Effect of Carbonation on the Leachability of Solidified/Stabilized Lead-Contaminated Expansive Soil

Lime, fly ash, and alkaline residue are used widely as effective binders to solidify/stabilize heavy metal-contaminated expansive soil. Carbonation, however, may influence the effectiveness of solidification/stabilization (S/S) by decomposing hydration products and decreasing pH, which would seriously damage the long-term durability of stabilized soils. ,is study focused on the variation of leaching characteristics of solidified/stabilized lead-contaminated expansive soils before and after accelerated carbonation under the leachant of pH 3 and 5 by the semidynamic leaching test. After semidynamic leaching, leaching indexes such as the effective diffusion coefficient (De), leachability index (Lx), and slope (rc) were used to reveal the ion leaching mechanism. ,e results indicated that the amount of Pb and Ca leached out under different pH conditions increased after carbonation, which confirmed that carbonate on solidified/stabilized lead (Pb) had a negative impact. Additionally, the De values of Pb and Ca varied in the range of 1.16E− 10 cm/s to 1.71E− 07 cm/s, which demonstrated that ion migration was low. ,e contaminated soil solidified by lime and AR could be used in “controlled utilization” as Lx was higher than 9, and the leaching process was controlled by a dissolution reaction according to the analysis of rc. Moreover, the strong acidic leachant (pH� 3) resulted in more ions leaching out and lower pH in leachate compared with a mildly acidic leachant. Finally, with literature and experimental results, we found that the main reason for the increase of lead ion filtration of the carbonation reduced the pH value of the matrix and made the hydration products denatured and decomposed.


Introduction
Industrial sites contaminated by lead (Pb) occur massively in many countries and seriously deteriorate the environmental quality and human health [1][2][3][4]. Expansive soil, a kind of special properties clay, contains a certain amount of hydrophilic minerals and has a specific structure, which makes lead-contaminated expansive soil more harmful than ordinary clay. e S/S technique has been proposed and widely accepted around the world. During this process, binders are mixed with the contaminated expansive soil to physically fix the harmful substances into a stable form, which chemically reduces the migration capacity in the soil and improves its engineering properties [2,4,5]. e cement-based binder was primitively regarded as a key binder in applications.
However, Lee et al. [6] stressed that cement could not effectively improve the strength of solidified lead-contaminated soil given high Pb 2+ concentrations. Furthermore, the solidification effect was limited significantly by lead concentration. Chen et al. [7] reported that when the lead concentration reached 7.5% (mass ratio), the strength of the sample solidified by cement was only about 0.7 MPa, which was much lower than the low-concentration lead-contaminated solidified soil (about 4.0 MPa). Although increasing the binder content could temporarily improve the S/S effect, the cost and production waste inevitably increased, and the unit efficiency ultimately was reduced [8].
Compared with cement-based binders, lime and AR could be a better choice. Research has indicated that lime could decrease the swelling potential as well as immobilize heavy metals when expansive soil is contaminated by lead or other heavy metals [9,10]. e swelling capacity of the expansive soil solidified by cement-lime improved significantly when 4%-6% wt lime content was guaranteed [11]. Sahoo and Pradhan [12] reported that the engineering properties, including the bearing capacity and the internal friction angle, strikingly increased when 8% wt lime was incorporated into the expansive soil. AR is the by-product generated by the alkaline manufacturing industry. A great deal of calcium compounds and a tremendous amount of fine particles are contained in AR, and this results in the adsorption of heavy metals and a decrease in the swelling potential of expansive soil. Some researchers [13][14][15] have suggested that the immobilizing heavy metal performance of AR is the result of its adsorption capacity. According to the research studies performed by Sun et al. [16], alkaline residue contains a large amount of calcium carbonate, which can be used as the skeleton of the modified soil and can form complexes with calcium hydroxide. erefore, the mechanical properties of soil solidified by alkaline residue will be significantly improved. And the author's earlier research studies [17,18] have also shown that the engineering performance of expansive soil treated with AR can be improved obviously.
