3.1. Analysis of the Mechanical Properties of the Cured Body
3.1.1. The Effect of Curing Agent Content on Mechanical Properties
The variation in unconfined compressive strength of the solidified body with the amount of curing agent is shown in
Figure 6. An experimental study on the relationship between the unconfined compressive strength of the solidified body and the amount of curing agent under four initial pollution degrees was carried out.
As is shown in
Figure 6, the relationship between the intensity and the amount of curing agent added can be concluded under the same initial pollution degree of heavy metal Pb, Cd, and Cu composite contaminated soil, that is, with the amount of curing agent increasing from 5% to 40%, the histogram is constantly rising, indicating that with the increase in the curing agent content, the unconfined compressive strength of the solidified body also increases, and the trend of the contaminated soil solidified body with different initial pollution degrees is consistent. The unconfined compressive strength can reach up to 13.38 MPa. This may be because, on the one hand, the new curing agent A is a small particle component, which can change the particle gradation of heavy-metal-contaminated soil macroscopically and make the overall structure dense. On the other hand, the formation of the whole strength of the curing agent and the contaminated soil is a long-term and complex process. The curing agent and the heavy-metal-contaminated soil are mixed and reacted by hydration and hydrolysis to form a whole with a cementing structure with higher strength. From a microscopic point of view, the main products of cement, fly ash, and silica fume in the curing agent after hydration are ettringite (AFT), calcium silicate hydrate (CSH), calcium aluminate hydrate (CAH), etc. [
17] With the further hydration of the curing agent material, the amount of hydration products continues to increase to form the intertwined crystalline packages on the surface of the soil particles, which play the role of bonding and filling, and increase the strength of the contaminated soil.
It can be seen from
Figure 6 that for the soil samples with the same initial pollution degree, when the dosage of curing agent increases from 5% to 40%, the increasing ranges of unconfined compressive strength of the solidified body are 25–41%, 39–57%, 18–60% and 4–14%, respectively. It can be found that after the curing agent content exceeds 30%, the unconfined compressive strength increases significantly with the increase in curing agent content. This may be due to the fact that too much curing agent material is added and cannot be fully involved in the hydration and hydrolysis reaction. As a result, a considerable part of the curing agent does not play its role. Therefore, simply adding more curing agent to improve the mechanical properties of the solidified body may not achieve the expected effect.
3.1.2. Influence of Initial Pollution Degree on Mechanical Strength
The variation in unconfined compressive strength of the solidified body with the initial pollution degree is shown in
Figure 7. An experimental study on the relationship between the unconfined compressive strength of the solidified body and the initial pollution degree under the dosage of five curing agents was carried out.
In
Figure 7, the relationship between the strength and the initial pollution degree of the heavy metal Pb, Cd, and Cu composite contaminated soil with the same dosage of curing agent can be obtained, that is, with the initial pollution degree rising from non-pollution to heavy pollution, the histogram is constantly decreasing, which shows that with the aggravation of the initial pollution degree, the unconfined compressive strength of the solidified body decreases, and the trend of solidified form of contaminated soil with different dosage of curing agent is consistent.
For the heavily polluted soil samples, the amount of curing agent increases from 5% to 40%, and the unconfined strength of the solidified body increases by 260%. When the dosage of curing agent is 20%, the unconfined strength is 6.32 MPa, which is higher than the geological landfill standard (5 MPa). When the content of curing agent is 30%, the unconfined strength is 10.12 mpa, which meets the requirements of building material strength (10 MPa). For moderately contaminated soil samples, the content of curing agent increases from 5% to 40%, and the unconfined strength of the cured body increases by 175%. When the content of curing agent is 10%, the unconfined strength is 5.42 MPa, which is higher than the geological landfill standard. According to the standard, when the curing agent content is 30%, the unconfined strength is 11.32 MPa, which meets the strength requirements of building materials. For lightly contaminated soil samples, the content of curing agent increases from 5% to 40%, and the unconfined strength of the cured body increases by 153%. When the content of curing agent is 5%, the unconfined strength is 5.04 MPa, which is higher than the standard. When the curing agent content is 20%, the unconfined strength is 10.07 MPa, which meets the strength requirements of building materials.
