stabilized zinc contaminated soil subjected to freezing–thawing cycles

Journal Pre-proof Strength and microstructure characteristics of cement-soda residue solidified/stabilized zinc contaminated soil subjected to freezing–thawing cycles

Jingjing Liu, Fusheng Zha, Xu Long, Bo Kang, Chengbin Yang, Qi Feng, Wei Zhang, Jiwen Zhang PII:

S0165-232X(19)30595-6

DOI:

https://doi.org/10.1016/j.coldregions.2020.102992

Reference:

COLTEC 102992

To appear in:

Cold Regions Science and Technology

Received date:

25 September 2019

Revised date:

3 December 2019

Accepted date:

15 January 2020

Please cite this article as: J. Liu, F. Zha, X. Long, et al., Strength and microstructure characteristics of cement-soda residue solidified/stabilized zinc contaminated soil subjected to freezing–thawing cycles, Cold Regions Science and Technology(2019), https://doi.org/10.1016/j.coldregions.2020.102992

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© 2019 Published by Elsevier.

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Strength and microstructure characteristics of cement-soda residue solidified/stabilized zinc contaminated soil subjected to freezing-thawing cycles

Jingjing LIUa, Fusheng ZHAa*, Long XUa, Bo KANGa, Chengbin YANGa, Qi FENGa,

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School of Resource and Environmental Engineering, Hefei University of Technology,

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a

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Wei ZHANGb, Jiwen ZHANGb

Shaanxi Key Laboratory for the Property and Treatment of Special Soil and Rock,

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Corresponding author

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Xi’an Shanxi, 710043, China

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b

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Hefei, 230009,China

*Prof. Fusheng ZHA

School of Resource and Environmental Engineering, Hefei University of Technology No.193, Tunxi Road, Hefei, 230009 P.R. China 1

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Email: [email protected]

Tel: +86 551 62901523 Mobile: +86 13696533099 Fax: +86 551 62901524

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Abstract: The remediation efficiency of the solidification/stabilization (s/s) method on soil contaminated with heavy metals may deteriorate when the treated soil is exposed to freezing-thawing (F-T) cycles. In this study, a series of laboratory tests were conducted to investigate the strength and microstructural characteristics of Zn-contaminated soil treated by cement-soda residue under cyclical F-T conditions.

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Test results showed that the stress-strain curves were able to be characterized by a strain softening model and the failure mode could be classified as brittle failure before

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subjecting to F-T cycles. After exposure to the F-T cycles, the stress-strain curves

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gradually began to resemble the hardening model, and the compressive strength

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decreased significantly. Unconfined compressive strength (UCS) of the specimens

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obtained the maximum value at Zn2+ concentration of 2000 mg/kg, regardless of their

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exposure to the F-T cycles; sample Zn0.2 was the most vulnerable. After being subjected to F-T cycles, micro-cracks formed in the specimens. These cracks would

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be enlarged by the continuous exposure to F-T cycles, which resulted in the reduction of strength. F-T cycles could significantly damage the crystalline structures of cement hydrates; the damage could also contribute to the deterioration of soil structure. Microstructural analysis results showed that the size of the samples’ micro-pores decreased due to the migration of pore water during freezing, while the size of larger pores increased due to ice lens formation and expansion. Keywords: Solidification/stabilization; Zn-contaminated soil; freezing-thawing cycles; unconfined compressive strength; microstructure characteristics

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1 Introduction In remediation of contaminated sites, it has been agreed upon that the engineering properties of soil contaminated with heavy metals can be effectively improved in cement-based solidification/stabilization methods, to a degree such that they can be reused as the subgrade material in road construction (Environment

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Canada, 1991; Wei et al., 2011; Saeed et al., 2015; Goodarzi and Movahedrad, 2017; Liu et al., 2019). However, the engineering performance of the treated soil tends to be

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deteriorated due to the long-term exposure to the externally complicated and

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changeable environment (Kogbara, 2014; Wang et al., 2014; Wang et al., 2016).

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Li et al. (2014) observed an obvious increase in cumulative mass loss when

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conducting cyclic drying-wetting test on the cement stabilized Pb-contaminated soil,

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which intended to induce a reduction in soil strength. Alam et al. (2019) reported that the unconfined compressive strength (UCS) of the ground granulated blast furnance

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slag (GGBS) treated red mud decreased attributed to the drying-wetting cycles, although the addition of GGBS can improve the UCS. Cao et al. (2013) and Zha et al. (2013) obtained the similar results with respect to both cement and fly ash treated heavy metal contaminated soils. Pandey et al. (2012) stated that carbonation would reduce the compressive strength of fly ash-geopolymer treated synthetic metal wastes. However, Du et al. (2016) indicated that the UCS of Pb and Zn contaminated soils stabilized with magnesium potassium phosphate cements (MPC) was increased by carbonation, which was predominantly attributed to the formation of MgCO3·3H2O. Besides, inorganic salts (such as chloride and sulfate) attack also have obviously

