Dynamic Compression Properties of Ni-Fe Slag Powder Soil Cement under Impact Load

Abstract

In this research, Ni-Fe slag powder and mineral powder are blended into mineral admixtures and added to soil cement, with the aim of investigating the mechanical property of soil cement under a dynamic environment, and the dynamic properties of Ni-Fe slag powder soil cement after impact compression are obtained by conducting split-Hopkinson pressure bar (SHPB) test. The results show that under the same age and different admixture conditions, the dynamic stress of Ni-Fe slag powder soil cement increases first and then decreases and reaches the maximum when the admixture ratio is 40%, and the dynamic stresses at 7 d, 28 d and 60 d were 5.10 MPa, 9.73 MPa and 13.51 MPa, respectively. Under the same admixture ratio, Ni-Fe slag powder soil cement shows an increasing trend in dynamic stress with age, and its growth rate at the curing age from 7 d and 28 d is significantly higher than that at the curing age from 28 d to 60 d. After comparison, it is concluded that the best admixture ratio for Ni-Fe slag powder is 40%, which is close to the maximum value of 45% for mineral admixtures to replace cement as specified in the national standard.

1. Introduction

With the advantages of high strength, low compressibility, low permeability, economy, environmental protection and easy construction, soil cement is widely used in the treatment of soft soil foundation [1,2,3,4,5,6]. However, under the actual working conditions, such as mixing piles subjected to vibration and roadbeds subjected to vehicle loads, soil cement not only bears the static load, but may also be affected by the impact forces caused by vibration and shock [7]. There have been many studies on the statics of soil cement [8,9,10]. For example, Wu et al. [11] used static triaxial tests to explore the effect of powdered clay on the strength and stiffness of cement-cured soil under different confining pressure and cement admixtures. Wang et al. [12] studied the mechanical properties of cement soil by undrained triaxial compression test and analyzed the effects of confining pressure and cement admixture ratio on the strength, pore pressure and stiffness of soil cement. Chen et al. [13] analyzed the soil cement by triaxial test, and the results showed that the stress-strain curve of soil cement in triaxial compression test was the stress-softening type, and the brittle damage of soil cement becomes more and more significant with the increase in cement admixture. Pongsivasathit S et al. [14] investigated the effect of cement content on the strength of soil cement samples by Unconfined Compressive Strength (UCS), California Bearing Ratio (CBR) test, 3 Point Load Test and Plate Load Test (PLT). S. Kolias et al. [15], by adding cement and fly ash to fine-grained clay, studied the effects of cement and fly ash on the strength and deformation characteristics of cement-soil. Mostafa A. Ismail et al. [16] conducted a series of triaxial test studies to obtain the effect of cement type on shear strength. Nevertheless, there are few comprehensive studies on the strength of soil cement under dynamic mechanics; in particular, little research has been reported on the dynamic characteristics of soil cement doped with Ni-Fe slag powder under dynamic mechanical action.

Ferronickel slag is a metal waste product produced by smelting steel or nickel. Generally, it is dumped as sand containing fine powder, which is nonbiodegradable. Nickel-iron slag can be used as a raw material for the preparation of slag cement, which can be used as a substitute for cement under specific conditions and has significant environmental benefits in the practice of mineral slag utilization [17]. Therefore, based on the previous research on the static strength of Ni-Fe slag powder soil cement [18,19,20,21], this study analyzes the strength characteristics of Ni-Fe slag powder soil cement under a dynamic environment using the Hopkinson pressure bar, instantly destroys the specimen by impact compression test, and records the damage process of the specimen under the impact, so as to obtain its dynamic stress-strain curve, analyze the influence of Ni-Fe slag powder admixture ratio and age on the dynamic stress of soil cement, and provide a more scientific basis for practical applications.

2. Test Materials and Scheme

2.1. Test Materials

(1) Soil: The soil was taken from the foundation pit of a real estate project in Fuzhou City, and the basic physical and mechanical indexes of the silt are shown in

Table 1. Basic Physical and Mechanical Indicators of Silt.