In summary, as the amount of curing agent increases, the unconfined compressive strength of the solidified body also increases, and the trend of solidified body of the contaminated soil with different initial pollution degrees is consistent. The maximum unconfined compressive strength of the solidified body can reach 13.38 MPa. After the curing agent content exceeds 30%, the unconfined compressive strength will decrease significantly with the increase in curing agent content. Simply adding more curing agent to improve the mechanical properties of the solidified body may not achieve the desired effect. In terms of mechanical properties alone, for heavily polluted soil samples, the content of curing agent must reach 20% to achieve safe disposal. To meet the strength requirements of building materials, the content of curing agent must reach 30%. For moderately contaminated soil samples, the content of curing agent must reach 10% to achieve the purpose of safe disposal, and to meet the strength requirements of building materials, the content of curing agent must also reach 30%. For lightly contaminated soils, the content of curing agent needs to reach 5% to achieve the purpose of safe disposal. To meet the strength requirements of building materials, however, the content of curing agent also needs to reach 20%.
3.2. Analysis of the Deformation Characteristics of the Solidified Body
Since the solidified body of heavy-metal-contaminated soil is nonlinearly deformed, the deformation modulus is not a constant. The deformation modulus E
50 is usually used to characterize the deformation characteristics of the material. E
50 refers to the secant modulus corresponding to 50% of the peak stress, also known as deformation coefficient [
3]. Therefore, according to the stress–strain curve, the failure strain and the deformation modulus E
50 are obtained to analyze the deformation characteristics of the solidified body of heavy-metal-contaminated soil.
Figure 8 shows the stress–strain curve, failure strain curve, and deformation modulus of the solidified body of different heavy metal pollution levels with different curing agent content.
It can be seen from
Figure 8 that the change trend of stress–strain curves of different initial contamination degree is similar under different dosage of curing agent. If the dosage of curing agent is low (5% and 10%), the peak value of stress–strain curve is not obvious. If the dosage of curing agent is high (20%, 30% and 40%), the stress increases obviously with strain at the beginning of compression, and there is a significant peak after reaching the ultimate strength. After the peak, the structure of the solidified body is destroyed, and the stress–strain curve decreases rapidly. With the increase in the amount of curing agent, the stress–strain curve shows a peak left-up shift, which represents the weakening of the plasticity of the solidified body and the enhancement of the resistance to deformation.
It can be seen from the regular curves of curing agent dosage, failure strain and deformation modulus E
50 in
Figure 8 that for solidified bodies of different initial pollution degrees, with the increase in curing agent dosage, the failure strain of solidified body gradually decreases, the peak value in the corresponding stress–strain curve shifts to the left. Moreover, with the increase in curing agent dosage, the extent of the reduced failure strain gradually decreases, and the corresponding curve of the relationship between the failure strain and the dosage of curing agent tends to flatten out gradually. It can be seen from the relationship curve between the deformation modulus E
50 and the dosage of curing agent that with the increase in the dosage of curing agent, the deformation modulus E
50 has a significant nonlinear increasing trend. When the content of curing agent increases from 5% to 10%, the increasing range of E
50 is the largest. With the further increase in the content of curing agent, the increasing range of E
50 gradually decreases.
3.3. Analysis on Leaching Characteristics of Solidified Form
The leaching concentration diagram of heavy metals Pb, Cd, and Cu in the solidified body is shown in
Figure 9. According to the “Identification Standard for Hazardous Wastes Leaching Toxicity Identification” (GB 5085.3-2007), the safety standard limit for Pb leaching is 5 mg/L, the safety standard limit for Cd leaching is 1 mg/L, and the safety standard limit for Cu leaching is 100 mg/L.