e long-term performance of S/S-treated soil, however, may be considerably challenged when the restored contaminated sites are located in the near-surface strata because carbonation brings out damage. Carbonation, also named neutralization, is a common chemical reaction process occurred in the near-surface strata. It involves the reaction of hydration products contained in cementitious materials with carbon dioxide (CO 2 ) diffused into pores of concrete or solidified soil. When pH decreases, carbonation may contribute to the generation of calcium carbonate (CaCO 3 ) and the decomposition of hydration products, such as calcium silicate hydrate (CSH) and portlandite [19][20][21]. e five main prerequisite steps for the occurrence of carbonation are as follows [22]: (1) diffusion of gaseous CO 2 through the pores of the mortar, (2) dissolution of the CO 2 in the pore water, (3) dissolution of Ca(OH) 2 in the pore water, (4) solution reaction between Ca(OH) 2 and CO 2 , and (5) precipitation of solid CaCO 3 . e main reaction formulas involved are listed as follows [23][24][25]: Carbonation could affect the physical mechanics, chemical leachability, and microstructure of the matrix, and it also could decrease its pH [25]. According to research results [23,24,[26][27][28][29], carbonation significantly decreased the leaching of lead, copper, calcium, chromium, molybdenum, and arsenic under certain conditions. In contrast, Macias et al. [30] thought that there would be an increase in the leaching of chromium, and Gunning et al. [25] reported that carbonation would reduce the leaching of lead and barium but would increase antimony, chromium, arsenic, copper, and nickel. Lower leaching of lead, calcium, sodium, potassium, and fluoride after carbonation has been found, and in some cases, the leaching of lead, calcium, copper, arsenic, chromium, and molybdenum even increased [29,31]. e leachability of heavy metals depends on the carbonation method, carbonation degree, type of test sample, and pH of leachant, as Van Gerven et al. [32] reported. It remains unclear; however, how the long-term durability of S/S lead-contaminated expansive soil evolves when it undergoes carbonation, which may impede the S/S effect. In this study, the leaching characteristics of lime/ARsolidified/stabilized lead-contaminated expansive soil based on the semidynamic leaching tests were investigated, and the explanations for the change of the leaching mechanism as a result of carbonation were revealed as well.

Testing Materials.
We excavated the tested soil, typical Hefei expansive soil, from a construction site at a depth of 3-3.5 m in Hefei City, Anhui Province, which is located in the eastern part of China.
e basic physicochemical properties of tested clay are summarized in Table 1. e tested soil is a kind of weakly acidic clay soil with a free swelling ratio of 55%. According to previous research studies and X-ray powder diffraction (XRD) results conducted by the Bureau of Geology and Mineral Exploration of Anhui Province, the mineral content of montmorillonite, illite, kaolinite, and quartz was 15%-30%, 10%-22%, 2%-9%, and 30%-47%, respectively. e hydrophilic mineral contained in soil such as montmorillonite and illite was dominant, for which the characteristic of shrinkage and swelling had a noticeable effect on the structure of soil during the process of treatment.
e compaction curve of the tested soil and the curve of particle size distribution are shown in Figures 1 and 2. As can be seen, maximum dry density of 1.63 g/cm 3 and optimum water content of 24.05% can be determined, in accordance with ASTM D698-12e2 [38]. According to Figure 2, silt content (2-20 μm) of 59.40% and clay content (<2 μm) of 40% were clarified for the tested soil.
Lead, as one of the most common toxic heavy metals in contaminated sites worldwide, was selected as the target heavy metal in this study [40,41]. Lead nitrate (Pb(NO 3 ) 2 ) reagents (chemical analytical reagent) were selected as the source of contamination because the nitrate radical (NO 3− ) has the advantages of high solubility and low interference to the hydration process of binders [42,43]. And compared with phosphate or sulfate anions, nitrate anions have an insignificant effect on the engineering properties of clay soil [44].
On the basis of the previous research, the lime was selected as the binder, and the major chemical component was calcium oxide (CaO). Lime is not only a suitable way to modify expansive soil but also an effective and economical binder to amend heavy metal-contaminated soil [45,46]. e AR adopted in this work was from an ammonia alkali factory in Weifang, Shandong Province, which is located in the eastern part of China. e major chemical components determined by the X-ray fluorescence (XRF) technique are presented in Table 2. It is evident that CaO accounted for a dominant proportion of the AR, whereas MgO, SiO 2 , and Al 2 O 3 also accounted for a relatively large amount. As shown in Figure 3, the particle size of AR ranged from 1 to 10 μm, which confirmed the large specific surface area and strong adsorptive capacity of the AR adopted in this work. According to ASTM C1308 [47] and previous research [48], the acetic acid solution was chosen as the leachant in the semidynamic leaching test. In this study, this solution was prepared according to the specific amount of distilled water and acetic acid, and the leachant was adjusted to a specific pH of 3 and 5 to investigate the characteristics of leaching in the worst field condition of acidity.

