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solidified heavy metal contaminated soils with cement-fly ash based on electrical resistivity measurements
Measurement 141 (2019) 85–94
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Mechanism of stabilized/solidified heavy metal contaminated soils with cement-fly ash based on electrical resistivity measurements Liu Jingjing a, Zha Fusheng a,⇑, Xu Long a, Kang Bo a, Tan Xiaohui a, Deng Yongfeng b, Yang Chengbin a a b
School of Resource and Environmental Engineering, Hefei University of Technology, Hefei 230009, PR China Institute of Geotechnical Engineering, School of Transportation, Southeast University, Nanjing 210096, PR China
a r t i c l e
i n f o
Article history: Received 21 August 2018 Received in revised form 21 March 2019 Accepted 23 March 2019 Available online 11 April 2019 Keywords: Solidification/stabilization mechanism Heavy metal contaminated soil Electrical resistivity Unconfined compressive strength Microstructure
a b s t r a c t The present work aimed at investigating the S/S micro-mechanism for treating heavy metal contaminated soils by an electrical resistivity method. Results indicated that the electrical resistivity (q) increased as a function of curing time and decreased as the heavy metal concentration increased. Unconfined compressive strength (qu ) has an exponential correlation to the electrical resistivity. The variations of the electrical resistivity parameters (such as pore water electrical resistivity (qx ), formation factor (F), shape factor (f) and anisotropy coefficient (A)) indicated that increasing curing time led to a porosity reduction and an increase in the cementation degree, which resulted in a denser structure and a greater strength of the specimens. Finally, the X-ray diffraction (XRD) and scanning electron microscope (SEM) results confirmed that hydrates increasingly filled the soil pores as the curing time increased, while increasing the heavy metal concentration hindered the development of hydrated reactions. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Solidification/stabilization (S/S) is an effective method for the remediation of heavy metal contaminated soils, which is a profound challenge confronted industrially and agriculturally [17,33,35,62]. In the S/S method, the heavy metal contaminated soils are blended with specially designed binders (cement, fly ash, etc.), which reduces the mobility and toxicity of contaminants and improves soil engineering properties [23,27,49]. In recent decades, a great deal of research has examined the immobilization mechanism of S/S-treated soils [28,12,51]. The most accepted conclusions were that the formation of hydrates such as calcium silicate hydrate (CSH), calcium aluminate hydrate (CAH), calcium hydroxide (CH) and ettringite (AFt) not only enhanced the soil strength but also immobilized the contaminants [40,30,50]. In addition, Yoobanpot et al. [59] found that CSH was evenly distributed on clay clusters and filled the pore spaces between clay particles, resulting in a denser structure. The needle-like AFt crystal that was formed between clay and the CSH fabric resulted in a stiffer and clay packed structure. Du et al. [20] testified that the massive segregated prism Mg3(PO4)28H2O and MgKPO46H2O led to a denser and stiffer ⇑ Corresponding author at: School of Resource and Environmental Engineering, Hefei University of Technology, Tunxi Road 193#, Baohe District, Hefei 230009, PR China. E-mail address: [email protected] (F. Zha). https://doi.org/10.1016/j.measurement.2019.03.070 0263-2241/Ó 2019 Elsevier Ltd. All rights reserved.
structure of the KMP-stabilized soil. Zn2+ was immobilized in the form of Zn3(PO4)24H2O or CaZn2(PO4)24H2O, while Pb2+ was immobilized as Pb5(PO4)3F. Aldaood et al. [3] indicated that in lime-treated soils, the volume of small pores (diameter < 0.1 lm) increased as the gypsum content increased, which induced a homogenous pore size distribution in the stabilized soil. Meng et al. [38] reported that the addition of nano-CaCO3 into cement stabilized soils resulted in a higher quantity of Ca(OH)2 than that without addition in a marine environment. All the aforementioned conclusions can be analyzed by techniques such as isothermal titration calorimetry, X-ray diffraction, scanning electron microscope, mercury intrusion porosimetry and infrared spectroscopy [56,58,14,15,22]. However, the current methods for investigating the S/S mechanism are time-consuming, destructive, and expensive, and they involve severe preservation conditions for the samples [26,46,52,13,29,63]. Therefore, developing a time-saving, nondestructive and cost-effective testing technique to investigate the mechanism of stabilized/solidified heavy metal contaminated soils is a topic of intense study. Electrical resistivity is an inherent bulk physical property of soil and is a measure of the ability to resist the flow of electrical current. Archie [4] established the relationship between the electrical resistivity and microstructure of soils. The concept of Formation Factor was developed, and a resulting resistivity based formula, applied to pure argillaceous sandstone, was established. Subsequently, electrical resistivity was utilized in the geotechnical
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(Cd), lead (Pb), zinc (Zn), arsenic (As) and chromium (Cr(III)) pollution was the most common in surface soil. And the contaminants concentration in the sites varied from about 500 mg/kg to 50,000 mg/kg. In this paper, Pb, Zn and Cr contaminated soils were selected as the research object. Pb(NO3)2, Zn(NO3)26H2O and Cr (NO3)39H2O were selected as the contamination sources to artificially prepare the Pb, Zn and Cr contaminated soils due to their high solubility and low interference to the hydration process. The contaminant concentrations were designated as 1000, 5000, 10,000, and 15,000 mg/kg as a mass ratio to dry soil, which were in the general concentration range of Pb, Zn and Cr contaminated soil (denoted as M0.1, M0.5, M1.0 and M1.5, respectively; where M refers to heavy metal ions).
engineering field to evaluate, for example, the groundwater quality, the engineering properties of contaminated soil, and the effectiveness of stabilized soils [25,64,18,1,16,24,45,47]. Recently, several groups have focused on the study of hydration processes and microstructure evolution of the stabilized soil based on electrical resistivity [10,2,42]. Yim et al. [57] developed a certified method to determine the setting time of early-age mortar by using electrical resistivity. Yousuf et al. [60] also proved that electrical resistivity was a sensitive technique for monitoring the microstructure development of cement paste. In view of the abovementioned advantages of electrical resistivity measurements, this paper attempts to reveal the micromechanism of stabilized/solidified heavy metal contaminated soils based on electrical resistivity measurements. To illustrate the microstructure characteristics of the stabilized soil, electrical resistivity parameters such as pore water electrical resistivity (qx ), formation factor (F), shape factor (f) and anisotropy coefficient (A) were introduced in addition to the average electrical resistivity (q). A strength prediction model was established according to the results of an unconfined compressive strength and electrical resistivity test. Finally, X-ray diffraction (XRD) and scanning electron microscope (SEM) technology were utilized to confirm that the obtained observations depended on the electrical resistivity.
