Study on the Early Effect of Excitation Method on the Alkaline Steel Slag

Abstract

The change law of alkaline steel slag cementitious activity was investigated by mechanical excitation, alkaline excitation, and salt excitation methods. The effect of grinding time, chemical activators, and content of steel slag on the properties of cement replaced by steel slag was studied. The hydration products of cement replaced by steel slag were analyzed by XRD and SEM. The results show mechanical excitation can effectively improve the particle size distribution and cementitious activity of steel slag. The best mechanical properties are obtained when grinding for 20 min and adding 10% steel slag. Chemical excitation can further improve the cementitious activity of steel slag based on mechanical stimulation. The optimal mechanical properties are obtained when the dosage of sodium hydroxide is 1.0%, the dosage of early strength agent is 2.0%, and the dosage of steel slag is 25%. The main hydration products of cement replaced by steel slag are calcite (CaCO₃), calcium hydroxide (Ca(OH)₂), dicalcium silicate (Ca₂SiO₄), and C-S-H gel. The microstructure presents a fibrous network structure, laying the foundation for improving mechanical properties.

1. Introduction

Steel slag is a solid waste produced by smelting iron and steel, and its discharge accounts for about 12~20% of steel output [1]. China’s steel output has exceeded 800 million tons every year since 2013, and exceeded 900 million tons for the first time in 2018. With the continuous increase of steel production in China, and the annual addition of more than 100 million tons of steel slag, the accumulated steel slag stock has reached 2 billion tons. The large amount of steel slag storage has led to land occupation, environmental pollution, and resource waste. Therefore, it is a very urgent task to solve the problem of steel slag utilization. Currently, the utilization rate of steel slag in China is lower than 30%, much lower than that in developed countries such as Germany (close to 100%). The comprehensive utilization of steel slag resources can reduce pollution and produce benefits for iron and steel enterprises. Therefore, improving the utilization rate of steel slag is a major problem in promoting the sustainable development of enterprises, even in China [2,3,4]. The mineral composition of steel slag is determined by its chemical composition, reaction formation condition, and cooling process; and the physical properties of steel slag are related to its composition, mineral composition, and cooling granulation mode. The main chemical composition of steel slag includes CaO, SiO₂, Fe₂O_3, and Al₂O₃, which is close to the chemical composition of silicate cement clinker and has potential cementitious activity. Influenced by the steelmaking process, the type of raw materials, origin, and many other factors, the steel slag of different steel mills varies significantly in composition; even in the same steel mill, there are differences in the composition of steel slag. The effect of activating the activity of steel slag by different excitation methods varies greatly [5,6,7,8].

