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The rheological behavior of paste prepared from hemihydrate phosphogypsum and tailing
Construction and Building Materials 229 (2019) 116870
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The rheological behavior of paste prepared from hemihydrate phosphogypsum and tailing Guanzhao Jiang a,b, Aixiang Wu a,⇑, Yiming Wang a, Jianqiu Li c a
Key Laboratory of High-Efficient Mining and Safety of Metal, Ministry of Education, University of Science and Technology Beijing, Beijing 100083, China School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China c School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China b
h i g h l i g h t s The paste prepared from hemihydrate phosphogypsum and tailing (HTP) will be applied in mine filling. The hemihydrate phosphogypsum hydration was unfavorable to the HTP rheological stability. HTP was a new engineering paste with good flow ability, low resistance and proper rheological stability. This work may be a preliminary guide in the HTP mixing and transport process.
a r t i c l e
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Article history: Received 22 December 2018 Received in revised form 4 August 2019 Accepted 1 September 2019 Available online 9 September 2019 Keywords: Hemihydrate phosphogypsum Flow ability Rheological parameters Thixotropy Rheological stability
a b s t r a c t The paste prepared from hemihydrate phosphogypsum (HPG) and tailing (HTP) differs greatly from the cement tailings paste (CTP). The rheological behavior of HTP including the visual flow ability and rheology was uncovered in the present study. The suitable mass concentration of HTP was determined by fluidity and bleeding tests firstly. Then, the rheological parameters and thixotropy were analyzed based on rheological tests. Compared with CTP, HTP has higher ultimate yield stress, smaller yield stress and plastic viscosity, and shorter thixotropic time. Finally, the rheological properties with different resting time and shearing time were studied. The results showed that the rheological stability deteriorated with time, especially in the resting state. The special rheological behavior of HTP may be attributed to the stable mosaic structure, which was related to the particles size and shape. In addition, the HPG hydration will enhance the structure, resulting in the rheological stability of HTP weakening gradually. This work may pave ways to the understanding of the HTP mixing and transport process. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Phosphogypsum (PG) is a problematic industrial by-product of the wet process of phosphoric fertilizer and acid. Global production of this waste is from 100 to 280 million tons per year, but only 15% of worldwide PG is recycled as building materials, agricultural fertilizers, setting controller, etc [1,2]. It is attributed to impurities that degrade the performance of PG products and high utilization costs [3–6]. Hemihydrate phosphogypsum (HPG), the main phase being CaSO40.5H2O, is produced by hemi-wet process. In the process of CaSO40.5H2O hydrating into CaSO42H2O, HPG has an obvious self-hardening property. However, due to the effect of ⇑ Corresponding author at: School of Civil and Environmental Engineering, University of Science and Technology Beijing, No. 30 Xueyuan Road, Haidian District, Beijing 100083, China. E-mail address: [email protected] (A. Wu). https://doi.org/10.1016/j.conbuildmat.2019.116870 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
impurities such as acid soluble phosphorus, fluorine, the property is poor. The alkaline matters such as quicklime can neutralize the acid impurities to release the potential self-hardening property of HPG. A new low-cost ecological filling material (HFM) was prepared from quicklime, HPG and phosphorus tailings [7]. It has been successfully applied to mine filling to control surface subsidence. Jiang et al. [7] have conducted some research on the strength and hydration mechanism of HFM, but it did not involve the rheological behavior, which was crucial for mixing preparation and pipeline transport of HFM paste. The rheological behavior of cement tailings paste (CTP) usually focused on the rheological models, thixotropy. The former, used to characterize the relationship of shear stress vs. shear rate, were mainly a three-parameters Herschel–Bulkley model or a two-parameters Bingham model (special Herschel–Bulkley model) [8–11]. The hysteretic cycle was usually used to describe the latter,
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and it was useful as a preliminary indicator of behavior, but not providing a good basis for quantitative treatments [12,13]. In the early papers, researchers used a viscometer to study the rheological behavior of gypsum plaster and thought it was consistent with Casson model [14,15]. But after that, Caufin et al. [16] found the function of shear stress and shear rate of plaster fitted fairly well the Bingham model. The rheometer was more advanced in obtaining more rheological information and reducing the wall slip effect [17]. Using a rheometer, it was also found that the plaster, similar to CTP, fitted the Bingham or Herschel–Bulkley model [18,19]. As the filling binder, the HPG was very different from cement in some characteristics, such as coarser particle size, special particle shape and faster hydration. It may lead to significant changes in the rheological behavior of paste prepared from HPG and tailing (HTP), especially in the rheological stability. The latter had practical significance for equipment restart and long-distance pipeline transport, but it was rarely mentioned in the above research on plaster. Regardless of HTP or CTP, the yield stress should be less than 250 Pa, and had proper rheological stability to avoid pipeline blockage [20]. This paper conducted laboratory tests on HTP rheological behavior. The suitable mass concentration was first determined by the conventional flow ability and bleeding tests. Then, the rheological properties were studied by a rheometer to analyze the rheological parameters and thixotropy. In addition, this paper focused on the changes rheological properties with time to study the HTP rheological stability. This work paved the way to the HTP mixing and transport in engineering practice.
2. Experimental 2.1. Raw materials The HPG was taken from a chemical plant in Guizhou, China, and dried at 105 °C in a dry environment. The tailings were ungraded solid wastes obtained by filter pressing in the process of phosphorus ore dressing, and dried at 190 °C. The commercial grade quicklime containing 77.4% effective calcium was used to neutralize acid impurities in HPG. The physical and chemical properties of raw materials were shown in Table 1. The main phase of HPG was CaSO40.5H2O, accounting for more than 90%, and its pH was 3.4, mainly caused by 0.42% soluble P2O5. The tailings, main phase being dolomite, was neutral and the acidic impurities was not detected in laboratory. Due to no chemical reaction with quicklime and HPG, tailings acted as inert materials to improve particle size distribution (PSD) of HFM.
Fig. 1. The PSD of HPG, tailings and HFM.