2.2. Test methods 2.2.1. Sample preparation The oven-dried soil and binders (cement and fly ash) were initially pulverized and sieved with 2 mm and 0.5 mm sieves, respectively. Based on the optimum water content, a certain quality of distilled water was weighed and divided into two equal parts. The heavy metal contaminants were dissolved in one part of the water; those nitrate solutions were then thoroughly mixed with soil and allowed to stand for 24 h. The other part of the water was mixed with the binders and stirred until a slurry formed. The slurry was subsequently added to the contaminated soils and fully mixed. Cylindrical specimens (U38.1 mm H80 mm) and cutting ring specimens (U61.8 mm H20 mm) were prepared using a static compaction method, which depended on the 95% maximum dry density. Finally, the specimens were sealed in plastic bags and stored in the standard curing room (humidity 95 ± 5%, temperature 20 °C). The specimens for unconfined compressive tests were cured for 3, 7, 28 and 90 days (d). In order to monitor the hydration process of cement and fly ash by electrical resistivity method [56,53], curing time of specimens for electrical resistivity test was designed as 0 h (h), 1 h, 2 h, 3 h, 6 h, 12 h, 1 d, 3 d, 7 d, 14 d, 21 d, 28 d and 90 d.
2. Test materials and methods 2.1. Test materials 2.1.1. Soil sample The tested soil was obtained from a project site located in Hefei, China. The soil was sampled at a depth of 4–5 m, had a yellowishbrown, hard-plastic appearance. The indexes of the basic physical properties are listed in Table 1. The liquid limit and plastic index of the tested soil were 49.4% and 24.6, respectively. Thus, the tested soil can be classified as low liquid limit clay (CL) according to the Unified Soil Classification System (ASTM D2487-00) [7]. 2.1.2. Binders The binders used in the test included Portland blast furnace slag cement (P325) and fly ash (FA). The main composites of both cement and fly ash were CaO, SiO2, Al2O3 and Fe2O3, as determined by X-ray fluorescence, and are shown in Table 2. Based on ASTM C618 [8], the used fly ash can be classified as F for the total content of SiO2, Al2O3 and Fe2O3 in fly ash surpassed 70%. The stabilized soils were divided into two groups. One group was stabilized with 10% cement (denoted as C10), and the other was stabilized with 10% cement blended with 30% fly ash (denoted as C10 + F30).
2.2.2. Unconfined compressive strength test The unconfined compressive strength (qu ) test was conducted on the specimens with a strain rate of approximately 1% / min based on the ASTM procedure (D5102, [9]). The stress and strain of the specimens were recorded at 15 s intervals. 2.2.3. X-ray diffraction (XRD) XRD analysis was performed on the specimens to identify the products of the hydrated reactions. The specimens were freezedried and were then pulverized and passed through a 0.075 mm sieve. XRD analyses were performed with Cu-Ka radiation on a Rigaku D/Max-2005 V.
2.1.3. Heavy metal contaminants Based on the recent investigation related to heavy metal contaminated soils in China performed by Duan et al. [21], cadmium
Table 1 Basic physical properties of the tested soil. Density/g/cm3
Water content/%
Gravity
Liquid limit/%
Plastic limit/%
Plastic index
Optimum water content/%
Maximum density/g/cm3
1.92
25.5
2.669
49.4
24.8
24.6
21.2
1.663
Table 2 Composites and relative contents of cement and fly ash. Composites Content/%
C FA
CaO
SiO2
Al2O3
SO3
Fe2O3
MgO
TiO2
Na2O
CO3
43.798 1.062
27.209 48.793
9.895 29.814
3.013 /
2.880 2.920
1.565 0.641
0.468 1.755
0.442 0.462
/ 12.864
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2.2.4. Scanning electron microscope (SEM) SEM was carried out to analyze the microstructure of the specimens. The samples used for SEM analysis were freeze-dried and broken slightly to pieces. The samples with approximate dimensions of 5 mm 5 mm 5 mm were selected, and their surface was cleaned using a hairbrush. To achieve sufficient electrical conductivity, the sample surface was pre-treated by vacuum metal spraying technology. 3. Electrical resistivity theory and methods 3.1. Electrical resistivity theory A benchtop LCR meter (INSTEK LCR-816) was used in this study to measure sample resistance (the ratio of applied voltage to resultant current flow) in 2-terminal mode. The apparatus is illustrated in Fig. 2 (a) and (b). The electrical resistivity was then derived from the specimen/electrode dimensions using Eq. (1).
q¼
RS L
(a) Soil sample used for electrical resistivity measurement
ð1Þ
where R represents the resistance of soil sample (X); S represents the contact area between the electrode plates and the soil sample (m2); L represents the distance between the two electrode plates (m). Both the horizontal and vertical electrical resistivity were measured due to the anisotropy of soil. Then, the average electrical resistivity q, the average pore water electrical resistivity qx, the average formation factor F, the average shape factor f and the anisotropy coefficient A were calculated with Eqs. (2)–(5) [4]:
2qH þ qV 3 2FH þ FV q q F¼ FH ¼ H ; FV ¼ V 3 qx qx
q¼
f ¼ nF sffiffiffiffiffiffi FV A¼ FH
ð2Þ ð3Þ
(b)
Electrode used for the top or bottom surface of sample
ð4Þ ð5Þ
where qH represents the horizontal electrical resistivity (Xm); qV represents the vertical electrical resistivity (Xm); FH is the horizontal formation factor; FV is the vertical formation factor; and n represents the porosity.
(c)
Electrode used for the side surface of sample
Fig. 1. Sample used for electrical resistivity measurements.
3.2. Electrical resistivity tested method In general, soil electrical resistivity consists of the electrical resistivity of the soil particles and the pore water [31]. The electrical resistivity was determined by the two-electrode probe method. To overcome the polarization phenomenon of the electrodes, the frequency of alternating current was set to 2000 Hz [4]. 3.2.1. Average electrical resistivity Sample for electrical resistivity measurement is shown in Fig. 1 (a). Preparing four metal plates, two had shapes identical to the top and bottom surfaces of the samples, and the other two had the same shape as the side planes, as shown in Fig. 1(b) and (c). The vertical and horizontal resistance of sample can then be determined by apparatus LCR-816 based on the schematic diagram as shown in Fig. 2. A pressure of 25 kPa was applied to the electrode plates to ensure full contact between the electrodes and soil sample. The average electrical resistivity was then calculated using Eq. (2).
