Study on Mechanical and Microscopic Properties of Nickel–Copper-Contaminated Soil Solidified by Cement,Fly Ash and Desulfurization Gypsum Under Carbonization Condition

Abstract

The engineering characteristics of remediated soil are easily affected by CO₂ erosion in nature. However, there are limited investigations on the mechanical and microscopic properties of heavy metal-contaminated soil. This study introduces effect of accelerated carbonization on the mechanical and microscopic properties of nickel–copper-contaminated soil, and the soil has been treated with a novel curing agent, formed by mixing cement, fly ash and desulfurization gypsum (CFG). The objective of the study is to ascertain CO₂ erosion resistance of nickel–copper-contaminated soil solidified by CFG. Using unconfined compressive strength (UCS) tests, carbonization depth, X-ray diffraction, and scanning electron microscopy, the sample’s characteristics are investigated under different carbonization times and heavy metal ion concentrations. The results demonstrate that the UCS of samples of Ni0Cu0, Ni0.02, and Ni0.4 decrease with the increasing carbonization time, while that of Ni1, Cu1, and Ni1Cu1 increase initially and then decrease; in addition, when the concentration of heavy metals is lower, the effect of carbonization on UCS of samples is more significant. Moreover, the carbonization depth of samples increases with the increasing carbonization time, and the prediction model is given. Furthermore, the microscopic analysis demonstrates that calcium carbonate is the main carbonization product. The decomposition of hydrated calcium silicate gel leads to poor integrity of the structure and more pores produced in samples, which is the main reason for the decrease of the UCS in the process of carbonization. The outcomes of this investigation provide a reference for the durability in practical engineering of heavy metal-contaminated soil solidified by CFG.

Keywords

Aviation Environmental Impacts of Aviation Sustainability/Adaptation Infrastructure Geology and Geoenvironmental Engineering Mechanics and Drainage of Saturated and Unsaturated Geomaterials AKG40 Soil Improvement Soil and Rock Properties AKG20 Soil Compaction Stabilization of Geomaterials and Recycled Materials AKG90 Soils Materials Durability of Concrete AKM70 Alkali-Carbonate Reactivity Structures Culverts Buried Bridges and Soil-Structure Interaction AKB70 Soil Mechanics Soil Structure Interaction Planning and Analysis Environmental Analysis and Ecology Engineers

Soil contamination by heavy metal ions has become a major concern and also a persistent problem worldwide, owing to rapid industrialization and urbanization ( 1 ). This industrialization, coupled with human activities, has resulted in large amounts of waste containing heavy metal ions contaminating the soil, changing its the physical and chemical properties ( 2 , 3 ). Besides, heavy metals tend to accumulate in the soft tissues, and their toxicity can lead to deterioration of mental and the central nervous system, blood poisoning and deterioration of other vital organs, and inevitably pose a potential threat to the surrounding environment ( 4 ). Therefore, it is of great importance to remedy heavy metal-contaminated soil through a sustainable, efficient, and cost-effective approach which can minimize environmental impacts and improve engineering properties ( 3 ).

There are two typical remediation techniques used to treat contaminated soils: in situ and ex situ techniques. In general, in situ treatments have received limited attention because of their time-consuming processes and uncertainty in their long-term stability ( 5 ); it is easier to study the long-term characteristics and influencing factors of contaminated soil remediation with ex situ techniques. The solidification/stabilization method is a widely used ex situ remedy technology for contaminated soils. This technique uses a binder additive to hinder leaching of the heavy metal contaminants through the soil; it can enhance soil strength and reduce soil hydraulic conductivity. Cement-based stabilizer is one of the best and most commonly used solidifying agents, owing to its low cost, robustness, and easy adaptability for different soil types, as well as its effectiveness. Besides, soil solidified by cement has excellent long-term chemical and physical stability, and good mechanical and structural properties (6 –13). However, traditional cement-based stabilizers are also facing criticism because of the adverse environmental influence related to their production. According to statistics, the total amount of CO₂ produced by the process of cement production accounts for about 5% of the total global CO₂ emissions ( 14 ). Other binders used for solidifying contaminated soil have also been studied, such as dicalcium silicate ( 15 ), calcium silicate hydrate, natural zeolites, and metakaoline ( 16 ).