Fig. 1 showed the raw materials’ PSD obtained by the laser analyzer. The uniformity coefficient and curvature coefficient of HPG were 3.2 and 1.2, respectively, which meant that its PSD was poor. The mean particle size of HPG was 59.6 lm, obviously coarser than cement particles. In addition, the content of particles finer than 20 lm improving the water retention capacity of paste was only 9.77% in HPG, less than 15% recognized minimum value internationally [20]. The poor PSD, coarser size and less fine particles of HPG may inevitably lead to poor anti-segregation of HTP and increase the risk of pipeline blockage [20]. If the HPG is the only filling material, it will not be helpful to the HTP transport. The tailings had good PSD and increased the fine particles in HFM to 21.91%, meaning that tailings improved the anti-segregation. Fig. 2 showed the microscopic morphology of HPG particles. The hexagonal flat columnar crystals of 20–30 lm in diameter and 5– 15 lm in length were clustering together to form very irregular particles. The special particles shape led to rough surface, changing the particles’ contact from point to surface, and the friction between particles was greatly increased. In addition, the flat crystals was finer than a-hemihydrate, which may lead to a high water demand of HPG [21]. 2.2. Test methods 2.2.1. Slump and fluidity The visual flow ability of HTP was characterized by slump and fluidity. The slump was the fall height difference of slurry due to
Table 1 Physical and chemical properties of raw materials. Components (wt%)
HPG
Lime
Tailings
Main chemical composition SiO2 Al2O3 Fe2O3 CaO MgO SO3 P2O5 F Loss on ignition pH
4.20 0.46 0.45 37.86 0.28 44.82 1.37 1.43 8.12 3.4
1.42 0.98 – 84.57 1.75 – – – 6.37 –
2.59 0.38 0.42 34.44 17.91 0.82 6.45 0.64 34.81 7.1
Physical properties Specific gravity Bulk density (kg/m3) Porosity (%)
2.69 1270 52.95
2.62 800 70.69
2.82 1130 60.28
Fig. 2. The microscopic morphology of HPG.
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its own weight in the bucket, which was a trumpet-shaped iron container with a height of 300 mm, upper diameter of 100 mm and bottom diameter of 200 mm. The slump was determined according to the Chinese standard (GB/T 50080-2016). The fluidity of HTP was measured within 6 min from adding water to the end of expanded diameter measurement according to Chinese standard (GB/T 2419-2005). The results were the average of two perpendicular diameters.
Table 2 Comparison of some characteristics between HTP and CTP. Characteristics
HTP
CTP
Formula Mass concentration (wt%) Setting time (min) Slump (cm)
HPG:lime:tailings/62:1:37 65–71
Cement:tailings/20:80 72–78
100–130 24.5–28.5
＞480 21.0–29.0
2.2.2. Saturation degree The theoretical saturation degree (St) was defined as the ratio of liquid volume to void volume, and was calculated by Eq. (1) [22,23].
q Gw C m St ¼ 1 1 1þ Gs Gs ð100 C m Þ
ð1Þ
where Cm was the slurry mass concentration; Gs and q was the materials’ specific gravity and bulk density; Gw was the water specific gravity at the temperature of 20 °C. The real saturation (Sr) can be obtained by the conversion of bleeding rate (Br), as shown in Eq. (2).
Sr ¼ 1=ð1 Br Þ
ð2Þ
Referring to Chinese standard (GB/T 50080-2016), the paste was poured into a 500 mL glass measuring cylinder to carry out bleeding rate test. The initial mass of paste was recorded as M0. When the bleeding time was within 30 min, the bleeding water was absorbed by a dropper every 5 min, and then the frequency was 10 min. Per water mass was recorded to obtain the cumulative mass Mw, accurate to 1 g. Br was calculated by Eq. (3).
Br ¼
100Mw M0 ð1 C m Þ
ð3Þ
2.2.3. Rheological parameters The rheological parameters were determined by using a Brookfield rheometer fitted with a blade rotor. After weighing the raw materials, HTP was mixed for 1 min at the speed of 100 rmin1, then immediately transferred to rheological tests. The shear rate (c_ ) was increased linearly from 0 to 120 s1 within 90 s, and the shear stress (s) data was collected by the Rheo3000 software. 2.2.4. Thixotropy The HTP was prepared at the mass concentration of 69 wt% by the same methods as Section 2.2.3. Keeping c_ at 30, 60, 90, 120 s1, the s with shearing time in 90 s was obtained. The s as the function of c_ and shearing time was analyzed. The time when the initial s (si) reached an equilibrium value (se) was the thixotropic equilibrium time (te) [12]. The thixotropy was characterized by te and thixotropic rheological parameters. 2.2.5. Rheological stability The HTP rheological stability was the changes of rheological properties with resting time or shearing time, representing the case of mixing or pumping process stop and long pipeline transport, respectively. Before the resting rheological tests were carried out, the mixed HTP stayed in a beaker for 1, 5, 10 and 15 min without shearing. On the contrary, during the shearing rheological tests, the HTP kept mixing for 30, 60, 90, and 120 min at the same speed.
Fig. 3. Comparison of HTP and CTP strength.
tration of 65–71 wt% after HFM was mixed with deionized water. The main characteristics of HTP and CTP were shown in Table 2 and Fig. 3, and the data was obtained from Refs. [7] and [11]. Compared with CTP, the setting time of HTP was shortened by more than 4 times, and the strength curing for 3 days was higher. It indicated that the supports for wall can be formed in a short time, which was very important for the improvement of mining efficiency. At the design mass concentration, the HTP had better flow ability due to the greater slump. The lower mass concentration of HTP was due to the higher water demand for normal consistency of HPG. HTP’s setting time was obviously longer than building gypsum, generally less than 30 min, so it was considered to carry out rheological tests without retarders [16]. 4. Results and discussion 4.1. Suitable mass concentration determination 4.1.1. Changes of slump and fluidity It can be seen from Fig. 4 that the mass concentration was negatively correlated with slump and fluidity, indicating that the HTP’s visual flow ability became worse. When the concentration was lower than 69 wt%, the paste, slump being more than 27 cm, had good flow ability, and can consider self-flowing transport mode [24]. Both the slump and fluidity curves showed a steep fall at the concentration above 69 wt%, which may mean a change of flow pattern. When the concentration exceeded 71 wt%, it was found that the HTP mixing was difficult due to the poor plasticity in laboratory, so it should be less than 71 wt% in engineering practice.
