reconstructed Steel Slag

Abstract Improving the cementitious activity and volume stability of steel slag by thermal reconstruction is an innovative method for efficient utilization of steel slag. In this study, different amounts of CaF2 and CaSO4 were added to steel slag as a mineralizer based on the determining admixtures needed for the reconstruction reaction, and the effects of CaF2 and CaSO4 on the cementitious activity and volume stability of the reconstructed steel slag were investigated. The results show that when the CaF2 content is increased to 5 wt%, the cementitious activity index of the reconstructed steel slag gradually increases to 92%, which is 12% higher than the first level technical requirement specified by the national standards, and the free CaO (f - CaO) and MgO (f-MgO) contents gradually decrease to 0.35 and 0.13 wt%, respectively. With increasing CaSO4 content, the cementitious activity index first increases and then decreases, while the contents of f -CaO and f-MgO show the opposite trend. When the CaSO4 content is 2 wt%, the activity index is 105%,which is 25%higher than the first level technical requirement specified by the national standards, and the f -CaO and f-MgO contents reach minima of 0.44 and 0.35 wt%, respectively.


Introduction
The amount of steel slag accounts for 15%-20% of total steel production, and the annual output of steel slag in China is close to 100 million tons [1]. However, the comprehensive utilization rate of steel slag in China is only about 20% [2]. Large-scale emissions and accumulation of steel slag occupy land, pollute the environment, and cause waste of resources. By comparison, the utilization rate of steel slag in developed countries is above 95%, in which the in-plant circulation is more than 20% [3]. But steel slag from different countries is mostly used as road building materials, backfill materials, and so on, with low economic benefits. Steel slag is a potential cementitious material because it contains some minerals with cementitious activity, such as C 3 S, C 2 S, C 3 A, and C 4 AF [4]. However, there is less content of cementitious minerals in steel slag, and the mineral crystals are complete, the grains are coarse and the defects are few, which results in its low cementitious activity. Scholars have developed many kinds of steel slag activation technologies, such as mechanical activation, thermal activation, chemical activaton, and so on. But they can not fundamentally eliminate the effects of component fluctuations. Moreover, steel slag contains free CaO (f -CaO) and MgO (f -MgO), whose hydration reactions result in volume expansion of the steel slag [5]. Therefore, steel slag can not be widely used as cementing materials.
The temperature of liquid steel slag can reach 1450 ∘ C-1650 ∘ C, and the enthalpy is up to 2000 MJ/t [6], which is a high quality waste heat resource. At present, the sensible heat recovery technology of steel slag is still in its infancy.
Lots of scholars at home and abroad have studied the waste heat recovery of steel slag in vairous aspects, such as wind quenching [7], continuous casting-continuous rolling dry granulation [8], rotating-drum method, and so on. Although the researches make the waste heat of steel slag be recovered and utilized, the performance problems existing in the tailings are not solved. It is necessary to develop and popularize a technology in order to make steel slag treatment and recycling level reach the scale industrialization. In this paper, an on-line reconstruction technology of steel slag, that is, the composition and structure of    K0  50  17  33  0  L1  50  17  33  1  L2  50  17  32  2  L3  49  16  32  3  L4  48  16  32  4 the steel slag are modified by the residual heat is proposed. That is, suitable conditioning components are added to the molten steel slag and high-temperature phase reactions between the steel slag and conditioning components occur by the waste heat of the steel slag and the composition and structure of the steel slag are regulated. This is performed to improve the cementitious activity and volume stability of steel slag. In this process, it is important to improve the kinetic conditions of the reconstruction reaction at high temperature. Addition of a mineralizer, such as CaF 2 or CaSO 4 , can can effectively solve this problem. Mineralizers are widely used in cement industry, and some trace elements from industrial waste can be used as mineralizers for the purpose of solid waste utilization. Scholars [9][10][11] found that CaF 2 can decrease the temperature of phase transition and promote the formation of C 3 S in clinker. Kacimi, et al. [12] found that CaF 2 can, in some cases, improve the hydraulic properties of Portland clinker. Horkoss et al. [13,14] found that [SO 4 ] 2− can replace [SiO 4 ] 4− in belite, and activation of the C 2 S lattice can be promoted. Uda, et al. [15] suggested that SO 3 can promote the absorption of f -CaO and improve the burnability of raw materials.
In summary, many experiments have proven that CaF 2 and CaSO 4 contribute to formation of cementitious minerals to some extent. However, there is the lack of a quantitative study of their effects on the reconstructed steel slag cementitious activity and stability, and there is no systematic study of the mechanisms of action of fluoride and sulphate mineralizers. Therefore, in this study, different amounts of CaF 2 and CaSO 4 were added as mineralizers to reconstructed steel slag and the effects of CaF 2 and CaSO 4 on the cementitious activity and volume stability of the reconstructed steel slag were investigated.