Currently, there are three main methods for researchers to improve the activity of steel slag, namely mechanical excitation, chemical excitation, and thermal excitation. Physical excitation, also known as mechanical excitation, is the use of mechanical grinding equipment to grind the materials in order to improve their fineness and specific surface area [9]. Chemical excitation is a method to improve the activity of steel slag by adding certain admixtures. There are many kinds of chemical activators, including alkali excitation, acid excitation, salt excitation, and compound excitation [10]. Thermal excitation refers to providing a certain amount of heat in the cementitious system to the tetrahedral network structure of the glass body in the steel slag, and to improving the activity of the steel slag [11]. Some scholars have investigated the effect of mechanical excitation on the activity of steel slag [12]. As a result of this study, they selected the steel slag/slag based filling cementing material developed by physically excited steel slag, which could meet the requirements of mine filling strength and fluidity, and effectively reduce the filling cost. The optimal grinding time is 50 min, the steel slag and slag content are 35.5%, and the material cost of filling cementitious material is reduced by 43.95% [13]. In another study, Jiang et al. performed oxidative reforming of steel slag, which significantly improved the grindability of the reformed steel slag. The particle size distribution of unmodified steel slag and of modified steel slag has undergone significant changes (from 100 to 10 μm) after the secondary grinding [14]. The chemically stimulated steel slag activity method is now more widely studied. Dong used chemically excited steel slag to develop steel slag cementing material suitable for total tailings filling, which could meet the needs of mine filling [15]. Li et al. partially replaced sulfoaluminate cement with steel slag, fly ash, and silica fume to adjust the setting time of sulfoaluminate cement and improve its compressive strength. The results showed that steel slag at 5%, fly ash at 14.11%, and silica fume at 6% were the best ratios for the 28d strength of sulfoaluminate cement [16]. Xiao et al. stimulated the activity of steel slag based on the composite activator composed of slag and gypsum. The experimental results show that when slag is 40%, gypsum is 25%, steel slag is 35%, the mechanical property is the best, and the strength of 356d can reach 5.5 MPa [17]. Zhang et al. investigated the effect of Na₂SiO₃, NaOH, and Ca(OH)₂ on the activity of steel slag by preparing a composite alkaline exciter. The results showed that Na₂SiO₃ had a significant impact on the 7d compressive strength of alkaline steel slag cemented materials, NaOH had a considerable impact on the 3d compressive strength, and Ca(OH)₂ had a substantial impact on the 28d compressive strength [18]. Liu et al. prepared all-solid waste concrete by completely replacing cement clinker with steel slag powder, slag, and desulfurization gypsum as cementitious materials. The compressive strength of the prepared all-solid waste concrete was highest when steel slag powder, slag powder, and desulfurization gypsum powder accounted for 25%, 63%, and 12% of the total mass of cementitious materials [19]. Xiang et al. used steel slag, compound, fine sand, gravel, and admixtures to formulate cement-free steel slag cementitious material with mortar compressive strength up to 70 MPa [20]. Gao et al. used steel slag to produce cement clinker, and the maximum steel slag content was 14.30% [21]. Although there are many types of research on improving the comprehensive utilization of steel slag materials, most focus on one or two kinds of excitation methods, and few explore the effect of multiple excitation methods on the activation of steel slag activity.

In this paper, we use mechanical excitation, alkaline excitation, and salt excitation methods to systematically investigate the excitation effect of different excitation methods on the activity of steel slag, in order to maximize the potential activity of steel slag and enhance the blending of steel slag in cement. The steel slag was pulverized for 10 min, 20 min, 30 min, and 40 min;the steel slag powder was chemically excited by an early strength agent and NaOH; and the steel slag substituted cement strength test was carried out. The results showed that mechanical excitation, alkali excitation, and salt excitation could improve the activity of steel slag, and the substitute cement content was up to 25%. The article was aimed to improve the comprehensive utilization rate of steel slag, reduce the environmental pollution and land occupation caused by the stockpiling of steel slag, and lay a foundation for the resource utilization of industrial solid waste.

2. Materials and Methods

2.1. Materials

Steel slag is purchased from Ningxia Iron and Steel emissions solid waste (D50 = 36.85 μm, D90 = 195.6 μm). Figure 1 shows the XRD pattern and particle size distribution of the steel slag. The main chemical components of steel slag are shown in

Table 1. Composition of the steel slag (mass fraction).

Raw Material	SiO2/%	Al2O3/%	CaO/%	MgO/%	Fe/%
Steel slag	19.92	4.08	40.83	12.48	16.69

. Sodium hydroxide is an analytically pure reagent purchased from Shanghai Wokai Biotechnology Co., Ltd. in Shanghai, China. Sodium sulfate, triethanolamine, and anhydrous ethanol are all analytical pure-grade reagents produced by Sinopharm Chemical Reagent Co., Ltd. in Shanghai, China. P·O 42.5 Portland cement is supplied by Ningxia Saima Cement Co., Ltd. in Ningxia, China. Based on the chemical compositions, the alkalinity coefficient of the steel slag can be calculated. According to the alkalinity coefficient, steel slag is usually classified into three types, namely high basicity slag, medium basicity slag, and low basicity slag; the specific classification is shown in

Table 2. Classification of steel slag by basicity.

Steel Slag	Alkalinity Coefficient
high basicity slag	R < 1.8
medium basicity slag	1.8 < R < 2.5
low basicity slag	R > 2.5

.

By calculation from Equation (1) [22], the alkalinity coefficient of the steel slag is 2.22, which is higher than 1.8 lower than 2.5. This means that this kind of steel slag belongs to the category of medium basicity slag.

$$\mathrm{R}=\frac{{\mathrm{m}}_{\mathrm{CaO}}}{{\mathrm{m}}_{{\mathrm{Al}}_2{\mathrm{O}}_3}+{\mathrm{m}}_{{\mathrm{P}}_2{\mathrm{O}}_5}}$$

(1)

2.2. Test Design and Specimen Preparation

First, the steel slag was subjected to grinding using ball mills for 10, 20, 30, and 40 min to obtain steel slag powder of different particle sizes.