3. HPG-tailings paste (HTP) It had been determined that the formula of HPG, quicklime and tailings in HFM was 62%, 1% and 37% in laboratory based on the strength. The HTP was a solid-liquid mixture with a mass concen-
4.1.2. Changes of saturation degree Fig. 5 showed the theoretical and the conversion value of saturation degree was a function of mass concentration. The mixing water was completely filled in the particles’ pores at the saturation
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degree of 100%, when the HTP can be called ideal paste with the characteristic of no bleeding. However, due to the poor flow ability, the mixing and transport was very difficult. Many scholars believed that in order to ensure the good anti-segregation and flow ability, the saturation degree should be 101.5–105.3%, so that a small amount of water separated from the particles and formed a lubricating layer on the pipe wall, which helped the paste transport [23,25]. It can be seen from the shaded area in the Fig. 5 that the suitable concentration was 69.2–71 wt% in engineering practice, when the HTP evolved from a solid-liquid two-phase fluid to a structure fluid [26,27], consistent with the results in Section of 4.1.1.
4.2. Rheological parameters analysis
Fig. 4. Effect of mass concentration on the HTP slump and fluidity.
Fig. 5. Effect of mass concentration on the HTP saturation degree.
4.2.1. Rheological model As shown in Fig. 6, as the c_ increased, the shear stress at different mass concentration showed a similar change, which can be divided into two stages. In the area Ⅰ where the c_ was less than 34 s1, the s increased sharply to peak value just after being subjected to shearing, and then decreased rapidly. The peak value was known as the ultimate yield stress (smax), and the area was called the stress overshoot area [9,28,29]. The overshoot did not occur until the concentration increased to 67 wt%, and became more pronounced as the mixing water decreased. The main reasons for stress overshoot may be the combined effect of particles size and shape (Fig. 3). The coarser and irregular HPG particles may make it easy to form the stable mosaic structure, which increased the solid friction between particles and required a larger shear force to destroy. Once the mosaic structure was destroyed, the friction and shear force decreased rapidly. It was the mechanical description of stress overshoot. There was no overshoot at the concentration of 65 wt%, indicating that the increasing concentration was one of the important reasons for the phenomenon. When the concentration was high, the free water acting as a lubricant between particles decreased, leading to the friction increasing further, so the overshoot became more obvious. The stress overshoot may mean that the initial flow resistance, which the paste needed to overcome from resting to flowing, increased with smax. It was
Fig. 6. The curves of shear stress vs. shear rate of HTP with different concentration.
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very important for the restart resistance analysis of the mixing or pumping equipment. After the shear force overcame the friction, the mosaic structure changed into a network structure in which the free water was gradually filling particles, and the stress overshoot was weakened obviously. In the area II where the c_ was more than 34 s1, and the shear time exceeded 25 s, the overshoot disappeared and the rheological curves increased linearly. The Bingham model Eq. (4) can be used to analyze the rheological parameters in this area [18]. In the Bingham area, the network structure played a leading role in the HTP rheological properties. :
s ¼ s0 þ lB c
ð4Þ
where s0 was the yield stress (Pa); lB was plastic viscosity (Pas). According to the modified Rabinowich-Mooney equation, the relationship of wall shear rate between the non-Newtonian c_ w N and Newtonian fluid c_ w was shown in Eq. (5).
c_ w ¼
3n þ 1 N c_ w 4n
ð5Þ
Fig. 7. The curve of yield stress vs. mass concentration of HTP.
5
c_ Nw ¼
8v D
ð6Þ
where n was the flow index; D was the pipe diameter (m), v was the average flow velocity of fluid in the pipe (ms1). Generally, the filling pipe D was 0.1–0.25 m, and v was 1–2 m/s. N According to the Eq. (6), the c_ w was greater than 32 s1. HTP was similar to Bingham fluid based on the rheological curves, so n depending on the fluid pattern was not greater than 1 and c_ w was more than 32 s1. Thus, it was more valuable to study the Bingham area in engineering practice. It can be seen from Fig. 6 that the fitting degree of rheological curve and Bingham model was more than 95%. 4.2.2. Yield stress It can be seen from Fig. 7 that s0 was an exponential function of mass concentration, and the s0 was between 27.9 and 45.7 Pa at the suitable concentration. It can be concluded that the s0 of HTP meeting engineering practice was below 50 Pa, obviously less than CTP. The latter was generally 150–250 Pa [11,20,30]. This indicated that HTP was subject to less resistance in the process of transport. 4.2.3. Viscosity Fig. 8 showed that HTP had the characteristic of shear thinning that apparent viscosity (lA) decreased as the c_ increased and gradually tended to a stable value. In the stress overshoot area, the lA was at a high level, where the paste needed to overcome larger flow resistance. When the mixing or pumping equipment was restarted, the resistance value was 4–10 times or even higher than that under continuous flow conditions. The lB depended on the cohesion and friction between particles. If the lB is small, it will reduce the rheological stability and aggravate the segregation of paste, which may lead to the pipe blockage accidents. The lB increasing with mass concentration helped to improve the anti-segregation. However, high lB will increase flow resistance and weaken flow ability. The HTP lB was only 0.24–0.33 Pas, which was a lower value than CTP, generally 0.3–0.8 Pas [20]. Therefore, the anti-segregation of HTP was weaker than CTP, which may be attributed to the fact that there were fewer fine particles in the paste.
Fig. 8. The curves of apparent viscosity vs. shear rate of HTP.
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According to the rheological parameters, it was known that HTP had good flow ability under continuous flow conditions. In contrast, when the mixing or pumping equipment was restarted, it was necessary to exerting more pressure to make the paste flow again. The restart pressure needed to be considered during selecting equipment. At the same time, due to the poor anti-segregation, a higher flow speed was required to avoid the particles depositing on the pipe wall. The s0 and lB showed a good functional relationship with mass concentration. This was because as the mixing water decreased, the particles pores reduced. Further, the strengthened chemistry effect between particles, including hydration, ions dispersion or adsorption, increased the cohesion and friction, which was positively related to s0 and lB [31–34]. 4.3. Thixotropy As shown in Fig. 9, HTP had significant stress relaxation at a constant c_ , which was key feature of thixotropy. The stress relaxation reflected the destruction and reconstruction of particles structure [13,35,36]. As the c_ increased, the se increased, but the te was the same 81 s, obviously shorter than CTP [11]. After being sheared, the high elastic energy accumulated in the mosaic structure will be released quickly, resulting in a rapid decrease of s. Keeping the shearing effect, the closed free water between particles was released, forming a lubricating water film absorbing on the surface of particles, and it was the main reason why the s continued to decrease. The si were 105.7, 110.7, 120.9, and 130.6 Pa at the c_ of 30, 60, 90, and 120 s1, respectively, which were obviously higher than the data in Fig. 6. It indicated that the initial thixotropic process was affected by the stress overshoot. When the mosaic structure was completely destroyed by shearing force, the overshoot disappeared, and high shear rate can accelerate the destruction. It may be inferred that disappearance time of overshoot was less than 25 s at the constant shear rate of 30–120 s1 due to the shortened shear history. The data in Fig. 6 was considered as si to avoid the interference of stress overshoot. The scatter graph was plotted with si and se as ordinates. The rheological parameters before and after thixotropy were analyzed by fitting method and the results were shown in
Fig. 10. The curves of shear stress vs. shear rate of HTP before and after thixotropy.