Sampling.
In preparation of the specimens, according to the excellent stabilization efficiency reported by Sahoo et al. [12], Zha et al. [17], and Cheng et al. [49], the mass ratio of lime and AR to dry soil was set to 8% and 30%, respectively (denoted as L8 and AR30, respectively). In China, the common Pb 2+ concentration of industrial contaminated sites is approximately 5000 mg/kg according to Li et al. [50] and Liao et al. [51]. erefore, the Pb 2+ concentration of the soils was designed as 5000 mg/kg in this study (denoted as Pb0.5).
Soil samples are designated as LmPbn to denote a specimen with binders of m% and Pb concentration of n%. And ARmPbn naming rules are as same to LmPbn. First, the dry soil was contaminated by a solution that contained a specific amount of lead nitrate, and then, the contaminated soil was set for a few days. After oven-drying at 105°C for 24 h, the soil and binders were ground into powders to filter them through 2 mm and 0.5 mm sieves, respectively. e binders (lime and AR) were then mixed evenly with the Pbcontaminated soil at the designed proportion. After that, the mixtures were put into a compaction mold and were statically compacted into target cylindrical specimens with a dimension of 100 mm in height and 50 mm in diameter, and a dry density equaled 95% of the maximum dry density of 1.63 mg/cm 3 . en, the prepared specimens were extruded from the mold, sealed with a plastic bag, and cured under the standard condition with a temperature of 20°C ± 1°C and relative humidity of 95% for 28 days.

Carbonation
Procedure. When the specimens are cured to designated time, half of them were taken out and put into the carbonation chamber, which is denoted as the C specimen. e carbonation conditions were designated with a temperature of 20°C ± 3°C, carbon dioxide concentration of 20% ± 3%, and relative humidity of 70% ± 4%, according to the Standard for Test Methods of Long-Term Performance and Durability of Ordinary Concrete GB/T50082-2009. e duration of carbonation was 3 days to achieve the thoroughgoing carbonation of the specimen. As the control group, the remaining half of the specimens were kept in the original curing chamber for the same time, which is denoted as the S specimen.   Water content (%)

Leaching Test.
e semidynamic leaching test, in which a sample is exposed to the leachant renewed periodically, is usually employed to elucidate the dominant leaching mechanism from an S/S specimen [5]. e semidynamic leaching tests were conducted according to the ASTM C1308 [47]. e leaching specimen with a diameter-to-height ratio of 1 : 2 was used. And the leachant volume was determined based on the specimen surface area and an estimate of the leach rate. On the basis of the preliminary experiment, large volumes of the leachant, such as those that are 10 times the surface area of the specimen (in cm 2 ), can make analysis challenging, even for major constituents of the specimen, and create unnecessary waste disposal costs. Under these circumstances, a higher specimen surface area-to-leachant volume ratio may be used. e leach rates of some waste from materials are low enough that a specimen surface area-toleachant volume ratio higher than 0.1 cm −1 could be used to generate measurable solution concentrations. us, a specimen surface area-to-leachant volume ratio of 0.5 cm −1 , namely, 1000 ml acetic acid solution, is appropriate [47].
A plastic and transparent sink was used as a leaching container, and the specimen was placed in the sink after cleaning the surface dust. en, a prepared 1000 ml acetic acid solution with a specific pH was poured into the sink. e test duration was 11 days, and the environmental temperature in the lab was 20°C ± 3°C. To begin with, the leachant was replaced periodically at intervals of 2, 5, 17, and 24 h. After 24 hours, the leachant was replaced every other day for 10 days. At each replacement of the leachant, the leachant was filtered through a 0.02 μm membrane, and the pH was measured using a ZDJ-4A pH meter. en, 50 ml of this leachate was acidified with 1 ml of concentrated HNO 3 for analytical measurement using an AA800 Flame Atomic Absorption Spectrometer. To ensure accuracy, triplicate measurements were made for the concentration of lead and calcium.
