Impact of Steel Slag Ratio on Strength and Durability of Flowable Waste Soil for Foundation Pit Backfill

Abstract

In order to broaden the means of resource utilization of waste soil and steel slag produced in the process of urban construction, in this study, steel slag was used to replace part of the cement with waste soil, to prepare flowable waste soil. Through unconfined compressive strength (UCS), permeability, and dry–wet cycle tests, the mechanical properties of flowable waste soil under different steel slag replacement ratios and moisture contents were studied. The results show that the UCS of the flowable waste soil increases with the increase in curing age. When the steel slag replacement ratio is less than 66.7%, the UCS of the sample after 7 days of curing is more than 100 kPa. When the moisture content of the sample is 58% and the steel slag replacement ratio is 58.3%, the UCS can reach 101 kPa after 1 day of curing, which can meet the requirements of rapid construction. The UCS, resistance to the dry–wet cycle, elastic modulus, failure stress, and failure strain of flowable waste soil all decrease with the increase in the moisture content and steel slag substitution ratio. The permeability coefficient of the steel slag mixed sample decreased from 2.1 × 10⁻⁶ cm/s to 7.4 × 10⁻⁷ cm/s, and the permeability coefficient of the flowable waste soil after 28 d curing extended below 6 × 10⁻⁶ cm/s, indicating that the flowable waste soil has good impermeability and can be applied well in engineering.

1. Introduction

Improving the industrial and construction usage of solid waste may save energy and cut emissions in the context of national carbon neutrality. The Chinese building materials industry has proposed a full carbon peak by 2025, with cement and other industries taking the lead in the 2023 Carbon Peak. The CO₂ emission from cement production accounts for about 8% of the carbon emission from human activities [1]. At present, the main method of addressing this issue is to find solid waste that can partially replace cement to reduce the carbon emission [2,3]. Steel slag is a kind of industrial solid waste produced in the steel manufacturing process, and its discharge accounts for about 15% of the crude steel output. However, only 30% of steel slag can be reused effectively. Steel slag is rich in Ca, Mg, Fe, Si, and other oxides, and it has proven to be an environmentally friendly raw material for cement replacement. Waste soil is a construction solid waste produced in the excavation process during urban development [4]. China’s accumulation of waste soil has reached 10 billion tons, but the utilization ratio is quite low. Massive steel slag and waste soil accumulation occupy valuable land resources and pose a threat to the surrounding ecological environment, and to resolve this, we need to recycle waste for the sustainable development of society [5]. The excavation of a foundation pit is always required at a construction site, and a large number of foundation pit sidewalls need to be backfilled, but the narrow sidewalls of the working face usually limit the use of large mechanical equipment. It is difficult to achieve layered backfill and layered tamping. Therefore, the material requires both high mobility for placement and sufficient strength for structural support when used for backfilling. According to the American Concrete Association (ACI), Controlled Low-Strength Material (CLSM) is a self-compacting gel material with a compressive strength of 8.3 MPa or less, which is mainly used as backfill [6]. It is the best choice for backfilling the side wall of foundation pit.

In previous studies, it is demonstrated that the strength of a steel slag mixture will be greatly enhanced when steel slag partially replaces cement, functioning as a skeleton material. Therefore, steel slag is widely used as the raw material in the preparation of subgrade, flowable soil and backfill materials [7]. Behiry [8] found that when 70% steel slag is mixed with 30% limestone, the optimum density and strength of the pavement subbase can be achieved at the lowest construction cost. Karolina et al. [9] substituted both coarse and fine aggregates in concrete with steel slag, demonstrating the feasibility of producing high-strength, environmentally friendly concrete. Wang et al. [10] found that in flowable soil prepared with steel slag replacing fine aggregate, slump decreased while the bleeding ratio increased as the steel slag replacement ratio rose. Ghanbari and Bayat [11] found that the partial replacement of cement with steel slag as a curing agent for soil has economic, environmental, and technical advantages, especially when the steel slag is combined with polypropylene fiber to obtain a good unconfined compressive strength (UCS).

