Investigation on the hydration of hemihydrate phosphogypsum after post treatment

Construction and Building Materials 229 (2019) 116864

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Investigation on the hydration of hemihydrate phosphogypsum after post treatment Xuemei Chen, Jianming Gao ⇑, Yasong Zhao Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing 211189, China

h i g h l i g h t s
Adjusting the pH of HPG would lead to abnormal setting and strength degradation.
Excessive SL can promote the rehydration of HPG resulting in dense microstructure.
The formation of Ca3(PO4)2 under alkaline condition influence the hydration process of HPG and gypsum morphology.
A saturated solution of calcium hydroxide is needed for HPG during hydration in order to achieve better performance.

a r t i c l e

i n f o

Article history: Received 14 February 2019 Received in revised form 8 August 2019 Accepted 1 September 2019 Available online 26 September 2019 Keywords: Hydration Slaked lime Hemihydrate phosphogypsum Setting time Strength

a b s t r a c t Pretreatment is very important for the application of phosphogypsum (PG), but it is seldom adopted in practise due to high investment and costs. In order to promote the application of PG, post treatment of hemihydrate phosphogypsum (HPG) by addition of slaked lime (SL) was investigated. As an alkaline additive, SL can neutralize HPG. In addition, the interaction of HPG and SL can not possibly be avoided, because SL is generally presented in composite binder and it is also the hydration product of cement based materials which are widely used. Therefore, the hydration process, setting and hardening characters of HPG with different SL contents at a constant water/powder ratio were studied in detail. It was found that small amounts of SL can neutralize HPG but lead to super retardation and strength degradation due to the formation of calcium phosphate which would hinder the dissolution of HPG and alter the morphology of gypsum from needle-like to tiny crystal with little interlocking. Surprisingly, the addition of excessive SL regardless of pH would promote the rehydration of HPG and eliminate the influence of calcium phosphate contributing to the comparable strength and normal setting possibly due to the preferential precipitation mechanism of calcium phosphate. This important finding could be translated into specific design guidelines for post treatment. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Phosphogypsum (PG) is produced by the wet phosphoric acid process, which is the main byproduct resulting from the phosphate fertilizer industry. About five tons of the PG are generated for every ton of the phosphoric acid made [1–5], and the chemical reaction can be described as Ca5 ðPO4 Þ3 F þ 5H2 SO4 þ 10H2 O ! 3H3 PO4 þ 5CaSO4  2H2 O þ HF. PG contains mainly of CaSO42H2O but also involves some impurities such as P2O5 as H3PO4, Ca (HPO4)2H2O, CaHPO4H2O, Ca3(PO4)2, fluorides (NaF, Na3AIF6, Na2SiF6, Na3FeF6 and CaF2), residual acids, trace metals (e.g. Zn, Cu, Cr and Cd), sulfate ions and organic matters which adhere to ⇑ Corresponding author at: Southeast University, School of Materials Science and Engineering, Nanjing 211189, China. E-mail address: [email protected] (J. Gao). https://doi.org/10.1016/j.conbuildmat.2019.116864 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

the surface of the gypsum crystals [6,7]. What’s more, fresh PG is highly acidic (pH = 1.5–4.5) and contains more than 20% free water in weight [8]. Over 70 million tons of PG are produced annually in China, and the reusing proportion of PG is even less than 10% compared to 15% of worldwide [9]. At present, the surface stockpile in China is estimated to be above 300 million tons which cause serious environmental contamination of soil, water, and atmosphere [10–12]. With the enhancement of social protection to the environment, it is essential to recycle and utilize PG in building materials. Efforts have been made to use original and calcined PG as a part of building materials [13–18]. Nowadays, more than 65% recycled PG in China are used as the retarder in Portland cement. It is also reported that the impurities in PG especially the phosphorus have a negative effect on the hydration kinetics of cement, which would retard setting time unfavorably and decrease strength [19].