In recent years, scholars have created more and more new composite cement or composite materials as stabilizers, and their properties have been studied, such as the influence of CaO on the strength of lead-contaminated soil solidified by ettringite ( 17 ), the mechanical and micro-mechanism properties of lead-contaminated soil solidified by magnesium phosphate cement ( 18 , 19 ), static triaxial mechanical properties of new cement soil reinforced with polypropylene fiber ( 20 ), and the influence of a new type of limestone-calcined clay-cement (LC³) on zinc-contaminated soil ( 21 ); the materials mentioned above all have the potential for excellent solidification effects.

In response to the strategy of sustainable development in China, the development and application of green environmental protection materials are increasingly advocated. China is a major consumer of coal, and a large amount of fly ash and desulfurized gypsum—unused industrial wastes—is produced in China every year. There are many research findings about the utilization and properties of desulfurized gypsum and fly ash ( 22 – 24 ). To better combine the advantages of cement curing agent with the concept of environmental protection and energy saving, a new green composite material, formed by mixing cement, fly ash, and desulfurization gypsum (CFG), was developed by Wang et al.; the results show that there is an optimal ratio of CFG composite curing agent ( 25 ), and the new binder is beneficial to the strength and impermeability of heavy metal-contaminated soil (26 –30). However, the durability of the CFG-solidified contaminated soil should be further studied to provide a reference for the specific application of CFG in engineering practice.

With an increase in curing time CO₂ in the air can penetrate into the CFG-solidified contaminated soil, which can cause the carbonization of the solidified soil. Therefore, it is necessary to study the carbonization mechanism of heavy metal-contaminated soil solidified by CFG. (In this research, carbonization means the changes in engineering properties caused by a series of physical and chemical reactions of solidified soil exposed to CO₂). The effect of nickel ion concentration, copper ion concentration, and carbonization time on the strength and carbonization depth of the solidified soil are analyzed through unconfined compressive strength (UCS), carbonization depth, scanning electron microscopy (SEM), and X-ray diffraction (XRD) tests in this study.

Materials and Methods

Materials

Soil samples were collected from 2–5 m of the foundation pit of Huainan, Anhui University of Science and Technology, Shannan Campus. Remolded soil was obtained after wind drying and milling, and was then passed through a 2 mm standard round-hole sieve. The soil was classified as clay and the main physical and mechanical parameters are shown in Table 1, in accordance with “Test Method of Soils for Highly Engineering” (JTG E40 2007).

Table 1.

Physical and Mechanical Properties of Soils ( 28 )

Soil sample classification	Natural moisture content/%	Plastic limit/%	liquid limit/%	Plasticity index/%	Optimum moisture content/%	Maximum dry density/(g•cm⁻³)
Clay	24.132	20.814	41.438	20.624	19.127	1.933

According to XRD, as shown in Figure 1a, the main chemical composition of the soil sample is SiO₂, followed by Fe₂O₃ and Al₂O₃.

Figure 1.

X-ray diffraction (XRD) patterns of experimental materials ( 28 ): (a) XRD of test soil, (b) XRD of fly ash, and (c) XRD of desulfurized gypsum.

P. O.42.5 ordinary Portland cement was employed in this study. The desulfurization gypsum and fly ash were taken from Huainan Pingyuan Power Plant, and their XRD patterns are shown in Figure 1, b and c , respectively. Besides, the test screening curve is shown in Figure 2. The compaction curve of the test soil without addition of other materials is shown in Figure 3. The fineness content of fly ash was 10%. The significant oxides of the cementitious materials are shown in Table 2. The contaminated soil was prepared in the laboratory. NO^3- was inert and had little influence on hydration ( 28 ). Therefore, nickel ions were supplied by nickel nitrate hexahydrate and copper ions were supplied by copper nitrate rehydrate. Local tap water was used in the test.

Table 2.

Chemical Composition of Cement, Desulphurization Gypsum and Fly Ash, Mass% ( 28 ).

Composition	Cement	Desulphurization gypsum	Fly ash
SiO₂	26.12	1.94	51.70
AL₂O₃	6.02	0.90	32.1
Fe₂O₃	0.54	0.23	7.75
CaO	59.98	31.31	5.02
MgO	0.98	0.29	0.84
SO₃	2.89	43.49	0.66

Figure 2.

Cumulative curve of diameter soil ( 28 ).

Figure 3.

Compaction curve of test soil.

Sample Preparation and Mix Design

The flow chart indicating the methodology of sample preparation is shown in Figure 4. The diameter and height of the mold were 50 mm and 100 mm, respectively.

Figure 4.

Sample preparation flow chart ( 28 ).