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detrimental impacts on the behaviors of cement-based matrix (Zhang et al., 2014; Liu et al., 2018; Zhang et al., 2019). To sum up, it can be concluded that the long-term performance of the stabilized contaminated soils based on cementitious materials is likely to be susceptible to the external environment. In China, the frozen soil and seasonal frozen soil are extensively distributed,

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which accounts for 77 % of China’s land area (Zhou et al., 2000). When the treated soil is re-used in the many seasonally frozen regions, the efficiency will be challenged

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by the effect of the freezing-thawing (F-T) cycles on the engineering properties

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(including the compressibility, strength, permeability, etc.) (Kamei et al., 2012;

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Eskişar et al., 2015; Hotineanu et al., 2015; Starkloff et al., 2017; Zhang et al., 2019).

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Therefore, the remediation efficiency of heavy metal contaminated soil experiencing

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F-T cycles should be assessed more comprehensively from a durability perspective. In particularly, the related research has been rarely carried out so far.

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For soil subjected to F-T cycles, the evolution in permeability accompanied with changes in microstructures has been extensively investigated in previous research (Yıldız and Soğancı, 2012; Jamshidi and Lake, 2014; Hirosea and Ito, 2017; Wang et al., 2020). It has been commonly concluded that the cyclic F-T process increases the permeability of fine-grained soil owing to the formation of cracks with each successive cycle. The mechanisms which triggered these cracks were found to be (1) ice lens formation due to the temperature gradient in the soil, and (2) desiccation caused by pore water migration, namely, unfrozen water moved toward the frozen fringe attributed to the negative pore pressure (Aubert and Gasc-Barbier, 2012; Sterpi,

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2015; Tang and Yan, 2015; Lu et al., 2016; Zhang et al., 2016). Otherwise, the mechanical behavior of soils subjected to F-T cycles has also been attracted extensive attentions (Davis et al., 2007; Lu et al., 2018; Tang and Li, 2018). Eskişar et al. (2015) investigated the engineering properties of cement-treated clayey soil with consideration of F-T cycles, and indicated that the Atterberg limits and the unconfined compressive strength of the cement-treated soil decreased after 5 F-T cycles. Yıldız

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and Soğancı (2012) also found that the compressive strength of clays decreased after

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experiencing successive F-T cycles, although the addition of lime can effectively

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enhance the resistance to the freezing-thawing cycles. Several researches explained

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that the tensile stress generated during the freezing of pore water damaged the original

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structure of soil pores, resulting in the formation of cracks, which caused a decrease

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in soil stiffness and the resultant occurrence of macroscopic deformation after a further propagation (Hori and Morihiro, 1998; Penttala and Al-Neshawy, 2002;

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Yarbaşı et al., 2007; Hotineanu et al., 2015; Lai et al., 2017). Furthermore, Shibi and Kamei (2014) compared the microstructure of the cement-stabilized clayey soil blended with basanite or coal ash before and after F-T cycles, the results upheld that the integrity of ettringite crystal was destroyed by F-T cycles, resulting in a decrease in soil strength. As mentioned above, the effect of F-T cycles on the engineering properties of original clay or cement-based solidified/stabilized soils has been extensively investigated during past decades. However, previous research has reported that the presence of heavy metal ions in soil will change the microstructure and chemical

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components, leading to variation in its engineering properties (Sunil et al., 2009; Kogbara and Al-Tabbaa, 2011; Kogbara et al., 2013; Goodarzi and Movahedrad, 2017). On this basis, the effect of F-T cycles on the mechanical behavior of the solidified/stabilized heavy metal-contaminated soil becomes more complicated, and has rarely been reported in previous research. With the aim of solving this problem,

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the strength and microstructural characteristics of the solidified/stabilized heavy metal-contaminated soil under cyclic F-T conditions are investigated in this study.

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The mechanical behavior and causal mechanisms of the material properties are

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predicted based on the results of this investigation.

