Preparation and Performance Testing of Steel Slag Concrete from Steel Solid Waste

Abstract

In this paper, steel slag powder was used to replace part of the cement in road concrete, and group tests were carried out on coarse aggregates with different water–cement (W/C) ratios, different steel slag parameters, and different particle sizes. A sample of 100 mm × 100 mm × 100 mm was prepared and cured for 3 days, 7 days, and 28 days. In addition, the fluidity and compressive properties of the samples were also tested. The outcomes of this study revealed that, at a constant W/C ratio, increasing the proportion of steel slag improved the concrete’s fluidity but reduced its compressive strength; the 3-day (3 d) compressive strength of 40% steel slag was lower because the early activity of steel slag was lower than that of cement; steel slag also decreased the early-hydration rate of concrete. Comparisons across different W/C ratios demonstrated that steel slag made a more significant contribution to lower W/C ratios than higher ones. The water requirement for steel slag was relatively moderate, and the compressive strength of steel slag concrete with a high W/C ratio notably improved in the later stages. Based on the experimental conditions, the optimal content of steel slag was found to be 35%. Reusing steel slag as a replacement for sand in coarse aggregate can effectively lower costs and offer an innovative approach to steel slag treatment.

1. Introduction

Steel slag, a by-product of steel production, amounts to approximately 12–20 tons per 100 tons of steel produced [1]. Its disposal poses significant environmental challenges, including space consumption, water pollution, and other concerns [2]. In mainland China alone, over 400 million tons of slag are deposited annually, with a utilization rate of less than 30% [3,4]. Steel slag can be categorized into three types based on the steelmaking process: basic oxygen furnace (BOF), electric arc furnace (EAF), and ladle furnace slag. BOF slag constitutes about 70% of China’s total production [5]. Recycling BOF slag can reduce waste and alleviate pressure on industrial landfills, benefiting environmental conservation.

BOF slag is characterized by high porosity and density, being 15–25% heavier than natural coarse aggregate (NCA) due to its higher iron content. Chemically, it predominantly consists of CaO, SiO₂, Fe₂O₃, MgO, and Al₂O₃ [6]. Concrete, the most widely used artificial material globally, relies heavily on ingredients like NCA and river sand. However, the uneven distribution of NCA and its environmental impact from quarrying pose challenges [7]. Policies limiting NCA supply underscore the need for alternative materials in concrete production.

Exploring viable applications for surplus steel slag in construction, such as using it as a supplementary cementitious material or as fine/coarse aggregate, holds promise given the concrete industry’s substantial demand.

A large amount of steel slag will be emitted in the process of crude steel smelting, but due to the constraints of the stability of steel slag, the annual output of steel slag in China is high but the utilization rate is very low [8,9,10,11]. Steel slag, like cement, has similar activity to gelling materials, and the main components of the two minerals are similar, so steel slag can be applied to concrete [12,13,14,15]. Concrete with better performance can be prepared by using steel slag as coarse aggregate. The surface of steel slag is rough and has good hardness, and the bonding effect with mortar is significantly improved, thus improving the strength and durability of concrete [16,17,18,19,20,21,22,23,24].

J.C.M. Ho conducted an experimental investigation into substituting natural coarse aggregate (crushed granite) with five types of steel slag in concrete mixtures. Curing was performed at temperatures ranging from ambient (25 °C) to as high as 1000 °C for durations spanning from 28 to 91 days. Their findings indicate that replacing broken granite with steel slag can enhance the thermal insulation properties of concrete significantly. Additionally, Lai examined how different fillers impact both mechanical characteristics and long-term durability in steel slag-based concretes; it was found that incorporating SF powder instead of cement further enhances their strength. Moreover, under consistent conditions regarding aggregate composition and filler type, increasing the water-to-powder ratio leads to reduced total electrical charge permeability and chloride ion penetration depth in steel slag concretes owing to their denser microstructure, which improves resistance against chloride ions [25,26]. Zou Min analyzed the influence of steel slag powder on the properties of cement-based materials from the aspects of setting time, workability, mechanical properties, durability, and volume stability, and put forward the existing problems and future research and development directions of the application of steel slag powder in cement-based materials [27]. Ali Serdar Ecemis’ findings indicate that the inclusion of scrap rubber in concrete leads to a decrease in both the mechanical characteristics and weight of the material. This is mostly attributed to the lower strength and stiffness of the rubberized concrete [28]. Mohammad Alharthai studied the production and performance characteristics of structural concrete containing pumice in different proportions (0%, 25%, and 50% by volume) and aluminum powder of 0%, 1%, 2%, and 3% as additives under fire conditions [29]. Yasin Onuralp Ozkılıc studied the effect of waste aluminum on reinforced concrete shear beams. By adding 12 reinforced concrete beams with different waste aluminum ratios and different shear bar spacing, it was found that the bearing capacity of reinforced concrete beams decreased with an increase in the amount of aluminum slag added [30]. Liu Mo artificial steel slag aggregates were prepared by carbonating mixes of steel slag, fly ash, and Portland cement, of which their microstructure, properties, and impact on the performance of concrete were investigated [31].