The composite binder CFG, composed of cement, fly ash, and desulfurization gypsum, was used as a stabilizer. Taking the properties of solidified soil into account (31 –34), the mass ratio of cement, fly ash, and desulfurization gypsum was designed as 5:3:2, and the CFG content was 15% (the ratio of CFG content to dry soil quality), abbreviated as CFG15 (35 –37). To ensure the CFG in soil has sufficient hydration reaction, the moisture content of the test was determined to be 35% through pre-experiment.

In accordance with the environmental quality standard for soils (GB 15618 2008), the design for total nickel was 0, 1, 20 and 50 times the national standard, that is, 0%, 0.02%, 0.4%, and 1% (the ratio to dry soil quality), as control, the amount of copper was 0% and 1%, respectively. Therefore, there were total six levels of heavy metal content, abbreviated as Ni0Cu0, Ni0.02, Ni0.4, Ni1, Cu1, and Ni1Cu1.

As the effect of a high concentration of heavy metal ions on the hydration reaction of solidified soil cannot be ignored and hydration products are the main participants in carbonization, a sample with a concentration of 0.02% nickel ion was selected for a microtest. Specifically, samples of Ni0.02 with different carbonization time were taken out by the reaction of phenolphthalein reagent, to identify the three stages of carbonization: not carbonized, being carbonized, and completely carbonized, and then tested by SEM and XRD.

Specific Experimental Steps

UCS Test

According to the Standard for soil test method (GB/T50123-1999), the UCS tests were carried out on specimens in different carbonization time and concentration of heavy metals, using a microcomputer control electronic universal testing machine. The preloading load was 0.05 kN, the sampling interval was 0.1 s, and the strain rate was 1.0%/min. The test conditions were as follows: the strain increases by 3.0% after the peak stress, and the strain exceeded 20.0%.

Accelerated Carbonization Test

The purpose of accelerating carbonization was to study the influence of the carbonization effect on the physical, chemical, and mechanical properties of solidified soil. Samples of the solidified soil were placed under certain conditions of temperature, humidity, and CO₂ concentration for a period of time. The test was conducted in reference to the Standard for test methods of long-term performance and durability of ordinary concrete (GB/T 50082-2009). The specific steps for the carbonization test are as follows:

Put samples with standard curing for 28 days in the carbonization box, and the interval of each sample is not less than 50 mm.

Make sure the carbonization box is well sealed.

Set the carbonization condition to CO₂ concentration at 20%, the temperature at 20 ± 2°C, and the relative humidity at 70 ± 5%.

Check the CO₂ concentration, temperature, and humidity in the carbonization box every 4–8 h.

Take out the sample that reaches carbonization time to carry out unconfined compression test.

Phenolphthalein Test

This is a method used to determine the carbonization depth. The phenolphthalein was melted into anhydrous ethanol at a proportion of 1% to make phenolphthalein reagent. As the alkalinity of the solidified soil gradually decreases after carbonization, the non-carbonated solidified soil appears purple-red when dropped into phenolphthalein reagent, and the color of the completely carbonated solidified soil does not change; therefore, the carbonization depth can be distinguished.

SEM and XRD Tests

The specimen with 15% CFG curing agent and concentration of heavy metals as Ni0.02 was tested by SEM and XRD at three different stages of carbonization.

Result and Discussion

Analysis of Strength Characteristics

By observing the scattered points distribution of carbonization time and UCS as shown in Figure 5, it can be seen that with the increase of carbonization time, samples with different concentrations of heavy metals show different change rules of UCS. To further analyze the change trends, a polynomial regression model with carbonization time can be established by regression analysis according to the change rule of the scattered points, and formula 1 is selected for fitting. The fitting line is shown in Figure 5 and the fitting formulas are given in formulas 2–7 in Table 3; convergence of the fitting results and the determinant coefficient R² in formulas indicate the high fitting degree of models, which can provide the corresponding parameter guidance for the UCS prediction within a certain range of carbonization time.

Table 3.

Relational Formulas of Unconfined Compressive Strength (UCS) and Carbonization Time

Formula	R ²	Numbering of equation
UCS_Ni0Cu0 = 0.027t²–0.425t + 2.937	0.964	2
UCS_Ni0.02 = 0.081t²–0.517t + 2.661	0.979	3
UCS_Ni0.4 = 0.115t²–0.515t + 1.716	0.969	4
UCS_Ni1 = −0.085t² + 0.285t + 0.872	0.910	5
UCS_Cu1 = −0.029t² + 0.102t + 1.102	0.862	6
UCS_Ni1Cu1 = −0.084t² + 0.284t + 0.985	0.936	7

UCS = a t^{2} + bt + c

(1)

Note: t represents carbonization time, unit: day; a, b, c are fitting parameters.