3.2.2. Pore water electrical resistivity Based on the method introduced by the Japanese Geotechnical Society (2000), the pore water electrical resistivity was measured. The tested sample was oven-dried and then pulverized. The dried soil was mixed with distilled water at a mass ratio of 1:5 and was stirred for 30 min. This mixture was poured into a polymethyl methacrylate box and was allowed to stand for two hours. Finally, the measured electrical resistivity of the suspension was defined as pore water electrical resistivity. A diagram of the tested setup is shown in Fig. 3. 4. Results and discussion 4.1. Effects of curing time on electrical resistivity Fig. 4 shows the relationships between the average electrical resistivity and the curing time. The samples were contaminated
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Fig. 3. Schematic for: (a) the cell for pore water electrical resistivity measurement; and (b) diagram of pore water electrical resistivity measurement. Fig. 2. Schematic of the two-electrode probe method: (a) apparatus used for resistance measurement; (b) diagram of determining the resistance of soil sample.
with Pb0.1, Pb0.5, Pb1.0 and Pb1.5 and were then treated with C10 and C10 + F30. As shown in Fig. 4, during the initial curing periods (7 d), the average electrical resistivity fluctuated slightly. As the curing time increased from 7 days to 90 days, the average electrical resistivity clearly increased as well. Compared with the C10 treated samples, the samples treated with C10 + F30 had lower electrical resistivity. The class F fly ash used had low hydraulicity and contained some metal cations, which can increase the electrical conductivity of soils. As shown in Fig. 4, we also observed that for lower Pb2+ concentrations, the electrical resistivity obviously increased after a 7 days curing time. However, for higher Pb2+ concentrations, the electrical resistivity gradually increased until the curing time was more than 21 days. Thus, it can be seen that the increasing heavy metal concentration may delay the development of hydrated reactions. 4.2. Effects of heavy metal species and concentrations on electrical resistivity The variations of average electrical resistivity with heavy metal concentrations are shown in Fig. 5. The specimens were stabilized with C10 and C10 + F30 and were cured for 28 days. It can be seen from Fig. 5 that increasing heavy metal concentrations led to a reduction in electrical resistivity. Different heavy
Fig. 4. Effect of curing time on the average electrical resistivity of the stabilized Pbcontaminated soil.
metal species also caused different influences on the electrical resistivity behavior of the stabilized soils. As shown in Fig. 5, the electrical resistivity of the three types of contaminated soils was Pb-contaminated soil > Zn-contaminated soil > Cr-contaminated soil.
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Fig. 5. Relationship concentration.
between
electrical
resistivity
and
heavy
metal
ion
Fig. 7. Relationship between qu/qu0 and q/q0.
4.3. Relationships between qu and q Based on the unconfined compressive strength and electrical resistivity test results, a correlation analysis was performed. The relationships between qu and q of the Pb-contaminated soils treated with C10 and C10 + F30 were proposed. As shown in Fig. 6, qu was the exponential function of q, regardless of the Pb2+ concentration. To predict qu by q more simply, qu and q of the specimens were normalized by dividing the corresponding initial value without curing. The calculated qu =qu0 and q=q0 values were plotted in Fig. 7, and a linear correlation between qu =qu0 and q=q0 was observed. The relationship is expressed by Eq. (6):
^ q=q þ 1:082; R2 ¼ 0:8954 qu =qu0 ¼ 0:418A 0
ð6Þ
where qu represents the unconfined compressive strength (MPa) tested at a certain curing time; qu0 represents the strength without curing; q is the electrical resistivity (X m) tested at a certain curing time; and q0 represents the electrical resistivity without curing. Thus, qu of the stabilized specimens can be predicted based on q.
Fig. 6. Relationship between electrical resistivity and unconfined compressive strength.
Fig. 8. Pore water electrical resistivity: (a) effects of curing time; (b) effects of heavy metal concentration and species.
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4.4. Immobilization mechanism based on electrical resistivity 4.4.1. Pore water electrical resistivity (qx ) Fig. 8 shows the effects of curing time and heavy metal concentrations on the average pore water electrical resistivity. As shown in Fig. 8(a), the pore water electrical resistivity increased as the curing time increased for the stabilized Pbcontaminated soils. Compared with C10 specimens, the C10 + F30 samples had higher pore water electrical resistivities. Liu et al. [37] proposed that a conductive path of soil contained soil particles and pore solutions. During the initial phase of hydration, there were two types of reactions taking place: ① OH, Na+, K+, Ca2+, SO24 etc., existed in the fly ash dissolved and released into the pore solution attributed to contact with water [54–55], ② dissociative ions in the pore solution participated in the early hydration reactions and formed hydrated products (such as ettringite, Ca (OH)2, etc.) that can fill in soil pores. The first type of reaction reduced the electrical resistivity, while the second increased it. The combined actions of these two reaction types initially negated each other, resulting in a slight change in soil electrical resistivity during the initial curing time. It is well known that the electrical conductivity of a solution depends on the concentration and charge of the dissolved ions. For a specific ionic species increasing the concentration will increase the conductivity (and reduce the resistivity). As seen in Fig. 8(a), increasing the curing time caused an increase in the pore water electrical resistivity. This result indicated that the concentration of dissociative ions in the pore solutions decreased, because some of them participated in the hydrated reactions. For instance, Ca2+ participated in the hydrated reaction to form CSH and Ca(OH)2 which had low electrical conductivity in the relatively alkaline environment provided by cement and fly ash. Pb2+ can be immobilized by precipitation, adsorption, encapsulation and isomorphous substitution, which reduced the electrical conductivity of pore solutions. Furthermore, the addition of fly ash accelerated the immobilization of Pb2+ according to adsorption and pozzolanic reactions, which resulted in a higher pore water electrical resistivity of the specimens stabilized with C10 + F30. As shown in Fig. 8(b), a reduction in the pore water electrical resistivity was observed as the heavy metal concentration increased. The pore water resistivity of the three contaminated soils exhibited different electrical conductivities: Pbcontaminated specimens < Zn-contaminated specimens < Crcontaminated specimens. As above-mentioned, the electrical conductivity of the pore solution is primarily controlled by ion concentration and ionic charge. Thus, the decrease in pore water electrical resistivity revealed that the ion concentration in the pore water increased. Moreover, because of the differences in the electrical charges and ionic radii of Pb2+, Zn2+ and Cr3+, the types of specimens exhibited obvious discrepancies in the electrical resistivity.
4.4.2. Formation factor (F) The variations in the average formation factors with curing time and heavy metal concentrations are presented in Fig. 9. Fig. 9(a) shows that increasing curing time led to an increase in the formation factors. Previous research groups proposed that the formation factor can reflect the porosity and porous structure characteristics [5]. The increased formation factor revealed that the porosity decreased and that the structure connecting strength increased as the curing time increased. During the development of the hydrated reactions, more and more hydrates were produced and filled in the soil pores, which then resulted in a decrease in pore volume. Thus, the amount of conductive paths was reduced. In addition, the dispersive soil particles were aggregated by the
Fig. 9. Average formation factor: (a) effects of curing time; (b) effects of heavy metal concentration and species.
cementitious materials, and they enhanced the structure connecting strength of the stabilized specimens. Besides, as shown in Fig. 9(a), the formation factor of the specimen C10 increased slightly during the initial curing period and then clearly increased as the curing time varied from 28 days to 90 days. The formation factor of the specimens treated with C10 + F30 started increasing after the curing time surpassed 7 days. These results demonstrated that the addition of fly ash accelerated the hydrated reactions. As shown in Fig. 9(b), the formation factor decreased as the heavy metal concentration increased. The decreased formation factor indicated that increasing heavy metal concentration contributed to a larger soil pore volume. Previous studies reported that the presence of Pb2+ hindered the formation of CAH. Pb2+ can also precipitate as Pb(OH)2, which would encapsulate the binder particles in the alkaline environment offered by cement and fly ash Qiao [43,44,11]. Thus, cement hydration can be delayed. Furthermore, the presence of Zn2+ also limited the generation of hydrated products by encapsulating the cement particles with the products formed by reactions between Zn2+ and cement components [61,19]. Accordingly, the quantity of hydrates filled in the soil pores decreased due to the increase of heavy metal concen-
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tration, and the connective strength of the soil structure decreased as well.