Limited research has been conducted on determining the optimal proportion of steel slag in concrete when utilizing it as a replacement for cement as a coarse aggregate. To fill this research gap, in this paper, the steel slag produced in the production process of a converter steel making plant in Wuhan was crushed and ground, and fine powder with more than 100 μm particles was screened out as the research object. The chemical composition, phase composition, water content and particle size composition of the fine powder were analyzed, and it was used as an admixture to replace part of cement to prepare steel slag concrete. Group experiments were conducted under different W/C ratios (mass ratios of water to cement), steel slag contents, and particle sizes of coarse aggregate. The fluidity and compressive properties of the samples were also tested. In this paper, steel slag concrete was prepared by adding different steel slag content under different water–cement ratios, and the optimal steel slag content was found by testing its compressive strength and fluidity, which brings a new idea for iron and steel enterprises to solve the problem of solid waste.

2. Raw Materials and Analysis

2.1. Steel Slag

The steel slag was taken from a steelmaking converter of a steel enterprise in Wuhan. After being crushed and ground, fine powder with more than 100 μm particles was screened out, and its chemical composition is shown in

Table 1. Chemical composition analysis of steel slag.

Ingredient	$\mathrm{CaO}$	${\mathrm{Fe}}_2{\mathrm{O}}_3$	${\mathrm{SiO}}_2$	$\mathrm{MgO}$	${\mathrm{Al}}_2{\mathrm{O}}_3$	${\mathrm{P}}_2{\mathrm{O}}_5$
Content/%	44.90	20.51	13.55	5.83	4.01	1.39
Ingredient	$\mathrm{MnO}$	${\mathrm{TiO}}_2$	${\mathrm{Na}}_2\mathrm{O}$	${\mathrm{SO}}_3$	${\mathrm{V}}_2{\mathrm{O}}_5$	${\mathrm{Cr}}_2{\mathrm{O}}_3$
Content/%	1.28	0.55	0.23	0.19	0.15	0.10

and Figure 1. It can be seen that the steel slag was mainly composed of CaO, SiO₂, and Al₂O₃.

The particle size composition analysis of steel slag is shown in

Table 2. Particle size composition analysis of steel slag.

Fraction/mm	>10	6 < x < 10	3 < x < 6	1 < x < 3	0.45 < x < 1	0.15 < x < 0.45	<0.15
Productivity/%	22.54	34.41	16.76	10.73	4.72	3.69	7.15

. The energy spectrum analysis of steel slag is shown in Figure 2. The steel slag had relatively coarse particle sizes, with 22.54% being over 10 mm, and could serve as a skeleton during the molding process.

2.2. Cement

The Huaxin Cement (Huaxin Cement Co., Ltd., Wuhan, China) used in this experiment had a strength grade of 42.5, and its chemical composition and technical indicators are shown in

Table 3. Chemical composition of Huaxin Cement.

Place of Origin	Chemical Composition					Mineral Composition
	${\mathbf{SiO}}_2$	${\mathbf{Al}}_2{\mathbf{O}}_3$	${\mathbf{Fe}}_2{\mathbf{O}}_3$	$\mathbf{CaO}$	$\mathbf{MgO}$	${\mathbf{C}}_3\mathbf{O}$	${\mathbf{C}}_3\mathbf{S}$	${\mathbf{C}}_2\mathbf{S}$	${\mathbf{C}}_3\mathbf{A}$	${\mathbf{C}}_4\mathbf{AF}$
Huaxin Cement Co., Ltd.	21.06	6.04	3.63	63.98	2.67	2.25	48.13	24.04	9.87	11.04

and

Table 4. Technical indicators of Huaxin Cement.