2

X. Chen et al. / Construction and Building Materials 229 (2019) 116864

Gypsum plaster-an ancient material which maybe the most promising application for PG is widely used in the dry wall. In fact, PG plaster is restricted due to its lower performance because of impurities in comparison to the natural gypsum [20]. It is obvious that impurities play a vital role in the application of PG, thus pretreatment of PG is highly recommended by scientists. Lime water washing or neutralization is the most commonly used PG pretreatment method, because it can reduce, eliminate and remove impurities from raw PG due to the fact that the soluble impurities (P2O5 and F2) can be converted to insoluble matters (Ca3(PO4)2 and CaF2). Nevertheless, pretreatment of raw PG before calcination is not adopted in practice due to the high cost [21–23]. Therefore, commercial HPG is still acidic and contains impurities. In order to improve the performance of PG production, alkali material like cement, granulated slag, fly ash and hydrated SL were added to prepare PG based cementitious binders [24–28]. It is noticeable that most of the application condition for PG is accompanied by the presence of calcium hydroxide. What’s more, it is also used for pretreatment. Naturally, post treatment of HPG by adding SL into the mix design can be a economical and effective way to improve the performance of HPG in theory. However, to the best knowledge of the authors, the influence of post treatment on the hydration of HPG is seldom discussed. It is not known that whether the interaction of SL and impurities especially the phosphorus substituted in the gypsum crystal lattice affect the hydration of HPG. This is very important for the application of PG under alkaline condition. In this paper, the detailed hydration of HPG after addition of SL was studied which can contribute to an in-depth understanding of the hydration of HPG under alkaline condition. It is also the key to develop consistent guidance at the technical level regarding the design of alternative environment-friendly PG based composite cement of better performance. Therefore, SL was directly added to HPG, and the effect of SL content on the hydration process, microstructure, and properties of HPG paste were investigated. It was interesting to find that a small amount of SL had an adverse effect on the hydration of HPG paste but it can set and harden well beyond a certain content.

into the holder and then followed by retarder and SL powder, respectively. After stirring in the planetary mixer for 30 s, gypsum powder was added and mixed for another 1 min. After that, the paste was used for property test. 2.2. Setting time and compressive strength The setting time of HPG plaster is examined by Vicat apparatus. The initial setting time of the sample is recorded from the time when the sample contacts water to the time when the steel needle fails to touch the glass bottom plate for the first time. The final setting time of the sample is also recorded from the time when the sample contacts water to the time when the depth of the steel needle inserted into the slurry for the first time is not more than 1 mm. Prismatic specimens of size 40  40  160 mm3 were prepared according to the mix proportions in Table 3. The samples were cured at 20.0 ± 1 °C, 60 ± 5% relative humidity for 7 days and dried at 40 °C until constant mass (the weight change of two followed weighing is limited in 0.01 g) was reached. After that, mechanical strength is conducted. For bending strength test, one group of three specimens were examined at the loading rate of 30 N/S using the three-point method and the average was determined as the final experimental result. The specimens after bending test were used for compressive strength test at the loading rate of 0.6 KN/S and the compressed area is 40 mm  40 mm. The compressive strength was the average value of six test results. 2.3. pH and hydration degree testing The powder of samples B1-4 and C1-4 before hydration without adding water after dry mixing and after hydration prepared by grinding hydrated samples after 7 days curing and drying at 40 °C were diluted by water at the ratio of 1:10. Mix the solution for 5 min and let it stand for 25 min before filtration. The pH was tested from the filtrate by a Digital pH Meter Model PHS-25 (Leici, Shanghai). Furthermore, 5 g of the mixture (B1, B3, B4, C1, C2, C3) diluted by water at the ratio of 1:10 were put in a 200 ml cup which was placed on the magnetic stirrer and then pH detector equipped with ZETA potentiometer (DT310) was inserted in the paste immediately. Thus, the variation of pH with time was recorded by ZETA potentiometer every two minutes. It should be noticed that the samples used for continuous pH monitoring were stirred all the time. The actual water for HPG hydration were different in above two pH testing methods and retarder was not added in continuous pH monitoring samples in order to short the monitoring time. So, the pH value may be a little different from that before and after hydration tested by above mentioned method. The hydration reaction of hemihydrate (CaSO40.5H2O) to dehydrate gypsum (CaSO42H2O) leads to the increment of crystal water (1.5H2O), so the hydration degree (DCS) can be calculated according to the following equation:

DCS ¼


M CS  W 2;H2 O - W 1;H2 O  100% 27  W CS

W H2 O ¼ 2. Experimental methods 2.1. Materials and mix proportions HPG is a commercial product, and it is obtained by thermal dehydration from PG after three times water washing. The chemical composition and physical properties of HPG were listed in Tables 1 and 2 respectively. The chemical composition of HPG is complicated, because it only has 75.72% CaSO40.5H2O concerning the chemically combined water. The physical properties of HPG shown in Table 2 meet the level 2 requirements in Chinese standard for calcined gypsum (GB/T 97762008). The radioactivity of HPG was described in previous study [29]. SL powder purchasing on the market and containing more than 96% Ca(OH)2 are used to neutralize HPG. Retarder (Retardant-200P) is added to acquire the desired operation time at the rate of 0.06% by mass of HPG powder and the water to powder ratio is 0.64 which is the water requirement of HPG for normal consistency. Twelve mix proportions of hemihydrate phosphogypsum paste shown in Table 3 are designed and all materials are weighted by mass percent. 0.1%-0.4% SL powder can change the pH of HPG from being acidic (5.85) to being alkaline (11.08), thus the effect of minor SL with respect to pH on the properties of HPG paste were investigated by mixtures of A1-A4. The mixtures of B1-B4 were used to investigate the effect of minor SL in the presence of retarder. The effect of extra lime regardless of pH were also studied through the mixtures of C1-C4. At first, water was poured