4.4.3. Shape factor (f) Fig. 10 presents the effects of curing time and heavy metal concentration on the average shape factor of the stabilized Pbcontaminated soils. Thevanayagam [48] suggested that the shape factor was a parameter related to the shape of soil particles and the cementation degree. As shown in Fig. 10(a), the shape factor increased with curing time. With hydrated reactions proceeding, the cementitious hydrated products continuously formed. The gaps among the soil particles were filled with the hydrates, which enlarged the size of aggregates and enhanced the cementation degree of the specimens as well. Fig. 10(b) shows that the shape factor decreased as the heavy metal concentration increased. It can be concluded that the presence of a heavy metal or the generation of some components containing heavy metals decreased the cementitious properties of the hydrates. For instance, Cr3+ can replace Ca2+, Al3+, Si4+ and Fe3+, which all exist in the hydrated products and generate Cr-bearing compounds without gelatinization[34,32,41].
Fig. 10. Average shape factor: (a) effects of curing time; (b) effects of heavy metal concentration and species.
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4.4.4. Anisotropy coefficient (A) The relationships between the anisotropy coefficient of the stabilized specimens and the curing time are shown in Fig. 11. Arulanandan and Muraleetharan [6] suggested that the anisotropy coefficient can describe the orientation of soil particles. With curing time increased, the shape and size of the soil particles, the porosity, and the structure of the specimens changed based on the results of the formation factor and the shape factor. Therefore, the arrangement of soil particles varied with the curing time. As shown in Fig. 11(a), the anisotropy coefficient decreased as the curing time increased. This result confirmed that the directional permutation of soil particles decreased that the connectivity weakened, which hindered the conductive path. Increasing the heavy metal concentration led to a growth in the anisotropy coefficient as shown in Fig. 11(b). The increased anisotropy coefficient revealed that the presence of a heavy metal impeded the development of cement hydration and affected the arrangement of the soil particles. 4.4.5. Mineralogy and microstructure 4.4.5.1. X-ray diffraction analysis (XRD). The XRD patterns of Zncontaminated soils stabilized with C10 curing for 1, 7, 28 and
Fig. 11. Anisotropy coefficient: (a) effects of curing time; (b) effects of heavy metal concentration and species.
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Fig. 12. X-ray diffractograms of Zn-contaminated specimens stabilized with C10: (a) curing for 1 day; (b) curing for 7 days; (c) curing for 28 days and (d) curing for 90 days.
90 days are shown in Fig. 12. The XRD patterns indicated that the primary hydrated products of cement included calcium aluminate hydrate (CAH), calcium silicate hydrate (CSH), ettringite (AFt), calcium hydroxide (CH) and calcium monosulfoaluminate hydrate (AFm). In addition, some Zn2+ ions were incorporated into the crystal lattice of CH and formed as CaZn2(OH)62H2O based on the peaks that appeared at 2h of 28° and 36.5°. Previous studies reached similar conclusions [39,36]. Compared with Fig. 12(a) and (b), the intensity of these hydrates increased slightly as the curing time increased from 1 to 7 days. When the curing time increased to 28 days, the intensity of the hydrates obtained clearly and rapidly increased at 90 days, as shown in Fig. 12(c) and (d). In particular, the intensity increase was more pronounced for Aft and CSH at 2h of 29.5°, 40.3°, 50.2° and 60°. In conclusion, the XRD results agreed with the development of electrical resistivity as a function of the curing time.
4.4.5.2. Scanning electronic microscopy analysis (SEM). Fig. 13 shows the microstructure of the Zn-contaminated soils treated with C10 at 7 days and 90 days of curing time. As shown in Fig. 13(a), some cracks and pores are observed in the specimens cured for 7 days. There were few hydrates filling in the soil pores, and the structure exhibited a low cementation degree. For the specimens cured for 90 days, as illustrated in Fig. 13(b), many hydrated products formed as the cement hydration fully developed. A large amount of fibrous and flocculent CSH was distributed throughout the samples, occupied the soil pored and resulted in a denser soil structure. The dispersive aggregates were connected with needle-like Aft, and a higher cementation degree was obtained. Furthermore, some hexagonal CAH or AFm were observed and enhanced the soil structure as well. The observation of CSH, AFt, CAH and AFm confirmed the results revealed by XRD analysis. Changes in the microstructure of specimens contaminated with 10000 mg/kg Zn2+ were analyzed by SEM. As shown in
Fig. 13(c), very little AFt was observed in these samples. The quantity of hydrated products was insufficient to fill the soil pores and led to a higher void volume compared with 1000 mg/kg Zn2+ specimens. Therefore, it can be concluded that increasing the Zn2+ concentration retards the development of cement hydration and that the structure of hydrated products was damaged. Compared with the variations in the micro-structure characteristics and the electrical resistivity parameters, a good correlation was observed. This correlation indicated that this electrical resistivity methodology can be applied to the assessment of immobilization effectiveness of heavy metal contaminated soils treated with cement and fly ash.
5. Conclusions This paper investigated electrical resistivity behaviors and utilizing the variations in electrical resistivity parameters revealed the immobilization mechanisms of stabilized heavy metal contaminated soils. The primary conclusions are as follows: (1) As the curing time increased, the electrical resistivity, pore water electrical resistivity, formation factor, and shape factor of the specimens also increased. (2) Increasing the heavy metal concentration led to an obvious decrease in the electrical resistivity, pore water electrical resistivity and shape factor. Different electrical resistivity behaviors were observed for the specimens contaminated with different heavy metals. (3) A linear correlation between qu =qu0 and q=q0 was found, which can be used to predict the UCS of the stabilized specimens. (4) The XRD results indicated that, with increased curing time, the quality of hydrates such as CSH and ettringite also increased and that the soil pores gradually filled. Increasing the Pb2+ concentration retarded cement and fly ash hydration.
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Acknowledgments This research is financially supported by the National Natural Science Foundation of China (grant No. 41672306, 41877262, 41807239, 41372281), the Special Project for Major Science and Technology in Anhui Province, China (18030801103). References
(a) Zn.0.1, C10, curing 7 days
(b) Zn0.1, C10, curing 90 days
(c) Zn1.0, curing 90 days Fig. 13. Graphs of the microstructure of the solidified zinc contaminated soil.
(5) The variations in the microstructure characteristics of the solidified specimens were consistent with that of the electrical resistivity parameters when the curing time and heavy metal concentration increased.
