Hydration characteristics and microstructure of magnesium phosphate cement in presence of Cu2+

Construction and Building Materials 225 (2019) 234–242

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Hydration characteristics and microstructure of magnesium phosphate cement in presence of Cu2+ He Xin a, Lai Zhenyu a,⇑, Yan Tao a, Wu Jie a, Lu Zhongyuan a,⇑, Lv Shuzhen a, Li Feng b, Fan Xiaoling b a Southwest University of Science and Technology, School of Materials Science and Engineering, State Key Laboratory of Environment-Friendly Energy Materials, Mianyang, Sichuan 621010, China b Chengdu Jiangong Precast Concrete Technology Co. Ltd, Chengdu, Sichuan 610065, China

h i g h l i g h t s
The effect of Cu

2+

on hydration and strength of magnesium phosphate cement is significant. 2+ is increased. 2+
The rate of change of pH and the crystallinity of the main hydration product are reduced due to the presence of Cu . 2+
Cu are mainly solid soluble in hydration products the MKPCs.


The final setting time of MKPC is prolonged as the amount of Cu

a r t i c l e

i n f o

Article history: Received 26 April 2019 Received in revised form 7 July 2019 Accepted 19 July 2019

Keywords: Cu2+ Magnesium phosphate cement Hydration Microstructure Leaching properties

a b s t r a c t Heavy metal pollution is one of the severe problems facing the environment, and magnesium phosphate cement (MKPC) is increasingly used for the solidification/stabilization (S/S) treatment of heavy metals due to their excellent properties, the hydration characteristics and microstructure of MKPCs plays a key role in the S/S treatment of heavy metals, but this has not been sufficiently studied. In this study, the effects of Cu2+ on the hydration characteristics of MKPCs were analyzed using compressive strength, setting time, pH, heat of hydration, the phases and microstructure of samples were investigated by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy measurements and Cu2+ leaching rate of samples were also measured. The results indicated that the addition of Cu2+ reduced the compressive strength of MKPCs and also delayed the setting time at high magnesia to potassium dihydrogen phosphate (M/P) ratios. When the addition of Cu2+ was 4%, the compressive strength of the sample cured 12 h decreased by 73% and the final setting time of the sample was prolonged by 94% with the M/P ratio of 4:1. Cu2+ did not affect the main hydration products of MKPCs, but lowered the change rate of the pH, got the smaller exothermic hydration peak, and reduced crystallinity of the main hydration product. Moreover, the leaching concentration (1.7 mg/L) of Cu2+ measured using the toxicity characteristics leaching procedure was lower than the TCLP (Toxicity Characteristic Leaching Procedure) requirement (15 mg/L). A lower M/P ratio is beneficial to improve the S/S performance of Cu-incorporated MPC. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Owing to the rapid development of industrialization, toxic heavy metals such as Cd2+, Pb2+, Cr3+, Zn2+, Cu2+, etc. are ubiquitous in water and soil, not only polluting the environment but also posing severe threats to plants, animals, and even human beings [1,2]. Among them, Cu2+ has been widely used for petroleum, paper, and paperboard processing, ceramics manufacturing, electroplating,

⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Lai), [email protected] (Z. Lu). https://doi.org/10.1016/j.conbuildmat.2019.07.184 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

glass manufacturing, fertilizer production, etc., which also induced severe environmental pollution such as soot emitted from copperzinc ore smelting and wastewater discharged from electroplating and metal processing [3,4]. For human beings, higher concentrations (>5 mg/L) of Cu in the body has been associated with severe renal failure and liver diseases [5]. Therefore, there is an urgent need to develop technologies that effectively immobilize Cu2+ to reduce environmental pollution. Among the available technologies, the S/S technology is a conventional and effective method for treating various wastes, particularly heavy metal contaminants [6–9]. The US Environmental