Compressive Strength (MPa)		Flexural Strength (MPa)		Setting Time (min)		Stability	Fineness	Density (${\mathbf{kg}/\mathbf{m}}^{\mathbf{3}}$)
3 d	28 d	3 d	28 d	Initial Solidification	Final Solidification
27.9	48.0	7.2	9.7	2.32	3.40	Up to standard	0.98	3.1

.

2.3. Fine Aggregate

The fine aggregate used in this experiment was natural river sand, and the specific technical indicators are shown in

Table 5. Technical indicators of fine aggregate.

Test Item	Moisture Content	Packing Density	Apparent Density	Fineness Modulus
Result	5%	1158.5	2660	2.78

.

2.4. Coarse Aggregate

The coarse aggregate used in this experiment was gravel, and the specific technical indicators are shown in

Table 6. Technical indicators of coarse aggregate.

Test Item	Moisture Content	Packing Density	Apparent Density	Slime Content
Result	1.6%	1199	2650	1%

.

2.5. Water

The water used in this experiment was tap water.

3. Experimental Methods

3.1. Experimental Ratio

1.

Determination of the benchmark mix ratio

(1)

Calculation of the trial strength.

$${\mathrm{f}}_{\mathrm{cu},0}={\mathrm{f}}_{\mathrm{cu},\mathrm{k}}+1.645\sigma =30+1.645\times 5=38.2 \mathrm{MPa}$$

In this formula, ${\mathrm{f}}_{\mathrm{cu},0}$ is the concrete preparation strength (MPa), ${\mathrm{f}}_{\mathrm{cu},\mathrm{k}}$ is the standard value for the compressive hardness test of cement concrete cubes (MPa), and $\sigma$ is the standard deviation of concrete strength (MPa).

(2)

Calculation of the water–cement ratio (water–binder ratio).

a.

The important parameters in the design of the concrete binding ratio were determined;

b.

The W/C ratios of concrete were determined to be 0.35 and 0.5;

c.

The calculation parameter was determined to be 1.16 based on the latest cement surplus coefficient.

2.

The standard specification for compressive strength specimens of steel slag concrete was 100 mm × 100 mm × 100 mm. Their maintenance required a temperature of 20 ± 1 °C and a relative humidity of 95 ± 5%, and their maintenance times were 3 days, 7 days, and 28 days. When the W/C ratio was 0.35 and 0.5, the results of the mix ratio design under different slag production were as shown in

Table 7. Results of mix ratio design of concrete with W/C ratio of 0.35.

Steel Slag Content/%	Composition of Concrete/(kg/m3)
	Cement	Steel Slag	Coarse Aggregate	Fine Aggregate	Water
0	400	0	1353.59	582.04	140
20%	400	16	1353.59	582.04	140
35%	400	20	1353.59	582.04	140
40%	400	24	1353.59	582.04	140
50%	400	40	1353.59	582.04	140

and

Table 8. Results of mix ratio design of concrete with W/C ratio of 0.5.

Steel Slag Content/%	Composition of Concrete/(kg/m3)
	Cement	Steel Slag	Coarse Aggregate	Sand	Water
0	400	0	1159.13	614.34	200
1.26	395	5	1159.13	614.34	200
2.6	390	10	1159.13	614.34	200
5	380	20	1159.13	614.34	200
6	376	24	1159.13	614.34	200
7	372	28	1159.13	614.34	200
8	368	32	1159.13	614.34	200
10	360	40	1159.13	614.34	200
20	320	80	1159.13	614.34	200
40	240	160	1159.13	614.34	200

, respectively.