The raw materials for the preparation of flowable soil are not limited to steel slag. Scholars have found that sand, slag, and dredged silt are also good choices. Achtemichuk et al. [12] utilized the excitation effect of recycled concrete aggregate (RCA) residual slurry and the cementitious activity of slag or fly ash to achieve cementless CLSM and realize the resource utilization of waste. Lee et al. [13] found that flowable soil produced by reclaimed soil and marine dredged soil can be used as a backfill material for underground pipelines. Gemperline et al. [14] found that by controlling the water–cement ratio and water–solid ratio, respectively, at 0.5–0.7 and 0.7, the waste soil excavated in the field can be used to prepare flowable soil suitable for structural backfilling. de Medeiros et al. [15] used marble and granite residues in 50% and 100% weight ratios instead of the commonly used filler, hydrated lime. Asphalt mixtures containing marble and granite waste were found to have superior mechanical properties to mixtures containing 0% residue. If steel slag is used as the backfill, it will inevitably be affected by the external environment after construction, which will cause the expansion or contraction of the backfill soil and produce destructive dry and wet stress, resulting in a decline in durability. Therefore, the effect of the dry–wet cycle on the strength of the steel–slag mixture cannot be ignored. Guo and Xiong [16] added fiber to a fly ash–steel slag-based geopolymer to improve its performance under drying–wetting conditions. Wu et al. [17] found that the strength degradation of expansive soil treated with steel slag and cement was not prominent under drying–wetting.

It is beneficial to manufacture green construction materials using waste soil and steel slag. Currently, extensive research has been performed on the secondary usage of steel slag and waste soil as well as the study of backfill materials for flowable soil. The use of steel slag to partially replace cement and waste soil to create flowable soil for backfilling the sidewall of the foundation pit has not received much attention. Unlike previous studies that focused on single waste systems such as fly ash CLSM [12] or steel slag concrete [9], our method achieves the collaborative utilization of waste. This study uses waste soil, steel slag, and Portland cement to prepare flowable waste soil as a backfill material for the sidewalls of foundation pits. It studies the mechanical properties of different ages, durabilities under wet–dry cycles, and permeabilities of flowable waste soil. Waste soil is used to replace most of the sand and gravel, and steel slag is used to partially replace cement to reduce construction costs. Ultimately, it is found that this mobile waste soil meets strength and permeability requirements in engineering.

2. Test Materials and Methods

2.1. Test Materials

The waste soil is selected from the 3–8 m deep silty clay layer (gray–yellow, containing silt lenses) in Hongxin Square, Zhenjiang City, China. It belongs to the widely distributed river lake sediment in the Yangtze River alluvial plain. After drying and grinding, slag with a particle size of less than 2 mm is selected. To enhance the activity of the steel slag, it is ground into steel slag powder of 200–400 mesh for subsequent tests. The basic physical properties of the soil samples were determined according to ASTM D4318-17 (ASTM D4318-17 Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils [S]. West Conshohocken, PA: ASTM International, 2017), as shown in

Table 1. Physical properties of waste soil.

Bulk Density (g/cm3)	Specific Gravity	Moisture Content (%)	Maximum Dry Density (g/cm3)	Optimum Moisture Content (%)
1.71	2.69	12.8	2.07	20.94

.

The steel slag used in the experiment is from a steel plant in Zhengzhou, Henan Province. Steel slag with a particle size of less than 4 mm is selected The curing agent is 32.5 composite Portland cement. The composition of steel slag and cement is analyzed by an X-ray fluorescence spectrometer. The results are shown in

Table 2. Chemical composition of experimental materials (% mass).

Name of Material	Major Chemical Constituents (%)
	CaO	Fe2O3	SiO2	Al2O3	MgO	SO3	MnO	Cr2O3	Na2O	P2O5	TiO2
Steel slag	34.30	26.22	19.64	7.72	4.14	0.493	2.49	2.08	1.06	0.763	0.523
Cement	61.39	2.46	20.92	6.41	3.01	3.41	-	-	-	-	-

.

Mason B [18] classified the alkalinity of the steel slag used in this study. The calculation method was P = Cao/(SiO₂ + P₂O₅). Based on the data in Table 2, it can be calculated that the alkalinity of the steel slag is 1.68. However, the higher the alkalinity of the steel slag, the greater its activity, and the steel slag used in this study has relatively low activity.

The particle size distribution of waste soil and steel slag was determined by using a laser particle size analyzer (Mastersizer 3000 (Malvern Instruments Limited, Malvern, UK)), as presented in Figure 1.