ð1Þ

m0  m1  100% m0

ð2Þ

where, MCS is the relative molecular weight of CaSO40.5H2O; W 2;H2 O is the crystal water content of samples after hydration; W 1;H2 O is the crystal water content of samples before hydration; W CS is the initial content of CaSO40.5H2O (75.72%) and 27 is the relative molecular weight of 1.5H2O, respectively. The crystal water was measured by drying difference subtraction with regarding to Chinese standard GB/T 5484–2012. About 1 g powder that have been dried to a constant quantity varying in the range of 0.001 g at 40 °C were dried at 230 °C ± 5 °C in a drying oven for 2 h. Then, the samples were removed and cooled to room temperature for weighing. The drying process was repeated until the weight of samples keep constant (the weight change of two followed weighing is limited in 0.001 g). The crystal water (W H2 O ) was calculated regarding to Eq. (2). Where, m0 and m1 were the weight of samples before and after drying respectively. 2.4. Other testing methods The hydration heat evolution rate as well as the cumulative hydration heat were measured through an eight-channel TAM Air Isothermal Calorimeter. The tests were performed at 20 °C, within 72 h and the water/powder ratio by weight was fixed at 0.64. The paste of samples cured for 7 days were crushed and immersed in ethanol for 48 h to stop hydration. The fragments after drying at 40 °C were used for SEM observation and ground for XRD testing.

Table 1 Chemical composition (by mass %) of HPG. Compositions (%)

SO3

CaO

SiO2

MgO

Cl

Fe2O3

Al2O3

P2O5

SrO

K2O

TiO2

Chemically combined water

HPG

50.3

38.1

5.48

0.461

0.187

1.31

1.84

0.956

0.902

0.213

0.168

4.7

3

X. Chen et al. / Construction and Building Materials 229 (2019) 116864 Table 2 Physical properties of HPG. Group

pH

Water requirement of normal consistency/%

Setting time/min

Compressive strength/MPa (2 h)

Initial

Final

Flexural strength

Compressive strength

HPG Standard

5.85 –

0.64 –

6.6 >3

10.67 <30

2.4 2.0 (level 2)

5.6 4.0 (level 2)

Table 3 Mix proportions and characteristic values of HPG paste. Sample

Mix proportion/%

A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4

HPG powder

SL

retarder

water

100 99.9 99.8 99.6 100 99.9 99.8 99.6 98.6 97.6 96.6 95.6

0 0.1 0.2 0.4 0 0.1 0.2 0.4 1.4 2.4 3.4 4.4

0 0 0 0 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06

64 64 64 64 64 64 64 64 64 64 64 64

3. Results 3.1. Characteristics of hydration heat evolution of HPG paste The hydration heat evolution curves of HPG with minimal SL without retarder were shown in Fig. 1. An intense and sharp initial heat evolution peak (referred as peak I) is observed immediately once powder mixed with the water, which is thanks to the wetting of particles and the initial dissolution of HPG, indicating the beginning of hydration. Few minutes after peak I, a stage of decreased heat evolution rate (HER) is noted in all samples. This stage has been commonly considered as an induction period during which gypsum nucleated and ions of calcium and sulfate came to supersaturation. The induction period allows the manipulation of the pastes while it is very short for pure HPG. It ended after 4.87 min (Table 3) with the onset of the subsequent peak (Peak II), which reached a maximum value 56.33 mW/g at 16.41 min and a steady