Raw materials
The steel slag comes from Tangshan Iron and Steel Company, the cement is Portland cement from a cement plant in Hubei Province, the conditioning components are quicklime and slag, and the mineralizers are CaF 2 and CaSO 4 . The chemical compositions of the raw materials are given shown in Table 1.

Experimental methods
The lime saturation coefficient (KH) of the reconstructed steel slag was adjusted according to the cement clinker. The KH value of cement clinker is generally between 0.88 and 0.96. The KH value of the reconstructed steel slag was determined to be 0.9, and the specific proportions are given in Tables 2 and 3. The steel slag, quicklime, slag, and mineralizer were mixed in different proportions. The mixed raw materials were then placed in a mold for molding and forming, and the reconstructed steel slag samples were obtained. Finally, the reconstructed steel slag samples were calcined at 1400 ∘ C in a high-temperature box furnace and then cooled to about 1000 ∘ C for water quenching after holding at 1000 ∘ C for 30 min.
The reconstructed steel slag was ground into a fine powder with a specific surface area of 400 ± 10 m 2 /kg. The reconstituted steel slag was then mixed with cement at a mass ratio of 3:7 to prepare a steel slag-cement slurry. The compressive strength of the slurry was measured after curing in a standard maintenance room for 7 or 28 days. The cementitious activity index of the steel slag was calculated according to the relevant regulation in "Steel Slag Powder Used for Cement and Concrete" (GB/T 20491-2017) [16]: where A is the activity index of the steel slag (%), R t is the strength of the tested mortar at the corresponding age (MPa), and R 0 is the strength of the cement mortar at the corresponding age (MPa). The f -CaO and f -MgO contents in the reconstructed steel slag samples were measured by ethylenediaminetetraacetic acid (EDTA) chemical titration.
The stability of the reconstructed steel slag samples was tested according to the relevant regulation in the "Cement Standard Consistency, Setting Time and Stability Test Method" (GB/T 1346-2011) [17].
The microstructure, composition, and morphology of the reconstructed steel slag were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and the lithofacies test.

Effects of fluoride and sulphate mineralizers on the cementitious activity of the reconstructed steel slag
The reconstructed steel slag samples with different CaF 2 and CaSO 4 contents were aged at room temperature for curing times of 7 and 28 days, respectively. The compressive strengths of the reconstructed steel slag samples aged for different times were determined and the cementitious activity index values were calculated. The results are shown in Figures 1 and 2.
From Figures 1 and 2, the early activity index of the reconstructed steel slag without mineralizer (K0) is 40% and the late activity index is 54%. With increasing CaF 2 content, the early activity index of the reconstructed steel slag first increases and then decreases ( Figure 1). When 3 wt%  CaF 2 is added, the early activity index reaches the highest value of 70%, which is 16% higher than that of the K0 sample and 5% higher than the first level technical requirement stipulated in "Steel Slag Powder used for Cement and Concrete" (GB/T 20491-2017). When the CaF 2 content exceeds 3 wt%, the early activity index gradually decreases. Because an increase in the total fluorine content in the C 3 S solid solution leads to a decrease in the C 3 S early hydration activity. The late activity index of the reconstructed steel slag gradually increases with increasing CaF 2 content. When 5 wt% CaF 2 is added, the late activity index reaches 92%, which is 22% higher than that of the K0 sample and 12% higher than the first level technical requirement specified in GB/T 20491-2017. However, the CaF 2 content should not be too high, because when the concentra-tion of CaF 2 in the high-temperature liquid phase reaches supersaturation, it recrystallizes and increases the viscosity of the liquid phase [18]. In addition, as CaF 2 volatilizes under high temperature, HF is generated by the action of high-temperature steam, which will not only pollute the environment, but also damage and corrode the refractory materials of the electric furnace used in the experiment and the containers used in in the on-line steel slag reconstruction technology [19]. Therefore, the maximum amount of CaF 2 is determined to be 5 wt%. With increasing CaSO 4 content, the early activity index of the reconstructed steel slag gradually increases ( Figure 2). When 4 wt% CaSO 4 is added, the early activity index reaches the highest value of 84%, which is 44% higher than that of the K0 sample and 19% higher than the first level technical requirement stipulated in GB/T 20491-2017. The late activity index increases and then decreases. When 2 wt% CaSO 4 is added, the late activity index reaches the highest value of 105%, which is 51% higher than that of the K0 sample and 25% higher than the first level technical requirement stipulated in GB/T 20491-2017.
Mineralizers are beneficial to improve the cementitious activity of the reconstructed steel slag mainly because it promotes a decrease of viscosity. Amphoteric element Me (mainly Al 3+ and Fe 3+ in this system) presents different forms to maintain the acid-base equilibrium of the high temperature liquid phase. can promote the equilibrium to move to the right. Therefore, the viscosity of the liquid phase is reduced, and the mobility of all ions is improved [20,21].
The cementitious activity index values of the reconstructed steel slag samples with different proportions of CaF 2 and CaSO 4 are higher than those of the K0 sample. The cementitious activity index of the reconstructed steel slag with CaSO 4 as a mineralizer fluctuates in the range 94% to 105%. This range is higher than that of the reconstructed steel slag mixed with CaF 2 , which has a highest value of 92%. The results show that CaSO 4 is a better mineralizer to improve the cementitious activity of steel slag than CaF 2 .