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Measurement journal homepage: www.elsevier.com/locate/measurement
Mechanism of stabilized/solidified heavy metal contaminated soils with cement-fly ash based on electrical resistivity measurements Liu Jingjing a, Zha Fusheng a,⇑, Xu Long a, Kang Bo a, Tan Xiaohui a, Deng Yongfeng b, Yang Chengbin a a b
School of Resource and Environmental Engineering, Hefei University of Technology, Hefei 230009, PR China Institute of Geotechnical Engineering, School of Transportation, Southeast University, Nanjing 210096, PR China
a r t i c l e
i n f o
Article history: Received 21 August 2018 Received in revised form 21 March 2019 Accepted 23 March 2019 Available online 11 April 2019 Keywords: Solidification/stabilization mechanism Heavy metal contaminated soil Electrical resistivity Unconfined compressive strength Microstructure
a b s t r a c t The present work aimed at investigating the S/S micro-mechanism for treating heavy metal contaminated soils by an electrical resistivity method. Results indicated that the electrical resistivity (q) increased as a function of curing time and decreased as the heavy metal concentration increased. Unconfined compressive strength (qu ) has an exponential correlation to the electrical resistivity. The variations of the electrical resistivity parameters (such as pore water electrical resistivity (qx ), formation factor (F), shape factor (f) and anisotropy coefficient (A)) indicated that increasing curing time led to a porosity reduction and an increase in the cementation degree, which resulted in a denser structure and a greater strength of the specimens. Finally, the X-ray diffraction (XRD) and scanning electron microscope (SEM) results confirmed that hydrates increasingly filled the soil pores as the curing time increased, while increasing the heavy metal concentration hindered the development of hydrated reactions. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Solidification/stabilization (S/S) is an effective method for the remediation of heavy metal contaminated soils, which is a profound challenge confronted industrially and agriculturally [17,33,35,62]. In the S/S method, the heavy metal contaminated soils are blended with specially designed binders (cement, fly ash, etc.), which reduces the mobility and toxicity of contaminants and improves soil engineering properties [23,27,49]. In recent decades, a great deal of research has examined the immobilization mechanism of S/S-treated soils [28,12,51]. The most accepted conclusions were that the formation of hydrates such as calcium silicate hydrate (CSH), calcium aluminate hydrate (CAH), calcium hydroxide (CH) and ettringite (AFt) not only enhanced the soil strength but also immobilized the contaminants [40,30,50]. In addition, Yoobanpot et al. [59] found that CSH was evenly distributed on clay clusters and filled the pore spaces between clay particles, resulting in a denser structure. The needle-like AFt crystal that was formed between clay and the CSH fabric resulted in a stiffer and clay packed structure. Du et al. [20] testified that the massive segregated prism Mg3(PO4)28H2O and MgKPO46H2O led to a denser and stiffer ⇑ Corresponding author at: School of Resource and Environmental Engineering, Hefei University of Technology, Tunxi Road 193#, Baohe District, Hefei 230009, PR China. E-mail address: [email protected] (F. Zha). https://doi.org/10.1016/j.measurement.2019.03.070 0263-2241/Ó 2019 Elsevier Ltd. All rights reserved.
structure of the KMP-stabilized soil. Zn2+ was immobilized in the form of Zn3(PO4)24H2O or CaZn2(PO4)24H2O, while Pb2+ was immobilized as Pb5(PO4)3F. Aldaood et al. [3] indicated that in lime-treated soils, the volume of small pores (diameter < 0.1 lm) increased as the gypsum content increased, which induced a homogenous pore size distribution in the stabilized soil. Meng et al. [38] reported that the addition of nano-CaCO3 into cement stabilized soils resulted in a higher quantity of Ca(OH)2 than that without addition in a marine environment. All the aforementioned conclusions can be analyzed by techniques such as isothermal titration calorimetry, X-ray diffraction, scanning electron microscope, mercury intrusion porosimetry and infrared spectroscopy [56,58,14,15,22]. However, the current methods for investigating the S/S mechanism are time-consuming, destructive, and expensive, and they involve severe preservation conditions for the samples [26,46,52,13,29,63]. Therefore, developing a time-saving, nondestructive and cost-effective testing technique to investigate the mechanism of stabilized/solidified heavy metal contaminated soils is a topic of intense study. Electrical resistivity is an inherent bulk physical property of soil and is a measure of the ability to resist the flow of electrical current. Archie [4] established the relationship between the electrical resistivity and microstructure of soils. The concept of Formation Factor was developed, and a resulting resistivity based formula, applied to pure argillaceous sandstone, was established. Subsequently, electrical resistivity was utilized in the geotechnical
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(Cd), lead (Pb), zinc (Zn), arsenic (As) and chromium (Cr(III)) pollution was the most common in surface soil. And the contaminants concentration in the sites varied from about 500 mg/kg to 50,000 mg/kg. In this paper, Pb, Zn and Cr contaminated soils were selected as the research object. Pb(NO3)2, Zn(NO3)26H2O and Cr (NO3)39H2O were selected as the contamination sources to artificially prepare the Pb, Zn and Cr contaminated soils due to their high solubility and low interference to the hydration process. The contaminant concentrations were designated as 1000, 5000, 10,000, and 15,000 mg/kg as a mass ratio to dry soil, which were in the general concentration range of Pb, Zn and Cr contaminated soil (denoted as M0.1, M0.5, M1.0 and M1.5, respectively; where M refers to heavy metal ions).
engineering field to evaluate, for example, the groundwater quality, the engineering properties of contaminated soil, and the effectiveness of stabilized soils [25,64,18,1,16,24,45,47]. Recently, several groups have focused on the study of hydration processes and microstructure evolution of the stabilized soil based on electrical resistivity [10,2,42]. Yim et al. [57] developed a certified method to determine the setting time of early-age mortar by using electrical resistivity. Yousuf et al. [60] also proved that electrical resistivity was a sensitive technique for monitoring the microstructure development of cement paste. In view of the abovementioned advantages of electrical resistivity measurements, this paper attempts to reveal the micromechanism of stabilized/solidified heavy metal contaminated soils based on electrical resistivity measurements. To illustrate the microstructure characteristics of the stabilized soil, electrical resistivity parameters such as pore water electrical resistivity (qx ), formation factor (F), shape factor (f) and anisotropy coefficient (A) were introduced in addition to the average electrical resistivity (q). A strength prediction model was established according to the results of an unconfined compressive strength and electrical resistivity test. Finally, X-ray diffraction (XRD) and scanning electron microscope (SEM) technology were utilized to confirm that the obtained observations depended on the electrical resistivity.