According to ASTM C1308 [47] and Du et al. [2], the cumulative fraction leached (CFL) can be calculated by using the following equation: where A t � the cumulative mass of the heavy metal ion that leach out (mg), A 0 � the total initial amount contaminated in the specimen (mg), c i � the measured concentration of the leached heavy metal ion in the leachate at i th test interval (mg/L), and V L � volume of the leachant (L). Effective diffusion coefficient (D e , cm 2 /s) can be calculated using the following equation: where t � time (s), V � volume of the specimen (cm 3 ), and S � surface area of the specimen (cm 2 ). According to Environment Canada [52], the leachability index (Lx) can be a performance criterion for the utilization and disposal of S/S waste as given in the following equation: where n is the total number of individual leaching periods and m is the number of the particular leaching period. When Lx was higher than 9, the S/S waste could be used in "controlled utilization," which meant that S/S waste was acceptable for specific utilization, such as quarry rehabilitation, lagoon closure, and road-base material. If Lx was higher than 8, then it could be disposed of as segregate or in sanitary landfills, but when the value was lower than 8, it was not suitable for disposal [53]. According to EA NEN 7375 [52], the leaching mechanism can be determined by the following equations: where E i � heavy metal ion mass leached per area (mg/cm 2 ) in i th leaching interval, c i � ion concentration in leachate (μg/ L), f � 1000 μg/mg, A � surface area of the test piece (cm 2 ), ε n � cumulative leaching mass per area (mg/cm 2 ), and t i � total time after i th leaching interval (s). en, logε n -logt was plotted according to equations (5) and (6) which revealed the leaching mechanism using the slope (r c ) of the linear fitting. When r c was less than 0.35, the leaching mechanism was surface wash-off; when r c was between 0.35 and 0.65, diffusion controlled the leaching  mechanism; when r c was higher than 0.65, dissolution was the dominant leaching mechanism.

3.1.
e pH of Leachate. e leachate pH of specimen L8Pb0.5 and AR30Pb0.5 with a leaching time (d) is shown in Figure 4. As depicted in the figures, the pH of leachate increased on the initial two days and reached the peak on the second day. en, it decreased slightly but was still higher than the initial leachant pH. Comparing the leachate pH of the S (solidified) and C (carbonated) specimen, it can be found that the leachate pH of the S specimen was higher than that of the C specimen. In addition, the leachate pH of limesolidified soil was higher than that of AR-solidified soil. e specimens of L8Pb0.5 and AR30Pb0.5 are alkaline and contain hydration products such as calcium hydroxide (CH) and hydrated calcium silicate (CSH), which is alkaline and has an acid-buffering capacity because the hydrogen ions (H + ) are consumed by hydroxyl ions (OH − ) released from these alkaline hydration products [54,55]. us, all of the leachate pHs are higher than the leachant pHs. During the initial two days, large amounts of hydroxyl ions that released from the surface of the specimen reacted with hydrogen ions in the leachant, which led to an increase in the pH. en, the pH decreased because of the rate of the hydroxyl ions releasing and the reaction between hydroxyl ions and hydrogen ions reaching equilibrium. Carbonation is a process in which hydration products such as CH and CSH transform into carbonates as a result of their reaction with carbon dioxide (CO 2 ) and following a decrease of pH [56]. As a result, the carbonated specimen was much more vulnerable to the attack of the strong acidic leachant because of the few hydroxyl ions released from the hydration products, which contributed to a lowering of the acid-buffering capacity. In addition, because of the buffering capacity of acetic acid, the difference of the pH in the leachate of specimens S and C was insignificant. 2+ in Leachate. Figure 5 shows the evolution of the incremental concentration of Pb 2+ in leachate at time (d). Figures 5(a), 5(b), and 5(d) share the same variation trend, in which the incremental concentration of Pb 2+ increased in the first two days, then decreased in the following days, and reached the peak on the second day. In contrast, specimen AR30Pb0.5-C had an irregular evolution. e incremental Pb 2+ concentration of the C specimen on each day was higher than that of the S specimen, which is shown clearly in graphs. e results indicated that carbonation had an adverse effect on leachability. e incremental concentrations of Pb 2+ in the strong acidic leachant were about 16%-40% higher than that in the weak acidic leachant. Moreover, the evolution tendency of pH and incremental Pb 2+ concentration shared the same pattern, which meant that these two indexes had some connections, given that the evolution of the leachate pH was associated with the dissolving of hydration products and alkaline lead-precipitate compounds. When the acidic leachant attacked the alkaline matrix, the hydration products and the alkaline lead-precipitate compounds dissolved. erefore, the incremental concentrations of Pb 2+ had a parallel effect on the evolution of leachate pH.

