3.1 Characterisation
Material characterization for the solidify/stabilize (S/S) soils allowed better analysis of their intrinsic physicochemical properties. The SEM micrographs of ordinary Portland cement 325 and soda residue are shown in Fig. 1. It is obvious from Fig. 1(a)-(c) that the microstructure of OPC is characterized by large porosity and structural connection and arrangement. The nature and mineral composition of OPC itself lead to a looser spongy structure on the OPC surface, which is characterized by more pores and a looser structure. With the magnification of SEM, this sparse structure becomes more and more obvious. And there are many fine clay mineral particles attached to the OPC surface. Fig. 1(d)-(f) show the microscopic morphology of SR, which reveals many loose and irregular porous structures with different number and size of pores, similar to sponge-like textures. There are also fine particles on the surface, which are most likely crystals of C CaCO3, CaSO4, CaO, Al2O3, and Mg(OH)2. The SEM micrographs of the S/S soils (Sb/Sd=20%, 60% cement+40% soda residue, i.e. CSR) and S/S soils with Pb2+(CSR-Pb2+) are shown in Fig. 2. Fig. 2(a)-(c) shows the microscopic morphology of CSR, and it can be observed that CSR has the morphological characteristics of OPC and SR with a loose texture and is rich in porous structure. In addition, a large amount of flocculated hydrated calcium silicate production can also be observed, as well as fine particles, which may be the mineral crystals mentioned above. And the S/S soil has a dense structure with small pores, a fish-scale texture, and comes with multiple layers of flakes, which are the basis for the curing material. It is obvious by observing Fig. 2(d)-(f) that the hydration products of cement and soda residue are generated in large quantities by the reinforced specimens, which are interwoven and filled in the soil voids, making the porosity of the soil decrease and the structure become dense, and the macroscopic performance is increased in strength. And with the increase of the maintenance age, the hydration reaction in the soil is more complete, and the hydration products cover the wrapped soil particles or are scattered between the pores, which makes the strength of the soil further increase. Compared with Fig. 2(a)-(c), the particle distribution is more concentrated, with larger granular crystals present, smoother surface, tighter internal structure, and the appearance of a regular structured mesh, which may be the hydrated calcium silicate or -Si-O-A1-O- bonded interconnected gels produced during the curing process of CSR (Yang et al., 2021). These gels could bond the CSR particles to each other into one larger particle (Zhang et al., 2017). In particular, in Fig. 2(f), the reticular or needle-like structures of hydrated calcium silicate (CSH) and calcium aluminate hydrate (CAH) can be clearly observed due to the complex physical and chemical reactions between cement-alkali slag-soil, which generate gels and complexes such as calcium zeolite, C-S-H, and C-A-H.
The BET specific surface area and pore size analysis of the different cured/stabilized soil samples are listed in Table 3. According to the definition of the International Association of Pure and Applied Chemistry (IUPAC), those with a pore diameter less than 2 nm are called micropores, and those with a pore diameter greater than 50 nm are called macropores (Wang et al., 2021); those with a pore diameter between 2 and 50 nm are called mesopores. This shows that the pore structure of cement, soda residue, and CSR (S/S soil) are mainly mesoporous materials. It can be seen that the BET specific surface area of SR is the largest (28.6796 m2·g−1), and the doping of SR into OPC can significantly enhance the BET specific surface area (8 times increase in BET specific surface area and 16 times increase in Langmuir specific surface area) and pore volume (10 times increase in total pore volume, 7 times increase in BJH Adsorption cumulative volume) of the S/S soil (CSR). This implies the ability to adsorb more contaminants such as heavy metal ions, which leads to the cured soil providing more active adsorption sites and porous structure (Li et al., 2019). This result also corroborates with the analysis of SEM in Fig. 1, Which directly indicates that the composite of soda residue into cement is theoretically able to improve the physicochemical properties of the cured soil, especially the porous structure and pore volume. The specific surface area, pore volume, and pore diameter of the cured soil slightly decreased after infiltration of Pb2+. This is direct evidence that Pb2+ is solidified on the surface and internal porous structure of CSR (Xi et al., 2014), occupying a small number of active adsorption sites, leading to a decrease in specific surface area and pore volume, clogging the pore channels and reducing the pore size.