2.2. Preparation and Experimental Scheme of Flowable Waste Soil

Add the slag, cement, and steel slag to the container in sequence, with the cement and steel slag evenly covering the slag. Slowly inject water to prevent splashing, and continue stirring for 3–5 min until the mixture is uniform. Pour the mixed mixture into a three-axis mold (diameter 3.91 cm × height 8 cm) and a through mold (diameter 6.18 cm × height 2 cm) in three portions without vibration to demonstrate the compactness of the mixture. Add tape around the top of the mold to a certain height, then fill the mixture to prevent sinking, and use a scraper to remove excess parts when disassembling the mold. Finally, place the unsealed sample in a constant-temperature and -humidity curing chamber (temperature 20 ± 1 °C, humidity > 90%) until the desired age is reached, and then proceed with subsequent testing. The calculation of the water content and steel slag replacement ratio in

Table 3. Mixture proportions for experimental specimens.

Experiment	W (%)	F (%)	mc:mg:ms:mw (g)
UCS test, Dry–wet cycle test	58	50	60:60:450:330.6
		58	50:70:450:330.6
		67	40:80:450:330.6
		75	30:90:450:330.6
	60	50	60:60:450:342.0
		58	50:70:450:342.0
		67	40:80:450:342.0
		75	30:90:450:342.0
	62	50	60:60:450:353.4
		58	50:70:450:353.4
		67	40:80:450:353.4
		75	30:90:450:353.4
	64	50	60:60:450:364.8
		58	50:70:450:364.8
		67	40:80:450:364.8
		75	30:90:450:364.8
Penetration test	60	0	120:0:450:342.0
		50, 75	-
	62	50, 58, 67	-

is shown in Formulas (1) and (2):

$$W={\displaystyle \frac{m_w}{m_c{+m}_g{+m}_s}}$$

(1)

$$F={\displaystyle \frac{m_g}{m_c{+m}_g}}$$

(2)

In Formulas (1) and (2), W is the moisture content (%); m_w is the water mass (g); m_c is the cement mass (g); m_g is the steel slag mass (g); m_s is the waste soil mass (g); and F is the steel slag replacement ratio (%).

When adding cement and steel slag, they should evenly cover the residue as far as possible.

2.3. Test Methods

Unconfined compressive strength test

UCS is the most widely used indicator for evaluating the mechanical properties of specimens. This study measured the UCS of the samples according to the ASTM D2166 standard “Standard Test Method for Unconfined Compressive Strength of Soil”. The size of the sample is diameter 6.18 cm × height 2 cm, and a vertical load is applied at a ratio of 1 mm/min. The stress–strain data is automatically collected by an unconfined compressive strength meter. After the stress reaches its peak, an axial strain of about 4% is measured, and the test is completed. The average value is taken of three copies of the report.

Dry–wet cycle test

Refer to the American standard ASTM D559-03 and conduct wet–dry cycle testing in conjunction with research [19]. Take out the sample that has been cured for 28 days and conduct a dry–wet cycle test. Take a set of samples from each wet–dry cycle and soak them in water (pH = 7.2 ± 0.3) at room temperature (24 °C) for 24 h. Then, remove the samples from the sink, dry them with a dry cloth, and let them air dry in a clean indoor area for 24 h. Group according to W-F combinations (3 per group), repeat the above steps for 3, 6, or 9 dry–wet cycles, and perform a single UCS test on the samples. Calculate the strength loss ratio according to Formula (3):

$$K_{\mathrm{n}}={\displaystyle \frac{R_{\mathrm{n}}-R_0}{R_0}}\times 100\%$$

(3)

In Formula (3), K_n is the strength loss ratio of UCS (%); R₀ is the UCS before the dry–wet cycle, the UCS (kPa); and R_n is UCS after n dry–wet cycles (kPa).

Permeation test

The permeability coefficient was determined using an electronic consolidation odometer. The sample was placed in a curing box for one day and entered into the electronic consolidation odometer, and the loading pressure was 12.5 kPa. The permeation test was carried out according to ASTM D5084. We calculated the permeability coefficient according to Formula (4):

$$K=2. 3{\displaystyle \frac{aL}{At}}lg{\displaystyle \frac{H_1}{H_2}}$$

(4)

In Formula (4), k is the hydraulic conductivity coefficient (cm/s); a is the cross-sectional area of the scale tube (cm²); L is the height of the specimen (cm); A is the cross-sectional area of the specimen (cm²); t is the time for each penetration test (s); H₁ is the initial water head (cm); and H₂ is the final water head (cm).