2.2. Test methods 2.2.1. Sample preparation The oven-dried soil and binders (cement and fly ash) were initially pulverized and sieved with 2 mm and 0.5 mm sieves, respectively. Based on the optimum water content, a certain quality of distilled water was weighed and divided into two equal parts. The heavy metal contaminants were dissolved in one part of the water; those nitrate solutions were then thoroughly mixed with soil and allowed to stand for 24 h. The other part of the water was mixed with the binders and stirred until a slurry formed. The slurry was subsequently added to the contaminated soils and fully mixed. Cylindrical specimens (U38.1 mm H80 mm) and cutting ring specimens (U61.8 mm H20 mm) were prepared using a static compaction method, which depended on the 95% maximum dry density. Finally, the specimens were sealed in plastic bags and stored in the standard curing room (humidity 95 ± 5%, temperature 20 °C). The specimens for unconfined compressive tests were cured for 3, 7, 28 and 90 days (d). In order to monitor the hydration process of cement and fly ash by electrical resistivity method [56,53], curing time of specimens for electrical resistivity test was designed as 0 h (h), 1 h, 2 h, 3 h, 6 h, 12 h, 1 d, 3 d, 7 d, 14 d, 21 d, 28 d and 90 d.
2. Test materials and methods 2.1. Test materials 2.1.1. Soil sample The tested soil was obtained from a project site located in Hefei, China. The soil was sampled at a depth of 4–5 m, had a yellowishbrown, hard-plastic appearance. The indexes of the basic physical properties are listed in Table 1. The liquid limit and plastic index of the tested soil were 49.4% and 24.6, respectively. Thus, the tested soil can be classified as low liquid limit clay (CL) according to the Unified Soil Classification System (ASTM D2487-00) [7]. 2.1.2. Binders The binders used in the test included Portland blast furnace slag cement (P325) and fly ash (FA). The main composites of both cement and fly ash were CaO, SiO2, Al2O3 and Fe2O3, as determined by X-ray fluorescence, and are shown in Table 2. Based on ASTM C618 [8], the used fly ash can be classified as F for the total content of SiO2, Al2O3 and Fe2O3 in fly ash surpassed 70%. The stabilized soils were divided into two groups. One group was stabilized with 10% cement (denoted as C10), and the other was stabilized with 10% cement blended with 30% fly ash (denoted as C10 + F30).
2.2.2. Unconfined compressive strength test The unconfined compressive strength (qu ) test was conducted on the specimens with a strain rate of approximately 1% / min based on the ASTM procedure (D5102, [9]). The stress and strain of the specimens were recorded at 15 s intervals. 2.2.3. X-ray diffraction (XRD) XRD analysis was performed on the specimens to identify the products of the hydrated reactions. The specimens were freezedried and were then pulverized and passed through a 0.075 mm sieve. XRD analyses were performed with Cu-Ka radiation on a Rigaku D/Max-2005 V.
2.1.3. Heavy metal contaminants Based on the recent investigation related to heavy metal contaminated soils in China performed by Duan et al. [21], cadmium
Table 1 Basic physical properties of the tested soil. Density/g/cm3
Water content/%
Gravity
Liquid limit/%
Plastic limit/%
Plastic index
Optimum water content/%
Maximum density/g/cm3
1.92
25.5
2.669
49.4
24.8
24.6
21.2
1.663
Table 2 Composites and relative contents of cement and fly ash. Composites Content/%
C FA
CaO
SiO2
Al2O3
SO3
Fe2O3
MgO
TiO2
Na2O
CO3
43.798 1.062
27.209 48.793
9.895 29.814
3.013 /
2.880 2.920
1.565 0.641
0.468 1.755
0.442 0.462
/ 12.864
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2.2.4. Scanning electron microscope (SEM) SEM was carried out to analyze the microstructure of the specimens. The samples used for SEM analysis were freeze-dried and broken slightly to pieces. The samples with approximate dimensions of 5 mm 5 mm 5 mm were selected, and their surface was cleaned using a hairbrush. To achieve sufficient electrical conductivity, the sample surface was pre-treated by vacuum metal spraying technology. 3. Electrical resistivity theory and methods 3.1. Electrical resistivity theory A benchtop LCR meter (INSTEK LCR-816) was used in this study to measure sample resistance (the ratio of applied voltage to resultant current flow) in 2-terminal mode. The apparatus is illustrated in Fig. 2 (a) and (b). The electrical resistivity was then derived from the specimen/electrode dimensions using Eq. (1).
q¼
RS L
(a) Soil sample used for electrical resistivity measurement
ð1Þ
where R represents the resistance of soil sample (X); S represents the contact area between the electrode plates and the soil sample (m2); L represents the distance between the two electrode plates (m). Both the horizontal and vertical electrical resistivity were measured due to the anisotropy of soil. Then, the average electrical resistivity q, the average pore water electrical resistivity qx, the average formation factor F, the average shape factor f and the anisotropy coefficient A were calculated with Eqs. (2)–(5) [4]:
2qH þ qV 3 2FH þ FV q q F¼ FH ¼ H ; FV ¼ V 3 qx qx
q¼
f ¼ nF sffiffiffiffiffiffi FV A¼ FH
ð2Þ ð3Þ
(b)
Electrode used for the top or bottom surface of sample
ð4Þ ð5Þ
where qH represents the horizontal electrical resistivity (Xm); qV represents the vertical electrical resistivity (Xm); FH is the horizontal formation factor; FV is the vertical formation factor; and n represents the porosity.
(c)
Electrode used for the side surface of sample
Fig. 1. Sample used for electrical resistivity measurements.
3.2. Electrical resistivity tested method In general, soil electrical resistivity consists of the electrical resistivity of the soil particles and the pore water [31]. The electrical resistivity was determined by the two-electrode probe method. To overcome the polarization phenomenon of the electrodes, the frequency of alternating current was set to 2000 Hz [4]. 3.2.1. Average electrical resistivity Sample for electrical resistivity measurement is shown in Fig. 1 (a). Preparing four metal plates, two had shapes identical to the top and bottom surfaces of the samples, and the other two had the same shape as the side planes, as shown in Fig. 1(b) and (c). The vertical and horizontal resistance of sample can then be determined by apparatus LCR-816 based on the schematic diagram as shown in Fig. 2. A pressure of 25 kPa was applied to the electrode plates to ensure full contact between the electrodes and soil sample. The average electrical resistivity was then calculated using Eq. (2).