Table 3
BET specific surface area and pore size analysis of different solidify/stabilize soil samples.
Sample type
|
Surface Area (m2·g−1)
|
Pore Volume (cm3·g−1)
|
Pore Size (nm)
|
BET Surface Area
|
Langmuir Surface Area
|
Single point surface area (p/p0=0.255)
|
t-Plot Micropore Area
|
Single point adsorption total pore volume
|
BJH Adsorption cumulative volume (1.70~300 nm)
|
t-Plot micropore volume
|
Adsorption average pore diameter
|
BJH Adsorption average pore diameter
|
OPC
|
2.2167
|
15.7532
|
2.1802
|
0.3542
|
0.004145
|
0.010713
|
0.000158
|
7.4965
|
23.9432
|
SR
|
28.6796
|
296.9066
|
27.9132
|
0.1304
|
0.057920
|
0.081120
|
0.000194
|
8.0783
|
11.0466
|
CSR
|
16.3006
|
251.1340
|
15.8500
|
16.5202
|
0.041290
|
0.077745
|
0.000288
|
10.1320
|
17.5043
|
CSR-Pb2+
|
15.1157
|
218.3564
|
14.7137
|
15.2187
|
0.037594
|
0.074223
|
0.00012
|
9.9485
|
18.3182
|
FTIR Spectroscopy of the solidify/stabilize (S/S) soils are presented in Fig. 3. Fig. 3(a) obviously displays that this S/S soils (Sb/Sd=20%, 60% cement+40% soda residue, i.e. CSR) is rich in functional groups, which is mainly attributed to the presence of many -OH (3420 cm−1) in the soda residue and -CH3 or -C-C- (1000-1070 cm−1) in the cement. The other functional group positions and cements are essentially the same as the soda residues, including C-H (2500-2921 cm−1), -COOH (1800 cm−1), -C=O- (1450 cm−1), and heterocyclic group (400-900 cm−1). It is noteworthy that the -OH stretching vibration peak at 3420 cm−1 and the heterocyclic stretching vibration peak at 400-900 cm−1 of CSR are significantly enhanced, which implies that the soda residues is compounded into the cement and that the adsorption properties and oxygen-containing functional groups are improved, making the composite more stable (Li et al., 2017; Ge et al., 2020). The FTIR spectra of the composite CSR after infiltration of Pb2+ are exhibited in Fig. 3(b). The -OH (3420 cm−1), -COOH (1800 cm−1), and -C=O- (1450 cm−1) could be clearly observed, and these oxygen-containing functional group stretching vibrational peaks were significantly increased. This most likely suggests that these active sites adsorb Pb2+ or produced gels of hydration products, such as CSH and CAH (Ge et al., 2020; Yang et al., 2021), which corroborates with the SEM and BET characterization results.
X-ray diffraction is capable of detecting the mineral content of a sample and quantifying the mineral content based on the intensity and half-peak width of the diffraction peaks. The XRD patterns of the S/S soils is demonstrated in Fig. 4. It could be noticed from Fig. 4 that OPC, SR and CSR have many minerals, mainly including SiO2, CaCO3, CaSO4, CaCl2, CaO, Al2O3, Fe2O3, and Mg(OH)2. The presence of CSH and CAH crystals was also detected in the samples after the CSR was infiltrated by Pb2+. It meant that complex physicochemical reactions, i.e. hydration reactions, took place between cement and soda residues during the curing period (Zhang et al., 2017; Yang et al., 2021). And a large amount of colloids and complexes such as CSH (hydrated calcium silicate) and CAH (hydrated calcium aluminate) were generated. Moreover, the soda residue is strongly alkaline, and under the alkaline environment and the energy generated by cement hydration, the curing agent reacts with the active SiO2 and Al2O3 in the soil particles to produce the complex CaO2·SiO2·Al2O3·2H2O (Ge et al., 2020; Yang et al., 2021). This product increases with the age of curing and as the hydration reaction proceeds. It may also make the soil fissures, and some soluble salts or fine soil particles dissolve out of the soil, which leads to the thinning of the soil particle-binding water film, decreasing the attraction and making the soil structure unstable. At the same time, the dissolution of alkaline substances in the pore water will make the stability of hydration products in the soil weaken and gradually be destroyed and dissolved out of the soil, thus leading to the reduction of the unconfined compressive strength of the specimen (Xi et al., 2014; Ge et al., 2020; Yang et al., 2021). The detailed curing mechanism is discussed specifically in section 3.5.