Ending time of the induction period/min

Time of the peak II/min

Intensity of peak II/mW/g

Total heat emission/J/g

4.87 7.41 16.64 – 14.79 56.09 121.50 1169.65 122.80 120.90 120.20 48.16

16.41 23.56 53.56 – 34.48 208.60 554.34 1263.20 207.75 198.79 197.40 169.00

56.33 37.75 16.66 – 29.94 8.18 6.35 3.39 18.92 20.10 18.61 17.50

64.95 65.03 65.28 51.92 63.45 64.47 63.69 63.10 67.32 69.64 72.19 72.17

state with around zero heat evolution within a period of approximately 60 min. However, the induction period of hydration for sample A series are prolonged gradually with the increased dosage of SL. The more content of SL, the smaller value of peak II, which reduced from 56.33 mW/g to 16.66 mW/g. What’s more, the peak II is disappeared for sample A4 and the decreased HER after peak I is closer to zero after a relatively long time more than 360 min. The results advocate that the addition of minimal SL can change the hydration process of HPG generally. Note: The total heat emission of samples A, B and C series were taken at the hydration time of 24 h, 72 h and 72 h, respectively. As can be seen from Fig. 2, the hydration heat emission of samples A1-4 increases intensively at the beginning and then meets a constant value. Nevertheless, the hydration heat developed slower with an increase in SL content, especially for sample A4 which experienced a long time to reach the maximum 51.92 J/g decreased by 20% compared with samples A1-3 which were in the ranges of

70 30

50 20

A2

Hydration heat (J/g)

A1 A2

40

30

A4 A3

10

20

A3

0

10

0

2

4

6

8

A2 A1

50

A4

40 30 20

10

10

Time/min

A4

A3

60

A1

mW/g

Hydration heat evolution rate (mW/g)

60

0

0 0

50

100

150

200

250

300

350

Time/min Fig. 1. Hydration heat evolution rate of samples A1-4.

400

0

200

400

600

800

1000

1200

Time/min Fig. 2. Hydration heat of samples A1-4.

1400

1600

X. Chen et al. / Construction and Building Materials 229 (2019) 116864

35

28

B4

30

24

B3 B2

25

mW/g

Hydration heat evolution rate (mW/g)

32

20

70 60

E

Hydration heat (J/g)

B1 B2

D B3

50

B4 40 30

C 20

B

10

A

0

0

500

1000

1500

2000

2500

3000

Time/min Fig. 4. Hydration heat emission of samples B1-4.

36

35

32

30

mW/g

Hydration heat evolution rate (mW/g)

64.95–65.28 J/g. The results demonstrate that the hydration reaction progressed slower with increased SL content, which are in consistent with the HER in peak II. The results of samples B1-4 after the addition of 0.06% retarder were presented in Fig. 3. There is no big difference in the shape of the curves between Figs. 1 and 3 except sample B4 which had two peaks instead of one peak but the intensity of peak II was much lower than those in Fig. 1. As shown in Table 3, the intensity of peak II reduced from 56.33 mW/g for sample A1 to 29.94 mW/g for sample B1; 37.75 mW/g for sample A2 to 8.18 mW/g for sample B2; and then 16.66 mW/g for sample A3 to 6.35 mW/g for sample B3. Furthermore, the peak intensity of the sample B4 was only 3.39 mW/g. The addition of retarder delayed the ending time of induction period from 4.87 min (A1) to 14.79 min (B1), but it was interesting to note that the ending time of induction period increased significantly from 14.79 min to 1169.65 min as SL content increased from 0.1% to 0.4%. Accordingly, the time of peak II for samples B1-4 were extended from 34.48 min to 1263.2 min. In addition, the HER increased more slowly after the induction period during which it reduced to zero. Thus the width of peak II increased for increasing content of SL. The results in Fig. 4 also reconfirmed these changes. There are four stages in hydration heat emission curves: A-B represents the heat emission of peak I; B-C is corresponding to part of the induction period when HER was equal to zero; C-D responds to peak II when the accelerated hydration reaction contributes to the initial strength; D-E is responsible for the stable period when HER after peak II is close to zero indicating the complete hydration. From the four stages, it is clear that the length of stage B-C increased, the slope of stage C-D is slower and the time of stage A-E is longer with an increase in SL content. All results illustrate that minimum SL has retarding effect on the HPG paste and this effect can be even more significant when retarder was added, which contributes to the prolongation of the induction period and the slower transformation of hemihydrate to dihydrate gypsum. It makes sense that the duration of whole hydration reaction increases. The hydration heat evolution rate and accumulated heat emission of samples C1-4 (added 1%-4% extra SL) were displayed in Figs. 5 and 6, respectively. It was interesting to note that the time of induction period of samples C1-4 decreased from 122.8 min to 48.16 min which decreased by 89.5%, 89.7%, 89.7%, 95.9% separately compared to that of sample B4 although still longer than sample B1. Amazingly, the maximum HER at peak II of samples C1-4 fluctuated between 17.5 mW/g and 20.1 mW/g which were much higher than 3.39 mW/g (B4) and a little lower than that of sample B1 (29.94 mW/g). Furthermore, a shoulder was observed

25

C1

28

B4

24

C4 C3 C2

20

20

16 C3 C2

15

C18

12

4

10

0 0

3

6

9

12

15

Time/min

5

C4

B4

0 0

100

200

300

400

1200

1600

Time/min Fig. 5. Hydration heat evolution rate of samples C1-4.