3.2. Experimental Process

3.2.1. Preparation of Concrete Test Blocks

According to the mix ratio design in Table 7 and Table 8, three portions of the corresponding raw materials were weighed to make the corresponding proportion of concrete test blocks for maintenance. The weighed raw materials were stirred in the specified area. After stirring, the concrete was put into grinding tools of 100 mm × 100 mm × 100 mm, filling up more than half of the grinding tools for the first time. After pumping the concrete a few times with a cement knife, the grinding tool was put onto a vibrating table for vibration, so that the mixed concrete was fully ground and filled in the grinding tool. After the first vibration of the vibrator, the vacant space in the grinding tool was filled with the remaining concrete. Then, the grinding tool was put on the vibrating table again to vibrate until the surface of the test block was flat, the top of which was then covered with plastic wrap to allow the concrete to congeal. The next day, the grinding tool was opened, and the concrete was marked with a brush and then placed in a curing room for 3, 7, and 28 days. When the time was up, the test block was taken out for the compressive strength test.

3.2.2. Slump Test of Steel Slag Concrete

To test the slump, it was necessary to thoroughly mix the concrete and place it in a slump-measuring cylinder for measurement. After pouring the concrete into the cylinder, a vibrating rod was used to thoroughly vibrate the concrete inside, ensuring that there were no gaps in the internal space until the cylinder mouth was filled. The measuring tube was then lifted so that the concrete flew, and the falling height was the slump of the concrete.

4. Results and Discussion

4.1. Effect of Steel Slag Micro-Powder on the Slump of Concrete Mixtures

When the water–cement ratio was 0.35 and 0.5, steel slag with different contents was added. After detection, the collapse degree values of the concrete mix in each group were as shown in

Table 9. Slump of each concrete mixture.

Steel Slag Content (%)	Water–Cement Ratio
	0.5	0.35
0%	36 mm	25 mm
10%	45 mm	33 mm
20%	60 mm	49 mm
40%	68 mm	57 mm

. Change curves of the slump of the concrete mixtures are shown in Figure 3.

The following can be seen from Figure 3:

① The comparison of fluidity between regular concrete is as follows: the smaller the W/C ratio, the higher the compressive strength of the concrete; simultaneously, the fluidity performance becomes smaller, the height of the slump decrease corresponds to a smaller value, and the workability becomes worse.

② The effect of steel slag content on the fluidity of concrete is as follows: Because the early activity of steel slag is lower than that of cement, when steel slag replaces part of the cement, the water demand of steel slag concrete is less than that of regular concrete; in other words, the water consumption of the concrete decreases [29]. Therefore, in the case of constant water consumption, the inclusion of steel slag will increase the fluidity of the concrete. Therefore, the more steel slag is added, the higher the fluidity of steel slag concrete, and the lower the compressive strength of steel slag concrete. As the fluidity increases, the corresponding slump decrease will also increase, and the workability of the concrete is significantly better than that of regular concrete.

③ Adding steel slag to concrete with varying W/C ratios enhances the fluidity of concrete. Increased amounts of steel slag result in improved concrete fluidity.

④ When the content of steel slag is 10–20%, the fluidity increases greatly, and the fluidity of the steel slag concrete also increases. When the content of steel slag is 20–40%, the fluidity of the steel slag concrete will increase rapidly. When the content of steel slag is more than 40%, the fluidity of the steel slag concrete may experience stagnation or shrinkage.

4.2. Compressive Strength of Steel Slag Concrete

4.2.1. Analysis of Compressive Strength of Steel Slag Concrete with Different Steel Slag Contents

The compressive strength of concrete (W/C ratio = 0.35) with different steel slag micro-powder admixtures is shown in

Table 10. Compressive strength of concrete (W/C ratio = 0.35) with different steel slag micro-powder admixtures.

Steel Slag Content (%)	0	20	35	40	50	Standard Deviation
3 d compressive strength ($\mathrm{MPa}$)	25.5	25.1	25.3	28.5	24.7	3.07
7 d compressive strength ($\mathrm{MPa}$)	27.5	27.2	26.9	28.2	26.5	3.40
28 d compressive strength ($\mathrm{MPa}$)	35.3	34.9	34.2	36.8	33.7	3.84

. The compressive strength of concrete (W/C ratio = 0.5) with different steel slag micro-powder admixtures is shown in

Table 11. Compressive strength of concrete (W/C ratio = 0.5) with different steel slag micro-powder admixtures.