Second, the strength experiments of steel slag replacing cement were conducted with different particle sizes of steel slag powder. Where the water-cement ratio is 0.4, the doping of steel slag powder is 10%, 15%, 20%, and 25%, and the sample size is 40 mm × 40 mm × 160 mm [23]. In standard maintenance conditions up to 3 and 7d, the best grinding time for steel slag can be obtained.

Third, salt excitation of steel slag powder was performed using an early strength agent. The early strength agent dose tested for strength is 1%, 2%, 3% and 4%, and the other conditions are the same as the second step. The optimal early strength agent dosing can be obtained.

Fourth, alkaline excitation of steel slag powder was carried out by using NaOH, and the dose of the NaOH tested for strength is 0.5%, 1.0%, 1.5%, and 2.0%, and the other conditions are the same as the second step. The optimal alkaline exciter dosing can be obtained.

Fifth, the experimental group samples with the best mechanical properties were selected for XRD and SEM tests.

2.3. Methodology

The particle size of steel slag powder was tested by a laser particle size analyzer. The chemical composition of the pristine steel slag was tested by X-ray Fluorescence method. At the age of 3 and 7 days, specimens were tested by a universal testing machine for compressive strength and flexural strength. The original steel slag sample and the samples obtained from the experiments were analyzed by X-ray diffraction. A scanning electron microscope was used to examine the morphology.

3. Results and Discussion

3.1. Effect of Mechanical Excitation on the Particle Size of Steel Slag

The grinding time was controlled at 10 min, 20 min, 30 min, and 40 min by the ball mill. After grinding, the particle size distribution of steel slag was measured by a laser particle size analyzer, and the results are shown in

Table 3. Particle size parameters of steel slag after grinding.

Grinding Time	D10/μm	D30/μm	D50/μm	D60/μm	D90/μm
10 min	5.86	9.43	15.39	19.48	34.83
20 min	4.78	7.86	14.56	17.18	31.97
30 min	4.08	7.80	11.76	14.65	29.36
40 min	3.71	6.29	9.99	13.38	28.54

and Figure 2.

From Table 2 and Figure 2, we learn that with the increase in grinding time, due to the strong mechanical collision, the particle size of steel slag gradually becomes smaller, and the particle size becomes more concentrated, and the distribution curve gradually shifts to the left. The D10 decreased by 36.7%, the D50 reduced by 35.1%, and the D90 fell by 18.1% after grinding for 40 min compared with that after grinding for 10 min. Compared with the original steel slag, the D50 decreased by 72.9%, and the D90 decreased by 85.4%. This shows that after proper mechanical grinding, the coarse particles in steel slag decrease sharply and the change range of fine particles is smaller than that of coarse particles. This indicates that the grinding process gradually enters a dynamic equilibrium stage of crushing, refinement and agglomeration, and coarsening. The fine particles are more difficult to further refine after grinding to a certain extent. In fact, similar phenomena are observed in other studies regarding mechanical activation [24].

3.2. Effect of Mechanical Excitation on the Activity of Steel Slag

The strength tests were carried out on steel slag after grinding at different times and replacing the cement according to different blending amounts. The results are shown in Figure 3 and Figure 4 below.

3.2.1. Compressive Strength

It can be seen from Figure 3 that under the condition of different steel slag content proportions, the compressive strength of 3d and 7d age shows a gradually increasing trend with an increase in grinding time, and the strength increase is mainly concentrated in the period from 10 min to 20 min. The maximum 3d compressive strength is 25.42 MPa, and the 7d compressive strength is 30.75 MPa. At the same grinding time, the more steel slag is mixed, the lower the compressive strength at both 3d and 7d. After mechanical grinding, the particle size distribution curve of steel slag gradually shifted to the left, the large particles gradually decreased, the small particles gradually increased, and the compressive strength gradually increased with grinding time. This is because when the hydration reaction occurs inside the system, the specific surface area of small particles is larger, so they can participate more in the hydration reaction and continuously fill up the voids inside the system to promote the further occurrence of the hydration reaction. Then the strength continues to improve, indicating that mechanical grinding can effectively enhance the activity of steel slag.