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Protection Agency (EPA) has reported that the S/S technology was selected for 13% of the ex-situ source treatment procedures, in the decision documents of the Superfund Remedy report [10]. Ordinary Portland cement (OPC) is often used for the S/S technique. The S/S technique mixes wastes with cement to form a solidified body of a certain strength, to reduce the leaching of hazardous components from the waste. Researchers have improved the S/S technology by using low-carbon and green materials [11–14]. However, some studies [15–17] have demonstrated that heavy metals can prolong the setting time of OPC and decrease its compressive strength. Moreover, the diameter of the OPC pore is higher in the presence of heavy metals contaminants. FS Hashem et al. [18] reported the presence of Cu (II) ions could retard to the hydration of C3S. These effects are harmful when addressing heavy metal waste using OPC. Recently, more studies have been performed on the S/S of heavy metals using an alternative, such as calcium sulfoaluminate cement [19–21] and magnesium phosphate cement [22–29]. MPCs, also known as chemically bonded phosphate ceramics, are cementitious materials obtained through acid-base reactions between dead-burned magnesia and soluble acid phosphates. The main constituent of MPCs is MgNH4PO46H2O (struvite) or MgKPO46H2O (K-struvite) [30–34]. Ammonia can, however, be released when ammonium phosphate is used to produce MPCs. Hence, potassium MPC (MKPC), which is produced using potassium dihydrogen phosphate (KH2PO4) has been studied by many scholars in recent years [35–43]. For example, Hongyan Ma et al. [44] confirmed by modeling that if the w/c of the MKPC-based material remains constant, there is an optimum m/p resulting in the highest compressive strength. Yanshuai Wang et al. [45] found that the use of reactive magnesia sands to replace inert quartz sands could improve the M/P molar ratio of the paste and the performance of the MKPC mortar. Owing to its excellent properties, such as strong bonding, fast-setting, low porosity, and high early strength, MKPC has been extensively used for the S/S of heavy metals and even radioactive waste [34,46–50]. Although MKPC presented an excellent performance on S/S treatment, few researchers have systematically studied the mechanism of heavy metals ions on the hydration process and microstructure of MPCs. However, the analysis of hydration mechanism and microstructure has a crucial role in improving S/S performance, but different heavy metal ions have significant differences in hydration mechanism. For example, Pb2+ [51] have a weaker influence on the hydration process, while Zn2+ [22] and Cd2+ [48] have a relatively significant influence, which their presence will prolong the setting time of MKPC due to reaction with phosphate ions. Therefore, in order to improve the S/S effect, it is necessary to carry out research on hydration characteristics and microstructure of different heavy metal ions to MPCs, which will provide more reliable data support for the treatment and disposal of hazardous wastes, it also provides a basis for the use of MPCs in certain specific environments. The main objective of this study was to analyze the effects of Cu2+ on the hydration characteristics and microstructure of MKPC. The compressive strength, setting time, pH, and hydration heat were investigated, along with the phases and microstructures of the specimens. Furthermore, the leaching concentration of Cu2+ was measured using the toxicity characteristics leaching procedure (TCLP), to determine the stability of Cu2+ stabilized by the MKPC. 2. Materials and methods 2.1. Materials The raw materials for this study included KH2PO4 (Analytical Reagent, 99.5%), dead-burned magnesia (MgO, Industrial purity, 90%), copper nitrate (Cu(NO3)2, Analytical Reagent, 99.7%), borax powder (Analytical Reagent, 99.5%), and water. The Liaoning Xinrong Mining Group supplied the dead-burned magnesia. The

Table 1 Chemical composition of dead-burned magnesia. MgO

SiO2

CaO

Fe2O3

Al2O3

SO3

Others

89.89

3.54

2.58

1.91

1.06

0.61

0.41

chemical composition of the dead-burned magnesia was determined using X-ray fluorescence spectrometry (XRF), and the results are listed in Table 1. The borax and KH2PO4 used in this study were industrial purity and were provided by the Chengdu Kelon Chemical Reagent Factory and Fine Chemical Plant of Shifang, Sichuan province, China, respectively. The Cu(NO3)2 employed as an analytical grade chemical reagent.