3. Results

3.1. Effect of Curing Age on UCS

As shown in Figure 2a–d, the UCS of flowable waste soil changes with age at four moisture contents, of 58%, 60%, 62%, and 64%, respectively. It can be seen that the strength of the flowable waste soil increases as the age increases; when the moisture content is 58% and the replacement ratio of steel slag is 50%, the maximum UCS of the sample is 398 kPa after curing for 28 d, while when the moisture content is 64% and the steel slag replacement ratio is 75%, the minimum UCS of the sample for 28 d curing is only 90 kPa. The increment of UCS is greater from 1 to 7 days as compared to that from 7 to 14 days or from 14 to 28 days. This is due to the rapid hydration reaction of cement after contact with water in the early stage of curing to produce hydration products such as calcium silicate hydrate to cement soil and steel slag particles, forming a stable structure. However, with the increase in curing age, the hydration reaction of cement is gradually completed, so the strength increase in the later period is small or negligible.

When using mobile soil as a backfill material, the UCS value of the sample after 7 days should meet the standard requirement of a temporary foundation support structure > 100 kPa (JGJ 120-2012) [20]. Statistical correlation shows that the UCS value of the 28-day sample is 1.4 times that of the 7-day sample. Samples exceeding 100 kPa within 7 days remain above 140 kPa after 28 days, ensuring long-term stability against lateral soil pressure [21]. It can be seen from Figure 2 that when the steel slag replacement ratio of flowable waste soil is less than 75%, it can essentially meet the minimum strength requirements during the backfilling of the foundation pit sidewall at 7 days, so the steel slag replacement ratio should not exceed 75%.

3.2. Stress–Strain Curve of Flowable Waste Soil

Figure 2 shows that the strength of flowable waste soil reaches its peak at 28 days. Therefore, this section provides a detailed analysis of the stress–strain relationship at this time point. The stress–strain curves of flowable waste soil with different steel slag substitution ratios at 28 days are shown in Figure 3, which allows for further investigating the changes in the mechanical properties of flowable waste soil with the steel slag substitution ratio. It can be seen that the stress–strain curve of the flowable waste soil exhibits strain-softening characteristics, and the peak stress value will be reached at 2% strain. When the water content of the sample is the same, the curve of the sample before reaching the stress peak is steeper and has a smaller slag replacement ratio, showing a clear elastic growth trend, and then the curve shows a rapid decline, that is, clear brittle failure characteristics. Then, when the replacement ratio of steel slag is high, the curve increases slowly before reaching the peak stress, and the curve declines slowly after reaching the peak stress, showing clear plastic failure characteristics.

3.3. The Influence of Dry–Wet Cycle on UCS

The UCS of flowable waste soil after different wet–dry cycles is shown in Figure 4a–d. It can be seen that when the number of wet–dry cycles is less than or equal to six, the UCS value of the sample slowly increases with the increase in wet–dry cycles, and after six cycles, the UCS begins to decrease. This may be due to the fact that in the early stages of the wet–dry cycle, when the sample is placed in air, the internal moisture content decreases, causing the bound water film to thin, resulting in shrinkage of the dry matrix of the sample and promoting its UCS growth [22]. In addition, the carbonation of silicates and Ca(OH)₂ will generate denser calcium carbonate that precipitates within the pores. This process reduces the porosity of the sample and increases cohesion. Therefore, carbonization can increase the strength of the sample during early wet–dry cycles. In particular, in samples with a low steel slag replacement ratio, cement can provide more Ca(OH)₂ for carbonation [23,24,25,26,27]. At the same time, it was found that repeated saturation expansion during wet–dry cycles and shrinkage during drying can lead to progressive microcrack propagation at the interface between waste soil and steel slag. This type of damage accelerate after six cycles, which may be due to the leaching of Ca²⁺ from steel slag reducing the bonding of the sample, and the expansion and cracking of free compounds leading to increased pore connectivity and permeation damage. This leads to a decrease in UCS value. The sample with F = 50% has a higher content of cement, forming a dense C-S-H network to resist crack propagation. After nine cycles, the strength loss is about 10%. Due to the low cement content, the compactness of the sample with F = 75% decreases. The hydration activity of steel slag is poor, which weakens the unreacted particles. After nine cycles, the UCS value decreased by about 25%. Compared with samples with steel slag replacement ratios of 67% and 75%, samples with steel slag replacement ratios of 50% and 58% showed more significant changes in UCS values. In particular, when the sample with a moisture content of 58% and a steel slag substitution ratio of 50% was recovered six times, the UCS reached 469 kPa, which was 17.84% higher than before the wet–dry cycle. Although the UCS of the samples decreased after wet–dry cycles, samples with a steel slag substitution ratio of 75% were excluded. The UCS of other samples still exceeded 100 kPa after nine wet–dry cycles, meeting the project requirements.