3.2 Unconfined compressive strength
Figure 5 shows the curve of unconfined compressive strength of solidified soil with curing age under different conditions of Sb/Sd and WP, when the curing agent is single cement. It can be seen that the strength of solidified soil gradually increases with the increase of curing age from 0 day to 90 days. Moreover, from Figure 5(a)-1(d), it can be demonstrated that the curing age is from 0 to 14 days, the curve is very steep, which implying a rapid increase in unconfined compressive strength from mean 0.32 Mpa to mean 1.56 Mpa. Then curing age from 28 to 90 days, the curve becomes flat, indicating that the unconfined compressive strength grows slowly and basically remains stable from the average value of 1.41 Mpa to 1.87 Mpa average. Furthermore, when the UCS test was performed at a curing age of 0 days, i.e., just after the preparation of the test cylinder samples, it was observed that the initial unconfined compressive strength kept increasing with the increase of the different Sb/Sd ratio. The initial unconfined compressive strength was maximum when the Sb/Sd ratio was 20%, with an average value of 0.73 Mpa at different Pb2+ concentration levels. However, with the increase of Pb2+ concentration levels (WP), the growth trend of solidified soil unconfined compressive strength gradually slows down, and Figure 5(a)-1(d) all have this same phenomenon, which means that the Unconfined compressive strength of the cured soil slowly decreases by 16.3–21.5% on average as the Pb2+ concentration increases from 0 mg·kg−1 to 50,000 mg·kg−1. Fortunately, although the level of Pb2+ concentration in the soil was increased, the decrease of unconfined compressive strength was basically the same at different Sb/Sd ratios without much difference. The strength of contaminated soil solidified by cement is lower than that of uncontaminated soil at different curing ages, and with the increase of Pb2+ concentration levels (WP), the degree of reduction is greater, which is consistent with the research results of Tashiro et al (1979), the main reason is that heavy metal oxides such as Pb react with cement slurry, which affects the hardening and strength development of cement at the initial hydration stage. In addition, the Unconfined compressive strength of the cured soil increased consistently with increasing single cement (OPC) addition Sb/Sd in lead-contaminated soil at different concentration levels of Pb2+, from 7.5–10% with an average increase of 6.8% UCS (MPa), an average increase of 13.3% UCS (MPa) from 10–15%, an average increase of 18.7% UCS (MPa) from 15–20%. This suggests that the Unconfined compressive strength increases as the addition of OPC (Sb/Sd) to the cured soil increases from 7.5–20%, with a total increase of 32.8%. It also indirectly shows that the curing effect is best when Sb/Sd =20, and this addition ratio (Sb/Sd =20) will be applied later in the analysis.
Figure 2 shows the curve of unconfined compressive strength of solidified soil with curing age when the curing age is cement and soda residue, with Sb/Sd=20%, WP=50000 mg·kg−1 and different Ss/Sb. The test results show that the addition of soda residue improves the early strength of cement-solidified soil, but reduces the long-term strength of solidified soil. The strength of solidified soil increases by 10% ~ 25% in 14 days and decreases by 16.3%~23.5% in 90 days. This turning point occurred near 20 days compared to the 100% single OPC control group. With the increase of the amount of soda residue, the degree of improvement and reduction is also greater. The reason is that soda residue is a material with high water absorption and high alkalinity (Wang et al., 2021a). The addition of soda residue can improve the hardening speed of cement at the initial hydration stage and the early strength of cement-solidified soil. However, at the same time, soda residue contains NaCl and other easily soluble salts, which affects the long-term strength development of cement-solidified soil (Zha et al., 2020a). Nevertheless, compared to Fig. 5(d), Fig. 6 clearly shows that the addition of soda residue to the composite curing agent enhances the initial unconfined compressive strength by an average of 23.1% Mpa. Moreover, the initial unconfined compressive strength increases gradually as the composite proportion of soda residue increases from 0 to 40%, and reaches the maximum when the composite proportion of soda residue is 40%, at which time the unconfined compressive strength is 0.96 Mpa. It indicates that the choice of 40% soda residue and 60% OPC is ideal up to 20 days of curing age.