Fig. 10. The s0 and lB of thixotropic HTP were 23.0 Pa and 0.20 Pas, decreasing by 17.7% and 18.0%, which reflected the destruction of particle structure and friction reduction during the shearing process. In the process of transport, HTP was subjected to continuous shearing, so the flow resistance based on the thixotropic rheological parameters was more realistic. The flow ability was better after thixotropy, which was beneficial for pipeline transport.
4.4. Rheological stability 4.4.1. Effect of resting time HTP contained more than half of HPG. After contact with mixing water, the CaSO40.5H2O will gradually hydrate into CaSO42H2O and the crystals also grew from flat column and plate-like to more stable short column and block [7]. The initial setting time of HTP, when it lost plasticity, was only 100 min and significantly shorter than CTP. Therefore, the influence of HPG hydration on HTP rheological stability must be considered [9,10,37–39].
Fig. 9. The curves of shear stress vs. shearing time of HTP.
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As shown in Fig. 11, the HTP rheological curves at the mass concentration of 69 wt% were very different as the resting time increased. After resting for 1 min, the smax increased from 74.8 to 197.2 Pa, increasing by 163.6%. However, the boundary between stress overshoot and Bingham area was clear, and the Bingham curve trend had no obvious changes. When the resting time was within 5–10 min, the curve trend changed from flat to descending and the boundary disappear in the range of 0–120 s1. The smax increased to 434.5 Pa at the resting time of 15 min. The black arrows in Fig. 11 clearly indicated the Bingham curve trend, and it can be seen that the resting time was negatively correlated with the curve slope. The trend changes can be analyzed by the particles structure and HPG hydration. In the resting state, the hydration process was not disturbed by shear force, which caused the rapid increase of friction between particles. On the one hand, the chemical adsorption force, produced in the hydration of CaSO40.5H2O transforming to CaSO42H2O, increased the cohesion. On the other hand, the crystals’ growth reduced the particles’ pores and increased the contact chance [39]. In addition, some free water will bleed from particles without any shearing, resulting in settling phenomenon and further increasing the friction [20]. The disappearance of boundary may be attributed to the hydration and particles settling, but the main role of both will be studied further. Undoubtedly, both of them will make the mosaic structure more stable, and more energy was required to destroy it, which was the intrinsic reason for high smax and Bingham curves descending. It can be inferred that the rising curve will appear again at the higher shear rate or longer shearing time, when the mosaic structure was completely destroyed. The changes of rheological properties with resting time indicated that the HTP rheological stability decreased, it was very unfavorable for the paste mixing or transport. The smax was very important for the problem of whether the restart of mixing or pumping equipment can be successful. The paste resting led to the restart pressure increasing. In addition to
7
smax, the blades shape and structure of mixing apparatus, the diameter and length of pipeline, etc. may be needed to further determine the restart pressure in engineering practice. When the pressure was too small, the paste was still stationary and if not cleaned up in time, it will crush the mixing blades or cause the pipe blockage accidents. 4.4.2. Effect of shearing time As shown in Fig. 12, under the continuous shearing conditions, the curves were obviously different from that in the resting state. When the shearing time was in the range of 30–90 min, the curves can still be clearly divided into stress overshoot and Bingham area, and the s0 increased from 25.2 to 54.8 Pa with the increase of shearing time. After the HTP was mixed for 120 min, the curves began to show a descending trend in the range of 0–120 s1. Although the smax was much lower than the value in the resting state, there was still a positive correlation with the shearing time. The results showed that the HTP rheological stability can be significantly improved in the shearing state, but it was not effective for a long time. During long-distance pipeline transport, the paste was subjected to continuous shearing of pipe wall, and the changes of rheological stability were critical for the flow resistance calculation. Generally, the paste flow time in pipeline was less than 30 min, when the rheological properties were relatively stable, and the effect of HPG hydration can be neglected during transport process. The HPG hydration had an obvious effect on the rheological stability at the flow time of 120 min, and the flow resistance calculation should consider it. Although the shear force can destroy the particles structure, they did not prevent the HPG hydration [31]. Therefore, the particles structure was in the dynamic process of shearing destruction and hydration reconstruction [37,40]. As the shear time increased, the CaSO40.5H2O continuously grew into CaSO42H2O, which led to a loss of mixing water in the paste, so the particles had more
Fig. 11. The curves of shear stress vs. shear rate of HTP at different resting time.
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Fig. 12. The curves of shear stress vs. shear rate of HTP at different shearing time.
chance for contact and collision [9]. As a result, the friction increased, and so did the s0 and smax. In general, the rebuilding of particles structure was slightly faster than its breaking at a shearing speed of 100 rmin1 until the shearing time reached 120 min, and the speed difference was clearly presented on the rheological curve.
Acknowledgments The work was financially supported by the Key Research and Devlopment Program of China (2017YFC0602903), the Key Program of National Natural Science Foundation of China (51834001) and the Young Scientists Fund of National Natural Science Foundation of China (51804015).
5. Conclusions Due to the difference in materials gradation and chemical properties, the rheological behavior of HTP was very different from that of CTP. HTP had larger slump, smaller s0 and lB, and better flow ability. The existence of stress overshoot made smax nonnegligible in the restart resistance calculation of mixing or pumping equipment. In addition, HTP had obvious thixotropy and short thixotropic equilibrium time. In the resting state, the HTP rheological stability decreased rapidly, and the boundary between stress overshoot and Bingham area disappeared at the resting time of 5 min. In contrast, the continuous shearing prolonged the boundary disappearance to 120 min, far more than the paste flow time in pipeline. In the process of transport, it was not necessary to consider the resistance increase caused by HPG hydration until the flow time exceeded 30 min. The resting or shearing time can be clearly determine based on more engineering parameters. The mosaic structure of particles may be one of main reasons for the HTP special rheological behavior. In addition, the HPG hydration should be taken into account during analyzing the rheological properties. HTP can be described as a new engineering paste with good flow ability, low resistance and proper rheological stability. 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.