3.4. The Influence of Dry–Wet Cycle Times on UCS Loss Ratio

Figure 5 shows the relationship between the UCS loss ratio and the moisture content after nine dry–wet cycles. It can be seen that the strength loss ratio of all samples is less than 0%. According to the definition of this ratio, it can be seen that the UCS of the sample decreases to different degrees after nine cycles. The higher the replacement ratio of steel slag, the greater the strength loss ratio, with the maximum strength loss at 26.21%. When the replacement ratio of steel slag is less than 67%, the sample will not be damaged, while when the replacement ratio of steel slag is greater than 75%, the sample will be damaged, as shown in Figure 6. This is because before sample preparation, the particle size of steel slag is reduced by grinding, but the activity is still low, and it cannot undergo a hydration reaction to produce hydration products like cement in order to resist the dry–wet cycle. The ability of the sample to resist wet–dry cycles is mainly provided by the hydration products of cement, but as the wet–dry cycle progresses, the hydration products react or dissolve in water, decreasing the amount of hydration products, which leads to surface damage of the sample and a sharp decrease in UCS. This degradation mechanism was intuitively confirmed by the surface cracks observed after nine dry–wet cycles in samples with high steel slag replacement ratios (Figure 6).

3.5. Effect of Age on Permeability

An important index to test is the permeability of backfill materials. Previous studies have shown that, in general, the required permeability of flowable soils is in the range of 10⁻⁴–10⁻⁵ cm/s, while it can be as low as 10⁻⁷ cm/s if a higher strength of backfill material is required [20]. Based on the test results presented in previous sections, the replacement ratios of three steel slags at a 60% water content (0%, 50%, 75%) and three steel slags at a 62% water content (50%, 58%, 67%) were selected, and the relationship between permeability and age is shown in Figure 7. For the convenience of recording, 0 + 60% means the moisture content is 60% and the steel slag replacement ratio is 0%. With the increase in age, the permeability coefficients of the six ratios significantly decreased from 2 days to 12 days, slowly decreased from 12 days to 20 days, and tended to stabilize after 20 days, remaining essentially unchanged, while the permeability coefficients of the specific selected samples were all less than 6 × 10⁻⁶ cm/s. It can be seen from the figure that the permeability coefficient will increase with the increase in steel slag substitution ratio. This is because the increase in steel slag replacement ratio will lead to a decrease in cement content and a decrease in the amount of hydration product generated, and it is unable to effectively connect and fill the pores between soil and steel slag particles, so the permeability coefficient will increase.

4. Discussion

Failure Mechanism of Flowable Waste Soil in Elastic and Elastoplastic Stage

It can be taken from Figure 3 in Section 3.2 that the stress–strain curves of the specimens vary with different steel slag replacement ratios, which may be related to the pore structure inside the specimens. Previous studies have found that the dosage of cement will directly affect the pore structure and strength of the sample [24]. When the replacement ratio of steel slag increases, the dosage of cement decreases, resulting in a reduction in the amount of hydration products, which leads to an increase in the internal pores of the sample and a decrease in strength. So, when the sample is destroyed, it shows different forms of degradation.

In order to clarify the failure law of fluid residue with different moisture contents and steel slag replacement ratios, it is necessary to explore the change law of the elastic modulus (E_c) in the elastic stage and the failure stress (σ_f) and failure strain (ε_f) in elastoplastic stage. Figure 8 shows the E_c in the elastic stage of flowable waste soil. From the data in the table, it can be seen that when the moisture content is constant, with the increase in the steel slag replacement ratio, the elastic modulus of the flowable waste soil in the elastic stage decreases. E_c is essentially a concentration between 20 and 70 MPa, which is similar to the research of Lirer [28] and Temuujin [29].