3.3 Strength prediction methods under different Sb/Sd conditions
In the previous section (section 3.2), the relationship between the unconfined compressive strength of solidified soil and the content of different factors was given. Regression analysis was carried out on the above data, and a calculation method was established to predict the unconfined compressive strength of solidified soil according to the content factors. Processing the data in Fig. 5, taking the test without Wp as an example, Fig. 7 shows the law of unconfined compressive strength increasing with Sb/Sd at different ages. Moreover, as the curing age increases from 1 to 90 days, the unconfined compressive strength also increases gradually as presented in Fig. 7. However, the unconfined compressive strength increased slowly when the curing age was from 1 to 3 days and from 58 to 90 days, and increased rapidly when it was from 3 to 28 days, with an average increase of 50.4%.
It can be seen from Fig. 7 that the non-lateral compressive strength gradually increases with the increase of Sb/Sd, showing a linear growth law, and the growth laws under different ages are basically the same. Then the non-lateral compressive strength q at a certain age (t) can be expressed as:
(1)
In formula (1), q is the value of non-lateral compressive strength, t is the age, and a and b are the fitting parameters.
After obtaining the influence relationship of strength with Sb/Sd, it is necessary to add age factor, so the concept of strength ratio R is introduced, namely
(2)
Type of qt for the unconfined compressive strength under each age, t is different age; d is the unconfined compressive strength of the specimen at a specific age, qd is the strength base. Taking the test data without Wp as an example, Fig. 8 shows the change rule of strength ratio and age under four different strength standards of 1d, 7d, 28d and 90d. It can be seen from the results in Fig. 8 that the strength ratio is approximately logarithmic with the age, which can be expressed as:
R = Cln(t)+D (3)
In which C and D are fitting constants. It can be seen from the fitting results in Fig. 8 that the unconfined compressive strength of 90 d age is obviously better in convergence and the error between the fitting curve and the data points is smaller, so the compressive strength of 90 d is selected as the compressive strength value, C= 0.45 and D = 0.11. According to formula (1), the fitting parameters A and B under different Wp contents are summarized as shown in Table 2. Through the regression analysis of the parameters in Table 2, the strength base can be correlated with Wp content, which can be expressed as:
(4)
Table 2
Fitting parameters of different Wp content conditions q90
Wp
|
A
|
B
|
0.0%
|
3.24
|
1.29
|
0.1%
|
3.10
|
1.24
|
0.2%
|
2.89
|
1.19
|
0.5%
|
2.91
|
1.13
|
2.5%
|
2.70
|
1.08
|
5.0%
|
2.49
|
1.03
|
The formula for predicting the strength of solidified soil under different Sb/Sd and different Wp conditions can be obtained by introducing formula (4) into formula (3), namely:
(5)
Therefore, Formula (5) is the prediction model of unconfined compressive strength of fixed-line soil at any curing age, which will provide more convenience for future in-depth research. Contessi et al., (2020) reported similarly, but did not make predictions and numerical simulations. In the view of Contessi et al. (2020), it is important to be able to predict the unconfined compressive strength in the future direction of S/S technology research.