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2 Materials and Methods

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2.1 Materials

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The tested soil was collected from a construction site in Hefei, China at a depth of 5~6 m. Basic physical indexes of the soil are summarized in Table 1. The liquid

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limit and plastic index of the natural soil are 51.5 % and 25.6, respectively, which can be classified as high liquid limit clay (CH) based on ASTM-2487 (2000). The particle size distribution of the tested soil is also determined by the laser particle analyzer, as shown in Fig. 1. Moreover, the chemical compositions of the tested soil were analyzed by X-ray fluorescence (XRF) technology, and the results are shown in Table 2. Portland cement and soda residue were adopted as the binding materials in this study, which were obtained from commercial companies located in Hefei and Weifang City (China), respectively. The chemical compositions of the cement and soda residue

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are also listed in Table 2. It is worth noting that soda residue is the by-product of the alkali plant, which is commonly characterized by relatively high alkalinity (pH = 9~12) and specific surface area (1.13 m2/g~31 m2/g) (Wang, 2003; Xu et al., 2014). Considering the extensive distribution of zinc contaminated soil and its extremely high hazardous influences on the ecological environment and human

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beings, such as decreasing the microbial activity, retarding the growth of crops and inducing carcinogenesis, mutagenesis and teratogenesis (Spear, 1981; Goyer, 1991;

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Berger et al., 2011; Milosavljević et al., 2011), Zn(NO3)2·6H2O was selected as the

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heavy metal contaminant used to prepare the Zn-contaminated soil artificially. Note

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that the nitrate salts has little impact on the cement haydration due to its inert

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2.2 Specimen preparation

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character (Cuisinier et al., 2011; Du et al., 2014).

Zn-contaminated soil with Zn2+ concentrations of 500, 1000, 2000, 5000 and

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10000 mg/kg (at mass ratios of Zn2+ to dry soil) were prepared beforehand artificially. The detailed steps can be found in the previously published research by Liu et al. (2018). Then, the pre-prepared Zn-contaminated soil was treated by the binders based on solidification/stabilization (S/S) method. Note that specimens without Zn2+ were also stabilized with binders to work as the experimental control samples. The program for sample preparation is presented in Table 3, and the S/S procedure of Zn-contaminated soil with cement and soda residue is listed as follows: (1) Soil samples (uncontaminated and contaminated) and binders (cement and soda residue) were oven-dried at 105 ℃ for 24 h, pulverized and subsequently passed

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through the 2 mm and 0.5 mm sieves, respectively. (2) The binders (cement and soda residue) were mixed evenly with the soil samples at a content of 20% by weight. The cement was mixed homogeneously with soda residue at the mass ratios of 3:7, 4:6 and 5:5 (denoted as C3SR7, C4SR6 and C5SR5, respectively) in the binder mixture.

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(3) Deionized water was sprayed into the soil-binder mixtures based on the optimum moisture content obtained by compaction testing, and stirring thoroughly.

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(4) The prepared mixtures were axially compacted in a cylindrical steel mold

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(with dimensions of Ф 39 mm × H 80 mm) at a displacement rate of 0.5 mm/min.

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The dry density of sample was controlled at 95% of the maximum dry density.

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(5) After demolding, specimens were sealed in the plastic bags and placed in the

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standard curing chamber (temperature of 22 ± 1 °C; humidity of 95 ± 2%) for 90 days. Previous studies proved that the cement hydration was a time-dependent process,

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which tended to terminate at 90 days of curing (Cocke and Mollah, 1993; Horpibulsuk et al., 2010; Wang et al., 2015). Thus, the impact of cement hydration on the strength and microstructure characteristics of cement-soda residue treated soil with curing time of 90 days can be avoided in the present study. It should be noted that adding binders into the tested soil will change its compaction characteristics. As shown in Fig. 2, the optimum water content increases with the mass ratio of cement to soda residue decreases, while the maximum dry density is reduced. Thus, the corresponding optimum moisture content and maximum dry density adopted in the specimen preparation varied with the binder proportions.

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2.3 Test methods 2.3.1 Freezing-thawing procedure Freezing-thawing (F-T) test was performed according to ASTM D560 (2003). A prepared specimen was wrapped with a plastic film in order to prevent water loss and placed in a closed system, in which there was no available water to enter into the

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system during F-T process. The F-T process was performed in an apparatus with two chambers: a freezing chamber, and a thawing chamber. At the beginning of the F-T

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test, a constant temperature of -23 ℃ was set for the frozen system, and the thawing

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system was set as 21 °C. After the temperature reached the target value, samples were

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first placed into the freezing chamber for 24 h, and subsequently placed into the

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thawing chamber for 24 h. These two steps together constituted one F-T cycle. In this

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study, 0, 2, 4, 6, 8, and 10 F-T cycles were tested for various samples. 2.3.2 Unconfined compressive strength test

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The unconfined compressive strength (UCS) test was conducted on the specimens with a strain rate of approximately 1 %/min based on the ASTM D5120 (2004). The stress and strain of the specimens were recorded at intervals of 15 s to analyze the strength characteristics of the specimen. 2.3.3 Microstructural analysis Specimens being subjected to their designated numbers of F-T cycles were cut into small pieces, which were immediately immersed in liquid nitrogen and freeze-dried in the Alpha 1-4 LDplus Freeze Dryer for 24 h. The soil sample was broken into two pieces by hand, leaving the fracture surface undisturbed.