Water Content w (%)	Weight R (kN/m3)	Void Ratio e	Liquid Limit WL (%)	Plastic Limit WP (%)	Plasticity Index IP	Liquidity Index IL
58.5	15.67	1.729	53	28.7	21.53	1.61

. The main components contained in the soil are SiO₂, Al₂O₃, Fe₂O₃, and these components account for more than 50%. In addition, it also contains a small amount of CaO, MgO, Na₂O, K₂O and other components;

(2) Cement: Ordinary Portland cement (PO 42.5) was used, and the cement quality conformed to the relevant regulations of Ordinary Portland Cement (GB175-2007);

(3) Ni-Fe slag powder: An admixture of blast furnace Ni-Fe slag powder and granulated blast furnace ore powder was used. The mass ratio of blast furnace Ni-Fe slag powder to mineral powder was 2:1, and the chemical composition of the compound is shown in

Table 2. Chemical Composition of Compound.

Composition	SiO2	Al2O3	CaO	MgO	TiO2	MnO	Fe2O3	SO3	LOI
Ni-Fe Slag Powder (%)	35.41	21.57	29.31	9.57	0.69	0.61	1.27	0.19	2.38
Slag (%)	32.11	16.79	36.07	10.62	0.96	0.81	2.31	0.18	0.19

. The particle size distribution of nickel-iron slag powder and ore powder are 0.27 μm~4.38 μm and 0.65 μm~5.23 μm, respectively;

(4) Fresh water: The fresh water was purified from tap water by an ultra-pure water machine.

2.2. Test Scheme

2.2.1. Test Design

In this test, the wet soil material was prepared according to the water content of the as-built soil (58.5%), the water-cement ratio of soil cement was 0.5, and the admixture ratio of cementing material was 15%. Meanwhile, the Ni-Fe slag powder admixture ratio was designed in equal step increments of 10%, and three curing ages at 7 d, 28 d and 60 d levels were selected. The specific test scheme is shown in

Table 3. SHPB Test Scheme.

No.	Soil	Mixing Ratio-a (%)	Water-Cement Ratio	Mixing Ratio-b (%)	Quantity of Specimen (pcs.)
					7 d	28 d	60 d
Ayn	silt	15	0.5	0	3	3	3
Byn				10	3	3	3
Cyn				20	3	3	3
Dyn				30	3	3	3
Eyn				40	3	3	3
Fyn				50	3	3	3
Gyn				60	3	3	3

Notes: Mixing ratio-a: Admixture Ratio of Cementing Material and mixing ratio-b: equal Mass Substitution Ratio for Ni-Fe Slag Powder.

.

2.2.2. Test Instrument and Procedure

In this test, the Hopkinson pressure bar from the Structural Mechanics Laboratory, Collaborative Innovation Center, Fujian Jiangxia University was used, with a diameter of 50 mm, and the specimen mold adopted a customized cylinder with a diameter of 50 mm and a height of 50 mm. As for the specimen of Ni-Fe slag powder soil cement, its preparation was divided into stages such as mixing of Ni-Fe slag powder soil cement, molding, mold removal, and curing of specimens.

The specific steps of the SHPB test are as follows:

(1)

Fully apply petroleum jelly on the front and back sides of the specimen that has been cured to age;

(2)

Clamp the specimen between the incident bar and the transmission bar to make the specimen face in full contact with the bar;

(3)

Apply petroleum jelly on the contact surface between the incident bar and the impact bar and affix the strain gauge;

(4)

Turn on the data acquisition device;

(5)

Set the air pressure to 0.25 MPa and open the air valve so that the air pressure pushes the impact bar to move at high speed, hit the incident bar and push it to hit the specimen;

(6)

Collect the incident wave, transmitted wave and reflected wave post-processing data through a data acquisition device;

(7)

Collect specimen fragments, take photos and perform termination hydration;

3. Results and Analysis

3.1. Impact Compression Test Results

After curing for 7 d, 28 d and 60 d, respectively, under standard curing conditions, Ni-Fe slag powder mixed with 0% to 60% of Ni-Fe slag powder soil cement was subjected to impact compression tests. After several pre-tests, the impact pressure was finally determined to be 0.25 MPa. For the same test group of Ni-Fe slag powder soil cement peak stress, the peak strain was averaged. The results of the impact compression test of Ni-Fe slag powder soil cement are shown in

Table 4. Impact Compression Test Results for Ni-Fe Slag Powder Soil Cement.