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The rheological behavior of paste prepared from hemihydrate phosphogypsum and tailing Guanzhao Jiang a,b, Aixiang Wu a,⇑, Yiming Wang a, Jianqiu Li c a
Key Laboratory of High-Efficient Mining and Safety of Metal, Ministry of Education, University of Science and Technology Beijing, Beijing 100083, China School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China c School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China b
h i g h l i g h t s The paste prepared from hemihydrate phosphogypsum and tailing (HTP) will be applied in mine filling. The hemihydrate phosphogypsum hydration was unfavorable to the HTP rheological stability. HTP was a new engineering paste with good flow ability, low resistance and proper rheological stability. This work may be a preliminary guide in the HTP mixing and transport process.
a r t i c l e
i n f o
Article history: Received 22 December 2018 Received in revised form 4 August 2019 Accepted 1 September 2019 Available online 9 September 2019 Keywords: Hemihydrate phosphogypsum Flow ability Rheological parameters Thixotropy Rheological stability
a b s t r a c t The paste prepared from hemihydrate phosphogypsum (HPG) and tailing (HTP) differs greatly from the cement tailings paste (CTP). The rheological behavior of HTP including the visual flow ability and rheology was uncovered in the present study. The suitable mass concentration of HTP was determined by fluidity and bleeding tests firstly. Then, the rheological parameters and thixotropy were analyzed based on rheological tests. Compared with CTP, HTP has higher ultimate yield stress, smaller yield stress and plastic viscosity, and shorter thixotropic time. Finally, the rheological properties with different resting time and shearing time were studied. The results showed that the rheological stability deteriorated with time, especially in the resting state. The special rheological behavior of HTP may be attributed to the stable mosaic structure, which was related to the particles size and shape. In addition, the HPG hydration will enhance the structure, resulting in the rheological stability of HTP weakening gradually. This work may pave ways to the understanding of the HTP mixing and transport process. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Phosphogypsum (PG) is a problematic industrial by-product of the wet process of phosphoric fertilizer and acid. Global production of this waste is from 100 to 280 million tons per year, but only 15% of worldwide PG is recycled as building materials, agricultural fertilizers, setting controller, etc [1,2]. It is attributed to impurities that degrade the performance of PG products and high utilization costs [3–6]. Hemihydrate phosphogypsum (HPG), the main phase being CaSO40.5H2O, is produced by hemi-wet process. In the process of CaSO40.5H2O hydrating into CaSO42H2O, HPG has an obvious self-hardening property. However, due to the effect of ⇑ Corresponding author at: School of Civil and Environmental Engineering, University of Science and Technology Beijing, No. 30 Xueyuan Road, Haidian District, Beijing 100083, China. E-mail address: [email protected] (A. Wu). https://doi.org/10.1016/j.conbuildmat.2019.116870 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
impurities such as acid soluble phosphorus, fluorine, the property is poor. The alkaline matters such as quicklime can neutralize the acid impurities to release the potential self-hardening property of HPG. A new low-cost ecological filling material (HFM) was prepared from quicklime, HPG and phosphorus tailings [7]. It has been successfully applied to mine filling to control surface subsidence. Jiang et al. [7] have conducted some research on the strength and hydration mechanism of HFM, but it did not involve the rheological behavior, which was crucial for mixing preparation and pipeline transport of HFM paste. The rheological behavior of cement tailings paste (CTP) usually focused on the rheological models, thixotropy. The former, used to characterize the relationship of shear stress vs. shear rate, were mainly a three-parameters Herschel–Bulkley model or a two-parameters Bingham model (special Herschel–Bulkley model) [8–11]. The hysteretic cycle was usually used to describe the latter,
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and it was useful as a preliminary indicator of behavior, but not providing a good basis for quantitative treatments [12,13]. In the early papers, researchers used a viscometer to study the rheological behavior of gypsum plaster and thought it was consistent with Casson model [14,15]. But after that, Caufin et al. [16] found the function of shear stress and shear rate of plaster fitted fairly well the Bingham model. The rheometer was more advanced in obtaining more rheological information and reducing the wall slip effect [17]. Using a rheometer, it was also found that the plaster, similar to CTP, fitted the Bingham or Herschel–Bulkley model [18,19]. As the filling binder, the HPG was very different from cement in some characteristics, such as coarser particle size, special particle shape and faster hydration. It may lead to significant changes in the rheological behavior of paste prepared from HPG and tailing (HTP), especially in the rheological stability. The latter had practical significance for equipment restart and long-distance pipeline transport, but it was rarely mentioned in the above research on plaster. Regardless of HTP or CTP, the yield stress should be less than 250 Pa, and had proper rheological stability to avoid pipeline blockage [20]. This paper conducted laboratory tests on HTP rheological behavior. The suitable mass concentration was first determined by the conventional flow ability and bleeding tests. Then, the rheological properties were studied by a rheometer to analyze the rheological parameters and thixotropy. In addition, this paper focused on the changes rheological properties with time to study the HTP rheological stability. This work paved the way to the HTP mixing and transport in engineering practice.
2. Experimental 2.1. Raw materials The HPG was taken from a chemical plant in Guizhou, China, and dried at 105 °C in a dry environment. The tailings were ungraded solid wastes obtained by filter pressing in the process of phosphorus ore dressing, and dried at 190 °C. The commercial grade quicklime containing 77.4% effective calcium was used to neutralize acid impurities in HPG. The physical and chemical properties of raw materials were shown in Table 1. The main phase of HPG was CaSO40.5H2O, accounting for more than 90%, and its pH was 3.4, mainly caused by 0.42% soluble P2O5. The tailings, main phase being dolomite, was neutral and the acidic impurities was not detected in laboratory. Due to no chemical reaction with quicklime and HPG, tailings acted as inert materials to improve particle size distribution (PSD) of HFM.
Fig. 1. The PSD of HPG, tailings and HFM.