3.2.2. Pore water electrical resistivity Based on the method introduced by the Japanese Geotechnical Society (2000), the pore water electrical resistivity was measured. The tested sample was oven-dried and then pulverized. The dried soil was mixed with distilled water at a mass ratio of 1:5 and was stirred for 30 min. This mixture was poured into a polymethyl methacrylate box and was allowed to stand for two hours. Finally, the measured electrical resistivity of the suspension was defined as pore water electrical resistivity. A diagram of the tested setup is shown in Fig. 3. 4. Results and discussion 4.1. Effects of curing time on electrical resistivity Fig. 4 shows the relationships between the average electrical resistivity and the curing time. The samples were contaminated
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Fig. 3. Schematic for: (a) the cell for pore water electrical resistivity measurement; and (b) diagram of pore water electrical resistivity measurement. Fig. 2. Schematic of the two-electrode probe method: (a) apparatus used for resistance measurement; (b) diagram of determining the resistance of soil sample.
with Pb0.1, Pb0.5, Pb1.0 and Pb1.5 and were then treated with C10 and C10 + F30. As shown in Fig. 4, during the initial curing periods (7 d), the average electrical resistivity fluctuated slightly. As the curing time increased from 7 days to 90 days, the average electrical resistivity clearly increased as well. Compared with the C10 treated samples, the samples treated with C10 + F30 had lower electrical resistivity. The class F fly ash used had low hydraulicity and contained some metal cations, which can increase the electrical conductivity of soils. As shown in Fig. 4, we also observed that for lower Pb2+ concentrations, the electrical resistivity obviously increased after a 7 days curing time. However, for higher Pb2+ concentrations, the electrical resistivity gradually increased until the curing time was more than 21 days. Thus, it can be seen that the increasing heavy metal concentration may delay the development of hydrated reactions. 4.2. Effects of heavy metal species and concentrations on electrical resistivity The variations of average electrical resistivity with heavy metal concentrations are shown in Fig. 5. The specimens were stabilized with C10 and C10 + F30 and were cured for 28 days. It can be seen from Fig. 5 that increasing heavy metal concentrations led to a reduction in electrical resistivity. Different heavy
Fig. 4. Effect of curing time on the average electrical resistivity of the stabilized Pbcontaminated soil.
metal species also caused different influences on the electrical resistivity behavior of the stabilized soils. As shown in Fig. 5, the electrical resistivity of the three types of contaminated soils was Pb-contaminated soil > Zn-contaminated soil > Cr-contaminated soil.
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Fig. 5. Relationship concentration.
between
electrical
resistivity
and
heavy
metal
ion
Fig. 7. Relationship between qu/qu0 and q/q0.
4.3. Relationships between qu and q Based on the unconfined compressive strength and electrical resistivity test results, a correlation analysis was performed. The relationships between qu and q of the Pb-contaminated soils treated with C10 and C10 + F30 were proposed. As shown in Fig. 6, qu was the exponential function of q, regardless of the Pb2+ concentration. To predict qu by q more simply, qu and q of the specimens were normalized by dividing the corresponding initial value without curing. The calculated qu =qu0 and q=q0 values were plotted in Fig. 7, and a linear correlation between qu =qu0 and q=q0 was observed. The relationship is expressed by Eq. (6):
^ q=q þ 1:082; R2 ¼ 0:8954 qu =qu0 ¼ 0:418A 0
ð6Þ
where qu represents the unconfined compressive strength (MPa) tested at a certain curing time; qu0 represents the strength without curing; q is the electrical resistivity (X m) tested at a certain curing time; and q0 represents the electrical resistivity without curing. Thus, qu of the stabilized specimens can be predicted based on q.
Fig. 6. Relationship between electrical resistivity and unconfined compressive strength.
Fig. 8. Pore water electrical resistivity: (a) effects of curing time; (b) effects of heavy metal concentration and species.
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4.4. Immobilization mechanism based on electrical resistivity 4.4.1. Pore water electrical resistivity (qx ) Fig. 8 shows the effects of curing time and heavy metal concentrations on the average pore water electrical resistivity. As shown in Fig. 8(a), the pore water electrical resistivity increased as the curing time increased for the stabilized Pbcontaminated soils. Compared with C10 specimens, the C10 + F30 samples had higher pore water electrical resistivities. Liu et al. [37] proposed that a conductive path of soil contained soil particles and pore solutions. During the initial phase of hydration, there were two types of reactions taking place: ① OH, Na+, K+, Ca2+, SO24 etc., existed in the fly ash dissolved and released into the pore solution attributed to contact with water [54–55], ② dissociative ions in the pore solution participated in the early hydration reactions and formed hydrated products (such as ettringite, Ca (OH)2, etc.) that can fill in soil pores. The first type of reaction reduced the electrical resistivity, while the second increased it. The combined actions of these two reaction types initially negated each other, resulting in a slight change in soil electrical resistivity during the initial curing time. It is well known that the electrical conductivity of a solution depends on the concentration and charge of the dissolved ions. For a specific ionic species increasing the concentration will increase the conductivity (and reduce the resistivity). As seen in Fig. 8(a), increasing the curing time caused an increase in the pore water electrical resistivity. This result indicated that the concentration of dissociative ions in the pore solutions decreased, because some of them participated in the hydrated reactions. For instance, Ca2+ participated in the hydrated reaction to form CSH and Ca(OH)2 which had low electrical conductivity in the relatively alkaline environment provided by cement and fly ash. Pb2+ can be immobilized by precipitation, adsorption, encapsulation and isomorphous substitution, which reduced the electrical conductivity of pore solutions. Furthermore, the addition of fly ash accelerated the immobilization of Pb2+ according to adsorption and pozzolanic reactions, which resulted in a higher pore water electrical resistivity of the specimens stabilized with C10 + F30. As shown in Fig. 8(b), a reduction in the pore water electrical resistivity was observed as the heavy metal concentration increased. The pore water resistivity of the three contaminated soils exhibited different electrical conductivities: Pbcontaminated specimens < Zn-contaminated specimens < Crcontaminated specimens. As above-mentioned, the electrical conductivity of the pore solution is primarily controlled by ion concentration and ionic charge. Thus, the decrease in pore water electrical resistivity revealed that the ion concentration in the pore water increased. Moreover, because of the differences in the electrical charges and ionic radii of Pb2+, Zn2+ and Cr3+, the types of specimens exhibited obvious discrepancies in the electrical resistivity.
4.4.2. Formation factor (F) The variations in the average formation factors with curing time and heavy metal concentrations are presented in Fig. 9. Fig. 9(a) shows that increasing curing time led to an increase in the formation factors. Previous research groups proposed that the formation factor can reflect the porosity and porous structure characteristics [5]. The increased formation factor revealed that the porosity decreased and that the structure connecting strength increased as the curing time increased. During the development of the hydrated reactions, more and more hydrates were produced and filled in the soil pores, which then resulted in a decrease in pore volume. Thus, the amount of conductive paths was reduced. In addition, the dispersive soil particles were aggregated by the
Fig. 9. Average formation factor: (a) effects of curing time; (b) effects of heavy metal concentration and species.
cementitious materials, and they enhanced the structure connecting strength of the stabilized specimens. Besides, as shown in Fig. 9(a), the formation factor of the specimen C10 increased slightly during the initial curing period and then clearly increased as the curing time varied from 28 days to 90 days. The formation factor of the specimens treated with C10 + F30 started increasing after the curing time surpassed 7 days. These results demonstrated that the addition of fly ash accelerated the hydrated reactions. As shown in Fig. 9(b), the formation factor decreased as the heavy metal concentration increased. The decreased formation factor indicated that increasing heavy metal concentration contributed to a larger soil pore volume. Previous studies reported that the presence of Pb2+ hindered the formation of CAH. Pb2+ can also precipitate as Pb(OH)2, which would encapsulate the binder particles in the alkaline environment offered by cement and fly ash Qiao [43,44,11]. Thus, cement hydration can be delayed. Furthermore, the presence of Zn2+ also limited the generation of hydrated products by encapsulating the cement particles with the products formed by reactions between Zn2+ and cement components [61,19]. Accordingly, the quantity of hydrates filled in the soil pores decreased due to the increase of heavy metal concen-
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tration, and the connective strength of the soil structure decreased as well.