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Subsequently, the samples were vacuum metalized before examination. Finally, both morphology observation and chemical analysis were carried out simultaneously using a JSM-6490LV scanning electron microscope (SEM) equipped with energy dispersive spectroscope (EDS). Pore size distribution characteristics were investigated by mercury intrusion

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porosimetry (MIP) with a Quantachrome porosimeter (AutoPore IV 9500 V1.09) made in America. Two pressure systems were applied in the MIP test: a low-pressure

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system with working pressure ranging from 3.6 to 200 kPa; a high-pressure system

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with working pressure ranging from 0.2 to 241.3 MPa. Then the corresponding pore

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diameters were calculated by the capillary pressure equation: 4𝜏𝑐𝑜𝑠𝜃 𝑝

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𝐷=−

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in which, D presents pore diameter, in μm; τ presents the surface tension of mercury, 0.485 N/m; θ presents the contact angle between the mercury and soil particles, equal

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to 130°; and p represents the intrusion pressure, in MPa. Note that specimens used for MIP determination should first be cut into small cubes with side lengths of approximately 7 mm, and subsequently freeze-dried as previously mentioned. 3 Results and discussion 3.1 Soil stress-strain characteristics In order to investigate the effects of freezing-thawing (F-T) cycles on the deformation properties of cement-soda residue stabilized Zn-contaminated soil, the stress-strain relations of the C3SR7 and C5SR5 stabilized samples subjected to

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varying F-T cycles are plotted in Fig. 3 and Fig. 4, respectively. As shown in Fig. 3 and Fig. 4, the result of sample subjected to non-cycle of F-T demonstrates the highest failure stress, and the corresponding failure strain is at the minimum value. The stress-strain curves are characterized as strain softening, and the failure mode was brittle. As the number of F-T cycles increases, the failure stress

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decreases, and the corresponding strain increases significantly. Therefore, the stress-strain curves intend to show more characteristics of strain-hardening as the

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destroyed by F-T cycles (Qi et al., 2006).

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number of F-T cycles increases. It is because that the inter-particle bonding is

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Free water in the soil pore begins to freeze when the internal temperature of a

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specimen drops below 0 ℃. Some ice lenses form during the freezing process, which

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imposes a considerable stress to the soil particles or soil aggregates (Konrad, 1989; Ghazavi and Roustaie, 2010). As a result, some irreversible changes in soil structure

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occur when the ice lens melt during the thawing process. With the increase of the number of F-T cycles, more micro-cracks are generated. At the same time, the micro-cracks keep propagating and expanding. In addition, the rearrangement of soil particles controlled by the sequential F-T cycles will result in a loose soil structure (Eskişar et al., 2015). When an axial stress is applied on the specimen, macro-cracks or macro-pores are first compressed. As a consequence, the soil structure achieves a denser state. At this stage, the deformation of soil is not elastic but plastic, which is reflected by the stress-strain curves. It can be seen from Fig. 3 and Fig. 4 that the stress-stain curves of all cases are

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approximately linear before failure, which appears to be the elastic deformation. Thus, the Young’s modulus is adopted here to describe the elastic deformation behavior of specimens involved in F-T cycles. As can be seen from Fig. 5, the Young’s modulus decreases with the number of F-T cycles increases. This indicates that specimens are more deformed after exposure to F-T cycles. On this basis, the shear failure of

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samples subjected to F-T cycles can be said to take place at a larger strain than that of the sample without F-T cycles.

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From Fig. 3 and Fig. 4, it can be observed that the initial concentration of Zn2+

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also has an obvious influence on the stress-strain characteristics of the sample

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subjected to F-T cycles. As the initial concentration of Zn2+ increases from 0 to 2000

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mg/kg, the failure stress increases significantly and the corresponding strain decreases.

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While increasing the initial concentration of Zn2+ up to 10000 mg/kg results in a remarkable decrease in the failure stress and a considerable increase in failure strain.