Fig. 1 showed the raw materials’ PSD obtained by the laser analyzer. The uniformity coefficient and curvature coefficient of HPG were 3.2 and 1.2, respectively, which meant that its PSD was poor. The mean particle size of HPG was 59.6 lm, obviously coarser than cement particles. In addition, the content of particles finer than 20 lm improving the water retention capacity of paste was only 9.77% in HPG, less than 15% recognized minimum value internationally [20]. The poor PSD, coarser size and less fine particles of HPG may inevitably lead to poor anti-segregation of HTP and increase the risk of pipeline blockage [20]. If the HPG is the only filling material, it will not be helpful to the HTP transport. The tailings had good PSD and increased the fine particles in HFM to 21.91%, meaning that tailings improved the anti-segregation. Fig. 2 showed the microscopic morphology of HPG particles. The hexagonal flat columnar crystals of 20–30 lm in diameter and 5– 15 lm in length were clustering together to form very irregular particles. The special particles shape led to rough surface, changing the particles’ contact from point to surface, and the friction between particles was greatly increased. In addition, the flat crystals was finer than a-hemihydrate, which may lead to a high water demand of HPG [21]. 2.2. Test methods 2.2.1. Slump and fluidity The visual flow ability of HTP was characterized by slump and fluidity. The slump was the fall height difference of slurry due to
Table 1 Physical and chemical properties of raw materials. Components (wt%)
HPG
Lime
Tailings
Main chemical composition SiO2 Al2O3 Fe2O3 CaO MgO SO3 P2O5 F Loss on ignition pH
4.20 0.46 0.45 37.86 0.28 44.82 1.37 1.43 8.12 3.4
1.42 0.98 – 84.57 1.75 – – – 6.37 –
2.59 0.38 0.42 34.44 17.91 0.82 6.45 0.64 34.81 7.1
Physical properties Specific gravity Bulk density (kg/m3) Porosity (%)
2.69 1270 52.95
2.62 800 70.69
2.82 1130 60.28
Fig. 2. The microscopic morphology of HPG.
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its own weight in the bucket, which was a trumpet-shaped iron container with a height of 300 mm, upper diameter of 100 mm and bottom diameter of 200 mm. The slump was determined according to the Chinese standard (GB/T 50080-2016). The fluidity of HTP was measured within 6 min from adding water to the end of expanded diameter measurement according to Chinese standard (GB/T 2419-2005). The results were the average of two perpendicular diameters.
Table 2 Comparison of some characteristics between HTP and CTP. Characteristics
HTP
CTP
Formula Mass concentration (wt%) Setting time (min) Slump (cm)
HPG:lime:tailings/62:1:37 65–71
Cement:tailings/20:80 72–78
100–130 24.5–28.5
＞480 21.0–29.0
2.2.2. Saturation degree The theoretical saturation degree (St) was defined as the ratio of liquid volume to void volume, and was calculated by Eq. (1) [22,23].
q Gw C m St ¼ 1 1 1þ Gs Gs ð100 C m Þ
ð1Þ
where Cm was the slurry mass concentration; Gs and q was the materials’ specific gravity and bulk density; Gw was the water specific gravity at the temperature of 20 °C. The real saturation (Sr) can be obtained by the conversion of bleeding rate (Br), as shown in Eq. (2).
Sr ¼ 1=ð1 Br Þ
ð2Þ
Referring to Chinese standard (GB/T 50080-2016), the paste was poured into a 500 mL glass measuring cylinder to carry out bleeding rate test. The initial mass of paste was recorded as M0. When the bleeding time was within 30 min, the bleeding water was absorbed by a dropper every 5 min, and then the frequency was 10 min. Per water mass was recorded to obtain the cumulative mass Mw, accurate to 1 g. Br was calculated by Eq. (3).
Br ¼
100Mw M0 ð1 C m Þ
ð3Þ
2.2.3. Rheological parameters The rheological parameters were determined by using a Brookfield rheometer fitted with a blade rotor. After weighing the raw materials, HTP was mixed for 1 min at the speed of 100 rmin1, then immediately transferred to rheological tests. The shear rate (c_ ) was increased linearly from 0 to 120 s1 within 90 s, and the shear stress (s) data was collected by the Rheo3000 software. 2.2.4. Thixotropy The HTP was prepared at the mass concentration of 69 wt% by the same methods as Section 2.2.3. Keeping c_ at 30, 60, 90, 120 s1, the s with shearing time in 90 s was obtained. The s as the function of c_ and shearing time was analyzed. The time when the initial s (si) reached an equilibrium value (se) was the thixotropic equilibrium time (te) [12]. The thixotropy was characterized by te and thixotropic rheological parameters. 2.2.5. Rheological stability The HTP rheological stability was the changes of rheological properties with resting time or shearing time, representing the case of mixing or pumping process stop and long pipeline transport, respectively. Before the resting rheological tests were carried out, the mixed HTP stayed in a beaker for 1, 5, 10 and 15 min without shearing. On the contrary, during the shearing rheological tests, the HTP kept mixing for 30, 60, 90, and 120 min at the same speed.
Fig. 3. Comparison of HTP and CTP strength.
tration of 65–71 wt% after HFM was mixed with deionized water. The main characteristics of HTP and CTP were shown in Table 2 and Fig. 3, and the data was obtained from Refs. [7] and [11]. Compared with CTP, the setting time of HTP was shortened by more than 4 times, and the strength curing for 3 days was higher. It indicated that the supports for wall can be formed in a short time, which was very important for the improvement of mining efficiency. At the design mass concentration, the HTP had better flow ability due to the greater slump. The lower mass concentration of HTP was due to the higher water demand for normal consistency of HPG. HTP’s setting time was obviously longer than building gypsum, generally less than 30 min, so it was considered to carry out rheological tests without retarders [16]. 4. Results and discussion 4.1. Suitable mass concentration determination 4.1.1. Changes of slump and fluidity It can be seen from Fig. 4 that the mass concentration was negatively correlated with slump and fluidity, indicating that the HTP’s visual flow ability became worse. When the concentration was lower than 69 wt%, the paste, slump being more than 27 cm, had good flow ability, and can consider self-flowing transport mode [24]. Both the slump and fluidity curves showed a steep fall at the concentration above 69 wt%, which may mean a change of flow pattern. When the concentration exceeded 71 wt%, it was found that the HTP mixing was difficult due to the poor plasticity in laboratory, so it should be less than 71 wt% in engineering practice.
3. HPG-tailings paste (HTP) It had been determined that the formula of HPG, quicklime and tailings in HFM was 62%, 1% and 37% in laboratory based on the strength. The HTP was a solid-liquid mixture with a mass concen-
4.1.2. Changes of saturation degree Fig. 5 showed the theoretical and the conversion value of saturation degree was a function of mass concentration. The mixing water was completely filled in the particles’ pores at the saturation
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degree of 100%, when the HTP can be called ideal paste with the characteristic of no bleeding. However, due to the poor flow ability, the mixing and transport was very difficult. Many scholars believed that in order to ensure the good anti-segregation and flow ability, the saturation degree should be 101.5–105.3%, so that a small amount of water separated from the particles and formed a lubricating layer on the pipe wall, which helped the paste transport [23,25]. It can be seen from the shaded area in the Fig. 5 that the suitable concentration was 69.2–71 wt% in engineering practice, when the HTP evolved from a solid-liquid two-phase fluid to a structure fluid [26,27], consistent with the results in Section of 4.1.1.