Steel Slag Content (%)	0	20	35	40	50	Standard Deviation
3 d compressive strength ($\mathrm{MPa}$)	27.9	25.8	28.7	25.2	23.2	3.68
7 d compressive strength ($\mathrm{MPa}$)	29.6	27.1	30.2	26.4	25.2	3.93
28 d compressive strength ($\mathrm{MPa}$)	37.3	34.9	38.3	33.4	31.8	5.23

.

According to the comparison of the compressive strength between ordinary cement with a W/C ratio of 0.5 and cement with different contents of steel slag, it can be seen from Figure 4 that when the steel slag content is 40%, its 3 d compressive strength is very low. This is because the early activity of steel slag is much lower than that of cement; steel slag reduces the early-hydration rate of concrete. If the amount of steel slag increases, the compressive performance of steel slag concrete will be lower. When the content of steel slag is 35%, the compressive performance of the steel slag concrete is close to that of regular concrete.

According to Figure 4a, after replacing natural fine aggregate with steel slag fine aggregate, the compressive strength of concrete is lower than that of ordinary concrete, and the compressive strength increases first and then decreases with an increase in the addition of steel slag fine aggregate. When the steel slag content is 40–50%, the compressive strength of concrete exceeds or approaches that of ordinary concrete, so it can be seen that the optimal steel slag replacement amount is 40%.

It is evident that the early strength of steel slag concrete generally exhibits lower values compared to ordinary concrete. This can be attributed to several factors. Firstly, the high water absorption of steel slag fine aggregate results in variable bonding strength between the aggregate and cement slurry, impacting the mechanical properties and microstructure. Additionally, the porous and rough surface of steel slag leads to lower compactness than natural fine aggregate, further contributing to reduced compactness in steel slag concrete compared to ordinary concrete. Lastly, due to the low activity of C₃S and C₂S in steel slag fine aggregate, a longer period is required for hydration product formation through a reaction with water. Consequently, at seven days old, the transition interface zone between the aggregate and cement slurry in steel slag concrete may not exhibit tightness. These three factors collectively contribute to the observed lower early strength of steel slag concrete when compared with ordinary concrete.

Figure 4b illustrates that at a water–cement ratio of 0.5, the addition of steel slag initially decreases the compressive strength of concrete, which then gradually improves with increasing steel slag content. The compressive strength of steel slag concrete at different ages reaches or exceeds that of ordinary concrete when the replacement percentage is between 35% and 40%, particularly at a replacement rate of 40%, where it surpasses ordinary concrete by approximately 3%. However, when the steel slag content reaches 50%, there is a decline in strength, indicating an optimal replacement rate of 35% for steel slag.

In terms of early strength, steel slag concrete generally exhibits lower performance compared to ordinary concrete. This is primarily due to the rough porosity and high water absorption of steel slag, which results in reduced hydration by the cement surrounding the steel slag. Consequently, the degree of cement hydration is diminished, leading to poor bond performance and a significant impact on the concrete strength resulting from the bond between the aggregate and cement slurry. Furthermore, the low activity of C₂S and C₃S contained in steel slag causes delayed reactions, resulting in a less compact transition interface between the aggregate and cement slurry during the early stages of steel slag concrete. Additionally, the increased incorporation of steel slag aggregates enhances mechanical interlocking with cement slurry due to its rough multi-angle surface. As a result, higher levels of steel slag incorporation lead to improved interlocking and increased compressive strength in steel slag concrete.

4.2.2. Analysis of the Effect of Steel Slag Particle Size on the Compressive Strength of Concrete

When the water–cement ratio was 0.35 and 0.5, respectively, the compressive strength after 3, 7, and 28 days of curing was measured after adding coarse aggregates of different particle sizes, and the specific values are shown in

Table 12. W/C ratio of 0.5 for compressive strength of steel slag concrete with coarse aggregates of different particle sizes.

Coarse Aggregates of Different Particle Sizes (mm)	0.125	<0.15	0.15–0.45	0.45–1	1–3	3–6	6–10	10–20	20–40	>40
3 d compressive strength ($\mathrm{MPa}$)	30.1	24.2	26.7	31.3	24.3	28.7	23.6	21.6	19.5	17.9
7 d compressive strength ($\mathrm{MPa}$)	38.7	32.5	35.6	39.3	33.3	34.6	33.7	31.5	30.3	24.4
28 d compressive strength ($\mathrm{MPa}$)	46.9	42.1	41.9	51.3	42.5	43.7	46.9	40.5	37.4	32.7

and

Table 13. W/C ratio of 0.35 for compressive strength of steel slag concrete with coarse aggregates of different particle sizes.