Specimen	Peak Stress (MPa)			Peak Strain (10−3)
No.	7 d	28 d	60 d	7 d	28 d	60 d
Ayn	3.43	6.00	7.70	0.00995	0.0071	0.00649
Byn	3.77	6.60	8.54	0.00761	0.00717	0.00691
Cyn	4.14	7.45	9.74	0.00809	0.00623	0.00628
Dyn	4.57	8.45	11.35	0.00753	0.00672	0.00488
Eyn	5.10	9.73	13.51	0.00615	0.00559	0.00467
Fyn	4.64	8.74	11.33	0.00737	0.00578	0.00484
Gyn	3.89	6.82	8.21	0.00797	0.0085	0.00848

.

3.2. Analysis of the Stress-Strain Curve

The dynamic stress-strain curve of soil cement with different amounts of Ni-Fe slag powder admixture at the curing ages of 7 d, 28 d and 60 d is shown in Figure 1.

According to Figure 1, the peak dynamic stress of Ni-Fe slag powder soil cement is higher than that of ordinary soil cement at different ages. With the increase in curing age, the dynamic stress of Ni-Fe slag powder soil cement with different amounts of admixture and varied soils all increase significantly, and its peak value of dynamic stress also increases with age. Meanwhile, the strength of the cement soil shows an increasing trend with the amount of Ni-Fe slag powder when the amount of Ni-Fe slag powder is mixed in the range of 0% to 40%. When the admixture ratio exceeds 40%, the strength decreases significantly. Especially when the admixture ratio is 60%, the soil cement specimens prepared also show that the mortar is more diluted, and the initial setting time is longer. This is due to the hydration reaction that occurs with the admixture of cement and nickel slag powder. Minerals on the surface of cement particles react quickly with water in the soil to produce numerous hydration products. At the same time, the activity of nickel slag is stimulated by ore powder, and a hydration reaction similar to cement is produced [22]. Under the joint action of cement and nickel slag, the integrity of Ni-Fe slag powder soil cement is greatly enhanced, and its strength is also improved with the hydration reaction. However, when the admixture ratio exceeds a certain limit, its hydration activity decreases more, which in turn affects the hydration reaction and thus causes a decrease in strength.

3.2.1. Effect of Admixture Ratio on the Soil Cement Dynamic Stress

The relationship between the peak dynamic stress of Ni-Fe slag powder soil cement and the Ni-Fe slag powder admixture ratio is shown in Figure 2. From the figure, it is possible to make a general judgment on the overall trend of the cyclic growth rate of nickel-iron slag powder soil cement. The strength growth rate of Ni-Fe slag powder soil cement shows an increasing trend in the range of 0% to 40% and a decreasing trend in the range of 50% to 60% of admixture ratio, in which the extreme of peak dynamic stress is reached at the admixture ratio of 40%.

Table 5. Growth Rate of Peak Dynamic Stress of Ni-Fe Slag Powder Soil Cement.

	Age	7 d	28 d	60 d
Growth Rate
Ratio of Byn to Ayn		9.89%	9.99%	10.89%
Ratio of Cyn to Byn		9.83%	12.89%	14.07%
Ratio of Dyn to Cyn		10.32%	13.44%	16.48%
Ratio of Eyn to Dyn		11.80%	15.16%	19.07%
Ratio of Fyn to Eyn		−9.14%	−10.22%	−16.11%
Ratio of Gyn to Fyn		−16.20%	−21.94%	−27.57%

compares the growth rates of the peak dynamic stress in the specimens under different admixture ratios. The trend of the peak dynamic stress growth rate of Ni-Fe slag powder soil cement is analyzed in Figure 2 and Table 5 as follows.