2.2. Sample preparation and test methods To study the effects of the Cu2+ content on the setting time and compressive strength of MKPCs, all sample mixtures prepared were divided into three series, which were named A, B, and C. Six samples were tested to get the average values. For each series, the weight ratio of Cu(NO3)2 to solids (KH2PO4, borax, and MgO) was controlled at 0, 1, 2, 3, and 4%. Furthermore, the weight ratio of borax to dead-burned magnesia (B/M) was 0.1, and the weight ratio of water to solid (w/s) was 0.14. Besides, the weight ratios of dead-burned magnesia to KH2PO4 (M/P) of the three series were 2:1, 3:1, and 4:1 which the molar ratios were about 7:1, 10:1 and 13:1, respectively [22,27,34,52]. The mixing proportions of all samples are listed in Table 2. The setting time referred to the final setting time due to rapid hardening of the MKPCs, and the consistency of the slurry gradually decreased as the stirring time was extended. All samples were stirred at the temperature of 20 ± 2 °C and relative humidity greater than 50%, and the final setting time of the samples was measured using a modified Vicat needle. The powders were first slowly premixed in a blender for 60 s, then slowly stirred with water for 10 s, and finally rapidly stirred for 60 s. The slurry was cast in 20  20  20 mm molds for measuring the compressive strength. All samples were cured at a temperature of 20 ± 1 °C and relative humidity of 60%. The compressive strength of the samples was tested at 12 h, 1 day, 3 days, 7 days, 14 days and 28 days using a SANS-CMT5105 (China) machine at the loading rate of 500 N s1. During the pH value test, the phosphate was first dissolved in deionized water. The pH of the solution was tested after the raw material was entirely dissolved, and was considered to be the initial pH value of the system. Then, the pH of the system was tested and recorded by using a PHS-3C acidometer (Shanghai Instrument and Electrical Science Instrument Co., Ltd., China) after dead-burned magnesia and copper nitrate were added. The formula design of pH testing is presented in Table 3. The hydration heat of samples was monitored using an isothermal conduction calorimeter (TAM Air, TA Instruments, USA) and the formula design for the heat of hydration testing is presented in Table 4. The phases of the specimens were analyzed using a PANalytical X-ray diffraction (XRD, X’Pert Pro, Netherlands) machine. The XRD patterns were collected using Cu-Ka1 radiation (kKa1 = 1.5406 Å) and a 0.02° step. The morphological changes and micro-region composition of the hydration products of the samples were measured using a MAIA3 scanning electron microscopy (SEM, TESCAN, Czech Republic) instrument and an Octane Super energy dispersive spectrometry (EDS, AMETEK, USA) device. The Cu2+ leaching concentration test data were compared to the standard TCLP results reported by the EPA (USEPA. EPA Test Method 1311-TCLP) [53]. After curing for 28 days, dried 20 g samples were extracted in a horizontal oscillator using a

Table 2 Formula design of strength and setting time. Series

Cu(NO3)2 (wt%)

M/P (weight ratio)

M/P (molar ratio)

B/M

w/s

A0 A1 A2 A3 A4

0 1 2 3 4

2/1

7/1

0.1

0.14

B0 B1 B2 B3 B4

0 1 2 3 4

3/1

10/1

0.1

0.14

C0 C1 C2 C3 C4

0 1 2 3 4

4/1

13/1

0.1

0.14

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X. He et al. / Construction and Building Materials 225 (2019) 234–242

Table 3 Formula design of pH testing.

0% 1% 2% 3% 4%

11

Series

Cu(NO3)2 (wt%)

M/P (weight ratio)

M/P (molar ratio)

w/s

A0 A1 A2 A3 A4 B0 C0

0 1 2 3 4 4 4

4/1 4/1 4/1 4/1 4/1 3/1 2/1

13/1 13/1 13/1 13/1 13/1 10/1 7/1

10:1 10:1 10:1 10:1 10:1 10:1 10:1

10 9

pH

8 7 6

Table 4 Formula design for heat of hydration testing.

5

Series

Cu(NO3)2 (wt%)

M/P (weight ratio)

M/P (molar ratio)

w/s

A0 A1 A2 A3 A4 B0 C0

0 1 2 3 4 4 4

4/1 4/1 4/1 4/1 4/1 3/1 2/1

13/1 13/1 13/1 13/1 13/1 10/1 7/1

5:1 5:1 5:1 5:1 5:1 5:1 5:1

4 3 0

20

40

60

80

time (min)

(a) 10

4:1 3:1 2:1

9

liquid-solid ratio of 20:1 and pH of 2.88 ± 0.05 obtained utilizing glacial acetic acid and sodium hydroxide at the oscillator speed of 120 r/min. After the extraction, the supernatant was separated, and the concentration of Cu2+ was detected using an iCAP6500 inductively coupled plasma optical emission spectrometry (ICP-OES, ThermoFisher, USA) instrument.