It can be seen from
Figure 9a that after the heavy-metal-contaminated soils of three pollution degrees are solidified with different dosages of curing agent and cured for 7 days, the leaching concentrations of Pb in the solidified body are mostly lower than the safety limit. The leaching concentration of heavily polluted soil is 7.547 mg/L when the curing agent content is 5%, which is slightly higher than the standard safety limit for leaching. The leaching concentration of Pb in all other solidified bodies is lower than the limit. Additionally, the Pb leaching concentration of lightly polluted soil solidified by 40% curing agent is lower than the minimum detection limit of equipment 0.001 mg/L, and it was expressed as 0.001 when the data were collated. It can be seen from
Figure 9 that with the increase in the curing agent content, the leaching Pb concentration decreases significantly, and the leaching concentration of Pb in lightly and moderately contaminated soil solidified by 5% curing agent is lower than the safety limit. The leaching concentration of Pb in three kinds of contaminated soil solidified by 10% curing agent is lower than the safety limit. After the curing agent content reaches 20%, the leaching concentration of Pb drops significantly, which is far below the safety limit.
It can be seen from
Figure 9b that after the heavy-metal-contaminated soils of three pollution degrees are solidified with different dosages of curing agent and cured for 7 days, the leaching concentrations of Cd in the solidified body are all lower than the safety limit of 1 mg/L. Similar to the leaching concentration of Pb in the solidified body, the leaching concentration of Cd in the solidified body decreases significantly with the increase in the amount of curing agent. For the heavily polluted soil, when the content of curing agent increases from 5% to 40%, the leaching concentration of Cd decreases by 94.7%, which is far below the leaching safety standard limit.
It can be seen from
Figure 9c that after the heavy-metal-contaminated soils of three pollution degrees are solidified with different dosages of curing agent and cured for 7 days, the leaching concentrations of Cu in the solidified body are all lower than the safety limit of 100 mg/L. Although the leaching concentration of Cu in the contaminated soil solidified by 5% curing agent is lower than the leaching safety standard limit, the leaching concentration continues to decrease with the increase in the curing agent content. However, with the increase in the curing agent content, the leaching concentration of Cu does not decrease as obviously as that of Pb and Cd, especially in lightly polluted soil. When the content of curing agent increases from 5% to 20%, leaching concentration of Cu decreases to only 9.5%.
In summary, it can be seen from the analysis of the leaching performance test results of the optimized solidified soil that with the increase in the amount of curing agent, the leaching concentrations of three heavy metals Pb, Cd and Cu in the solidified soil also decrease, and the trend of solidified soil with different initial pollution degrees is consistent. Only in heavily polluted soil, the leaching concentration of Pb slightly exceeds the limit after leaching test of the solidified body with 5% curing agent, and the leaching concentrations of Cd and Cu in the same group reach the standard. The leaching concentrations of three heavy metals Pb, Cd and Cu decrease with the increase in the amount of curing agent. The decrease in Pb and Cd is relatively significant, while Cu decreases slightly. The leaching concentration of Pb in slightly and moderately polluted soils is lower than the safety limit if the amount of the curing agent is 5% and the leaching concentrations of Pb in three polluted soils are all lower than the safety limit if the dosage is 10%. As for Cd and Cu, the leaching concentrations of the three contaminated soils after solidification are all lower than the safety limit with curing agent of 5%. Therefore, in terms of leaching safety of the solidified body, 10% curing agent is required for the heavily polluted soil to achieve safe disposal, whereas for other contaminated soils, the dosage of curing agent of 5% can be satisfying.
3.4. Microstructure Analysis of Solidified Body
The SEM/EDS diagram and distribution diagram of main elements (including lead, cadmium, copper, calcium, iron and silicon) of heavy-metal-contaminated soil before solidification are shown in
Figure 10. The content of Pb, Cd and Cu in the heavy-metal-contaminated soil samples were 10,000 mg/kg, 40 mg/kg and 6000 mg/kg, respectively. Among a large number of scanning images, the images with a magnification of 5000 times were selected to observe its micro morphology. It can be seen from its microstructure distribution that the internal structure of the contaminated soil sample is loose, the particles are small and dispersed obviously. The bonding characteristics between the soil particles are damaged to a certain extent, and the whole structure is relatively loose, which may be due to the addition of heavy metals to change and destroy the original structure of the soil particles.