(1) At the standard curing age of 7 d: The dynamic stress curve and peak at the 7 d showed an upward trend with the increase in Ni-Fe slag powder admixture ratio; the growth rate also increased to a certain extent. The peak dynamic stress and the growth rate of soil cement with an admixture ratio of 40% reached the maximum. At the 7 d, the role of admixture of Ni-Fe slag powder was mainly to fill the pores and increase the compactness of soil cement, but its activity was not fully activated, so it had limited influence on the cementation of soil cement particles.

Within the admixture range from 0% to 40%, the peak dynamic stress and its growth rate gradually increased with the increase in the admixture ratio and reached the extreme value of the peak dynamic stress when the admixture ratio was 40%. With the further increase in admixture ratio, the growth rate turned from positive to negative, and the peak dynamic stress started to decline, indicating that the admixture ratio of Ni-Fe slag powder was not better as it was higher. Instead, it would start to generate side effects after reaching the maximum admixture ratio. Based on the overall trend, it can be inferred that there may be an optimal content of Ni-Fe slag powder, which makes the peak dynamic stress of Ni-Fe slag powder on soil cement close to the maximum.

(2) At the standard curing age of 28 d: The increasing trend of the curve at 28 d was more significant in the admixture range of 0% to 40% of Ni-Fe slag powder, and the growth rate of peak dynamic stress in the admixture range of 20% to 40% was higher than that of admixture ratio in the range of 0% to 20%. A decrease in the peak dynamic stress of Ni-Fe slag powder soil cement was observed when the admixture ratio of Ni-Fe slag powder reached 50% to 60%. This indicates that the influence of Ni-Fe slag powder on the dynamic stress of the hydrated soil is significantly increased at the age of 28 d compared to the age of 7 d. This physically fills the pores, and chemical reactions similar to the hydration of cement occur under the excitation of mineral powder. Chemical substances such as calcium alumina, hydrated calcium silicate and iron hydroxide, which can increase the dynamic stress of the hydrated soil, begin to be produced.

Portland cement in soil-cement contains a large number of mineral components, such as C₃S, C₂S, C₃A, and C₄AF. So, the cement undergoes hydration reactions upon contact with water, resulting in the formation of a weak alkaline environment within the cement soil. At the same time, ferronickel slag powder contains a large number of potentially active vitreous substances, including SiO₂, Al₂O₃, and Fe₂O₃. Under the weak alkali environment, vitreous substances can give better play to the chemical activity effect and produce hydration products such as C-S-H, C-A-H, CaO·Fe₂O₃·mH₂O, Aft, and AFm [20]. At 28 d, the dynamic stress of soil cement mixed with Ni-Fe slag powder showed a more significant increase compared with that of soil cement without Ni-Fe slag powder at the earlier age. Since the cement had been more fully hydrated at 28 d, the hydration products such as C-S-H gel and calcium alumina began to fill the pores of the structure, which enhanced the cohesion of the soil cement. Meanwhile, as a low-activity mineral admixture, the Ni-Fe slag powder played the effect of filling the pores at an early age; as the age increased, the Ni-Fe slag powder started to increase its activity after being activated by the mineral powder and underwent a hydration reaction similar to that of cement, thus also generating substances that enhance the dynamic stress of soil cement.

(3) At the standard curing age of 60 d: Compared to the curing ages of 7 d and 28 d, the dynamic stresses in the Ni-Fe slag powder soil cement of different soils at 60 d increased significantly with the admixture ratio of Ni-Fe slag powder.

There was significant growth in the admixture range of 0% to 20% of Ni-Fe slag powder compared with the early ages, and the most significant growth effect was observed in the admixture range of 20% to 40% of Ni-Fe slag powder. When the admixture ratio exceeded the maximum limit, the growth rate of dynamic stress in soil cement changed from positive to negative and showed a decreasing trend, and the decrease was greater compared with the early age.

The data shows that the peak dynamic stress and the quarter-on-quarter growth rate of the Ni-Fe slag powder soil cement are optimal at 60 d when the admixture ratio is 40%. At 60 d, the growth rate of the dynamic stress of nickel-iron slag powder soil cement is enhanced compared with the earlier age, the hydration reaction of cement has been basically completed, and its influence on the dynamic stress of soil cement is gradually decreasing. Ni-Fe slag powder starts to play a more important role, and its activity is further improved under the excitation of mineral powder, and its hydration reaction occurs more fully, generating more substances that enhance the dynamic stress of soil cement, so that the peak dynamic stress and growth rate of Ni-Fe slag powder soil cement are rapidly increased.