3.4 Toxic Leaching Characteristics
Figure 9 shows the solidified soil TCLP and SPLP leachate with curing a age of 28 days under different Sb/Sd and Pb2+ concentration levels when the curing age is single OPC. The experimental test results revealed that that the leaching concentration of Pb2+ gradually decreased with the increase of Sb/Sd in the leachate of different extraction methods for Pb2+, meanwhile, the initial Pb2+ concentration level was the same. When the initial concentration of Pb2+ in contaminated soil (Wp) was 800, 5000 and 50,000 mg·kg-1, the average reduction of Pb2+ concentration in leachate was 17.5%, 34.3% and 39.2%, respectively. And with the increase of the initial Pb2+ concentration level of the soil, the leaching concentration of Pb2+ also increased gradually in the leachate of different extraction methods for Pb2+, when the same Sb/Sd ratio. The concentration of Pb2+ in the leachate increased on average by 7.2, 8.1, 5.7 and 4.3 times when the percentage of OPC added to the contaminated soil (Sb/Sd ratio) was 7.5%, 10%, 15% and 20%. It is noteworthy that when the initial concentration of lead ions in the soil is 50,000 mg·kg-1, the concentrations are all greater than 5 mg·L-1, which exceeds the allowable value of the Standard for Pollution Control of Hazardous Waste Landfill in China. But such high Pb2+ concentrations in contaminated soils are generally rarely seen in real life. It suggests that the higher Pb2+ leaching concentration in the experiments with a single OPC and therefore a composite model is needed to enhance the S/S process (Kundu et al., 2016). By comparing the experimental results of three different extractants, it is obvious that when the extractant is glacial acetic acid, the concentration of Pb2+ in the leaching solution of cement-cured soil is the lowest, when the extractant is deionized water the concentration of Pb2+ in the leaching solution is the highest, and the mixture of nitric acid and sulfuric acid is in the middle. The concentration of Pb2+ in the mixed solution of nitric acid and sulfuric acid is slightly higher than that of deionized water, which is consistent with the research results in reference (Sinegani et al., 2018; Ge et al., 2020; Zha et al., 2020b).
Figure 10 provides the variation of leachate Pb2+ concentration under different cement-soda residue complex ratio (Ss/Sb) conditions for TCLP with curing age of 28 days at Sb/Sb =20% and initial Pb2+ concentration (WP) is 50,000 mg·kg-1. It clearly demonstrated that the Pb2+ concentration in the leachate did not exceed 5 mg·L-1 in all treatment groups; which means that the cement-soda residue complex was more effective in the toxic percolation rate of the lead-contaminated soil compared to the single cement curing agent (Fig. 9c). Indirectly, it illustrated that the solidification and stabilization effect of cement-soda residue complex on lead-contaminated soil was satisfied with the Standard for Pollution Control of Hazardous Waste Landfill in China. Moreover, with the increase of Ss/Sb ratio from 10–40%, the average concentration of Pb2+ in the leachate gradually decreased from 4.16 to 1.87 mg·L-1. On the whole, the average concentration of Pb2+ in the leachate will gradually decrease by 74.2% for each 10% addition of soda residue. In addition, the results of the three extractants exhibited the same trend at different cement-soda residue complex ratio (Ss/Sb) conditions, i.e., glacial acetic acid was the smallest, deionized water was the largest, and the mixture of nitric acid and sulfuric acid was in the middle, with the concentration range not exceeding 0.7 mg·L-1. When the addition dosage of soda residue reached 40% of the total amount, the Pb2+ concentration decreased on average by 3.28 times compared to the single cement curing agent (Fig. 9c) under the conditions of three extractants, which implied a significant effect of soda residue on the immobilization of heavy metal Pb2+, especially when the initial Pb2+ concentration in the contaminated soil was higher. The adsorption performance for heavy metal Pb2+ ions was greater than that of the single cement control, under channel conditions. This was mainly attributed to the fact that the addition of soda residue increased the pH of the cured soil powder leachate, stimulating the chemical reaction of heavy metal ion exchange between cement, soda residue and soil, which in turn led to the replacement and consequent immobilization of Pb2+ (Zha et al., 2020a; 2020b; 2021).