3.2.2. Flexural Strength

It can be seen from Figure 4 that under the condition of the same steel slag admixture, with an increase in grinding time, the change curves of 3d and 7d flexural strength are relatively flat. Under the same grinding time conditions, the flexural strength gradually decreases with an increase of steel slag admixture. Comprehensive 3d, 7d compressive strength and flexural strength change law and improve the utilization rate of steel slag, the best grinding time is for 20 min, with steel slag dosing of 10%.

3.3. Effect of Salt Excitation on the Activity of Steel Slag

Based on 20 min of mechanical excitation, the varying regularity of compressive strength and flexural strength of steel slag replacement cement with an early strength agent (salt content) is shown in Figure 5 and Figure 6 below.

3.3.1. Compressive Strength

It can be seen from Figure 5 that under the condition of different steel slag content ratios, the compressive strength at the age of 3d and 7d fluctuates significantly with the change of early-strengthening agent. When the content of steel slag is 10%, the strength shows a gradual increase with the content of the early strong agent, and the compressive strength increases more significantly in the range of 1–2% of early strength agent. With steel slag dosing at 15%, the strength shows a changing trend, the content of early strong agent first decreases and then increases, and the compressive strength reaches the maximum when the amount of early strength agent is 3%. When the content of steel slag is 20% and 25%, the strength shows a changing trend, the content of early strong agent first increases and then decreases, and the compressive strength reaches the maximum when the amount of early strength agent is 2%. At the same early strength agent dosing, the more steel slag dosing, the lower the 3d and 7d compressive strength. This shows that for the active excitation effect, there is an optimal ratio of steel slag dosing and early strength agent. When the steel slag content is within a specific range, the excitation effect of the early strength agent is better with the increase of the doping amount. When the steel slag content is beyond a specific range, the excitation effect of the early strength agent is not obvious. Because the early strength agent contains more SO₄²⁻, it promotes the chemical reaction of calcium aluminate in the slurry to produce more ettringite, and improves the strength of the system [25].

3.3.2. Flexural Strength

It can be seen from Figure 6 that under the condition of the same steel slag admixture, with an increase of early strength agent, the 3d and 7d flexural strength curve showed a trend of rising and then falling. When the content of early strength agent is 2% and steel slag is 20%, the maximum flexural strength of 3d and 7d is 5.84 MPa and 6.54 MPa. Under the same grinding time conditions, the flexural strength gradually decreases with an increase of steel slag admixture. For the combined 3d and 7d compressive strength and flexural strength, the best effect is achieved when the content of early strength agent is 2% and steel slag is 20%.

3.4. Effect of Alkaline Excitation on the Activity of Steel Slag

Based on 20 min of mechanical excitation and 2.0% early strength agent dosing, the varying regularity of compressive strength and flexural strength of steel slag replacement cement with alkaline excitation is shown in Figure 7 and Figure 8 below.

3.4.1. Compressive Strength

It can be seen from Figure 7 that when the steel slag content is 10%, the compressive strength of 3d and 7d gradually decreases with an increase in NaOH content, and the maximum compressive strength at 3d and 7d when NaOH dosing is 0.5%. At the proportions of 15%, 20%, and 25% steel slag content, the 3d and 7d compressive strength increases first and then decreases with the content of NaOH, and the 3d and 7d compressive strength was at its maximum at 1.0% NaOH dosing. In the same NaOH dosing, the more steel slag dosing, the lower the 3d and 7d compressive strength. This indicates that NaOH has a pronounced stimulating effect on the activity of steel slag when the content of steel slag is significant. This is because the addition of NaOH increases the alkalinity of slurry and promotes the depolymerization of the vitreous body in steel slag. More C-S-H gels and ettringites are generated in the system, which cross and lap with each other to form a network structure, promoting the improvement of strength [26].