3.2.2. Effect of Age on Soil Cement Dynamic Stress

According to Figure 3 and

Table 6. Growth Rate of Peak Dynamic Stress with Age for Nickel-Iron Slag Powder Soil Cement.

	Age	7 d to 28 d	28 d to 60 d
Admixture Ratio
Ayn		74.95%	28.36%
Byn		75.11%	29.40%
Cyn		80.00%	30.75%
Dyn		85.09%	34.25%
Eyn		90.65%	38.81%
Fyn		88.38%	29.70%
Gyn		75.46%	20.35%

, the dynamic stress-age relationship plot of Ni-Fe slag powder soil cement shows a trend of increasing dynamic stress with age, and the growth rate in the age range of 7 d to 28 d is greater than that in the age range of 28 d to 60 d. The error bars are shown in Figure 4.

At the curing age of 7 d, the dynamic stress of Ni-Fe slag powder soil cement was still at a low point, mainly because the hydration reaction of cement inside the soil cement was still in the initial stage and there were few hydration products generated. At the same time, the Ni-Fe slag powder in the soil cement was still in a low activity state and had not been fully activated by the mineral powder; it still played the role of filling the pores, with low participation in the hydration reaction.

The Ni-Fe slag powder cement achieved a significant increase in strength when the curing age increased from 7 d to 28 d. The main reason for the significant increase is that the cement in the soil cement achieved a sufficient hydration reaction in the age range of 7 d to 28 d. As the age increased, the Ni-Fe slag powder started to increase its activity after being activated by the mineral powder and underwent a hydration reaction similar to that of cement, thus generating a large number of hydration products. Under the dual action of cement and Ni-Fe slag powder, the dynamic stress of Ni-Fe slag powder soil cement gets a rapid rise.

In the age range of 28 d to 60 d, the growth rate decreases compared to that of 7 d to 28 d, mainly because the curing effect of cement on soil cement starts to decrease at this time, so the increasing effect is not as large as that of the early age.

4. SEM Test

4.1. Principle of the SEM Test

Due to the complexity of the internal structure of Ni-Fe slag powder soil cement, the completion of hydration reaction, the denseness of the material structure, the filling of pores, the location of Ni-Fe slag powder in the soil cement and the changes in the internal structure of the soil cement with the increase in admixture ratio and age were observed by microscopic imaging of the surface layer of the specimen through the scanning electron microscopy (SEM), in order to explain more reasonably the changing trend of the strength of Ni-Fe slag powder soil cement from both macroscopic and microscopic perspectives. The SEM is shown in Figure 5.

The principle of SEM is to scan the specimen by a focused, high-energy electron beam, in which the electron wave forms the light source, and the electromagnetic field becomes the lens. The specimen surface is bombarded with the high-energy electron beam, so that the specimen interacts with the incident beam and excites the secondary electron emission. Microscopic imaging was performed by converging and receiving secondary electrons in each direction with a detector to observe the surface morphological characteristics and internal material structure of the specimen.

In this SEM test, the samples were taken from the range of 2 cm~3 cm from the surface of the test block, and the observation surface of the selected samples was as flat as possible. Two test groups with the most typical Ni-Fe slag powder admixture ratio of 0% and 45% in cured sandy soil were selected, and the curing age was set at 7 d and 60 d.

4.2. Analysis of SEM Test Results

The following results can be observed through SEM images.

C-S-H gel: C-S-H gel is a non-unique form of hydration product. At the early stage of cement hydration reaction, C-S-H gel has no specific morphological characteristics, and it is difficult to identify its morphology. When the curing age reaches over 3 d, C-S-H gel shows mesh, fibrous, irregular, and other large-particle morphological forms.

Ca(OH)₂ crystals: A hexagonal laminated structure with an angle of 120° and a thin plate.