3.5 Solidification mechanism
Based on the macroscopic test results in this work, the curing and leaching mechanism of Cement-Soda residue composites for solidified/stabilized heavy metal lead-contaminated soil was visualized and speculated by referring to the relevant literature (Zha et al., 2020a; 2020b; 2021; Wang et al. 2021a) and depending on the basic chemical theoretical knowledge. Firstly, when the cement-soda residue solidified soil encounters water, it will undergo hydration reaction and forming a lot of gel (Fig. 1-Fig. 2), which will solidify/stabilize lead ions into the interior of soil particles through the effects of encapsulation, precipitation, complexation and adsorption. Secondly, part of Pb2+ could replace the high-valent Ca2+, Mg2+, Al3+ and Fe3+ in the hydration products, and ion-exchange reaction may occur, thereby making the whole mixed system more stable (Fig. 3). During the curing process, cement hydration produces hydration products, such as hydrated calcium silicate (CSH), calcium aluminate hydrate (CAH), and Ca(OH)2, which combine the soil particles and increase the strength of the total soil (Li et al., 2019; Zha et al., 2021). Moreover, in the application of Cement-soda residue composites as curing agents, the soda residue could provide a stable alkaline environment in the S/S process (Fig. 4), and the surface of soda residue enriched with anions needs to adsorb a large number of cations to meet the charge balance. Furthermore, Ca(OH)2 in pure soda residue could complex with SiO2 and Al2O3 in clay particles to produce CSH and CAH as well. Specifically, the main components of soda residue are CaCO3 and CaO, complex and combine with reactive SiO2 in soil particles upon contact with water to produce calcium silicate complexes with large specific surface area, such as CaSiO3·CaCO3·Ca (OH)2·nH2O (CSH gel), which will increase the strength of unconfined compressive strength test early in the curing age (e.g., within 20 days in this study, Fig. 2). In our opinion, complex physicochemical reactions between cement-soda residue composites and soil particles produce a great deal of complexes and gel cements in the soil, which cause the soil samples to become honeycomb aggregates and enhance the strength of the samples (Zhang et al., 2017). The presence of lead ions leads to the reaction with cement to form Pb(OH)2 and lead white (Pb(OH)2·2PbCO3), which wrap around the unreacted curing agent particles and separate the binder from the pore water. In addition, soda residue contains many aluminas, magnesium hydroxide and sodium silicate, which will also generate aluminum silicate or magnesium silicate gel during the hydration process. Sodium silicate solution itself curing also provides certain strength for the S/S system later, and the addition of sodium silicate can slow down the rate of water absorption of soda residue-cement-water S/S system, increase the fluidity and prevent segregation (Ouhadi et al., 2021). In this study, the function of deionized water is to dissolve the cement and soda residue, facilitate the transfer between various anions and cations, provide chemical activation energy for hydration and participate in hydration reactions (e.g., H+, OH−). At a later stage, ionized water may be converted into binding water for the binder and provide an aqueous environment for the polymerization reaction (Fig. 1-Fig. 2). Therefore, the quantity of water directly affects the curing rate and strength magnitude of the crystalline body.
Additionally, Zha et al. (2020a) found that under the heat generated by cement hydration, the reaction between soda residue-cement-soil can produce calcium zeolite with a large specific surface area, which is known to be an efficient adsorbent with a large specific surface area. In summary, the main reaction equation is presumed to be as follows:
(1) In the first stage, the S/S reaction process produced hydrated calcium silicate gel (C-S-H). The process occurs on the surface where the soda residue and cement are in contact with the aqueous solution, uniformly distributed in the system (Fig. 1-Fig. 4). The reaction process is expressed as follows:
CaCO3+Ca(OH)2→CaCO3·Ca(OH)2↓ (6)
CaCO3+2CaO+SiO2+(n+1)H2O→CaSiO3·CaCO3·Ca(OH)2·nH2O↓ (7)
SiO2+Ca(OH)2+nH2O→CaO·SiO2·(n+1)H2O ↓ (8)
Al2O3+Ca(OH)2+nH2O→CaO·Al2O3·(n+1)H2O ↓ (9)
Na2SiO3 + CaCl2 + nH2O→CaSiO3·nH2O↓ + 2NaCl (10)
Na2SiO3 + Ca(OH)2 + nH2O→CaSiO3·nH2O↓ + 2NaOH (11)
Na2SiO3 + CaSO4 + nH2O→CaSiO3·nH2O↓ + Na2SO4 (12)
(2) In the second stage, the S/S reaction process produces silica-aluminate polymers (N-A-S-H, also known as CAH). The NaOH generated by the above reaction and the alkaline OH- of the soda residue itself act on the surface of the lime and are excited to form silica-aluminate polymers of different polymerization degrees (N-A-S-H) by a process of dissolution and polymerization (Sobiecka et al., 2014; Ge et al., 2020). The reaction rate of this process is closely related to the basicity of the system. The reaction process can be expressed as follows:
n(SiO2·Al2O3) + nSiO2 + 4nH2O (Na+)·(-Si-O-Al-O-Si-O)n (13)
(3) In the third stage, as the calcium-containing component of the soda residue reacts with sodium silicate solution to generate hydrated calcium silicate gel. The strength in the later stage is further improved by the cement being excited by alkali and generating a silica-aluminate polymer gel on its surface through the process of dissolution and re-polymerization, and the increase is large. At this time, the products in the system are C-S-H gel and N-A-S-H gel coexisting.