3.4.2. Flexural Strength

It can be seen from Figure 8 that when the steel slag content is 10%, the flexural strength at 3d and 7d slowly decreases with an increase of NaOH content, and the maximum flexural strength at 3d and 7d when NaOH dosing is 0.5%. At the proportions of 15%, 20%, and 25% steel slag content, the 3d and 7d flexural strength increases sharply and then decreases slowly. When the steel slag dosing is 15% and NaOH is 1.0%, the 3d flexural strength obtains its maximum. When the steel slag dosing is 25% and NaOH is 1.0%, the 7d flexural strength reaches its maximum. For comprehensive 3d and 7d compressive strength and flexural strength, the optimum NaOH content, and steel slag content are 1.0% and 25%, respectively.

3.5. X-ray Diffraction Analysis of Steel Slag Substitute Cement

After the standard curing for 3d and 7d, appropriate samples were prepared and dried, and ground to powder for the XRD test. The test results are shown in Figure 9, where the curing time in Figure 9a is 3d and in Figure 9b is 7d.

Figure 9 shows the phase diffraction pattern with 25% steel slag content. It can be seen that partial hydration has begun in the slurry at the age of 3d, and the hydration products are mainly calcite (CaCO₃) and calcite hydroxide (Ca(OH)₂). The addition of NaOH in the system, so that the reaction conditions gradually alkaline, the steel slag vitreous dispersion, and dissolution created the conditions. With the increase in maintenance time, the calcium oxide in the steel slag is gradually hydrolyzed, and the hydroxide calcium stone diffraction peak is gradually strengthened. Silicon oxide and alumina continuously react chemically with calcium hydroxide stone to produce dicalcium silicate (Ca₂SiO₄) and C-S-H gel, which promotes the further improvement of the mechanical properties of the system.

3.6. Microstructure Analysis of Steel Slag Substitute Cement

The microstructure diagram of cement hydration products replaced by steel slag was obtained by scanning electron microscopy (SEM), as shown in Figure 10, where the curing time in Figure 10a is 3d and that in Figure 10b is 7d.

It can be seen from Figure 10 that the hydration reaction has begun inside the system at 3d, and the hydration products that can be observed are hexagonal plate-shaped Ca(OH)₂ crystals and amorphous C-S-H gels. In addition, there are needle and rod-like hydration products of calcium alumina generated on the surface, but the amount and types of hydration products are few. It is obvious that the structure of the specimen at this time is loose, and the gap is large. At the age of 7d, the structure of the slurry gradually becomes dense, the gaps are significantly reduced, and the true rod-like ettringite becomes coarser and interwoven, forming an apparent fibrous network structure. This indicates that the active components in the system further react with Ca(OH)₂ for hydration, which promotes the system becoming denser [27]. The mechanical properties of test blocks are further enhanced on a macro level.

4. Conclusions

This study investigated the effects of mechanical activation, salt excitation and alkali excitation on steel slag, including particle size distribution, compressive strength, flexural strength, hydration products and microstructures. The following conclusions can be drawn.

(1)

Through the mechanical grinding experiment, the particle size distribution curve of steel slag gradually shifted to the left. After grinding for 40 min, D50 decreased by 72.9%, and D90 by 85.4%. The change range of coarse particles in steel slag was larger than that of fine particles, and it was more difficult for fine particles to be further refined.

(2)

Mechanical excitation can significantly improve the activity of steel slag. After grinding for 10 min, the 3d compressive strength can reach 19.29 MPa, and the 7d compressive strength can reach 23.32 MPa. The comprehensive excitation effect, economic cost, the optimal grinding time is 20 min, and the content of steel slag is 10%.

(3)

Salt excitation can further enhance the activity of steel slag based on mechanical excitation. The optimal excitation content is 2.0%, and steel slag content is 20%.

(4)

Alkali excitation can further enhance the activity of steel slag based on salt excitation and mechanical excitation. The optimal excitation content is 1.0%, and steel slag content is 25%.

(5)

The hydration products replaced by steel slag are mainly calcite (CaCO₃), calcareous hydroxide (Ca(OH)₂), and dicalcium silicate (Ca₂SiO₄). With the increase of curing age, the hydration products become coarser and interwoven, forming a fibrous network structure, which further enhances the mechanical properties.

Through our research, we can reflect on the effects of different excitation methods on the activity of steel slag, which can help maximize the activity of steel slag. This has significance for further improving the utilization rate of steel slag and reducing pollution of the environment.