8

7

pH

3. Results and discussion 3.1. Effect of Cu2+ on pH of MKPC paste The changes in the pH of the samples are illustrated in Fig. 1. It can be seen from Fig. 1(a) that the addition of Cu2+ does not change the pH change trend of the hydration reactions of MKPCs. The pH levels of all samples indicate acid-to-base changes. Moreover, two distinct inflection points in the reaction process can be observed. This indicates that in the presence of Cu2+, the hydration reaction of MKPCs mainly occurs as follows:

H2 PO4



þ MgO þ 7H2 O ! MgHPO4  7H2 O þ OH

2MgHPO4  7H2 O þ Kþ þ OH ! Mg2 KHðPO4 Þ2  15H2 O

ð1Þ ð2Þ

6

5

4

3 0

20

40

60

80

100

120

140

time (min)

(b) Fig. 1. Effect of Cu2+ on pH of hydration reaction of MKPC samples. (a) M/P = 4 with various copper nitrate contents; (b) 4% copper nitrate content with various M/ P ratios.

Mg2 KHðPO4 Þ2  15H2 O þ Kþ þ OH ! 2MgKPO4  6H2 O þ 4H2 O

ð3Þ

According to Chau [54], the formation of MgHPO47H2O and Mg2KH(PO4)215H2O is the main reasons for the inflections in the reaction process, while Eq. (1) of the hydration process is responsible for the pH change from acid to alkaline. Besides, borax also plays a role in adjusting pH. Its water solution is weak alkaline + as the borax releases B4O2 7 and Na after contact with water, while KH2PO4 is a polybasic weak acid salt and can hydrolyze to produce H+ in water. Therefore, when they are mixed in water, B4O2 7 from borax easily combines with H+ from KH2PO4 to form boric acid, which reduces the amount of H+ and thereby increases the pH in the early mixed solution [55]. From the time, the change rate of the pH gradually decreased as the Cu2+ content increased, in this study. When the Cu(NO3)2 content of the MKPCs was 0 wt%, the reaction that produced the intermediate hydration products was completed quickly, approximately 10 min after the beginning of the experiment, and the pH of the system increased from 4.47 to more than 10.50 when the reaction proceeded to 60 min. When the content of Cu(NO3)2 increased to 4 wt%, the change rate of the pH was decreased, and the hydration reaction of the

intermediate products was completed in approximately 25 min. When the reaction that produced the intermediate hydration products lasted up to 80 min, the pH of the system only increased from 3.80 to 8.48. Also, the second inflection point of the hydration reaction almost disappeared when the content of Cu(NO3)2 was 4 wt%. From Fig. 1(b), it can be seen that when the content of Cu(NO3)2 of the MKPCs was 4 wt%, the pH change rate of the MKPCs with different M/P ratios present different trends: for the MKPCs with the M/P ratio of 2:1, the hydration reaction of the intermediate products was completed approximately 25 min after the beginning of the experiment, and when the experiment was extended to 100 min, the pH of the system increased to 8.92. For the MKPCs with the M/P ratio of 4:1, the pH of the system was 8.42 at 100 min, and the change rate of the pH was lower than that of samples with the M/P ratio of 2:1 and 3:1. Generally speaking, the change trends of the pH of MKPCs with different M/P ratios were similar for the MKPCs containing 4 wt% Cu(NO3)2, but the change rate increased as the M/P ratio of the MKPCs decreased. The above phenomenon may be attributed to the addition of Cu (NO3)2 not changing the main hydration reaction type of the

X. He et al. / Construction and Building Materials 225 (2019) 234–242

system for MKPCs with different M/P ratios. However, the Cu2+ in the systems could combine with phosphate ions and other anions to form a specific copper complex. The formation of this complex would reduce the amount of phosphate ions involved in the main hydration reaction and hinder the rate of OH generation in Eq. (1), thereby lowering the change rate of the pH of the system. For specific M/P ratios, the influence of Cu2+ increased as the amount of Cu2+ in the system increased. When the M/P ratio decreased, the amount of phosphate ions in the system slightly increased, and the influence of Cu2+ was relatively weakened. 3.2. Effect of Cu2+ on the hydration heat of MKPC paste Fig. 2 illustrates the effect of Cu2+ on the hydration heat of MKPCs. As can be seen, the increase in Cu2+ content does not change the exothermic hydration trend of MKPCs, and two exothermic peaks are identified. The first exothermic peak is sharper than the second one, which indicates that the system releases a large amount of heat in a short time when the first exothermic peak appears. Fig. 2(a) illustrates that the peak value of the first hydration exothermic peak decreases significantly as the Cu2+ ions content increases for MKPCs with M/P ratios of 4:1. However, the time of occurrence of the first peak does not change, while the peak value of the second hydration exothermic peak is reduced, and its occurrence is delayed. 0.055