80 70 60

Hydrartion heat (J/g)

4

C4

C3

C2 C1

50 40 30

B4

20 10

20

B1 0

15

16

0

B1 10

12

500

1000

1500

2000

2500

3000

Time/min

5

8

Fig. 6. Hydration heat emission of samples C1-4.

B2

4

0

3

6

9

12

Time/min

B3

15

B4

0 0

200

400

600

800

1000

1200

1400

Time/min Fig. 3. Hydration heat evolution rate of samples B1-4.

1600

after peak II in samples C2, C3, and C4 which increased the duration of hydration reaction indicating the adequate hydration of hemihydrate and that’s the reason why the total heat emission of samples C2, C3 and C4 in Fig. 5 were much higher than other samples. Besides, the HER after peak I in the induction period of the

5

X. Chen et al. / Construction and Building Materials 229 (2019) 116864

samples C1-4 were higher than zero portraying that the hydration reaction was not dormant. In a word, an increase in SL content beyond 1.4% would greatly accelerate the hydration reaction, reducing the setting time.

The effect of SL content on setting time of HPG paste with retarder including samples B1-4 and C1-4 were given in Fig. 7. It followed that the setting time was delayed with increased SL below 0.4% and the time span between the initial and final setting time was also increased. On the contrary, the setting time of sample C1 was greatly shortened which decreased by 71.12% compared with that of sample B4 when SL content increased from 0.4% to 1.4%, and then the setting time decreased slightly and the time span was approximately 15 min when SL further increased from 2.4% to 4.4%. These findings well agreed with the results in Section 3.1. Indeed, the addition of the SL delayed the setting time for all samples, but a small amount of SL resulted in the abnormal setting which would set for about 1 day.

10

Strength (MPa)

3.2. Effect of slaked lime on setting time of HPG paste

12

Bending strength (Left) Compressive strength (Right)

8

6

4

2

0

B1: 0

B2: 0.1 B3: 0.2 B4: 0.4 C1: 1.4 C2: 2.4 C3: 3.4 C4: 4.4

SL content (%) Fig. 8. The effect of SL content on strength of HPG paste with retarder.

G

(a)

3.3. Effect of slaked lime on strength of HPG paste

G P

Fig. 8 showed the compressive and bending strength of samples with different contents of SL in the presence of the retarder. Compared with the reference sample without SL, a dramatic reduction in strength was observed after SL was added. The strength kept a slight variation around 3 MPa and 1 MPa for compressive and bending strength respectively when SL increased from 0.1% to 0.4%. However, both compressive and bending strength increased for increasing content of SL after the addition of 0.4%. The maximum compressive strength obtained at highest SL content was 9.1 MPa which was 3 times of sample B4 and 0.8 times of the reference (B1). It was clear that a small number of SL had a deleterious effect on the strength of HPG paste, while further increase in SL content could eliminate the negative effect and enhance the development of strength under alkaline condition. But, the strength is still a little lower than the reference sample.

G G G

G G

C4

GG

C3 C2 B

B

B

B

B C1 B4 B3 B1

5

10

15

20

25

30

35

40

45

50

55

60

2

3.4. Effect of slaked lime on hydration products of HPG paste According to above results, it can be learned that the hydration process of HPG was strongly affected by the addition of SL which would delay the hydration reaction resulting in super retardation and strength degradation at the low dosage and it will accelerate

GG B

(b)

C4

1400

C3

Setting time (min)

1200 1000

C2

Initial setting time (Left) Final setting time (Right)

800

C1 600

17.5 400

17.7

17.8

17.9

18.0

18.1

18.2

18.3

18.4

18.5

2 Fig. 9. XRD patterns of samples with different SL content. G: gypsum, B: bassanite (calcium sulfate hemihydrate), P: portlandite.

200 0

17.6

B1: 0

B2: 0.1 B3: 0.2 B4: 0.4

C1:1.4 C2: 2.4 C3: 3.4 C4:4.4

SL content (%) Fig. 7. The effect of SL content on the setting time of HPG paste with retarder.

the hydration reaction when further increase the SL content. This finding is very interesting and important to guide the development of HPG based composite cementitious materials. Therefore, it is necessary to find out the influence mechanism of SL.