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The stress-strain curves are characterized as strain hardening curves, especially for the specimen experienced F-T cycles. By comparing the stress-strain curves of samples C3SR7 and C5SR5, the F-T resistance of samples can be effectively improved by increasing the mass ratio of cement to soda residue, as shown in Fig. 3 and Fig. 4. Compared with sample C3SR7, sample C5SR5 demonstrates a much higher failure stress, especially for the specimen Zn1.0. The results show that increasing the cement content is beneficial to the treatment of high concentration zinc contaminated soil. 3.2 Unconfined compressive strength (UCS)

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Fig. 6 shows the variations in UCS of the C3SR7 treated samples subjected to F-T cycles. The results indicate that increasing the number of F-T cycles can significantly decrease the soil strength. A relatively higher reduction rate in strength appears after the first two cycles of F-T. After experiencing 10 cycles of F-T, the UCS can decrease by 34.9-56.0 % (compared with specimen without F-T cycles) for

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specimens with different Zn2+ concentrations. When the sample is placed in the freezing chamber with temperature of -23℃,

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the surface temperature of the sample is the first part that drops below 0 ℃. A

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temperature gradient forms in the sample, which induces the migration of pore water

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from the high temperature zone (the central part of sample) to the low temperature

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zone (the surface layer of sample) (Benson and Othman, 1993; Talamucci, 2003). The

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migrated pore water gathers in the frozen fringe to form ice lens. The ice lens can break the soil aggregates, resulting in the formation and expansion of cracks (Kim and

cycles.

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Daniel, 1992). Therefore, the UCS decreases significantly after experiencing two F-T

After each F-T cycle, pore water redistribution and the rearrangement of soil particles will occur due to the temperature gradient induced by both freezing and thawing processes (Chamberlain and Gow, 1979; Benson and Othman, 1993; Makusa et al., 2016). However, with the number of F-T cycles increasing, a new dynamic equilibrium structure can be reached for the specimen (Wang et al., 2007; Zhang et al., 2016; Lu et al., 2018). Under this condition, freezing of pore water can’t induce any detrimental influences on the soil structure, so does the thawing process. Thus, the

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deterioration caused by F-T cycles is limited at 8 and 10 F-T cycles. In order to determine the effect of Zn2+ concentration on the UCS under cyclic F-T conditions, the UCS test was conducted on the C3SR7 stabilized soils with different Zn2+ concentration, as presented in Fig. 7. With the increase of the initial concentration of Zn2+, the strength first increases, and then decreases. The peak value

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of UCS appears at the Zn2+ concentration of 2000 mg/kg, whether the sample was exposed to the F-T cycles or not.

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During the cement hydration process, tricalcium aluminate (C3A) hydrates first

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and produces calcium aluminate hydrate (CAH) along with Ca2+ and OH- (Gougar et

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al., 1996). Meanwhile, Zn2+ contained in the pore solution reacts with the free OH-,

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and precipitates as Zn(OH)2. Note that this precipitate may encapsulate the cement

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grains and retards its early hydration (Arliguie et al., 1982; Li et al., 2001). With the prolongation of curing time, the other cement components hydrate gradually, giving a

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rise to the concentration of both Ca2+ and OH- in pore solution. Zn(OH)2 can react with Ca2+ and OH- to precipitate as calcium zincate (CaZn2(OH)6·2H2O) when the Ca2+ and OH- concentrations are high enough (Arliguie and Grandet, 1990; Du, et al., 2014). Although calcium zincate may encapsulate unhydrated cement grains, it will not hinder the long-term hydration of cement under a relatively low Zn2+ concentration (≤ 2000 mg/kg) (Chen et al., 2009). Because the generated calcium zincate can also contribute to the development of soil strength in addition to the typical hydrated products of cement. However, the excessively high Zn2+ concentration caused an extremely detrimental impact on the UCS of specimens. This

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concentration increases up to 10000 mg/kg, the pH value of pore solution will decrease significantly. The relatively low pH condition also has a negative influence

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on the stability of hydrated products, such as calcium silicate hydrates (CSH), which

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then hinders the strength development of the specimens.

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Specimens with different Zn2+ concentrations show different variations in UCS

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associated with F-T cycles (Fig. 7). The most notable changes in UCS induced by F-T

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cycles can be observed on the stabilized Zn0.2 sample, which decreases by approximately 54% after 10 cycles of F-T, while the stabilized Zn0 sample presents a

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more moderate change aroused by the F-T cycles. It is interesting to observe that the higher the initial UCS, the higher the reduction percentage in UCS when the samples undergo F-T cycles. This is because the stabilized Zn0.2 samples have a denser structure, resulting from the formation of cement hydrates and zinc precipitates between soil particles or aggregates. While for the sample with high Zn2+ concentration, the hydration process is retarded by the zinc precipitates, leading to a less compact structure. Viklander (1998) reported that fine-grained soil with an initially low void ratio was more vulnerable to F-T cycles than that with an initial high void ratio. Therefore, it can be explained why sample Zn0.2 suffers a higher

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reduction in strength than the other samples after exposure to F-T cycles. Fig. 8 shows the effects of binder proportions coupled with F-T cycles on the UCS of stabilized samples Zn0, Zn0.2 and Zn1.0. Results indicate that increasing the mass ratio of cement to soda residue can effectively improve the strength of specimens experiencing F-T cycles. Especially for the specimens stabilized with

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C5SR5, which demonstrates an obvious growth in strength comparing with that treated with C3SR7 or C4SR6.