0.06

0% 1% 2% 3% 4%

0.050

0.045

0.05

0.040

0.035

heat flow (mw)

0.030

0.04

80200

80400

80600

0.03

80800

81000

0.012

0.010

0.008

0.006

0.004

0.01

0.002 85000

90000

95000

100000

105000

0.00 80000

100000

120000

140000

160000

time (s)

(a) 0.04

4:1 3:1 2:1

0.036

0.034

0.032

0.03

Based on these results, we concluded that the dissolution of MgO mainly caused the first exothermic peak for the hydration process of MKPCs, while the second exothermic peak was mainly due to the formation of the hydration reaction products. This indicated that the addition of Cu2+ during the hydration reaction of MKPCs could affect the amount of dissolved MgO in the system to a certain, yet small, extent. On the other hand, from the changes in the second hydration exothermic peak in Fig. 2, the presence of Cu2+ in the system affected both the number of main hydration products and their formation rates. The above phenomena may be attributed to the formation of a Cu compound during the hydration process of MKPCs under experimental conditions. This compound would consume a certain amount of phosphate ions, thus resulting in a relative reduction in the amount of phosphate ions available for the formation of the major hydration products. Therefore, the heat released by the dissolving of MgO and the chemical reaction forming MgHPO47H2O was decreased. On the other hand, the copper complex may adhere to the surface of MgO and hinder the hydration reaction of MKPCs to a certain extent, which would eventually delay the appearance of the second exothermic peak during the hydration process of MKPCs. As the content of Cu (NO3)2 increased, the effect of the Cu2+ ions on the hydration reaction of MKPCs was also relatively increased. Fig. 2(b) illustrates that the hydration heat release curves of MKPCs of different M/P ratios containing 4 wt% Cu(NO3)2 are close. However, the second peak of the sample with the M/P ratio of 2:1 is significantly higher than those of the other samples. This indicated that the hydration products were easier to form when the M/P ratios of the MKPCs were lower. 3.3. Effect of Cu2+ on hydration phases of hardened MKPC

81200

0.014

0.02

237

0.030

Fig. 3(a) illustrates that for MKPCs with M/P ratios of 4:1, increasing the amount of Cu2+ does not change the type of the main hydration products. The primary diffraction peaks in the diffraction patterns of the hydration samples aged for 28 days are still attributed to K-struvite and MgO. This indicates that no apparent crystal phases of Copper were produced during the hydration reaction of MKPCs, and Cu2+ may exist in the system as certain compounds or in the hydration products in the solid solution state. From Fig. 3(b), it can be seen that for the MKPCs containing 4 wt% Cu (NO3)2, the change in the M/P ratio does not change the main crystalline substance in the system, and the main diffraction peaks are still those of K-struvite and unreacted MgO. In general, the changes in the M/P ratio and Cu(NO3)2 content did not change the main hydration products of MKPCs, but the formation of hydration products was better in lower M/P ratio. 3.4. Effect of Cu2+ on compressive strength of MKPC paste

heat flow(mw)

0.028

0.026 80200

0.02

80400

80600

80800

81000

81200

81400

0.01

0.00

80000

100000

120000

140000

160000

time (s)

(b) Fig. 2. Effect of Cu2+ on hydration heat of MKPC samples. (a) M/P = 4 with various copper nitrate contents; (b) 4% copper nitrate content with various M/P ratios.

The compressive strength of MKPC samples of different Cu2+ ions contents aged for 12 h, 1 day, 3 days, 7 days, 14 days, and 28 days is presented in Fig. 4. As can be seen in Fig. 4(a), for the MKPC samples with an M/P ratio of 2:1, as the amount of Cu2+ increased, the compressive strengths of the samples aged for different times have increased to some extent. Compared with the sample with the Cu(NO3)2 content of 0%, the compressive strength with the Cu(NO3)2 content of 4% increased by 39% after 12 h of aging and increased by 33% after 28 days of aging. As presented in Fig. 4(b), for the MKPCs with the M/P ratio of 3:1, the compressive strength of the samples aged for different times increased first and then decreased as the Cu2+ content increased. For the MKPCs with the M/P ratio of 4:1, Fig. 4(c) showed that the compressive strength of the samples of aged for different times decreased as the Cu2+ content increased. Specifically, when the amount of Cu (NO3)2 changed from 0% to 4%, the compressive strength of the