6

X. Chen et al. / Construction and Building Materials 229 (2019) 116864

The XRD patterns of samples with different SL content after 7 days of hydration were shown in Fig. 9. It can be seen that the main hydration products were dihydrate gypsum, bassanite (CaSO41/2H2O) and portlandite. The bassanite peaks increased at first and then became even stronger than that of gypsum when SL content came up to 0.4%. After that, the peaks of bassanite decreased and finally disappeared. In contrast, the peaks of gypsum firstly decreased and then increased for increasing content of SL. Since the peaks of portlandite were not invisible in Fig. 9(a), a much slower scanning speed of 1.5 s/step was used to increase the resolution from 17.5° to 18.5° (Fig. 9(b)). It can be seen that there was no peak of portlandite at about 18.1° in sample C1. However, a small peak of portlandite appeared in sample C2 and the peak area increased gradually in sample C3 and C4. As the hydration reaction occurs as: CaSO4  0:5H2 O þ 1:5H2 O ) CaSO4  2H2 O, it can be deduced that SL was consumed during hydration and the reaction inhibited the transformation of bassanite to gypsum at the same time. Furthermore, the effect of inhibition gradually vanished and the hydration of bassanite to gypsum continued when SL was excessive. 3.5. Effect of slaked lime on hydration degree of HPG paste

4. Discussion

The hydration degree of samples with different SL content were presented in Fig. 10. Just as indicated in Fig. 9, the hydration degree decreased gently as SL content increased from 0% to 0.2% and dropped dramatically when SL content reached 0.4% which reconfirmed the reduced inversion of bassanite into gypsum. When the addition of SL exceeded the threshold value between 0.4% and 1.4%, the hydration degree increased and even surpassed that of sample B1 without the addition of SL highlighting the rehydration of bassanite to gypsum. The results were also well depicted in Fig. 9. The decreased setting time and the enhanced strength in Figs. 7 and 8 have everything to do with the rehydration. The samples of C2-4 with comparative hydration degree had different strength indicating the deference in the microstructure. 3.6. SEM micrographs of HPG paste with slaked lime Fig. 11 shown the SEM images of the HPG pastes with different SL content. The gypsum crystal in sample B1 (reference) were mainly needle-like which interlaced with each other closely leading to a dense microstructure and higher strength in return. The crystal appearance changed significant with the addition of SL. The length-radius ratio of gypsum crystal decreased and thus bulky and granular gypsum crystal were observed in samples B2 and B3 which were well crystallized but with no interlocking between

20

90 16 80 70

12

60

10

50

8

40

B1:0

B2:0.1 B3:0.2 B4:0.4 C1:1.4 C2:2.4 C3:3.4 C4:4.4

SL content (%) Fig. 10. The hydration degree of samples with different SL content.

Hydration degree (%)

Chemically combined water (%)

100 18

14

each other. This is the reason responsible for the much lower strength even with relative high hydration degree. A looser microstructure filled with fluffy and floccus substances was presented in sample B4 which were consisted of dihydrate gypsum with low crystallinity and incompletely dissolved hemihydrate proving the inhibition effect of SL on hydration reaction. With a higher SL content in sample C1, a number of growing gypsum crystals interlaced with each other to some extent compared with the sample B4, but a few undissolved hemihydrates were still visible. It was clear that the gypsum crystal in samples C2-4 grew continuously, at the same time long column crystal with big lengthradius ratio was observed as well, which interlaced with each other more and more tightly with the increase of SL content. Meanwhile, the decreasing and later disappearing of the undissolved hemihydrate indicated that the further increase in SL content can promote the rehydration namely the dissolution of hemihydrate and contribute to the growth of gypsum crystal. The gypsum crystal in samples C3 and C4 were thick in comparison to the reference sample (B1), resulting in a higher porosity and a relative lower strength.