Figure 5.

The relationship between carbonization time and unconfined compressive strength.

According to Figure 5 and Table 3, there are two main trends of UCS with carbonization time. Specifically, when the sample’s heavy metal concentration is at a lower level, including Ni0Cu0, Ni0.02, and Ni0.4, the coefficient of quadratic term in strength models is positive in formula 2–4, and the UCS values of samples decrease with carbonization time, which is similar to the phenomenon in which the strength of concrete decreases after carbonization ( 38 ). Conversely, when the sample’s heavy metal concentration is at a higher level, including Ni1, Cu1, and Ni1Cu1, the coefficient of quadratic term in strength models is negative in formulas 5–7; the UCS values of samples increase first and then decrease with carbonization time. Furthermore, after standard curing for 28 days, samples with a lower heavy metal concentration show better performance in the UCS test. However, having undergone accelerated carbonization, samples with lower heavy metal concentration show a larger reduction in UCS, while those with higher concentration show a slight reduction or even an increase. As shown in the graph, after carbonization curing for 3 days, the UCS of the sample with Ni0Cu0 shows a 62% decrease, decreasing from 3.052 MPa to 1.894 MPa, whereas that of the sample with Ni1Cu1 increases 1.076 times, from 0.775 MPa to 0.834 MPa.

Moreover, the whole change of UCS, from standard curing for 3 days, standard curing for 28 days to complete carbonization with different heavy metal concentration, can be distinctly seen in Figure 6. It can be observed that all of the UCS values of samples with different heavy metal concentrations have seen a large increase from standard curing for 3 days to 28 days; basically, the UCS values of samples have more than doubled. Nonetheless, for samples with heavy metal concentration at Ni0Cu0, Ni0.02, and Ni0.4, the UCS values decrease rapidly within a few days of accelerated carbonization, after standard curing for 28 days; when carbonization is complete, the UCS values of samples are just slightly higher than that of standard curing for 3 days, which indicates that the carbonization reaction has a significant impact on the UCS values of these samples with lower heavy metal concentration. In contrast, for samples that have heavy metal concentrations at Ni1, Cu1, and Ni1Cu1, when carbonization is complete, the UCS values of the samples are slightly higher than that of the standard curing for 28 days but much greater than that of the standard curing for 3 days, which indicates that the carbonization reaction has a slight but perceptible impact on the UCS values of the samples.

Figure 6.

Unconfined compressive strength (UCS) values of different heavy metal concentration samples in different curing periods.

In fact, the large increase of UCS of the treated specimens from standard curing for 3 days to 28 days can be attributed to CFG, which fill the pores between the clay particles and coats the clay to form small aggregates situated between the clay particles. Furthermore, the UCS values of the samples contaminated by higher concentrations of heavy metal are significant smaller than those of the samples without heavy metal at the standard curing time of 28 days. The reason is that when Ni²⁺ and Cu²⁺ exist, Ca²⁺ and OH^- produced by the hydration of CFG would react to form amorphous or insoluble substances, such as CaNi₂(OH)₆•2H₂O, Cu(OH)₂, which implies that the hydrated products would precipitate heavy metal ions; these substances would encapsulate and hinder the further hydration of CFG to reduce the formation of skeleton, such as cement stone, and lead to the slowdown in the growth of the strength of samples. Besides, they would also reduce the alkalinity of binder and soil, thus reducing the production of CaO·SiO·H₂O (C-H-S) and AFt; as a result, the UCS of the specimens decreases with the reduction of cementation of clay ( 28 ). Moreover, it is proved that within a few days after standard curing for 28 days, the effect of the hydration reaction on UCS is very small, and can be ignored ( 28 ); therefore we mainly consider the influence of carbonization factors in the process of accelerating carbonization.