Concentration of Ca 2+ in Leachate.
e evolution of Ca 2+ incremental concentrations was similar to that of Pb 2+ , as presented in Figure 6. e Ca 2+ incremental concentrations of L8Pb0-C specimen were all 20%-55% higher than those of the S specimen during the condition of different pH conditions through the entire leaching time, which indicated that carbonation had an adverse effect on the leachability of Ca 2+ . In addition, the leached Ca 2+ concentration with the strong acidic leachant was nearly two times higher than that with the mildly acidic leachant, which meant that the pH also was a major factor influencing the leachability of Ca 2+ . For the leached Ca 2+ concentration evolution of the AR specimen, no considerable difference existed between specimens S and C. Because the AR contained a large amount of CaCO 3 , the effect of carbonation was not notable on the ARsolidified soil. Figure 7 presents the evolution of cumulative fraction leached (CFL) of Pb 2+ with time (s 1/2 ) for the S and C specimens of lime and AR-solidified soil under different pH conditions. e CFL-t 1/2 curves can be characterized approximately by a linear relationship. It was obvious that the Pb 2+ CFL of the C specimen was higher than that of the S specimen, which indicated that carbonation had a disadvantageous effect on the immobilization of lead. Under the strongly acidic leachant condition, the Pb 2+ CFL was two times higher than that of the mildly acidic leachant condition. e relationship between the cumulative fraction leached of Ca 2+ and time (s 1/2 ) is plotted in Figure 8. Also, the trend of Ca 2+ cumulative fraction leached was similar to that of Pb 2+ , which was depicted by the linear relationship. For the lime-solidified specimen, the Ca 2+ CFL of the C specimen was much higher than that of the S specimen. No significant change occurred, however, in Ca 2+ CFL of ARsolidified soil after carbonation. e Ca 2+ CFL of the lime specimen was much higher than that of the AR specimen, which indicated that the lime-solidified soil was easily affected by carbonation. A notable difference between lime and AR-solidified soil could be attributed to the diversity of components. Lime consists of a single chemical composition CaO, and its hydration product is Ca(OH) 2 . When Ca(OH) 2 mixes with soil, a pozzolanic reaction occurs and generates CSH. Both the CSH and Ca(OH) 2 reacted easily with CO 2 , which increased the Ca 2+ leaching. e AR had a complex chemical composition, in particular, the large proportion of CaCO 3 , which contributed to a lower carbonated influence on the Ca 2+ leaching. Obviously, the CFL of Ca 2+ under the pH of 3 had a nearly onefold increase compared with that under the pH of 5, which indicated that the strongly acidic leachant had a tremendous effect on the solidified soil.

Advances in Civil Engineering
As shown in Table 3, for Pb 2+ and Ca 2+ ions under different pH conditions and curing methods, the computed values of some of the related indexes revealed the mechanism of ion leaching. Under the same pH condition, carbonation had a notable effect on the indexes of D e , Lx, and r c based on an analysis of the experimental data.