The intrinsic reason for the hydration reaction of hemihydrate is due to its higher solubility in water concerning gypsum. There are three steps in the hydration of paste: the dissolution of hemihydrate (CaSO41/2H2O) leading to a supersaturated solution regarding gypsum, nucleation, and growth of gypsum crystals and a solid material is finally formed by the interlocking of gypsum crystals with complete consumption of hemihydrate. SL has a great effect on the hydration process of HPG. As can be seen from the results, the influence of SL can be described as: 1) negative effect of minor SL which would hinder the dissolution of hemihydrate and the growth of gypsum contributing to the prolongation of hydration reaction with lower HER. Eventually, HPG paste with lower hydration degree and loose microstructure was observed. 2) positive effect of SL beyond the threshold value which would promote the hydration of HPG with higher HER. Accordingly, the hemihydrate dissolved and gypsum crystal grew continually which finally improved the hydration degree and microstructure. 4.1. The pH evolution of HPG paste The solubility of calcium hydroxide is 0.166 g at 20 °C, and the pH of the saturated solution is 12.65 in theory. Therefore, the maximum amount of dissolved Ca(OH)2 for the hydration of 100 g hemihydrate in 64 g water is 0.106 g. Nevertheless, the pH before hydration (see Fig. 12) increased and kept constant at 12 when SL was beyond 1.4 which was 13.2 times of saturation dosage demonstrating the reaction of SL and residual acid which reacted as fast as they contacted. Meanwhile, the pH after hydration reduced and presented little variation when SL increased from 0.1 to 0.4% indicating the participation of SL during hydration. As shown in Fig. 13, the variation of pH with time illuminated that post treatment was a dynamic process. The acidic matters released during the dissolution of HPG particles and an increasingly lower dissolution rate were observed in sample B2 and B4, since the pH decreased slower and it took longer time to reach equilibrium proving that the addition of SL had influence on the dissolution of HPG particles. From the chemical composition of HPG in Table 1, it was clear that the main impurities in HPG used in this study were phosphatic impurities. As the raw PG was washed for three times, the 0.96% insoluble phosphorus detected in HPG could be the reason for the abnormal hydration of HPG with addition of SL. Furthermore, the phosphorus element was found in fluffy and

7

X. Chen et al. / Construction and Building Materials 229 (2019) 116864

B1

B2

B3

B4

C1

C2

C3

C4

Fig. 11. The SEM images of hydrated pastes.

floccus substances in Fig. 11 (sample B4) by EDS shown in Table 4, which confirmed our hypothesis. 4.2. The electrical resistance evolution of HPG paste The electrical resistance evolution with the time of hydration was shown in Fig. 14. It is clear that the reaction took place in three stages. (1) A minimum resistance-obtained in the first time, which represented the point of maximum solubility of the calcium and sulphate ions. (2) After that, the resistance increased steadily which was due to the precipitation of the dihydrate. (3) Finally, the resistance reached a relatively constant value at the end of

the hydration process; a further slight increase was due to the drying-out process. From the figure, it can well explain the effect of slaked lime on the hydration of HPG paste. It is also highly agree with the results of hydration heat evolution. 4.3. The proposed mechanism of slaked lime The phosphorus was presented as CaHPO40.5H2O which substituted in the crystal lattice of HPG particles, since it can not be eliminated and removed by washing or calcination at lower temperature. It dissolved during hydration and calcium phosphate precipitation was formed under alkaline conditions. Definitely, it is

8

X. Chen et al. / Construction and Building Materials 229 (2019) 116864

14

Before hydration (Left) After 1d hydration (Right)

12 10

pH

8 6 4 2 0 B1:0

B2:0.1

B3:0.2

B4:0.4

C1:1.4

C2:2.4

C3:3.4

C4:4.4

SL content (%)

Fig. 14. The electrical resistance of HPG paste as a function of time.

Fig. 12. The pH of hydrated HPG paste.

14

C3 13

C2 C1

12 11

pH

10 9

B4

8

B3

7 6

B1

5 4 0

1

2

3

4

5

6

7

8

Time/h Fig. 13. pH as a function of time.

calcium phosphate that affects the hydration of HPG. It may precipitate on the surface of HPG particles and gypsum crystal. When SL increased and completely dissolved in the system, calcium phosphate precipitation increased and the wrapping effect became stronger which affected the dissolution of HPG and the growth of gypsum and even stopped the hydration. The formation of Ca3(PO4)2 in cement was also reported when PG was used as retarder as it had an influence on the rheological, setting time and strength properties of cement [19,28]. To begin with, HPG could not be used under alkaline conditions despite many trials haven been conducted to promote the hydration of HPG with minor amounts of SL. Accidentally, excessive SL can accelerate the hydration of sample B4. It is assumed that there is a competitive mechanism of absorbing calcium phosphate between slaked lime and HPG particles. The calcium phosphate tends to precipitate on the surface of SL when SL is excessive (only parts of SL dissolved in the system). The more the SL content, the more calcium phosphate precipitating on the surface of SL.