Effects of fluoride and sulphate mineralizers on the volume stability of the reconstructed steel slag
To test the effects of CaF 2 and CaSO 4 on the volume stability of the reconstructed steel slag, EDTA chemical titrations and stability tests were performed. The f -CaO and f -MgO contents are given in Tables 4 and 5, and the trends are shown in Figures 3 and 4. From Tables 4 and 5, the f -CaO and f -MgO contents are all less than 2 wt%, which is within the standard range of good stability.    Addition of CaF 2 and CaSO 4 reduces the viscosity of the steel slag, which results in more f -CaO participating in the reactions to form C 2 S and C 3 S. In addition, CaF 2 and CaSO 4 promote the dissolution of f -MgO in gelling minerals, and the reasons are as follows. When S and F are dissolved into C 3 S crystal respectively, S 6+ displaces Si 4+ and F − displaces O 2− . In order to balance the electricity price, the vacancy reaction in C 3 S is likely to occur in the form of co-substitution of different price ions [22].  4 have similar effects on the f -CaO content in the reconstructed steel slag, but the f -MgO content in the reconstructed steel slag with CaF 2 is lower than that with CaSO 4 . The f -CaO and f -MgO contents in the reconstructed slag with CaF 2 are lower than those with CaSO 4 at the optimal amount of mineralizer, which indicates that addition of CaF 2 as a mineralizer to improve the stability of steel slag is better than addition of CaSO 4 .
Considering the effects of CaF 2 and CaSO 4 on the cementitious activity and volume stability of the reconstructed steel slag, the optimum contents of CaF 2 and CaSO 4 in the reconstructed steel slag are 5 and 2 wt%, respectively.
The stability of the reconstructed steel slag with CaF 2 and CaSO 4 was tested by boiling experiments, and the results are shown in Figure 5. The reconstructed steel slag test cake surfaces are smooth with no cracks, and the test cakes can be completely removed from the glass sheets. This shows that the volume stability of the reconstructed steel slag is up to standard.