The reasons for the above phenomenon are as follows: (1) for samples with heavy metal at a lower concentration, including Ni0Cu0, Ni0.02, and Ni0.4, contaminants have less effect on the hydration reaction, and sufficient hydration reaction occurred in the process of 28 d standard curing; thereby there is a large amount of C-S-H in samples, which is the primary source of the strength ( 39 ). Nevertheless, having undergone accelerated carbonization, a large amount of the C-S-H decomposed, leading to damage to the sample’s structure. Therefore, the carbonization reaction has a significant and adverse impact on the UCS of samples; (2) for samples that have a heavy metal concentration at a higher concentration, including Ni1, Cu1, and Ni1Cu1, the higher concentration of heavy metals obviously hinders the hydration reaction, resulting in fewer hydration products and the lower strength of samples after 28 days of standard curing ( 40 , 41 ). Meanwhile, there are many pores in the solidified soil samples as a result of the relatively low content of hydration products. Therefore, during the early stage of the accelerating carbonization, the unhydrated tricalcium silicate (C₃S), dicalcium silicate (C₂S), and partially hydrated calcium hydroxide (C-H) in the samples are carbonized to form calcium carbonate. Besides, C-H crystals reacts quickly with CO₂ to form CaCO₃ crystal that is considered to be embedded in the product of C-S-H ( 42 ), acting as a composite reinforcement instead of decomposition. These precipitates fill the intra-particle pores and also make soil structure denser to improve the UCS ( 43 ). However, as more and more CO₂ infiltrates into the soil, excessive CO₂ reacts with C-S-H with a slow reaction rate, leading to the decomposition of C-H-S, which makes the strength decrease gradually ( 44 ).

Analysis of Carbonization Depth Characteristics

To study the change rule of carbonization depth of solidified soil with carbonization time, the carbonization depth of samples was measured by phenolphthalein in the middle section at different carbonization times. Figure 7 shows the phenolphthalein effect photos with Ni0.02 at various carbonization durations. It is evident from the figure that the purple-red area decreases with the increase of carbonization time; this indicates that the longer carbonization time leads to greater carbonization depth, and the phenomenon is similar to the change rule of carbonization depth of concrete with carbonization time ( 45 ). With a carbonization curing for 0 h, there are many alkaline hydration products after standard curing for 28 days, leading to the completely purplish-red color of the sample’s cross-section as shown in Figure 7a. When carbonization curing for is for 6 h, the red area indicated by phenolphthalein on the cross-section of the sample decreases to a certain extent in Figure 7b, and the alkaline of the area without red color also decreases, which shows that the outside of the sample has been carbonized; thus the carbonization reaction can reduce the pH value of the sample. When carbonization curing is for 2 days, most of the area of the cross-section has been carbonized, as shown in Figure 7e. When carbonization curing is for 3 days, the sample has completely carbonized and no longer appears purple-red, as shown in Figure 7f.

Figure 7.

Effect photo of phenolphthalein with Ni0.02 at different carbonization times.

Figure 8 presents the carbonization depth curve of different heavy metal concentrations at different carbonization times. It can be seen from Figure 8 that the concentration of heavy metal has a certain effect on the growth rate of carbonization depth of samples; seen as a whole, the speed of the increasing trend is fast at first and then slightly slower, which indicates that the rate of carbonization is slowing down with carbonization time. Besides, the times of complete carbonization of samples with different heavy metal concentrations are different. Specifically, specimens with Ni0.4 have completely carbonized in less than 3 days of curing by accelerated carbonization, whereas the four groups of specimens with Ni0Cu0, Ni0.02, Ni1, and Cu1 have completely carbonized at the accelerated carbonization time of approximately 3 days, and specimens with Ni1Cu1 at more than 3 days of curing by accelerated carbonization.

Figure 8.

Carbonization depth at different carbonization times.

Moreover, the curves of carbonization depth with carbonization time in Figure 8 are approximately a power function. Among the carbonization depth research models proposed by scholars at home and abroad, the most recognized one is that the carbonization depth is in a positive proportion to the square root of time ( 46 – 48 ). Besides, as the specific time of complete carbonization of the sample is unknown, the data of complete carbonization should be excluded. Figure 9 shows a fitting model of the change of carbonization depth with the square root of time. The regression model of carbonization depth with different heavy metal concentrations and carbonization times is shown by formulas 8–13 in Table 4. The fitting results are convergent, and the fitting coefficient R² is both higher than 0.98, which implies a high degree of fitting. The complete carbonization time with heavy metal concentration is predicted according to the prediction function model.

Table 4.