4.2. Rheological parameters analysis
Fig. 4. Effect of mass concentration on the HTP slump and fluidity.
Fig. 5. Effect of mass concentration on the HTP saturation degree.
4.2.1. Rheological model As shown in Fig. 6, as the c_ increased, the shear stress at different mass concentration showed a similar change, which can be divided into two stages. In the area Ⅰ where the c_ was less than 34 s1, the s increased sharply to peak value just after being subjected to shearing, and then decreased rapidly. The peak value was known as the ultimate yield stress (smax), and the area was called the stress overshoot area [9,28,29]. The overshoot did not occur until the concentration increased to 67 wt%, and became more pronounced as the mixing water decreased. The main reasons for stress overshoot may be the combined effect of particles size and shape (Fig. 3). The coarser and irregular HPG particles may make it easy to form the stable mosaic structure, which increased the solid friction between particles and required a larger shear force to destroy. Once the mosaic structure was destroyed, the friction and shear force decreased rapidly. It was the mechanical description of stress overshoot. There was no overshoot at the concentration of 65 wt%, indicating that the increasing concentration was one of the important reasons for the phenomenon. When the concentration was high, the free water acting as a lubricant between particles decreased, leading to the friction increasing further, so the overshoot became more obvious. The stress overshoot may mean that the initial flow resistance, which the paste needed to overcome from resting to flowing, increased with smax. It was
Fig. 6. The curves of shear stress vs. shear rate of HTP with different concentration.
G. Jiang et al. / Construction and Building Materials 229 (2019) 116870
very important for the restart resistance analysis of the mixing or pumping equipment. After the shear force overcame the friction, the mosaic structure changed into a network structure in which the free water was gradually filling particles, and the stress overshoot was weakened obviously. In the area II where the c_ was more than 34 s1, and the shear time exceeded 25 s, the overshoot disappeared and the rheological curves increased linearly. The Bingham model Eq. (4) can be used to analyze the rheological parameters in this area [18]. In the Bingham area, the network structure played a leading role in the HTP rheological properties. :
s ¼ s0 þ lB c
ð4Þ
where s0 was the yield stress (Pa); lB was plastic viscosity (Pas). According to the modified Rabinowich-Mooney equation, the relationship of wall shear rate between the non-Newtonian c_ w N and Newtonian fluid c_ w was shown in Eq. (5).
c_ w ¼
3n þ 1 N c_ w 4n
ð5Þ
Fig. 7. The curve of yield stress vs. mass concentration of HTP.
5
c_ Nw ¼
8v D
ð6Þ
where n was the flow index; D was the pipe diameter (m), v was the average flow velocity of fluid in the pipe (ms1). Generally, the filling pipe D was 0.1–0.25 m, and v was 1–2 m/s. N According to the Eq. (6), the c_ w was greater than 32 s1. HTP was similar to Bingham fluid based on the rheological curves, so n depending on the fluid pattern was not greater than 1 and c_ w was more than 32 s1. Thus, it was more valuable to study the Bingham area in engineering practice. It can be seen from Fig. 6 that the fitting degree of rheological curve and Bingham model was more than 95%. 4.2.2. Yield stress It can be seen from Fig. 7 that s0 was an exponential function of mass concentration, and the s0 was between 27.9 and 45.7 Pa at the suitable concentration. It can be concluded that the s0 of HTP meeting engineering practice was below 50 Pa, obviously less than CTP. The latter was generally 150–250 Pa [11,20,30]. This indicated that HTP was subject to less resistance in the process of transport. 4.2.3. Viscosity Fig. 8 showed that HTP had the characteristic of shear thinning that apparent viscosity (lA) decreased as the c_ increased and gradually tended to a stable value. In the stress overshoot area, the lA was at a high level, where the paste needed to overcome larger flow resistance. When the mixing or pumping equipment was restarted, the resistance value was 4–10 times or even higher than that under continuous flow conditions. The lB depended on the cohesion and friction between particles. If the lB is small, it will reduce the rheological stability and aggravate the segregation of paste, which may lead to the pipe blockage accidents. The lB increasing with mass concentration helped to improve the anti-segregation. However, high lB will increase flow resistance and weaken flow ability. The HTP lB was only 0.24–0.33 Pas, which was a lower value than CTP, generally 0.3–0.8 Pas [20]. Therefore, the anti-segregation of HTP was weaker than CTP, which may be attributed to the fact that there were fewer fine particles in the paste.
Fig. 8. The curves of apparent viscosity vs. shear rate of HTP.
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According to the rheological parameters, it was known that HTP had good flow ability under continuous flow conditions. In contrast, when the mixing or pumping equipment was restarted, it was necessary to exerting more pressure to make the paste flow again. The restart pressure needed to be considered during selecting equipment. At the same time, due to the poor anti-segregation, a higher flow speed was required to avoid the particles depositing on the pipe wall. The s0 and lB showed a good functional relationship with mass concentration. This was because as the mixing water decreased, the particles pores reduced. Further, the strengthened chemistry effect between particles, including hydration, ions dispersion or adsorption, increased the cohesion and friction, which was positively related to s0 and lB [31–34]. 4.3. Thixotropy As shown in Fig. 9, HTP had significant stress relaxation at a constant c_ , which was key feature of thixotropy. The stress relaxation reflected the destruction and reconstruction of particles structure [13,35,36]. As the c_ increased, the se increased, but the te was the same 81 s, obviously shorter than CTP [11]. After being sheared, the high elastic energy accumulated in the mosaic structure will be released quickly, resulting in a rapid decrease of s. Keeping the shearing effect, the closed free water between particles was released, forming a lubricating water film absorbing on the surface of particles, and it was the main reason why the s continued to decrease. The si were 105.7, 110.7, 120.9, and 130.6 Pa at the c_ of 30, 60, 90, and 120 s1, respectively, which were obviously higher than the data in Fig. 6. It indicated that the initial thixotropic process was affected by the stress overshoot. When the mosaic structure was completely destroyed by shearing force, the overshoot disappeared, and high shear rate can accelerate the destruction. It may be inferred that disappearance time of overshoot was less than 25 s at the constant shear rate of 30–120 s1 due to the shortened shear history. The data in Fig. 6 was considered as si to avoid the interference of stress overshoot. The scatter graph was plotted with si and se as ordinates. The rheological parameters before and after thixotropy were analyzed by fitting method and the results were shown in
Fig. 10. The curves of shear stress vs. shear rate of HTP before and after thixotropy.
