1. Introduction
Over the past 30 decades, the rapid development of industry and agriculture have led to soils contaminated by heavy metals worldwide [
1,
2,
3,
4,
5,
6]. An investigation performed by Wei and Yang [
7] found that approximately 65% of cities had high or extremely high levels of contamination with heavy metals in urban soils. Likewise, Fan et al. [
8] reported that in some contaminated sites, the heavy metal content reached as high as 50~7000 mg·kg
−1, in particular, the lead content in some sites was as high as 424,000 mg·kg
−1, and the strontium content was as high as 1,700,000 mg·kg
−1. These reports demonstrated that heavy metal pollution has become the key factor in soil pollution in China.
If heavy metal contaminated soils remain untreated over time, heavy metal contaminants likely will percolate down to groundwater or discharge to rivers [
9]. As such, heavy metals could enter health systems through the food chain [
10] due to accumulation in plants, a process relatively unaffected by beneficial microorganism action [
11], which might result in adverse effects on the environment [
12] and human health [
13,
14]. Such contamination can also affect the basic physical properties and engineering mechanical properties of the foundation soil, such as the void ratio, compressibility, shear strength, and bearing capacity, etc. [
10,
15,
16,
17,
18]. Therefore, the effective treatment of heavy metal contaminated soils is urgently needed in terms of protecting the environment, improving the engineering properties of the foundation soil, and realizing the reuse of contaminated industrial sites [
19].
Among the existing remedial measures for heavy metal contaminated soils, the solidification/stabilization (S/S) technique stands out as being low-risk, relatively cost-effective, simple to operate, and widely applicable to engineering, as well as increasing comprehensive soil strength, and offering good resistance to biodegradation [
20,
21].
S/S is a remediation technology that reduces the toxicity of pollutants, by using physical or chemical methods to either immobilize harmful pollutants contained in the soil or to convert such pollutants into chemically inactive forms, preventing their migration and diffusion into the environment [
22,
23]. Solidification is defined as a process in which materials are added to waste to come into stable solids, while stabilization, on the other hand, refers to a chemical remediation process that uses chemical reaction to reduce the solubility and migration of pollutants in soils or converts them into chemically inert substances, reducing the hazards level of such wastes [
24,
25].
Du et al. [
26] confirmed that the main mechanisms of S/S technology are the chemical interaction of hydration reaction products (such as calcium silicate hydrate, CSH) with pollutants or their adsorption, which physically retains contaminants through the surfaces of various hydrates. These products are closely related to the binders [
20,
27], mainly involving cement, slag, fly ash, quicklime, modified clay (such as organic clay), lime, and some waste [
27,
28,
29]. For example, Wang et al. [
30] found that when the dosage of magnesium phosphate cement added to the soils increased from 30% to 70%, the leaching amount of lead seen in subsequent leaching was significantly reduced. Wang et al. [
31] further proved that an increase in the dosage of binders caused the integrity of a GMC (GGBS-MgO-CaO) mixture to be improved after several drying and wetting cycles. However, it remains inconclusive whether the engineering characteristics of the solidified contaminated soil will continue to improve if the dosage of the binder increases further.
On the other hand, researches, done by Aldaood et al. [
32], Du et al. [
33], Li et al. [
34], Zaimoglu [
35], and Xu et al. [
36], showed that extreme environmental changes, such as freeze–thaw cycles, high-salt groundwater, acid rain infiltration, or dry-wet alternation, could result in changes in the properties of solidified contaminated soils and even cause the S/S to fail. Regions in China subjected to freezing are widely distributed: For example, permafrost regions accounts for approximately 21.5% of the total land area, seasonal frozen soil regions about 53.5%, transient frozen soil regions approximately 23.9%, and frozen soil-free regions only 1.1% according to the report by Xia [
37]. Apparently, freeze–thaw alternation is an important abiotic force that affects the soil environment in China.