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In Europe, Portland cement blended with limestone is widely utilized. The

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maximum content of limestone can be up to 20–30% and still not modify the

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mechanical properties of the cement-based materials, because limestone powders can

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work as filler in the hydrating cement due to its fine-grained characteristics (Bentz et

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al., 2006; Lothenbach et al., 2008). However, if the limestone content measured appropriately, some positive chemical modification would occur, for the calcium

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carbonate in the limestone can work as a substitute of the calcium sulfate contained in the cement. As a consequence, ettringite formed in the early period of hydration is prevented from transforming into calcium monosulfoaluminate hydrates, which results in a low total porosity of the specimens (Matschei et al., 2007). Soda residue used in this study is predominantly composed of calcite (CaCO3), the composition of which is verified by the results determined by SEM technology as shown in Fig. 9. As shown in Fig. 8, the UCS of sample C4SR6 is slightly higher than that of C3SR7, which illustrates that the physical filling plays an important role in such binder proportions. As the soda residue content decreases, the chemical modification in the

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specimens is relatively enhanced. Thus, the sample C5SR5 demonstrates an obvious increase in UCS. Comparing the C3SR7 stabilized samples, a higher reduction percentage in UCS varying from 41.26% to 48.73% is found for the C5SR5 samples after 10 cycles of F-T. This result also verifies that the denser soil structure is more susceptible to the

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F-T cycles. 3.3 Visible changes in soil structure

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Visible changes of the specimens Zn1.0 stabilized with C3SR7 subjected to 0, 2,

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4 and 10 cycles of F-T were recorded by photos, as shown in Fig. 10. It can be

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observed that visible cracks emerge on the surface of specimens after experiencing 6

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cycles of F-T. The proceeding F-T cycles cause new cracks in addition to enlarge the

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previously formed cracks. According to Fig. 10(d), the macro-crack with a width of approximately 1 mm appears on the surface of the specimen subjected to 10 cycles of

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F-T. Furthermore, F-T cycles cause damage on the edge of samples. It can be concluded that the visible changes on the appearance of samples resulted from the F-T cycles is consistent with the variations in their mechanical properties. 3.4 Effects of F-T cycles on the microstructure characteristics To monitor the evolution of micro-cracks in the specimens after exposing them to the F-T cycles, images enlarged 500 times are taken by SEM technology. As shown in Fig. 11(a), the specimen has a dense structure before experiencing F-T cycles. After experiencing 2 cycles of F-T, several micro-cracks can be occasionally observed (Fig. 11(b)). Increasing the number of F-T cycles causes micro-crack propagation and

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typically wider micro-crack distribution in the specimens, as shown in Fig. 11(c) and (d). When the number of F-T cycles increase up to 8 or 10, the micro-cracks were enlarged gradually and some of them are transformed from microscopic cracks into macroscopic cracks (as shown in Fig. 11(e) and (f)). Fig. 12 and Fig. 13 show the microstructural characteristics of specimens

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subjected to no cycle and 10 cycles of F-T, respectively. For the specimens that did not experience F-T cycles (as shown in Fig. 12),

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cement hydrates, such as flocculent or fiber-like calcium silicate hydrates (CSH),

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needle-like ettringite (AFt), as well as the plate-like portlandite (CH) can be clearly

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observed in the specimens. These cement hydrates can effectively improve the

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mechanical properties by filling the pores and aggregating the particles. It should be

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noted that AFt is the early hydrated product. As curing time increases, AFt gradually transforms to calcium monosulfoalumiante hydrates (AFm) due to the reduction of

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sulfate concentration. However, Lothenbach et al. (2008) suggested that the presence of calcium carbonate can work as a substitution for calcium sulfate, which can prevent the AFt from transforming to AFm. Thus, AFt observed in the specimens is attributed to the presence of calcite contained in the soda residue. Moreover, calcite contained in the soda residue can fill in the gap between soil aggregates and work as a skeleton, which can contribute to strength development. After subjecting the specimens to 10 cycles of F-T, the intact crystalline structure of cement hydrates can hardly be identified by SEM technology, as presented in Fig. 13. On this basis, it can be preliminarily concluded that F-T cycles cause considerable

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physical damage on the structure of hydrates. For the sake of confirming the effect of the F-T cycles on the mineral compositions, a SEM test equipped with EDS is conducted on the specimens subjected to 10 cycles of F-T. Depending on the results presented in Fig. 13, the common cement hydrates, such as CSH, AFt and CH are detectable using EDS

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technology. However, both CSH and AFt appear as an atypical crystalline structure, and CH is obviously fractured by the F-T cycles as it typically has a plate-like

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structure. The changes induced by F-T cycles in the microstructure of hydrated

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products result in a considerable reduction in the soil strength. CSH containing Zn2+ is

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detected in Fig. 13(c). It can be speculated that Zn2+ is immobilized due to being

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incorporated into the interlayer of CSH under a lower Zn2+ concentration condition.