Regression Model of Carbonization Depth and Carbonization Time with Heavy Metal Concentration

NaOH content	Prediction model function	R ²	Formula	Prediction of complete carbonization time/d
Ni0Cu0	$y = 13.473 \sqrt{t}$	0.992	8	3.44
Ni0.02	$y = 14.251 \sqrt{t}$	0.993	9	3.08
Ni0.4	$y = 16.301 \sqrt{t}$	0.996	10	2.35
Ni1	$y = 13.490 \sqrt{t}$	0.989	11	3.43
Cu1	$y = 14.228 \sqrt{t}$	0.992	12	3.08
Ni1Cu1	$y = 12.309 \sqrt{t}$	0.989	13	4.12

Note :y represents carbonization depth, unit: mm; t represents carbonization time, unit: days.

Figure 9.

Fitting model of the relationship between carbonization time and carbonization depth.

According to the complete carbonization time predicted in Table 4, the specific UCS of samples, completely carbonated with different heavy metal ion concentrations, can also be predicted by putting them into the formulas 2–7, and Figure 10 shows the time and UCS of complete carbonization for samples. It is evident that the predicted UCS values based on the fitting model are very close to the practical UCS, which indicates that the functional relationship between carbonization depth and UCS comes into existence at the same carbonization time and heavy metal ion concentration, and the slight difference can be attributed to the completely carbonated time of practical UCS not being an exact time.

Figure 10.

Time and unconfined compressive strength (UCS) of complete carbonization.

Besides, it can be observed from Figure 9 and Figure 10 that the carbonization rates of samples are different with the increase of heavy metal concentration, that the carbonization rate of samples are different with the increase of heavy metal concentration, and the higher the carbonization rate, with greater slope, the shorter the time required for complete carbonization. Specifically, for samples contaminated by Ni, the carbonization rate of samples increases first and then decreases with the increase of heavy metal ion concentration, which means that carbonization resistance decreases first and then increases with the increasing heavy metal concentration. For samples that have a heavy metal concentration at a lower level, including Ni0Cu0, Ni0.02, and Ni0.4, the slope of the straight line increases gradually with the increase of ion concentration of Ni, and the higher concentration of Ni contributes to the faster carbonization rate as well as the shorter time of complete carbonization. However, when heavy metal concentration is at a higher level, the carbonization rate slows down and it takes longer to completely carbonize with the increase of heavy metal concentration; furthermore, the carbonization rate of Cu1 is slightly faster than that of Ni1 with the same heavy metal concentration; the carbonization rate of composite contaminated soil with Ni1Cu1 is the slowest, and the time required for complete carbonization is the longest.

The reasons for the above phenomenon are as follows: (1) the rate of carbonization for all samples slows down with carbonization time, which can be attribute to the formation of carbonized products, and the voids in the outer surface between the clay particles are filled by them, hindering the further infiltration of CO₂, thus slowing down the rate of carbonization; (2) for samples that have a heavy metal concentration at a lower level, hydration products and the compactness of samples decrease gradually with the increase of heavy metal concentration, leading to many voids in samples and the reduction of carbonization resistance, making the speed of CO₂ penetration into the sample faster, and improving the carbonization rate; thus it takes just 2.35 d to carbonate completely for samples with Ni0.4; (3) for samples that have a heavy metal concentration at a higher level, hydration of the CFG is highly inhibited, which results in a relatively high water content in samples (the fluidity of samples with a higher heavy metal content was obviously higher than that without heavy metal during sample preparation), and the effective migration of CO₂ gas is blocked, which is not conducive to the carbonization reaction. This greatly increases the time required for complete carbonization, thus it takes more than 4 d to carbonate completely for samples with Ni1Cu1. As a result, the carbonization resistance of samples with higher level of heavy metal concentration increases.

Analysis of Microcosmic Characteristics

The microstructure of contaminated soils solidified by CFG is investigated from the arrangement of particles, pore characteristics and cementation types. The microstructure characteristics determine the macro-UCS characteristics of solidified soil. The changes in microstructure and products brought by CO₂ in three different carbonization stages are characterized by techniques including XRD and SEM.

Analysis of Microcosmic Characteristics of XRD

XRD analysis was conducted to assess any mineralogical changes (2θ = 10–80°) which results from the interaction between the clay, CFG and heavy metals (Ni and Cu). The XRD pattern of sample with heavy metal concentration of Ni0.02 in three different carbonization stages, including not carbonized, being carbonized, and completely carbonized, is shown in Figure 11, and Table 5 shows the names, abbreviations, and chemical formulas of the main minerals in the XRD pattern.

Table 5.