In addition, it can be seen from the element distribution diagram that the distribution of heavy metals Pb, Cd, and Cu is significantly different from that of Ca, Fe, and Si. The distribution of elements Pb, Cd, and Cu are distributed throughout the entire image, and the distribution is uniform, without obvious regularity. The distribution of elements Ca, Fe, and Si has obvious regularity, and is related to the distribution of particles. The shape and edge of the distribution are consistent with the shape and edge of the soil particles. It can also be seen that the heavy metals Pb, Cd, Cu have no obvious dependence on mineral elements.
The SEM/EDS map and the distribution map of main elements (including lead, cadmium, copper, calcium, iron, and silicon) after solidification of heavy-metal-contaminated soil are shown in
Figure 10. The contents of Pb, Cd, and Cu of heavy-metal-contaminated soil samples artificially prepared in the experiment were 10,000 mg/kg, 40 mg/kg and 6000 mg/kg, respectively, with 20% curing agent. On the basis of a large number of scanning images, the images with a magnification of 5000 times were finally selected to observe its microscopic appearance. It can be seen from the microstructure distribution map that a large number of needle-like substances and flocculent substances appear on the microscopic image of the soil sample contaminated by heavy metals after solidification. A large number of previous studies have shown that this type of substance is the hydration product of the curing agent material in an alkaline environment [
18,
19,
20,
21]. Among them, needle-like material is ettringite (AFT), and flocculation material is mostly calcium silicate hydrate (CSH) gel. In addition, some structures similar to AFT or CSH gel hydrated products can also be detected, but there are some differences in structure and morphology. The distribution of heavy metals Pb, Cd and Cu is significantly different from that before solidification. It can be inferred that the structure changes after the hydration products absorb heavy metals and it is this kind of structure that plays the role of solidifying heavy metals.
3.5. Morphology Analysis of Solidified Metal
After solidification treatment and curing for 7 days, the occurrence forms of Pb, CD and Cu in the solidified body are shown in
Figure 11.
In the uncured original soil samples, heavy metal Pb mainly presents in the form of F1 (weak acid state), F2 (reducible state) and F3 (oxidizable state) with contents of 28.5%, 39.5%, and 31.1%, respectively. F4 (residual state) is extremely low, accounting for 0.9%. After solidification, the main forms of Pb in the contaminated soil are F2 (reducible), F3 (oxidizable) and F4 (residual). In addition, with the increase in the amount of curing agent, the content of F1 (weak acid state) gradually decreases, whereas the content of F2 (reducible state) does not change much, and the overall trend is gradually decreasing. The content of F3 (oxidizable state) also changes little, but the overall trend is gradually increasing. The content of F4 (residual state) increases greatly, which is 36 times higher than that without curing. This shows that with the increase in solidified agent content, Pb in contaminated soil transforms to F3 (oxidizable state) and F4 (residual state) and tends to be stable.
In uncured original soil samples, Cd mainly presents in the form of F1 (weak acid state) and F2 (reducible state) with contents of 43.8% and 34.2%. The content of F3 (oxidizable state) is 18.5% and F4 (residual state), 3.5%, is very low. After solidification, the main forms of Cd in contaminated soil are F2 (reducible), F3 (oxidizable) and F4 (residual). In addition, with the addition of curing agent, the content of F1 (weak acid state) decreases rapidly, the content of F4 (residual state) increases significantly, and the contents of F2 (reducible state) and F3 (oxidizable state) change little. With the increase in the dosage of curing agent, the content of F1 (weak acid state) constantly decreases. When the dosage of curing agent increases from 5% to 40%, the content of F1 (weak acid state) decreases by 85%. The content of F4 (residual state) increases with the increase in the dosage of curing agent. When the dosage of curing agent increases from 5% to 40%, the content of F4 (residual state) increases by 40.7%. This indicates that the addition of the curing agent can transform a considerable proportion of active heavy metal Cd in contaminated soil into a relatively stable form.