4.4.3. Shape factor (f) Fig. 10 presents the effects of curing time and heavy metal concentration on the average shape factor of the stabilized Pbcontaminated soils. Thevanayagam [48] suggested that the shape factor was a parameter related to the shape of soil particles and the cementation degree. As shown in Fig. 10(a), the shape factor increased with curing time. With hydrated reactions proceeding, the cementitious hydrated products continuously formed. The gaps among the soil particles were filled with the hydrates, which enlarged the size of aggregates and enhanced the cementation degree of the specimens as well. Fig. 10(b) shows that the shape factor decreased as the heavy metal concentration increased. It can be concluded that the presence of a heavy metal or the generation of some components containing heavy metals decreased the cementitious properties of the hydrates. For instance, Cr3+ can replace Ca2+, Al3+, Si4+ and Fe3+, which all exist in the hydrated products and generate Cr-bearing compounds without gelatinization[34,32,41].
Fig. 10. Average shape factor: (a) effects of curing time; (b) effects of heavy metal concentration and species.
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4.4.4. Anisotropy coefficient (A) The relationships between the anisotropy coefficient of the stabilized specimens and the curing time are shown in Fig. 11. Arulanandan and Muraleetharan [6] suggested that the anisotropy coefficient can describe the orientation of soil particles. With curing time increased, the shape and size of the soil particles, the porosity, and the structure of the specimens changed based on the results of the formation factor and the shape factor. Therefore, the arrangement of soil particles varied with the curing time. As shown in Fig. 11(a), the anisotropy coefficient decreased as the curing time increased. This result confirmed that the directional permutation of soil particles decreased that the connectivity weakened, which hindered the conductive path. Increasing the heavy metal concentration led to a growth in the anisotropy coefficient as shown in Fig. 11(b). The increased anisotropy coefficient revealed that the presence of a heavy metal impeded the development of cement hydration and affected the arrangement of the soil particles. 4.4.5. Mineralogy and microstructure 4.4.5.1. X-ray diffraction analysis (XRD). The XRD patterns of Zncontaminated soils stabilized with C10 curing for 1, 7, 28 and
Fig. 11. Anisotropy coefficient: (a) effects of curing time; (b) effects of heavy metal concentration and species.
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Fig. 12. X-ray diffractograms of Zn-contaminated specimens stabilized with C10: (a) curing for 1 day; (b) curing for 7 days; (c) curing for 28 days and (d) curing for 90 days.
90 days are shown in Fig. 12. The XRD patterns indicated that the primary hydrated products of cement included calcium aluminate hydrate (CAH), calcium silicate hydrate (CSH), ettringite (AFt), calcium hydroxide (CH) and calcium monosulfoaluminate hydrate (AFm). In addition, some Zn2+ ions were incorporated into the crystal lattice of CH and formed as CaZn2(OH)62H2O based on the peaks that appeared at 2h of 28° and 36.5°. Previous studies reached similar conclusions [39,36]. Compared with Fig. 12(a) and (b), the intensity of these hydrates increased slightly as the curing time increased from 1 to 7 days. When the curing time increased to 28 days, the intensity of the hydrates obtained clearly and rapidly increased at 90 days, as shown in Fig. 12(c) and (d). In particular, the intensity increase was more pronounced for Aft and CSH at 2h of 29.5°, 40.3°, 50.2° and 60°. In conclusion, the XRD results agreed with the development of electrical resistivity as a function of the curing time.
4.4.5.2. Scanning electronic microscopy analysis (SEM). Fig. 13 shows the microstructure of the Zn-contaminated soils treated with C10 at 7 days and 90 days of curing time. As shown in Fig. 13(a), some cracks and pores are observed in the specimens cured for 7 days. There were few hydrates filling in the soil pores, and the structure exhibited a low cementation degree. For the specimens cured for 90 days, as illustrated in Fig. 13(b), many hydrated products formed as the cement hydration fully developed. A large amount of fibrous and flocculent CSH was distributed throughout the samples, occupied the soil pored and resulted in a denser soil structure. The dispersive aggregates were connected with needle-like Aft, and a higher cementation degree was obtained. Furthermore, some hexagonal CAH or AFm were observed and enhanced the soil structure as well. The observation of CSH, AFt, CAH and AFm confirmed the results revealed by XRD analysis. Changes in the microstructure of specimens contaminated with 10000 mg/kg Zn2+ were analyzed by SEM. As shown in
Fig. 13(c), very little AFt was observed in these samples. The quantity of hydrated products was insufficient to fill the soil pores and led to a higher void volume compared with 1000 mg/kg Zn2+ specimens. Therefore, it can be concluded that increasing the Zn2+ concentration retards the development of cement hydration and that the structure of hydrated products was damaged. Compared with the variations in the micro-structure characteristics and the electrical resistivity parameters, a good correlation was observed. This correlation indicated that this electrical resistivity methodology can be applied to the assessment of immobilization effectiveness of heavy metal contaminated soils treated with cement and fly ash.
5. Conclusions This paper investigated electrical resistivity behaviors and utilizing the variations in electrical resistivity parameters revealed the immobilization mechanisms of stabilized heavy metal contaminated soils. The primary conclusions are as follows: (1) As the curing time increased, the electrical resistivity, pore water electrical resistivity, formation factor, and shape factor of the specimens also increased. (2) Increasing the heavy metal concentration led to an obvious decrease in the electrical resistivity, pore water electrical resistivity and shape factor. Different electrical resistivity behaviors were observed for the specimens contaminated with different heavy metals. (3) A linear correlation between qu =qu0 and q=q0 was found, which can be used to predict the UCS of the stabilized specimens. (4) The XRD results indicated that, with increased curing time, the quality of hydrates such as CSH and ettringite also increased and that the soil pores gradually filled. Increasing the Pb2+ concentration retarded cement and fly ash hydration.
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Acknowledgments This research is financially supported by the National Natural Science Foundation of China (grant No. 41672306, 41877262, 41807239, 41372281), the Special Project for Major Science and Technology in Anhui Province, China (18030801103). References
(a) Zn.0.1, C10, curing 7 days
(b) Zn0.1, C10, curing 90 days
(c) Zn1.0, curing 90 days Fig. 13. Graphs of the microstructure of the solidified zinc contaminated soil.
(5) The variations in the microstructure characteristics of the solidified specimens were consistent with that of the electrical resistivity parameters when the curing time and heavy metal concentration increased.
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