Coarse Aggregates of Different Particle Sizes (mm)	0.125	<0.15	0.15–3	3–6	6–10
3 d compressive strength ($\mathrm{MPa}$)	27.9	25.8	26.7	25.2	23.2
7 d compressive strength ($\mathrm{MPa}$)	36.3	33.6	35.2	31.6	33.6
28 d compressive strength ($\mathrm{MPa}$)	45.2	39.9	44.5	38.4	41.6

.

It can be seen from Figure 5 that steel slag concrete with different coarse aggregates will also reduce the compressive strength of concrete with an increase in steel slag. When the content of steel slag is 35%, the compressive strength of steel slag concrete is close to or higher than that of regular concrete.

The intensity fluctuation is relatively large, and there is a significant gap between the experimental results and theories. The reasons for these intensity fluctuations are as follows:

① During the experiment, a water content of 35% was taken as the average value. There might be a slight deviation in practice, resulting in uncertain water consumption for the concrete.

② Because the stones were divided into two batches during the experiment, the quality of the two kinds of stones was not the same, so the experiment had a slight deviation.

③ When conducting compressive tests, there might have been some deviation in the compressive performance of the concrete due to the non-standard operations.

④ Uncertain factors might have occurred during the curing process, resulting in different compressive performance of the concrete.

⑤ When the concrete test block was installed in the mold, the concrete was not fully filled, and the grinding tool was unstable, so the concrete was not filled tightly during vibration. When the mold was removed, the concrete test block appeared hollow, with many small holes on the surface, and the surface was incomplete, which impacted the compressive strength of the concrete.

⑥ When the W/C ratio was overly low due to a relatively small amount of water, the hydrated cement could not be fully hydrated, which was not conducive to the improvement of strength. In the same case, when the W/C ratio was too high, the cement particles in the concrete were relatively small, because the distance between the particles was relatively far, and the hydrated cement colloid was not enough to fill the gaps between the particles. During maintenance, some of the water evaporated, leaving more water holes for the concrete, which would reduce the strength of the concrete. Due to the low water requirement of steel slag in the concrete production process, steel slag can improve the compressive strength of steel slag concrete in the later stage under the condition of high W/C ratios.

5. Conclusions

In this research, the fluidity and mechanical properties of concrete are tested and analyzed by using steel slag powder instead of cement. The effects of coarse aggregates with different particle sizes on the mechanical properties of concrete are tested, and the following conclusions are drawn:

• As is widely recognized, steel slag can be used as a substitute for sand and stone in concrete aggregate. Owing to its high hardness and rough surface, steel slag has the potential to enhance the transition zone of the concrete interface (the binding area between cement and aggregate phase) and improve the mechanical properties of hardened concrete to a certain extent. However, incorporating steel slag may decrease the fluidity of the concrete mix, which is not conducive to achieving dense mixture formation and could consequently reduce the mechanical properties of hardened concrete.
• It is found that when the W/C ratio is 0.5, the higher the steel slag content, the lower the compressive strength of concrete, and the early compressive strength of concrete is more sensitive than the late compressive strength. The loss rate of concrete’s 3 d compressive strength is higher than the corresponding steel slag replacement amount. When the steel slag content is 35%, the compressive strength of steel slag 3 d concrete is extremely low, because the early activity of steel slag is far less than that of cement. It is also because the steel slag added to the concrete will reduce the early-hydration rate of concrete.
• Steel slag can effectively improve the performance of concrete in the proper dosage range; in particular, the appropriate amount of steel slag can enhance the compressive strength of concrete, and too much steel slag will reduce the flow of concrete, which is not conducive to the formation of the mix. And through this experimental study, it is found that under the experimental conditions, the optimal content of steel slag instead of cement is 40%.
• By incorporating an appropriate amount of steel slag into concrete, it is possible to enhance the compressive strength while maximizing the utilization of solid waste to achieve comprehensive treatment effects.