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3.5 Effects of F-T cycles on the pore size distribution The cumulative intrusion volume versus the logarithm of pore diameter of the

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specimens subjected to 0, 2 and 10 cycles of F-T is plotted in Fig. 14. As shown in Fig. 14, the cumulative intrusion volume of specimen gains an obvious increase after subjecting to 2 cycles of F-T compared with the one not exposed to F-T. It is indicated that the cyclic F-T causes an increase in the total pore volume of specimen, which is responsible for the considerable reduction of UCS at the end of the 2nd F-T cycles. While the number of F-T cycles varies from 2 to 10, the cumulative intrusion volume increases slightly. This is consistent with the strength evolution of specimen subjected to 4, 6, 8 and 10 F-T cycles. Moreover, increasing the mass ratio of cement to soda residue can effectively decrease the pore volume of

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specimen, whether subjected to F-T cycles or not. Thus, the durability of specimens subjected to F-T cycles can be enhanced. Fig. 15 shows the pore size distribution of specimens before and after subjecting them to the F-T cycles. As shown in Fig. 15, the pore size distribution curves are characterized by bimodal curves. Diameters of most micro-pores concentrated at

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approximately 0.3 μm before exposure to F-T cycles, which are around 0.15 μm after exposure to F-T cycles. Meanwhile, the general diameters of macro-pores increase

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from 10 μm to 20 μm as the number of F-T cycles ranges from 0 to 10. The reduction

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in the diameter of micro-pores confirms the shrinkage of soil particles caused by pore

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water migration during freezing process. It will further enlarge the gaps between the

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soil aggregates, or induce the generation of desiccation fissures in specimens, which

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imposed a negative impact on the soil strength. Besides, a frost heave resulted from the generation of ice lenses increases the diameter of macro-pores. After the ice lens

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thawing, the deformation of soil pore structure can’t entirely recover and subsequently weakened the structure of the specimens. Therefore, it can be concluded that the reduction in UCS has a closely association with the redistribution of both micro- and macro-pores. 4 Conclusions In order to investigate the durability of Zn-contaminated soil treated with cement-soda residue in cyclic freeze-thaw conditions, a series of laboratory tests were performed as recorded in the present paper. The main conclusions are listed as follows.

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(1) Before subjecting the soil to F-T cycles, the stress-strain curves are characterized as strain softening, and the failure mode is brittle failure. With an increasing number of F-T cycles, the stress-strain curves gradually begin to characterize as strain hardening. (2) UCS decreases obviously with the number of F-T cycles increasing, and a relatively higher reduction rate in strength appears at the end of the 2nd cycle of F-T.

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(3) The UCS of the specimens attain the maximum value at Zn2+ concentration

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of 2000 mg/kg, no matter whether the sample was exposed to F-T cycles or not.

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However, the sample Zn0.2 was the most vulnerable to F-T cycles due to its denser

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structure. Increasing the mass ratio of cement to soda residue can effectively improve

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the resistance of specimens to the F-T cycles.

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(4) After subjecting to 6 cycles of F-T, visible cracks can be occasionally be observed on the surface of specimens, while micro-cracks commonly appear in the

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specimen after 2 cycles of F-T. Besides, the proceeding F-T cycles leads to the crack propagation and expansion.

(5) F-T cycles can significantly damage the crystalline structures of cement hydrates. The porosity of specimens can be increased by F-T cycles. The micro-pores size will decrease due to pore water migration during the freezing process, while the macro-pores are further enlarged by the formation of ice lenses.

Acknowledgements Funding: This work was supported by the National Natural Science Foundation of

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China [Project numbers 41672306, 41877262, and 41807239] and the Special Project for Major Science and Technology in Anhui Province, China [Project number 18030801103].