Effects of fluoride and sulphate mineralizers on the composition and morphology of the reconstructed steel slag
The XRD patterns of the reconstructed steel slag with different CaF 2 and CaSO 4 contents are shown in Figures 6 and 7, respectively. The diffraction peaks of the three cementing minerals C 2 S, C 3 S, and C 3 A are present for the reconstructed steel slag samples ( Figure 6). However, the diffrac-tion peaks of these three minerals are weaker for the reconstructed steel slag without CaF 2 than for the other steel slag samples, and the peaks of the RO phase are present. For the reconstructed steel slag with CaF 2 , the diffraction peaks of the RO phase are absent but the diffraction peaks of C 4 AF and C 2 F are present, which indicates that CaF 2 is helpful to decompose the RO phase. FeOx from decomposition of the RO phase reacts with calcium and aluminum oxide to form C 4 AF and C 2 F. The diffraction peaks of C 2 S gradually decreases and the diffraction peaks of C 3 S and C 3 A gradually increases when the CaF 2 content exceeds 1 wt%, indicating that CaF 2 promotes the reactions of C 2 S and Al 2 O 3 with CaO to form C 3 S and C 3 A, respectively. As shown in Figure 7, the main cementitious minerals of the reconstructed steel slag with CaSO 4 are C 3 S, C 2 S, C 3 A, and C 2 F. The diffraction peaks of the RO phase are absent. FeOx from decomposition the RO phase mainly reacts with CaO to produce C 2 F. As the amount of CaSO 4 increases, the C 3 A content gradually increases, and the C 3 S and C 2 S contents first increase and then decrease. When 2 wt% CaSO 4 is added, the diffraction peaks of the main cementitious minerals, such as C 3 S and C 2 S, are the most acute. This is because of the excessive amount of SO 3 generated from CaSO 4 under high temperature stabilizes C 2 S and prevents absorption of f -CaO by C 2 S, which are not conducive to formation of C 3 S [24]. SEM images of the reconstructed steel slag with different CaF 2 and CaSO 4 contents are shown in Figures 8  and 9, respectively. The minerals in the reconstructed steel slag without CaF 2 are in the form of loose short rods. With increasing CaF 2 content, the minerals are gradually refined, and some minerals begin to exist in a melting state. When the CaF 2 content is 5 wt%, the minerals are in the  melting state. Some of them have fish scale or droplet shapes, which are determined to be mainly C 3 S, C 3 A and C 2 F according to energy spectrum analysis. The mineral structure is dense and uniform, and there are fewer pores and more liquid phase in the steel slag sample. When the CaSO 4 content is 2 wt%, the reconstructed steel slag has a better melting state and more liquid phase (Figure 9). Moreover, there is more C 2 S and C 3 S, whose grain shapes tend to be complete and the grain boundaries are clear. The reason why CaF 2 or CaSO 4 increases the amount of liquid phase is that the addition of them increases the composition of the system, and greatly reduces the minimum eutectic temperature and the liquid phase appearance temperature.
The lithofacies test images of the reconstructed steel slag with different CaF 2 and CaSO 4 contents are shown in Figures 10 and 11, respectively. The minerals in the steel slag without CaF 2 are coarse, varied in shape, and loose in arrangement ( Figure 10). After addition of CaF 2 , the main minerals in the reconstructed steel slag include cross bicrystalline, round-grained, or elliptical β-C 2 S, hexagonal or long-flake C 3 S, and gray acicular or dendritic C 2 F and C 4 AF. With increasing CaF 2 content, the particles of C 3 S and C 2 S increase and are significantly refined. When the CaF 2 content is 5 wt%, club-shaped C 3 S and roundgrained C 2 S show a uniform agglomeration distribution. When the content of CaSO 4 is 2 wt%, a large number of round or elliptical C 2 S and hexagonal C 3 S particles appear with a uniform distribution and high degree of crystallization (Figure 11b). In addition, scattered white sheet minerals are observed in the reconstructed steel slag doped with CaF 2 and CaSO 4 . An energy dispersive spectroscopy (EDS) test of this substance was performed ( Figure 12). From EDS    and XRD analysis, it is concluded that the white sheet mineral is MgFe 2 O 4 formed by the reaction of FeOx and MgO from decomposition of the RO phase solid solution. This is also a reason for reduction of f -MgO.

Mechanisms of the effects of fluoride and sulphate mineralizers on the hydration activity of the reconstructed steel slag
Strength development and the volume stability of steel slag are strongly affected by the hydration mechanisms of the cementitious minerals and the microstructures of the hydration products. The hydration reactions of the cementitious minerals at room temperature are C 3 S: C 3 S + nH → C-S-H + (3-x)CH C 2 S: 13 When CaSO 4 exists: C 4 AH 13 + 3CSH 2 + 14H → C 3 A·3CS·H 32 C 4 AF: C 4 AF + 4CH + 22H → 2C 4 (A,F)H 13 When CaSO 4 exists: C 4 AF + 2CH + 6CSH 2 + 50H → 2C 3 (A,F)·3CS·H 32 When steel slag-cement is mixed with water, C 3 A is hydrated first and the hydration reaction is severe. Hydration of C 4 AF and C 3 S then occurs, and hydration of C 2 S finally occurs because it has the slowest hydration rate. Hydrated calcium silicate (C-S-H), the hydration product of C 2 S and C 3 S, is the most abundant mineral in hardened steel slag-cement slurry and it plays a major role in its strength. Ca(OH) 2 , another hydration product of C 2 S and C 3 S, affects the strength by closely intertwining with C-S-H gel. The high hydration rate and high early strength of C 3 S make it the main material that determines the early strength of steel slag. In the presence of CaSO 4 , hydrated calcium sulphoaluminate (also known as AFt), the hydration product of C 3 A, also plays a role in the early strength. C 3 S not only has high early strength, but it also has good late strength development. The effect of C 2 S on the strength is not great until the late stage. Thus, the late strength is mainly affected by C 2 S and C 3 S. C 4 AF has a favorable effect on the sulfate resistance properties, but it has little effect on the strength.
The hydration process is the comprehensive action of mineral hydration, which is different from the general chemical reaction in solution or liquid. In particular, migration of ions is difficult and they cannot completely participate in the reaction in a short time. Instead, ions start from the surface and slowly migrate into the center by diffusion under the condition of constantly changing concentration. In this process, the change in the mineral composition of the reconstructed steel slag caused by participation of the mineralizer has a different effect on the hydration process and strength of the steel slag in different periods.
Mineralizer improves the early strength of the C 2 S slurry. This is because the C-S-H gel produced by early C 2 S hydration has a large specific surface area, which make F − or SO 2− 4 adsorb on the gel surface and precipitation of the gel on unhydrated particles surface is reduced, and C-S-H is immediately isolated once formed. The hydration reaction of C 2 S is then accelerated and the amount of C-S-H gel increases. The mineralizer can also decrease the distortion degree of the C 3 S crystal, and the symmetry of the C 3 S crystal structure increases. Moreover, with increasing F − or SO 3 content in C 3 S solution, Ca(OH) 2 supersaturation becomes slow, and crystallization and nucleation growth of Ca(OH) 2 is delayed. Therefore, mineralizer can reduce the early hydration rate of C 3 S. In the latter stages of hydration, the mineralizer also enables C-S-H to grow to elongated fibers, which cross and overlap, resulting in a denser and harder hydration product structure. The strength of C 3 A especially increases when CaSO 4 is added. This is because AFt can form a diffusion barrier on the surface of C 3 A particles to slow down the hydration reaction, smooth heat release, and greatly reduce the probability of crack generation.