Reference Component of X-ray Diffraction

Name	Chemical reaction	Abbreviation
Calcium silicate hydrate	CaO·SiO·H₂O	CSH
Quartz	SiO₂	Q
Calcium aluminate hydrate	CaO·Al₂O₃·10H₂O	CAH
Calcium aluminosilicate hydrate	CaO·Al₂O₃·SiO·10H₂O	CASH
Calcium hydroxide	Ca(OH)₂	CH
Ettringite	3CaO·Al₂O₃·3CaSO₄·32H₂O	AFt
Calcite	CaCO₃	Cal
Calcium plagioclase	2CaO·SiO₂	C₂S
Hexagonal calcium silicate	3CaO·SiO₂	C₃S
Gypsum	CaSO₄·2H₂O	NA

Note: NA represents not applicable.

Figure 11.

X-ray diffraction pattern of solidified soil in different stages of carbonization.

It can be observed in Figure 11 that the diffraction intensities increase with the increase of carbonization time. At the stage of no carbonization, obviously, apart from some strong quartz peaks detected, the CH (14.5°, 25°, 34°), CSH (32°), and AFt (19°, 22.5°, 27°) peaks are detected in the non-carbonated samples, revealing that the hydration products such as CH, CSH, and AFt are wrapped with soil particles, forming a skeleton to provide strength after standard curing for 28 days. At the stage of carbonization in progress, hydration products such as CH, AFt, CSH, and AFm as well as carbonization product Cal can be clearly observed; in particular, there are compound peaks of CSH and Cal in XRD, which proves that Cal crystal is considered to be embedded in the product of CSH, in accordance with the earlier analysis. At the stage of complete carbonization, the peaks of the hydration products CSH, CH, AFt, and AFm almost completely disappear, but more and higher peaks of Cal can be observed in XRD, demonstrating that that excessive CO₂ can consume CSH, CH, AFt, and other hydration products, and eventually convert to Cal. Moreover, there is no diffraction peak of heavy metal deposits existing in XRD; this is because the concentration of the heavy metal ion of Ni0.02 is relatively low so that it is difficult to form precipitation, or the CaNi₂(OH)₆·2H₂O crystallization is really poor.

Analysis of Microcosmic Characteristics of SEM

Figure 12, a–c, presents the SEM of samples with heavy metal concentration of Ni0.02 in three different carbonization stages, including not carbonized, being carbonized, and completely carbonized, at a magnification of 5,000 times. At the stage of no carbonization, the hydration products such as CH (six-corner plate columnar), CSH (floc or reticulate), and AFt (crystals are formed between soil particles) can be observed in Figure 12a, and the hydration products distribute in the inter-particle pores and bond soil particles together. Although the hydration reaction of samples with Ni0.02 is hindered to some extent, only a small amount of pores exist in the internal structure of the sample after standard curing for 28 days, still carrying a relatively high hydration degree. Therefore, the hydration products of CFG and soil particles contribute to denser microstructure and stronger inter-particle bonding, resulting in a high strength at this stage. At the stage of carbonization in progress, as shown in Figure 12b, the hydration products gradually disappear when specimens are exposed to CO₂ gas, and novel carbonization products Cal (rhomboid or pelletized) are generated; therefore not only hydration products but also carbonization products Cal can be observed, and some of the Cal crystals are embedded in the hydration product of CSH, consistent with the conclusion of the XRD analysis. Besides, having been decomposed, the original hydration products react with CO₂ to form carbonization products, leading to the change of the micro-morphology, and there are more and large pores between soil particles in this stage, resulting in the decrease of strength. At the stage of complete carbonization, hydration products are almost unobservable in Figure 12c but there is a large amount of Cal. Moreover, the micro-morphology has changed completely. Compared with the stage of no carbonization, there are more pores in samples and the structure of the soil is looser, instead of the tight connection between soil particles. At this stage, the internal structure of the sample is seriously damaged as a result of the decomposition of CSH; furthermore, as a result of the overall damage, the test block body breaks easily. Macroscopically, these findings could provide an essential explanation for why the UCS of specimens can be weakened significantly after carbonization. Generally, the hydration and carbonization products observed in the SEM micrograph images are very consistent with the XRD results.

Figure 12.

Scanning electron microscopy images of solidified soil in different stages of carbonization: (a) no carbonization, (b) carbonization in progress, and (c) complete carbonization.