Accordingly, the fewer calcium phosphate precipitated on the surface of gypsum and HPG particles. It makes sense that a small amount of SL had an adverse effect on the hydration of HPG paste and an further increase in SL content would promote the hydration of HPG under alkaline condition. This also explain the reason why the hydration of HPG with SL is little lower than that of HPG without SL. The proposed hydration mechanism of HPG with addition of SL was displayed in Fig. 15. To the best of our knowledge, it was the first time to focus on the hydration of HPG with SL in detail. The finding of the mechanism of SL laid the theoretical foundation for the application of HPG under alkaline condition. From the experimental results, it can be concluded that addition of SL to HPG as post treatment is feasible and the neutralization reaction is a complicated dynamic process. The key to HPG post treatment is to keep the system in a saturated solution of calcium hydroxide (the more the better) instead of adjusting the pH (see Figs. 12 and 13). Therefore, the threshold value and the optimal dosage of SL in this study are outside the scope of the paper since they are not universally suitable for all materials. Even the HPG based materials (e.g., HPG substituted by cement) is outside the scope of the present paper, it can be deduced that the abnormal setting and hardening characteristic of HPG substituted by cement would be observed if cement is not enough namely the calcium hydroxide from cement can not reach saturated solution and be excessive during hydration which would be studied further. 5. Conclusions The influence of SL on hydration of HPG was studied systematically in order to broader the application scope of HPG. It was found that a small amount of SL (0.1%–0.4%) can neutralize HPG changing pH from 5.85 to 11.08 but leading to abnormal setting and strength degradation. Addition of excessive SL regardless of pH can promote the rehydration contributing to a dense microstructure with higher strength. Therefore, adjusting the pH of HPG before hydration by addition of SL was not feasible since the neutralization reaction was a complicated dynamic process. CaHPO40.5H2O co-crystal with HPG particles was released during hydration. Meantime, the inactive calcium phosphate was formed

Table 4 The elements analysis of fluffy and floccus substances. Element

C

O

Al

Si

P

S

Ca

Wt% At%

11.99 19.83

46.34 57.53

1.33 0.98

1.54 1.09

1.92 1.23

8.54 5.29

28.34 14.04

X. Chen et al. / Construction and Building Materials 229 (2019) 116864

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Fig. 15. The proposed hydration mechanism of HPG with addition of SL. DH: CaSO42H2O.

under alkaline condition. It would affect the dissolution rate of HPG particles and the growth rate of gypsum through absorption and wrapping effect. The influence of Ca3(PO4)2 was outstanding in subalkalic environment but negligible in a saturated solution of calcium hydroxide during hydration. As CaHPO40.5H2O can not be eliminated and removed by washing or calcination at lower temperature, it would always company with HPG. Thus, mixing HPG with hydraulic material simply or controlling the pH of HPG can not obtain the composite binder with good performance. Post treatment of HPG is necessary and keeping a saturated solution of calcium hydroxide during hydration was the key factor to ensure the completely hydration of HPG and avoid the influence of impurities (corrosion, get mildew, etc). Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements National Natural Science Foundation of China (grant no. 51578141), the Major State Basic Research Development Program of China (2015CB6551002) and the Sino Japanese Project from MOST of China (2016YFE0118200) which give the financial support were gratefully acknowledged. References [1] T. Tian, Y. Yan, Z.H. Hu, Utilization of original phosphogypsum for the preparation of foam concrete, Constr. Build. Mater. 115 (2016) 143–152. [2] L. Yang, Y.S. Zhang, Y. Yan, Utilization of original phosphogypsum as raw material for the preparation of self-leveling mortar, J. Clean. Prod. 127 (2016) 204–213. [3] L. Yang, Y. Yan, Z.H. Hu, Utilization of phosphogypsum for the preparation of non-autoclaved aerated concrete, Constr. Build. Mater. 44 (2013) 600–606. [4] R.H. Moreiraa, F.S. Queirogaa, H.A. Paivaa, N.H. Medinab, G. Fontanaa, M.A. Guazzelli, Extraction of natural radionuclides in TENORM waste phosphogypsum, J. Environ. Chem. Eng. 6 (2018) 6664–6668. [5] V.N. Rychkov, E.V. Kirillov, S.V. Kirillov, V.S. Semenishchev, Recovery of rare earth elements from phosphogypsum. 196 (2018) 674-681. [6] M.I. Romero-Hermidaa, A.M. Borrero-Lópezb, F.J. Alejandrec, V. Flores-Alésc, Phosphogypsum waste lime as a promising substitute of commercial limes: a rheological approach, Cem. Concr. Comp. 95 (2019) 205–216. [7] A.M. Rashad, Phosphogypsum as a construction material, J. Clean. Prod. 166 (2017) 732–743.

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