(4) In the fourth stage, appropriate changes in environmental conditions can promote the transfer of Na+, the formation of C-S-H gels and N-A-S-H gels. In addition, the self-curing phenomenon of sodium silicate solution at room temperature for 20 d is due to the hydrolysis of sodium silicate itself to produce silica gel, which also provides some strength for the later stage. The reaction equation is as follows:
Na2SiO3 +(m+1)H2O→SiO2(active) + mH2O + 2NaOH (14)
In the toxic leaching process (TCLP), the leaching mechanism of cement-soda residue to the solution is mainly the consumption of acid ions by alkaline substances. Firstly, the hydration of cement soil produces a large number of hydration products calcium hydroxide, which can consume part of the H+ in the leachate (Li et al., 2017). Secondly, CaCO3, the main component of soda residue, can react with H+ in the acid solution, thus weakening the erosion damage of the acid solution on the gel (Bao et al., 2016; Cao et al., 2018). In addition, under the heat generated by cement hydration, the reaction between soda residue-cement-soil can generate calcium zeolite, which has a large specific surface area and can adsorb H+ in the acid solution and buffer the damage of the acid solution to the curing system.
3.6 Future industrial practice significance and evaluation
There are also some examples of engineering practices here, which are highly meaningful for us to inspire in-depth applied research. Zha et al. (2021) concluded that cement and soda residue (CSR) has been proven to be an effective binder for treating heavy metal contaminated soils, and its durability is its most important property. Zha et al. (2021) investigated the leaching behavior of the consolidated/stabilized CSR under acid rain conditions. The leaching behavior of Zn-contaminated soils was investigated under acid rain conditions, and it was found that the UCS of the cured soil samples decreased and the permeability coefficient increased, while the Zn concentration in the filtrate always met the applicable standard Chinese National Environmental Quality Standard III (<1 mg⋅L−1). Wang et al. (2021a) reported that soda residue and cement were used as limy materials for synthesizing four clinker binders, and then investigated the effects of temperature and number of washes on chloride ions in soda-cement, showing that the fracture and crushing strengths of clinker binders synthesized from soda-cement increased from 4.3 to 26.9 MPa to 7.2 and 52.7 MPa at 30 d, respectively, and reusing soda residue could reduce pollution emission and the management cost of enterprises. Zha et al. (2020b) evaluated the UCS strength, toxic leaching and microstructure of cement/soda residue treated Cr3+ contaminated soil (initial concentration up to 10,000 mg·kg−1), Zha believed that the unconfined compressive strength (UCS) increased with increasing curing time, binder content and binder ratio, and the leached Cr3+ concentration decreased to a minimum of 1.93 mg·L−1. It is very similar to our results, but the average leaching concentration was lower than 1.93 mg·L−1 for Pb2+ initial concentration up to 50,000 mg·kg−1 in our study, which shows that the cement-soda residue composite curing agent is more effective in curing Pb2+ in soil.
This work combines the latest research progress of current cement solidification/stabilization soil remediation technology with the actual treatment of industrial soda residue materials, and based on the characteristics of cement and soda residue, a new method of cement soda residue solidification/stabilization of soil contaminated by heavy metal lead is formed by using soda residue instead of partial cement for soil reinforcement based on the traditional cement reinforcement, which not only meets the actual requirements of engineering, but also this method not only meets the practical requirements of the project, but also solves the problems of soda residue piling and environmental pollution, and solves the problem of lack of high-quality filler in soft soil areas (Li et al., 2014; 2015). It provides some guidance for the application in engineering and the amount of admixture. Therefore, this study has very great economic and environmental benefits and has a broad application prospect, which is of great value and significance for industrial upgrading and transformation to a resource-saving society. Although the remediation of heavy metal contaminated soil sites in China is later than that in traditional developed countries, many large scale remediation projects for heavy metal contaminated sites have emerged in recent years due to the increasing emphasis on heavy metal contamination in soils on the one hand (Xi et al., 2014; Yang et al., 2020b), and the increasing scarcity of land resources and the change in national thinking about urban development on the other. This detailed study is a good demonstration and reference for the subsequent implementation of such technologies.