238

X. He et al. / Construction and Building Materials 225 (2019) 234–242 ——MgKPO4

O

2

——MgO

0% 1% 2% 3% 4%

60

50

comressive strength (MPa)

0%

1% 2% 3%

40

30

20

10

4% 0 12h 10

20

2 (°)

30

1d

40

3d

7d

——MgKPO4

O

2

60

——MgO

0 1% 2% 3% 4%

2:1 3:1 4:1 30

40

2 (°)

compressive strength(MPa)

50

20

28d

(a)

(a)

10

14d

curing time

40

30

20

10

(b) Fig. 3. XRD patterns of MKPC samples after 28 days of hydration. (a) M/P = 4 with various copper nitrate contents; (b) 4% copper nitrate content with various M/P ratios.

0 12h

1d

3d

7d

14d

28d

curing time

(b) 0% 1% 2% 3% 4%

50

comressive strength (MPa)

sample aged for 12 h decreased by 73%, from 18.77 to 5.08 MPa, and the strength of the sample aged for 28 days decreased by 52%, from 50.22 to 24.12 MPa. In general, when the M/P ratio was small, the addition of Cu2+ could increase the compressive strength of the MKPCs. As the M/P ratio increased, the addition of Cu2+ led to a decrease in the compressive strength of the MKPCs. This may occur because when the M/P ratio of the MKPC was low, the phosphate used to form the cementing phase in the system was rather excessive. As the curing age increased, the excess phosphate would gradually dissolve, resulting in an increase in the internal pores of the matrix and lowering growth in late compressive strength. After its incorporation, Cu2+ could react with phosphate ions and other anions to form different compounds during the hydration of MKPCs [56]. The formation of these compounds prevented the strength loss of MKPCs owing to the dissolution of the excess phosphate and increased the density of the solidified forms. The strength of the matrix was finally increased under the combined action of two factors. When the M/P ratio was high, the phosphate used to form the hydration phase of the system was relatively small. This was similar to the effect of Cd2+ and Zn2+ on the compressive strength of MKPC [22,48]. During the hydration and hardening of MKPCs, the reaction of Cu2+ and phosphate ions further reduced the hydration phase of the system. Compared with the contribution of the Cu-containing reactant to the denseness of the matrix, the effect of reducing the strength of the cementation phase was more pronounced. Therefore, Cu2+ might have exhibited a lowering effect on the compressive strength of MKPCs featuring higher M/P ratios.

40

30

20

10

0 12h

1d

3d

7d

14d

28d

curing time

(c) Fig. 4. Effect of Cu2+ content on strength of MKPCs of different M/P ratios: (a) 2:1, (b) 3:1, and (c) 4:1.

3.5. Effect of Cu2+ on the setting time of MKPC paste The effects of Cu2+ on the setting time of the MKPC samples of different M/P ratios are illustrated in Fig. 5. It can be seen from Fig. 5 that the setting time of MKPCs of different M/P ratios

X. He et al. / Construction and Building Materials 225 (2019) 234–242

1600 1400

setting time (s)

1200

0% 1% 2% 3% 4%

1000 800 600 400 200 0 2:1

3:1

M/P ratio

4:1

Fig. 5. Effect of Cu2+ content on setting time of MKPCs of different M/P ratios.

239

presented similar change trends in the presence of Cu2+. When the amount of Cu(NO3)2 was smaller than 2%, the addition of Cu(NO3)2 presented little effect on the setting time. However, when the amount of Cu(NO3)2 was higher than 3%, the final setting time was prolonged. When the amount of Cu(NO3)2 was 4%, the longest setting time of the MKPC specimens with an M/P ratio of 2:1 was approximately 1388 s, which was prolonged by more than 102% compared with the setting time of MKPCs containing no Cu2+. Moreover, the results obtained for the MKPC specimens with M/P ratios of 3:1 and 4:1 were similar with those of the MKPC samples with the M/ P ratio of 2:1, and the setting time of the samples containing 4 wt % Cu(NO3)2 was prolonged by 100 and 94% respectively. This indicated that during the hydration and hardening of MKPCs, a higher content of Cu2+ ions could play a more significant retarding effect. This may occur owing to the reaction of Cu2+ with phosphate to form a copper-containing substance during the hydration of MKPCs, which could adhere to the surface of dead-burned magnesia like borax and hinder the contact between phosphate ions and dead-burned magnesia, resulting prolongation of the setting time of the MKPCs.