Failure strain is also an important factor to measure the deformation characteristics of flowable waste soil when it is used as a backfill material for the sidewall of a foundation pit. When the sample reaches the peak stress, the greater the strain, the clearer the plastic characteristics of the sample, and vice versa, the clearer the brittleness characteristics of the sample. Figure 9 shows the σ_f and ε_f of flowable waste soil. It can be seen from the data in the table that when the water content is 58 to 64%, with the increase in the steel slag replacement ratio, the failure stress of flowable waste soil decreases continually, while the failure strain increases at first and then decreases, reaching a peak at a replacement ratio of steel slag of 58 to 67%.

The results show that the changes in σ_f and ε_f are related to the steel slag replacement ratio. Although the steel slag was ground into 200–400-mesh powder during the sample preparation process to improve its hydration activity, the steel slag used in this study had a low alkalinity and its hydration activity was not as good as that of cement. Although it can play a certain filling role, that role is not the main source of strength. Moreover, steel slag absorbs water during the reaction process. The more cement steel slag replaces, the more water it absorbs, meaning the cement is not fully hydrated to enhance the strength of the sample [30]. As can be seen from the above, steel slag cannot make up for the decrease in sample strength caused by the reduction in cement content and the decrease in hydration products. Therefore, with the increase in the steel slag replacement ratio, the overall structure becomes porous, and the strength of the specimens decreases and shows the characteristics of plastic failure.

5. Conclusions

The mechanical characteristics of the flowable waste soil used for foundation pit backfilling were examined in this paper, with particular attention paid to compressive strength, resistance to dry–wet cycling, and impermeability. The effects of the age, moisture content, and ratio at which steel slag is replaced on the mechanical characteristics of the flowable waste soil were also examined. The following are the primary conclusions:

(1)

The UCS of flowable waste soil first increases with the increase in curing age. The strength growth is the fastest at 1–7 days of curing and tends to stabilize at 28 days. The UCS decreases with the increase in moisture content and steel slag replacement ratio, and the growth ratio of UCS in the specimens with a lower moisture content is significantly greater than that of higher-moisture-content specimens. When the replacement ratio of steel slag is less than 66.7%, the UCS of the sample can reach 100 kPa after seven days, which can meet the requirements of the backfilling strength of the sidewall of the foundation pit.

(2)

The variation trends of UCS in flowable waste soils with different moisture contents with the dry–wet cycle are similar, both increasing first and then decreasing. When the moisture content is 58% or 60%, the UCS of the sample is the largest after the sixth dry–wet cycle. With the progress of the dry–wet cycle, the maximum value of UCS in the sample appears earlier and earlier. When the moisture content is 64%, the maximum value of UCS occurs in the third dry–wet cycle. After the ninth dry–wet cycle, the UCS of all samples was lower than the initial UCS, and the maximum strength loss was 26.21%

(3)

The permeability coefficient of the flowable waste soil gradually decreased with the increase in curing. When steel slag replacement ratio remained unchanged, the permeability coefficient of the sample increased with the increase in moisture content. After 20 days of permeation, the permeability coefficients of the specimens were all less than 6 × 10⁻⁶ cm/s. When the moisture content of the specimens was 50% and the steel slag replacement ratio was 60%, the permeability coefficient decreased to 7.4 × 10⁻⁷ cm/s, indicating that the specimens had good impermeability and could be applied well in engineering.

(4)

The strain of the specimen when it reaches the peak stress is around 2%, and the maximum does not exceed 3%. The specimens with low steel slag replacement ratios show brittle failure characteristics, while those with high steel slag replacement ratios show plastic failure characteristics. The elastic modulus, failure stress, and failure strain all decrease gradually with the increase in steel slag replacement ratio.

This study used Zhenjiang waste soil from a specific region, which is a typical soil of the Yangtze River Delta alluvial layer, but its mechanism is still transferable. For wider applicability, we suggest adjusting the adhesive ratio: for high-viscosity soils (such as Northeast China), increase the cement by 5–10% to offset expansion.