It is evident that the values of D e for Pb 2+ and Ca 2+ varied in the range of 1.16E − 10 to 1.71E − 7, which was much higher than the D e value reported in references [2,53,57] and increased by one order of magnitude for most of the lime specimens after being carbonated. According to Malviya and Chaudhary [58], the different D e values corresponded to the specific degree of mobility. When D e was less than 3E − 13 m 2 /s (3E − 9 cm 2 /s), the degree of mobility was low. When D e was between 3E − 13 and 1E − 11 m 2 /s (3E − 9 and 1E − 7 cm 2 /s), the degree of mobility was average. Figure 9 shows the D e values of Pb 2+ . It is obvious that all of the values were lower than 3E − 9 cm 2 /s regardless of the pH condition or curing method, so the mobility of Pb 2+ could be considered to be low. Furthermore, the values of D e of C specimens were nearly one order of magnitude higher than that of the S specimen at the pH of 3, but the values were two times higher than the S specimen at the pH of 5. is finding indicated that carbonation improved the mobility of Pb 2+ , and the strongly acidic leachant improved the mobility of Pb 2+ compared with the mildly acidic condition. As shown in Figure 10, the values of D e of Ca 2+ were higher than 3E − 9 cm 2 /s but varied in the range from 3E − 9 to 3E − 7 cm 2 /s, which meant that the mobility of Ca 2+ was average. Moreover, the D e value of the lime specimen increased by one order of magnitude after carbonation, which verified the degradation of carbonation on lime-solidified heavy metal-contaminated soil. No significant change occurred, however, in the AR specimen, and D e of the lime specimen was much higher than that of the AR specimen at nearly one order magnitude. e reason for this difference between the lime specimen and AR specimen was attributed to the difference of chemical compositions in these two binders, which resulted in a much weaker carbonated influence on the mobility of the Ca 2+ in the AR specimen. e leachability index (Lx) evaluated the S/S effect and revealed a slight drop after carbonation, which meant that carbonation had an adverse effect on the S/S of heavy metal contaminants. Most Lx values were over 9 except for the 8.99 of the carbonated lime specimen at a pH of 3 and 8.87 of the AR specimen at a pH of 3, which indicated that the amending effect met the Environment Canada standard [59]. erefore, this waste could be used for "controlled utilization," which indicated that the S/S waste was acceptable for specific utilization, such as quarry rehabilitation, lagoon closure, and road-base material [57]. r c is an index that revealed the leaching rate and mechanism of heavy metal ions. All of the r c values of Pb 2+ listed in Table 3 were more than 0.65 under the pHs of 3 and 5, and they decreased slightly after carbonation. In strong acidic leachant, the r c values of both S and C specimen were 0.1 higher than the mildly acidic leachant. According to tank leaching experiments, notably, European standard EA NEN 7375 [52], when r c was more than 0.65, the dissolution mechanism controlled the leaching of Pb 2+ . According to De Groot and Van der Sloot [60], at a slope of approximately 1, the dissolution mechanism controlled the leaching of Pb 2+ . In this study, the dissolution mechanism controlled the leaching of Pb 2+ for lime and AR specimens, regardless of the pH and curing method.

Discussion
It is evident that accelerated carbonation had an adverse effect on the long-term performance of S/S lead-contaminated expansive soil. As shown in previous studies [61,62], the pH of a matrix decreased after being carbonated. Past results have indicated that the pH of a matrix decreased from nearly 12 before carbonation to about 6.5 after carbonation, which was attributed to the reaction of hydration products such as CSH and Ca(OH) 2 with CO 2 . is dramatic decrease in pH could contribute to a significant change in the chemical properties of solidified lead-contaminated expansive soil, in particular for the pH of leachate and the leachability of Pb 2+ and Ca 2+ . Some researchers have focused on the immobilization mechanism of Pb 2+ and revealed that the pH of a matrix could be a controlling factor on the immobilization of Pb 2+ . According to Hale et al. [54] and Moon and Dermatas [57], in an alkaline matrix (normally pH > 10), in which hydration products were stable, the formation of insoluble lead hydroxide precipitates on the surface of CSH was a controlling mechanism of lead immobilization. Physical encapsulation was achieved by creating a solidified monolith, and chemical inclusion was achieved by incorporating lead in binder hydration products, such as ettringite by isomorphous substitution. is reaction, however, could take place only on the surface of CSH instead of inside CSH, and the formation of lead hydroxide would precipitate afterward [63][64][65]. Moreover, as reported by Jing et al. [65], both leaching tests and model simulations indicated that the leaching behavior of lead could be divided into three stages based on the leachate pH: (1) a high alkalinity leaching stage at pH > 12, in which lead formed soluble hydroxide anion complexes and leached out; (2) a neutral-to-alkaline immobilization stage in the pH range of 6-12, which was characterized by low Pb leachability caused by adsorption and precipitation; and (3) an acid leaching stage with pH < 6, in which the acid neutralizing capacity (ANC) of the S/S materials was consumed completely, and therefore, free lead ion leached out. Furthermore, Moulin et al. [66] reported that lead immobilization was effective through the formation Si-O-Pb bonds, which were barely attacked by acidity and could stabilize. As reported by Moon and Dermatas [57], lead immobilization in quick lime-fly ash-treated soils could be achieved effectively through the formation of lead silicate (Pb 2 SiO 4 ). Palomo and Palacios [67] confirmed that a different lead silicate (Pb 3 SiO 5 ) controlled lead immobilization in lead-contaminated fly ash. In these studies, the Advances in Civil Engineering concentration of the leached lead could be controlled within a secure level, although the carbonated specimen increased slightly.