Fig. 6. SEM images of MKPC samples of different Cu2+ contents with the M/P ratio of 4:1 after 28 days of hydration: (a) 0%, (b) 1%, (c) 2%, (d) 3%, and (e) 4%.

Fig. 7. SEM images of MKPC samples of different M/P ratios containing 4 wt% Cu(NO3)2 after 28 days of hydration: (a) 2:1, (b) 3:1, and (c) 4:1.

240

X. He et al. / Construction and Building Materials 225 (2019) 234–242

Fig. 8. EDS spectrum of MKPC sample containing 4 wt% Cu(NO3)2 with different M/P ratios:(a) 2:1;(b) 3:1;(c) 4:1. 2+

3.6. Effect of Cu

on the microstructure of hardened MKPC

The effect of Cu2+ on the morphology of the hydration products obtained after 28 days using MKPCs of different M/P ratios is illustrated in Figs. 6 and 7. Fig. 6 depicts the effect of the Cu2+ content on the morphology of the hydration products for an MKPC with the M/P ratio of 4:1. Fig. 7 presents the morphology changes of the hydration products obtained from MKPCs of different M/P ratios containing 4 wt% Cu(NO3)2. As can be seen from Fig. 6, for the samples containing no Cu2+, mainly plate-like and prismatic crystals formed, and these crystals are joined together to form whole structures. As the amount of Cu2+ increased, cluster-like structures appeared in the samples, while the fractions of plate-like and columnar crystals gradually decreased; the cluster-like substances were concentrated in the crystal gaps and at the surfaces of the samples. It can be seen from Fig. 7 that for the MKPC containing 4 wt% Cu(NO3)2, the amount of rod-like and plate-like crystals observed in the hydration product of MKPC after 28 days decreases as the M/P ratio increases, and the amount of cluster-like substances increases as the M/P ratio increases. Combining the results of the EDS analysis in Fig. 8 and Table 5 with those of the XRD analysis, it can be seen that the plate-like and prismatic materials presenting good crystallinity that were observed in the MKPC samples may be K-struvite and MgO and a small amount of Cu2+ existed in hydration products, but more Cu2+ existed in crystals with better crystalline state. This may be because Cu2+ was capable of substituting for Mg2+ in hydration products, and the ion radius of Cu2+ and Mg2+ is 0.073 nm and 0.072 nm respectively, which is almost the same, so the structure change was too small to be detected by XRD [57]. Also, as can be seen from Fig. 8 and Table 5, with the increase of M/P ratio, the finer crystals of hydrated products were gotten, and the content of Cu2+ in hydrated products decreased gradually. The results show that Cu2+ have more inhibitory effects on the formation of hydration products in samples with higher magnesium to phosphorus ratio. This also corresponded to the change of the compressive

strength of samples in Fig. 4, which gradually decreased with the increase of the M/P ratio. In general, during the hydration of MKPCs, Cu2+ might be mainly present in hydration products in the hardened MKPC matrices. To a certain extent, Cu2+ hindered the crystallization of the main hydration product, potassium K-struvite. The influence of Cu2+ on the morphology of the hydration products of MKPCs increased as the Cu2+ ion content increased, and decreased as the M/P ratio decreased.

3.7. TCLP leaching tests of hardened MKPC paste In this study, TCLP leaching tests of Cu2+ in solidified form were performed. Samples A1-A4, B4, and C4 in Table 1, which were hydrated for 28 days, were selected for the TCLP leaching tests, and the experimental results are presented in Fig. 9. Fig. 9(a) illustrates the effect of the Cu2+ content on the leaching toxicity of MKPC samples with M/P ratios of 4:1. It can be seen from Fig. 9 that the leaching concentration of Cu2+ increases as the Cu2+ ion content of the MKPCs increases, and the sample with the highest Cu(NO3)2 content (4 wt%) exhibited the highest Cu2+ ion leaching concentration of 1.7 mg/L which is much lower than the TCLP requirement (15 mg/L). The effect of the change in the M/P ratio on the leaching toxicity of the samples containing 4 wt% Cu(NO3)2 is presented in Fig. 9(b). It can be seen that the Cu2+ leaching concentration of MKPCs increases as the M/P ratio increases, which indicates that Cu2+ is more easily leached at higher M/P ratios. This phenomenon was consistent with the results of the EDS analysis. At higher M/P ratio, the hydrated product had less content of Cu2+ and was more susceptible to leaching. Analyzing the compressive strength and leaching toxicity of the MKPC samples after 28 days of hydration, we determined that the leaching concentration of Cu2+ increased as the sample strength decreased, which indicated that the density of the MKPC matrix and degree of hydration reaction presented significant effects on the solidification of Cu2+.