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List of Tables:

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Science Press. (in Chinese)

Table 1 Basic physical properties of tested soil Table 2 Main chemical compositions of the tested soil and binders

List of Figures:

Fig. 1 Compaction curves of the natural and treated soil Fig. 2 Stress-strain relationship of the C3SR7 stabilized specimens after subjecting to F-T cycles Fig. 3 Stress-strain relationship of the C3SR7 stabilized specimens after subjecting to F-T cycles Fig. 4 Relationship between UCS and the number of freeze-thaw cycles Fig. 5 Relationship between UCS and Zn2+ concentration under freeze-thaw conditions

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Fig. 12 Effects of F-T cycles on the porosity of specimens Zn0.05 subjected to 0, 2

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and 10 cycles of F-T

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to 0, 2 and 10 cycles of F-T

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Fig. 13 Effects of F-T cycles on pore size distribution of specimens Zn0.05 subjected

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Table 1 Basic physical properties of tested soil value

Water content (%)

25.3

Density (g/cm3)

2.1

Liquid limit (%)

51.5

Plastic limit (%)

25.9

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Parameter

25.6

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Plastic index

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Specific gravity

0.620

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Void ratio

2.72

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Table 2 Main chemical compositions of the tested soil and binders Soda Soil

Cement residue

Compositions

77.9

22.3

10.2

Al2O3

16.5

8.9

9

CaO

0.9

58

Fe2O3

6.7

3.8

1.3

TiO2

1.1

—

0.2

MgO

2.4

3.6

12.5

1

0.3

1.3

0.2

0.2

0.2

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K2O

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SO3

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Na2O

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SiO2

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(%)

62.8

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Table 3 Program for sample preparation Total binder content (%) 20 Zn2+ Water Soda Sample No. concentration Cement content residue (mg/kg) (%) content content (%) (%) C3SR7-Zn0 6 14 27.8 C4SR6-Zn0 0 8 12 23.0 C5SR5-Zn0 10 10 21.0 C3SR7-Zn0.05 6 14 27.8 C4SR6-Zn0.05 500 8 12 23.0 C5SR5-Zn0.05 10 10 21.0 C3SR7-Zn0.1 6 14 27.8 C4SR6-Zn0.1 1000 8 12 23.0 C5SR5-Zn0.1 10 10 21.0 C3SR7-Zn0.2 6 14 27.8 C4SR6-Zn0.2 2000 8 12 23.0 C5SR5-Zn0.2 10 10 21.0 C3SR7-Zn0.5 6 14 27.8 C4SR6-Zn0.5 5000 8 12 23.0 C5SR5-Zn0.5 10 10 21.0 C3SR7-Zn1.0 6 14 27.8 C4SR6-Zn1.0 10000 8 12 23.0 C5SR5-Zn1.0 10 10 21.0

Dry density (g/cm3) 1.507 1.524 1.577 1.507 1.524 1.577 1.507 1.524 1.577 1.507 1.524 1.577 1.507 1.524 1.577 1.507 1.524 1.577

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Fig. 1 Particle size distribution of the tested soil

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Fig. 2 Compaction curves of the natural and treated soil

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Fig. 3 Stress-strain relationship of the C3SR7 stabilized specimens after

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Fig. 4 Stress-strain relationship of the C5SR5 stabilized specimens after

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Fig. 5 Effect of F-T cycles on the Young’s modulus of the specimens

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Fig. 6 Relationship between UCS of specimen C3SR7 and the number of

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Fig. 7 Relationship between UCS of C3SR7 and Zn2+ concentration under

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Fig. 8 Relationship between the UCS and binder proportions under freeze-thaw

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Fig. 9 Image of the microstructure of the tested soda residue

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Fig. 10 Visible changes in specimens subjected to: (a) 0 cycles (b) 6 cycles; (c) 8

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Fig. 11 Evolution of cracks in the sample C3SR7-Zn0.2 subjected to F-T cycles: (a) F-T 0; (b) F-T 2; (c) F-T 4; (d) F-T 6; (e) F-T 8; and (f) F-T 10.

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Fig. 12 Microstructure of sample C3SR7-Zn0.2 subjected to 0 cycle of F-T

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Note: The Chinese words “图谱” in the images mean “spectrum” in English, which can’t be removed.

Fig. 13 Microstructure of sample C3SR7-Zn0.2 subjected to 10 cycles of F-T

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Fig. 14 Effects of F-T cycles on the porosity of specimens Zn0.05 subjected to 0, 2

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Fig. 15 Effects of F-T cycles on pore size distribution of specimens Zn0.05

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subjected to 0, 2 and 10 cycles of F-T

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interest in connection with the work submitted.

Journal Pre-proof  Freezing-thawing (F-T) cycles cause obvious deterioration in the UCS of the S/S treated Zn-contaminated soil.  Micro-cracks are commonly distributed in the specimen after experiencing F-T cycles, while the visible cracks are occasionally detectable after 6 F-T cycles.  F-T cycles significantly damage the crystalline structures of cement hydrates.

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F-T cycles result in the shrinkage of micro-pores and expansion of macro-pores.

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