As shown in Figure 11 (from Jing et al. [65]), although the soil was treated with different binders and tested with different leaching protocols, the lead concentrations in the leachate followed a similar concentration versus the pH pattern. e experimental and model calculation results indicated that the lead leachability was controlled mainly by adsorption at a pH < 9 and precipitation at a pH > 9, and it was obvious that the leaching behavior of lead was pHdependent. According to Van Gerven et al. [32], however, carbonation could lower the pH ultimately to 8.3 because of the reaction of portlandite and CSH turning into calcite. Lead is an amphoteric metal that has minimum solubility between pH 8 and 10. e most significant effect caused by carbonation is the decrease of pH owing to the reaction between CO 2 and hydration products, which transformed the alkaline matrix into acidic. Consequently, the main hydration products did not stabilize: portlandite developed into calcite, and CSH decomposed into calcite and silica gel. e silica had high permeability and was adverse to the immobilization of Pb 2+ [21,68]. With the decomposition of hydration products and a decrease in pH, the desorption of lead hydroxide precipitates from the surface of hydration products began, and the lead hydroxide transformed into the lead carbonate and free state of lead [29]. As a result, the lead immobilization mechanism of precipitation changed. According to Hale et al. [54], the precipitates of cerussite and hydrocerussite formed at a low pH. As reported by Van Gerven et al. [32], the stronger carbonation such as supercritical carbonation   turned carbonate into bicarbonate, which was more soluble than the carbonate. erefore, the concentration of Pb 2+ that leached increased compared with the noncarbonated sample. e leachability of Ca 2+ shared the same pattern as Pb 2+ . With a decrease in the pH caused by carbonation, the Ca compounds became unstable, which increased the concentration that leached. In summary, the decrease of pH caused by carbonation and strong acidic leachant enhanced the leachability of Pb 2+ and Ca 2+ .

Conclusion
is study investigated the effect of accelerated carbonation on the leachability of S/S lead-contaminated expansive soil through a semidynamic leaching test with different pH leachant. e pH of leachate, incremental leached concentration, and cumulative leached fraction of Pb 2+ and Ca 2+ were manifested to identify the adverse influence of carbonation on S/S. Meantime, the variation of indexes such as D e , Lx, and r c was calculated to evaluate the S/S effect and revealed the leaching mechanism of Pb 2+ and Ca 2+ . e following conclusions can be drawn: (1) e accelerated carbonation could result in a decrease in the pH of a solidified matrix, which decreased from about 12 to 6.5 as a result of the decomposition of the alkaline hydration products because of the CO 2 diffusion into the specimen. (2) Under different pH conditions, the pH of the leachate increased in the first two days and then decreased until it keeps stable. e leachate pH of the S specimen was higher than that of the C specimen because of the ANC of the alkaline hydration products. In other words, carbonation decreased the ANC. e increase of the S specimen's leachate pH was insignificant because of the buffering capacity of acetic acid.
(3) e incremental concentration of Pb 2+ and Ca 2+ leached followed the same pattern as the evolution of pH, and a linear correlation was found between the cumulative fraction leached of Pb 2+ and Ca 2+ with time (s 1/2 ). It was conspicuous that the accelerated carbonation contributed to the increase in the amount of Pb 2+ and Ca 2+ leached. As a result, the increase in D e and the decrease in Lx were evident. Moreover, the r c values, which were higher than 0.65, revealed that the dissolution mechanism controlled ion leaching. (4) Strong acidic leachant (pH � 3) resulted in a lower pH of leachate and significantly increased the leaching concentration of Pb 2+ and Ca 2+ compared with the mildly acidic condition (pH � 5), which indicated that the strong acidic condition had a dramatic influence on the leaching of Pb 2+ and Ca 2+ .

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest. Advances in Civil Engineering 11