Table 5 EDS data of MKPC hydration products. Graph

Element

NK

OK

Mg K

PK

KK

Cu K

Total

1

Wt% At% Wt% At% Wt% At% Wt% At% Wt% At% Wt% At%

0.000 0.000 1.925 3.183 0.902 0.138 0.746 1.284 1.442 2.575 0.000 0.000

27.240 42.421 33.303 48.183 39.240 51.539 29.719 44.764 25.084 39.188 32.194 47.573

15.983 16.593 16.554 15.967 46.220 40.471 16.785 16.855 13.979 14.559 20.311 20.009

29.372 24.395 25.776 19.890 6.586 4.613 27.643 22.207 30.516 25.426 23.695 18.674

23.729 15.160 20.095 11.928 4.386 2.363 22.519 13.916 27.693 17.749 20.913 12.678

3.675 1.431 2.347 0.849 2.666 0.875 2.587 0.974 1.285 0.502 2.886 1.066

100.000 100.000 100.000 100.000 100.000 100.000 100.000 100.000 100.000 100.000 100.000 100.000

2 3 4 5 6

X. He et al. / Construction and Building Materials 225 (2019) 234–242

hydration reaction delayed the change rate of the pH and destroyed the degree of crystallization of the main hydration products. The effect of Cu2+ on the hydration properties of MKPCs was enhanced by the increase in the Cu2+ content and decreased as the M/P ratio decreased. The highest leaching concentration of Cu2+ ions after 28 days of the sample was 1.7 mg/L. As the M/P ratio decreased, the stability of Cu2+ in MKPCs increased. These results indicated that MKPCs are sufficient for the S/S treatment of Cu2+, it presented a lower leaching concentration.

1.8 1.6 1.4 1.2 1.0

mg/L

241

0.8 0.6

Declaration of Competing Interest

0.4

The authors declare that there is no conflict of interests regarding the publication of this article.

0.2 0.0 1%

2%

3%

4%

content of Cu(NO3)2

(a)

Acknowledgments The authors would like to thank the projects supported by Sichuan Science and Technology Program (2018GZ0148) and Fund of Southwest University of Science and Technology (No. 13zxfk04, No. 17LZX544).

1 .8 1.6 1.4

References 1.2

mg/L

1.0 0.8 0.6 0.4 0.2 0.0 2:1

3:1

M/P

4:1

(b) Fig. 9. Leaching concentration of Cu2+. (a) M/P = 4 with various copper nitrate contents; (b) 4% copper nitrate content with various M/P ratios.

The experimental results of the leaching toxicity of Cu2+ demonstrated that the S/S effect of MKPCs on Cu2+ was obvious, and the leaching concentration of the samples hydrated for 28 days was the lowest overall. Also, the solidification effect of MKPCs on Cu2+ increased as the M/P ratio decreased, which indicated that appropriately increasing the amount of phosphate in MKPCs would be beneficial to the solidification of Cu2+ using MKPCs. 4. Conclusions In this study, the effects of Cu2+ on the strength, setting time, hydration process, and hydration products of MKPCs of different M/P ratios were studied. Based on those results, the leaching toxicity of Cu2+ was also discussed. The setting time of MKPC did not change significantly with a lower amount of Cu2+, while the setting time was prolonged as the amount of Cu2+ was increased. For the MKPCs with a smaller M/P ratio (2:1), the presence of Cu2+ could improve the compressive strength of the matrix, and the compressive strength of the matrix decreased as the Cu2+ content increased for MKPCs with a larger M/P ratio (4:1). During the hydration of MKPCs, Cu2+ mainly exist as a solid solution in the hydration products of the MKPCs. Moreover, the

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