Potential applications and prospects
It is found that the interior of the slag block is still red-hot when turning over slag and cracking the large slag block. It can be seen that the steel slag has a large thermal capacity, and the steel slag in the middle can maintain a high temperature state for a long time. However, the aim of steelmaking is to control the quality of steel, thus, the composition, the viscosity and the temperature range of liquid phase of steel slag in each furnace sometimes vary [25]. If the viscosity is large and the temperature range of the liquid phase is narrow, it is not conducive to the reconstruction reaction of steel slag. Addition of mineralizers based on the determining admixtures needed for the reconstruction reaction can reduce viscosity and increase the temperature range of liquid phase and make the reconstitution reaction more thorough.
The on-line reconstruction of steel slag makes use of residual heat of steel slag and improves its cementitious and stable properties at the same time. This study provides a theoretical basis for the high efficiency application of low activity metallurgical slag in building materials industry. It is beneficial to promote energy saving and emission reduction, and to reduce environmental pollution.

Conclusion
With increasing CaF 2 content, the early activity index of the reconstructed steel slag first increases and then decreases. When 3 wt% CaF 2 is added, the early activity index reaches the highest value of 70%, which is 5% higher than the first level technical requirement stipulated by the national standards. The late activity index gradually increases with increasing CaF 2 content, and it reaches the highest value of 92% with addition of 5 wt% CaF 2 . This value is 12% higher than the first level technical requirement. The f -CaO and f -MgO contents gradually decrease with addition of CaF 2 . When the CaF 2 content is 5 wt%, the f -CaO and f -MgO contents reach the lowest values of 0.35 and 0.13 wt%, respectively. After adding CaF 2 to the reconstructed steel slag, the RO phase decomposes and C 2 F and C 4 AF are generated. The C 3 S and C 3 A contents in the reconstructed steel slag gradually increase and the particles of the cementitious minerals are finer and more evenly distributed. The optimum dosage of CaF 2 is 5 wt%.
With increasing CaSO 4 content, the early activity index of the reconstructed steel slag gradually increases. When 4 wt% CaSO 4 is added, the early activity index reaches the highest value of 84%, which is 19% higher than the first level technical requirement. The late activity index of the reconstructed steel slag first increases and then decrease. When 2 wt% CaSO 4 is added, the late activity index reaches the highest value of 105%, which is 25% higher than the first level technical requirement. The f -CaO and f -MgO contents first increase and then decrease with increasing addition of CaSO 4 . When the CaSO 4 content is 2 wt%, the f -CaO and f -MgO contents reach the lowest values of 0.44 and 0.35 wt%, respectively. After adding CaSO 4 to the reconstructed steel slag, the RO phase decomposes and C 2 F is generated. The optimum dosage of CaSO 4 is 2 wt%.
The effect of CaSO 4 as a mineralizer to improve the cementitious activity of steel slag is better than that of CaF 2 . In contrast, the effect of CaF 2 as a mineralizer to improve the stability of steel slag is better than that of CaSO 4 .
The on-line reconstruction of steel slag is an efficient way to utilize steel slag by making full use of the residual heat of molten steel slag and modifying the steel slag. And the gelling activity and voluem stability of steel slag is improved. This paper provides a theoretical basis for the high efficiency application of steel slag. It is beneficial to reduce environmental pollution and waste of resources.