Under the action of freeze–thaw cycles, changes in liquid water cause changes in the three-phase composition ratio of the soil as well as changes in the size and stability of soil aggregates, leading to increases in permeability and decreases in soil stability. In addition, freezing and thawing causes aggregates to release a large amount of inorganic substances, which enhances the organic mineralization of soil, adsorption, and desorption of organics. The freezing and thawing also affect morphological transformation, microbial activity, and the free energy stored in the soil. These changes in the physical and chemical properties of the soil inevitably affect the occurrence, form, and migration of heavy metal pollutants, adsorbed within the soil and the particles of the solidifying agent or colloidal surface [
38,
39,
40], leading to changes in the structure and the physical and mechanical properties of solidified polluted soils. This causes the engineering mechanical properties of contaminated soil in such regions to differ from those seen in non-freeze–thaw regions and thus affects the reconstruction of sites abandoned due to pollution [
18,
41]. For instance, Roustaei et al. [
42] reported that the undrained triaxial compressive strength of both unreinforced and reinforced soil decreased as the number of freeze–thaw cycles increased. According to Wang et al. [
43], the dynamic modulus decreased significantly, and the damping ratio increased with an increase in the number of freeze–thaw cycles. However, there are few studies on how the secant modulus (
) and shear index (
and
) of solidified contaminated soil change when this is subjected to freeze–thaw cycles.
Based on the above evidences, taking the cement/quicklime/fly ash commonly used in soil S/S into consideration [
44], the aim of the present study was to systematically investigate the effects of the freeze–thaw cycle and binder dosage on the engineering properties of solidified/stabilized lead-contaminated soils remediated with cement/quicklime/fly ash alone or some mixture of these additives. For comparison purposes, the engineering properties of lead-contaminated soils without solidified/stabilized were also tested. A series of experiments, including unconfined compression strength (UCS) tests, direct shear tests, and permeability tests, were performed on soil samples subjected to freeze–thaw cycles for 0, 3, 7, or 14 days to investigate the effect of these freeze–thaw cycles and various binder dosages (0%, 2.5%, and 5%) on the values of UCS secant modulus (
), internal friction angle (
), cohesion (
), and permeability coefficient (
) in compound solidified/stabilized lead-contaminated soils. And combining with the direct trend chart of influencing factors, the dosage of binder in the solidified lead-contaminated soil under the freeze–thaw cycle, which could make engineering properties (UCS,
,
,
and
) better, was further studied.
4. Conclusions
In this work, unconfined compressive strength tests, direct shear tests, and permeability tests were performed to investigate the values of UCS, , , and in compound solidified/stabilized lead-contaminated soils under various freeze–thaw cycles and different binder dosages (0%, 2.5%, 5%). By adding an intuitive trend graph method to examine the influencing factors, this article further explored the issue of the best binder dosage for improving the properties of solidified/stabilized lead-contaminated soil under the action of short-term freeze–thaw cycles. The main conclusions are as follows:
Generally, the unconfined compressive strength (UCS), the secant modulus (), and the shear index ( and ) increased with increased dosages of binders, which confirmed the remediation effect of all tested binders on lead-contaminated soil.
The permeability coefficient () increased with the increase in binder to a certain extent, which suggesting that it is incorrect to think that the higher the dosage of binder, the better the curing effect; there appears to be a critical dosage of added binder for contaminated soils with certain heavy metal content, and excessive cementation of contaminated soil caused by adding too much binder can result in loss of strength and an increase in permeability.
The freeze–thaw cycle had an adverse effect on the engineering characteristics of solidified lead-contaminated soils, shown by decreases in UCS, , , and values and value increases with increases in freeze–thaw cycle time. For the solidified lead-contaminated soils with higher binder dosages, this degradation effect was even more obvious.
However, as the solidification/stabilization process was still in process, further reinforcement of the contaminated soil by the products of hydration reactions and adsorption sedimentation appeared to offset the deterioration caused by the freeze–thaw cycles, so that degradation was not severe in the early stage of the freeze–thaw cycle, and UCS, , , and values appeared to increase.
As excessive solidification and over cementation of the solidified contaminated soil was caused by an excess of binder, where the freeze–thaw cycle time was relatively short, the damage to the structure by the freeze–thaw cycle actually caused the solidified contaminated soil to cease being over cemented; similarly, the value consistently decreased with the increase of freeze–thaw cycle time.
Under the long term action of freeze–thaw cycles, adding 5% cement and 5% quicklime could show the better results for the UCS and shear index in lead-contaminated soils (1% lead), while the value of was best when the addition dosage of fly ash was 5%. However, to avoid excessive cementation of solidified contaminated soil, 2.5% fly ash and 2.5% cement/quicklime were sufficient to improve the permeability and of lead-contaminated soils to an excellent condition, respectively.