Carbonization Mechanism Analysis

Combining the results of the macroscopic and microscopic carbonization experiments, the carbonization mechanism of contaminated soil solidified by CFG can be inferred; first, the CO₂ gas in the carbonization tank infiltrates into the soil through the pores and capillary channels in the samples; then, CO₂ reacts with hydration products such as CH, CSH, and AFt, as well as non-hydrated C₃S and C₂S to form Cal, as shown in formulas 14–18; meanwhile, the alkalinity reduces ( 49 ).

C O_{2} + H_{2} O \to H_{2} C O_{3}

(14)

Ca {(OH)}_{2} + H_{2} C O_{3} \to CaC O_{3} + 2 H_{2} O

(15)

\begin{matrix} 3 CaO \cdot 2 Si O_{2} \cdot 3 H_{2} O + 3 H_{2} C O_{3} \to 3 CaC O_{3} \\ + 2 Si O_{2} + 6 H_{2} O \end{matrix}

(16)

\begin{matrix} 3 CaO \cdot A I_{2} O_{3} \cdot 3 CaS O_{4} \cdot 32 H_{2} O + 3 H_{2} C O_{3} \to 3 CaC O_{3} \\ + 2 Al {(OH)}_{2} + 3 CaS O_{4} + 32 H_{2} O \end{matrix}

(17)

The carbonization reaction of CSH and CH occurs almost at the same time, but the rate of carbonization reaction of CH is slightly higher than that of CSH. The CH carbonized to form calcium carbonate (Cal) increases the volume by 11.5% ( 50 ), and the Cal produced by CH fills in the pores between soil particles, resulting in a decrease in porosity and an increase in soil strength as well as a reduction of the alkalinity. By comparison, CSH carbonization can cause the decomposition of CSH, resulting in a reduction of the volume between the soil particles; the compactness of soil is weakened, and the soil’s internal structure is destroyed, thus reducing the strength of the soil. Carbonization can also decrease the pH value of the soil pore water solution, and the soil even becomes weakly acidic after complete carbonization, and the CH and CSH can hardly exist.

Conclusion

In this study, the UCS, carbonization depth, XRD, and SEM tests of nickel–copper ion-contaminated soil treated with CFG curing agent have been completed. The effects of carbonization on UCS, carbonization depth, and microstructure characteristics of contaminated soil were analyzed. The following conclusions can be drawn:

1. The carbonization reaction has a significant impact on the UCS of samples with a lower concentration of heavy metal nickel, whereas the carbonization reaction has a slight but perceptible impact on samples with higher concentrations of heavy metal nickel and copper.

2. The carbonization depth of samples increases with the increase of carbonization time. The carbonization depth is in a positive proportion to the square root of time. Besides, the carbonization process is gradually deepened from the surface to the inside of the test block. Therefore, in practical engineering, the change of engineering properties of the solidified soil caused by the role of CO₂ in natural conditions should be considered.

3. In a high concentration relative to a low concentration of heavy metals, the carbonization rate is slower, which also requires a shorter time of complete carbonization. Besides, the carbonization rate of Cu1 is slightly faster than that of Ni1. Therefore, the time of complete carbonization can be used as a significant parameter to judge carbonization rate or degree in engineering.

4. Micro test indicates that the hydration products are the main reasons for the enhancement of the UCS without carbonization. However, the internal structure of the sample is seriously damaged with the increase of carbonization time. There are more pores produced as a result of the decomposition of hydration products with the carbonization reaction, which results in the weakening of the compressive strength. Therefore, the adverse effects of carbonization in engineering need attention.

Novelty of This Work

The novelty of this study is as follows: (1) Using the composite material CFG to solidify nickel–copper-contaminated soil provides a new way for the recycling of industrial solid waste. (2) In this study, the mechanical properties and mechanism of the CFG-solidified nickel–copper-contaminated soil are discussed, which has reference value for practical engineering application. (3) The application of new composite curing agent CFG provides a new idea for the treatment of nickel–copper-contaminated soil.

Footnotes

Author Contributions

The authors confirm contribution to the paper as follows: study conception and design: Qiang Wang. Jingyang Cui. Xiaoliang Guo; data collection: Xiaoliang Guo. Jingyang Cui. Jingdong Yang. Wenjun Zhou, Man Li; analysis and interpretation of results: Man Li. Qiang Wang. Xiaoliang Guo; draft manuscript preparation: Qiang Wang. Man Li. Xiaoliang Guo. All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was funded by Research Foundation of the Institute of Environment-friendly Materials and Occupational Health (Wuhu), Anhui University of Science and Technology (ALW2020YF13).

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