AFt phase: mostly needle-like, with isometric columns visible after magnification, about 3–4 µm long. A few parts are radially clustered and much smaller in size than C-S-H.

Ni-Fe slag powder: smooth, gravelly cross-section, and dense structure.

According to Figure 6 and Figure 7, at the curing age of 7 d, the nickel-iron slag powder soil cement specimens have many inter-particle pores, discrete inter-particles, and fewer C-S-H gels, leading to the failed cementing of soil particles and weak dense structure. The SEM image can also provide a better explanation for the lower dynamic and static strength of Ni-Fe slag powder soil cement at the early curing age. A small amount of C-S-H gel, calcium alumina and Ca(OH)₂ crystals can be clearly observed in the image. Comparing specimen E with 45% Ni-Fe slag powder with specimen A without Ni-Fe slag powder, the denseness of the internal structure of specimen E is greater than that of specimen A. Ni-Fe slag powder not only participates in the reaction but also fills the pores between the soil particles.

It can be observed from the figure that at the curing age of 60 d, the content proportion of C-S-H gel increases greatly. As the C-S-H gel has strong adsorption, gels distributed throughout the structure bond the soil particles together, greatly improving the bond between the soil particles. Besides, the pores are covered by hydration products, and the internal structure becomes denser, forming a whole object with stronger integrity and higher strength. This indicates that at the curing age of 60 d, both the Ni-Fe slag powder and the cement have been fully hydrated, thus generating numerous cementing materials to strengthen the inter-particle bonding. As can be seen in the figure, the proportion of C-S-H gels has been greatly increased, while the proportion of pores has been significantly reduced. The SEM image also corroborates the aforementioned test results; in the age from 7 d to 60 d, in the range of maximum admixture ratio limit, the strength and integrity of the sample have been greatly improved.

5. Conclusions

Taking admixture ratio and curing age as influencing factors, this study investigates their impacts on the dynamic stress of Ni-Fe slag powder soil cement. The following conclusions can be obtained:

(1) Under the same age and different admixture conditions, the dynamic stress of Ni-Fe slag powder soil cement first increases and then decreases. The dynamic stress rises significantly in the admixture range from 0% to 40%, and its growth rate also increases with the admixture ratio. The dynamic stress begins to drop when the admixture ratio is between 50% and 60%. It can be inferred that the dynamic stress reaches the maximum value when the admixture ratio is 40%;

(2) The mineral powder in the nickel slag powder stimulates its activity, resulting in a hydration reaction similar to cement. Under the combined action of cement and nickel slag powder, the integrity of Ni-Fe slag powder soil cement is greatly improved, and its strength is also improved with the progress of the hydration reaction. However, when the admixture ratio exceeds a certain limit, its hydration activity is greatly reduced, affecting the hydration reaction and resulting in a decrease in strength;

(3) Under the same admixture ratio conditions, the dynamic stress of Ni-Fe slag powder soil cement increases with age. Since the hydration of Ni-Fe slag powder is obvious in the early stage, the growth rate of dynamic stress of the soil cement specimens from 7 d to 28 d is significantly higher than that from 28 d to 60 d;

(4) Elements such as C-S-H gel, AFt, calcium hydroxide and calcium carbonate can be observed from the SEM image. Comparing Ni-Fe slag powder soil cement with different ages, its overall structure becomes denser with age, the pores decrease obviously, and the hydration products generated also increase with age. This proves that the longer the curing time, the more perfect its properties will be. Compared with the Ni-Fe slag powder soil cement under different admixture ratios, the soil cement with the admixture ratio of 45% has more hydration products, and the pores are also reduced after the Ni-Fe slag powder fills the pores. This indicates that the appropriate amount of Ni-Fe slag powder can enhance the denseness of the internal structure of the soil cement;

(5) In this study, the effect on the dynamic stress of nickel-iron slag powder soil cement is investigated by controlling the two influencing factors of age and admixture ratio. It is concluded that when the admixture ratio of Ni-Fe slag powder is about 40%, the dynamic stress of soil cement shows an optimal state, which is close to the maximum value of 45% for mineral admixtures to replace cement as specified in the national standard.