In the uncured original soil samples, Cu mainly presents in the form of F1 (weak acid state), F2 (reducible state)/F3 (oxidizable state) with contents of 28%, 42%, and 25.7%, respectively. F4 (residual state) is relatively low; 4.3%. After solidification, the main forms of Cu in the contaminated soil are F2 (reducible), F3 (oxidizable) and F4 (residual). In addition, with the addition of curing agent, the content of F1 (weak acid state) decreases rapidly, the content of F4 (residual state) increases significantly, and the contents of F2 (reducible state) and F3 (oxidizable state) change little. With the increase in the content of curing agent, the content of F1 (weak acid state) decreases. When the content of curing agent increases from 5% to 40%, the content of F1 (weak acid state) decreases by 67.9%; The content of F4 (residual state) increases with the increase in curing agent dosage. When the dosage of curing agent increases from 5% to 40%, the content of F4 (residual state) increases by 66.7%, and the contents of F2 (reducible state) and F3 (oxidizable state) change little. However, with the increase in curing agent dosage, F2 (reducible state) decreases while F3 (oxidizable state) increases. This indicates that the addition of curing agent can transform a considerable proportion of active heavy metal Cu in contaminated soil into F3 (oxidizable state) and F4 (residual state) and tend to be stable.
To sum up, it can be seen that the new curing agent A can significantly reduce the content of the weak acid-extractable heavy metals after solidification of Pb, Cd and Cu composite contaminated soil, and effectively transform it into residual state. Increasing the amount of curing agent can further reduce the content of weak acid extraction in the solidified body and improve the content of its residual state. Meanwhile, the content of oxidizable state decreases slightly and the content of reducible state increases slightly.
3.6. Solid Phase Analysis
The phase analysis is a qualitative analysis based on the processing of the XRD spectrum. It can determine the mineral composition according to the diffraction angle and peak value of the mineral [
22,
23]. The contaminated soils combined with heavy metals Pb, Cd, and Cu before and after solidification were selected for X-ray diffraction test. The test results are shown in
Figure 12.
It can be seen from
Figure 12 that SiO
2 and CaCO
3 are the main minerals in the heavy-metal-contaminated soil before solidification, and Pb
5O
8, CdO
2 and CuO of Pb, Cd and Cu as well as other compounds Pb(NO
3)OH, Cd(OH)
2 and Cd(NO
3)
2 of Pb, Cd and Cu can be detected in the secondary minerals.
Figure 12b shows the XRD image of the solidified heavy-metal-contaminated soil. The figure also shows that the internal material composition of the heavy-metal-contaminated soil changes after solidification by the new curing agent, and some heavy metal compounds disappear due to the addition of the curing agent. At the same time, new phases are retrieved, mainly including hydrated calcium silicate hydrate (CSH), ettringite (Aft) and other stable compounds combined with heavy metals and Ca and Si ions. The principle of the formation process of the new phases is shown in
Figure 13.
New phases such as hydrated calcium silicate (CSH) and ettringite (AFT) can encapsulate the heavy metal ions. Some of them will undergo ion exchange or addition and combine with calcium and silicon ions to from a stable structure, so that lead, cadmium and copper ions can be stabilized in the contaminated soil. When the diffraction angle is 29.4°, the diffraction peak before solidification is CaCO3, and after solidification, the single peak is searched at the same diffraction peak position. It is found that there are a variety of Ca-Cd compounds, Cd-Si compounds and Pb-Si compounds with extreme similarity. It is determined that there should be a stable compound formed by the adsorption and encapsulation of calcium and silicon products produced by the hydration of curing agent on heavy metal ions.