Fig. 10. The s0 and lB of thixotropic HTP were 23.0 Pa and 0.20 Pas, decreasing by 17.7% and 18.0%, which reflected the destruction of particle structure and friction reduction during the shearing process. In the process of transport, HTP was subjected to continuous shearing, so the flow resistance based on the thixotropic rheological parameters was more realistic. The flow ability was better after thixotropy, which was beneficial for pipeline transport.
4.4. Rheological stability 4.4.1. Effect of resting time HTP contained more than half of HPG. After contact with mixing water, the CaSO40.5H2O will gradually hydrate into CaSO42H2O and the crystals also grew from flat column and plate-like to more stable short column and block [7]. The initial setting time of HTP, when it lost plasticity, was only 100 min and significantly shorter than CTP. Therefore, the influence of HPG hydration on HTP rheological stability must be considered [9,10,37–39].
Fig. 9. The curves of shear stress vs. shearing time of HTP.
G. Jiang et al. / Construction and Building Materials 229 (2019) 116870
As shown in Fig. 11, the HTP rheological curves at the mass concentration of 69 wt% were very different as the resting time increased. After resting for 1 min, the smax increased from 74.8 to 197.2 Pa, increasing by 163.6%. However, the boundary between stress overshoot and Bingham area was clear, and the Bingham curve trend had no obvious changes. When the resting time was within 5–10 min, the curve trend changed from flat to descending and the boundary disappear in the range of 0–120 s1. The smax increased to 434.5 Pa at the resting time of 15 min. The black arrows in Fig. 11 clearly indicated the Bingham curve trend, and it can be seen that the resting time was negatively correlated with the curve slope. The trend changes can be analyzed by the particles structure and HPG hydration. In the resting state, the hydration process was not disturbed by shear force, which caused the rapid increase of friction between particles. On the one hand, the chemical adsorption force, produced in the hydration of CaSO40.5H2O transforming to CaSO42H2O, increased the cohesion. On the other hand, the crystals’ growth reduced the particles’ pores and increased the contact chance [39]. In addition, some free water will bleed from particles without any shearing, resulting in settling phenomenon and further increasing the friction [20]. The disappearance of boundary may be attributed to the hydration and particles settling, but the main role of both will be studied further. Undoubtedly, both of them will make the mosaic structure more stable, and more energy was required to destroy it, which was the intrinsic reason for high smax and Bingham curves descending. It can be inferred that the rising curve will appear again at the higher shear rate or longer shearing time, when the mosaic structure was completely destroyed. The changes of rheological properties with resting time indicated that the HTP rheological stability decreased, it was very unfavorable for the paste mixing or transport. The smax was very important for the problem of whether the restart of mixing or pumping equipment can be successful. The paste resting led to the restart pressure increasing. In addition to
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smax, the blades shape and structure of mixing apparatus, the diameter and length of pipeline, etc. may be needed to further determine the restart pressure in engineering practice. When the pressure was too small, the paste was still stationary and if not cleaned up in time, it will crush the mixing blades or cause the pipe blockage accidents. 4.4.2. Effect of shearing time As shown in Fig. 12, under the continuous shearing conditions, the curves were obviously different from that in the resting state. When the shearing time was in the range of 30–90 min, the curves can still be clearly divided into stress overshoot and Bingham area, and the s0 increased from 25.2 to 54.8 Pa with the increase of shearing time. After the HTP was mixed for 120 min, the curves began to show a descending trend in the range of 0–120 s1. Although the smax was much lower than the value in the resting state, there was still a positive correlation with the shearing time. The results showed that the HTP rheological stability can be significantly improved in the shearing state, but it was not effective for a long time. During long-distance pipeline transport, the paste was subjected to continuous shearing of pipe wall, and the changes of rheological stability were critical for the flow resistance calculation. Generally, the paste flow time in pipeline was less than 30 min, when the rheological properties were relatively stable, and the effect of HPG hydration can be neglected during transport process. The HPG hydration had an obvious effect on the rheological stability at the flow time of 120 min, and the flow resistance calculation should consider it. Although the shear force can destroy the particles structure, they did not prevent the HPG hydration [31]. Therefore, the particles structure was in the dynamic process of shearing destruction and hydration reconstruction [37,40]. As the shear time increased, the CaSO40.5H2O continuously grew into CaSO42H2O, which led to a loss of mixing water in the paste, so the particles had more
Fig. 11. The curves of shear stress vs. shear rate of HTP at different resting time.
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G. Jiang et al. / Construction and Building Materials 229 (2019) 116870
Fig. 12. The curves of shear stress vs. shear rate of HTP at different shearing time.
chance for contact and collision [9]. As a result, the friction increased, and so did the s0 and smax. In general, the rebuilding of particles structure was slightly faster than its breaking at a shearing speed of 100 rmin1 until the shearing time reached 120 min, and the speed difference was clearly presented on the rheological curve.
Acknowledgments The work was financially supported by the Key Research and Devlopment Program of China (2017YFC0602903), the Key Program of National Natural Science Foundation of China (51834001) and the Young Scientists Fund of National Natural Science Foundation of China (51804015).
5. Conclusions Due to the difference in materials gradation and chemical properties, the rheological behavior of HTP was very different from that of CTP. HTP had larger slump, smaller s0 and lB, and better flow ability. The existence of stress overshoot made smax nonnegligible in the restart resistance calculation of mixing or pumping equipment. In addition, HTP had obvious thixotropy and short thixotropic equilibrium time. In the resting state, the HTP rheological stability decreased rapidly, and the boundary between stress overshoot and Bingham area disappeared at the resting time of 5 min. In contrast, the continuous shearing prolonged the boundary disappearance to 120 min, far more than the paste flow time in pipeline. In the process of transport, it was not necessary to consider the resistance increase caused by HPG hydration until the flow time exceeded 30 min. The resting or shearing time can be clearly determine based on more engineering parameters. The mosaic structure of particles may be one of main reasons for the HTP special rheological behavior. In addition, the HPG hydration should be taken into account during analyzing the rheological properties. HTP can be described as a new engineering paste with good flow ability, low resistance and proper rheological stability. 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.
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