Influence of time and temperature on rheology and flow performance of cemented paste backfill

Construction and Building Materials 231 (2020) 117117

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Influence of time and temperature on rheology and flow performance of cemented paste backfill Haiyong Cheng a,b, Shunchuan Wu a, Hong Li b,⇑, Xiaoqiang Zhang a a b

Faculty of Land Resources Engineering, Kunming University of Science and Technology, Kunming 650093, China Key Laboratory of Ministry of Education of China for Efficient Mining and Safety of Metal Mines, Beijing 100083, China

h i g h l i g h t s
A model was built for resistance calculation in view of time and temperature effects.
The time and temperature effects could be converted with an equation.
The distribution of the flow parameters in pipeline was obtained.
Obvious changes occurred in paste velocity distribution with time and temperature.

a r t i c l e

i n f o

Article history: Received 4 April 2019 Received in revised form 29 September 2019 Accepted 30 September 2019

Keywords: Cemented paste backfill Rheology Thixotropy Temperature Time Pipeline transportation

a b s t r a c t The Cemented Paste Backfill (CPB) technology can provide effective solutions to mining-related environmental and safety problems, and access to achievement of green mining and deep mining. Rheological properties of paste are important factors that affecting the flow characteristics and pipeline resistance calculation. The rheology of fresh cemented paste is influenced by time and temperature typically in pipeline transportation and is in dynamic evolution. By analyzing the effect of time and temperature on paste rheology, a calculation model of the resistance was established, which was implemented with the numerical software, COMSOL Multiphysics, thus the distribution of the flow parameters was obtained. The results show that, with increase in shear time, the yield stress decreases negative exponentially and the plastic viscosity decreases linearly. The rheological parameters gradually become stable after a certain time. The yield stress declines with increase in temperature, and the process could be described with a negative exponential function. Meanwhile, the variation of plastic viscosity is linear and slight. It is concluded that the rheological behavior of paste has time-temperature equivalent effect, and the transformation equation of time-temperature effect was established. Combined with the Buckingham equation, a calculation model of pipeline resistance with time-temperature effect was proposed, implemented in the numerical software, and a new equation of conservation of momentum was established. In this paper, the changes in paste rheological characteristics with shear rate, thixotropic time and temperature are clarified, and a calculation model of paste resistance considering the effects of time and temperature is established. Through three-dimensional simulation, the processes of velocity distribution of paste with time and temperature were displayed vividly. The mathematical model has been verified and is of guiding significance for pipeline transportation in paste backfill engineering. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Fresh cemented paste is a typical non-Newtonian fluid and has a higher yield stress in moving state [1,2]. Many researches have been conducted over rheology of non-Newtonian fluid which is essential for characterizing paste flowability [3,4]. The design and

⇑ Corresponding author. E-mail address: [email protected] (H. Li). https://doi.org/10.1016/j.conbuildmat.2019.117117 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

operation of pumping systems of CPB is based on the viscosity and yield stress values [5,6]. The yield stress is regarded as the transition stress between elastic solid-like behavior and viscous liquid-like behavior [7,8]. The rheological behavior of paste is affected by many factors which researchers have tried through many methods [9–11]. Kretser thought that the yield stress is significant in ensuring the successful start-up of a pumping system from a static shut down condition, and the viscosity is an indication for pumping requirements and ease of flow thereafter [12].

H. Cheng et al. / Construction and Building Materials 231 (2020) 117117

Merrill [13] used hyperspectral characterization to estimate rheological properties of mineral suspensions. Bobicki [14] found through experiments that the microwave pre-treatment greatly reduced the shear viscosity and direct yield stress of ultramafic nickel ore slurries. Yilmaz et al. studied the influence of superplasticizer on fluidity through experiments, and believed that the positive effect of superplasticizer dosage on fluidity behavior is more sensitive to solid content [15–20]. Besides, Guo [21] found that the distance between particles is key for flowability of fresh cemented paste, and to achieve a same yield stress, a larger distance is needed for coarse particles than fine ones. In the large amount of research works done on the rheology of CPB, a majority focused on the effects of one single factor, such as cement content, shear time, an addition of slag and/or fly ash, and pH value on the rheological properties of CPB [22]. As of equal importance, the factors, temperature and time, should not be ignored [23]. Mewis [24] discussed various experimental methods and considered the meaningful measurements of thixotropy, and thought [25] that the rheological manifestation of flow-induced structural changes is a variable viscosity and the microstructural changes due to flow are quite complex and not fully understood. Mujumdar [26] developed a nonlinear rheological model which accounts for the time-dependent elastic, viscous and yielding phenomena in order to describe the flow behavior of thixotropic materials. And Jarny et al. [27] pointed out that the hydration of cement in the first hour could be ignored by using Magnetic Resonance Imaging (MRI) velocimetry. It is well known that the backfill transportation system in every single underground mine is unique in terms of the difference in temperature [28]. Nehdi [29] estimated time and temperature dependent yield stress of cemented paste using oscillatory rheology and genetic algorithms, and thought that the genetic algorithm-based equations can depict the effect of mixing time and temperature on the yield stress of cemented pastes. Wang [30] found that both the shear stress and apparent viscosity of some particular tailings increased with temperature rising from 2 to 60 °C. Likewise, temperature has a significant impact on the Bingham yield stress of thickened tailings. Petit [31,32] studied the coupled influence of time and temperature on the variations of plastic viscosity of micro mortar, and found that the plastic viscosity varies linearly with the coupled effects of time and temperature for mixtures. Drouet [33] reported that water transport in cementitious materials was under strong influence of temperature. The influence is generally attributed to the variation in physical properties of water (density and surface tension) as well as the coarsening of pore structure in relation to ettringite dissolution and C-S-H alteration [34]. Jiang [35] studied the effect of a series of parameters (pH, W/C ratio, binder type, binder content, tailings type, sodium chloride concentration and different sub-zero temperatures) on the yield stress of fresh CPB in sub-zero environment, pointing out that CPB samples under a temperature of 6 °C had a slightly lower initial yield stress than those under room temperature, and the disparity increased with time. Computational Fluid Dynamics (CFD) simulation is an effective way in investigating the effect of rheology of non-Newtonian mineral slurries on the hydrodynamics [36]. Based on rheological tests, the pressure and velocity in pipeline and elbow are simulated by CFD over different mass concentrations and backfilling capacities [37]. The pipeline simulation was conducted using Fluent software, and the effects of velocity and mass concentration on the characteristics of gravity and bending parts were analyzed by Zhang [38]. As emphasized in the latest simulation results, the temperature and the cement hydration progress have impact on the rheological behavior and flowability [30,39]. However, no model is yet available to predict the coupled effects of temperature and time on the rheological behavior of fresh CPB flow [39,40].

The main objective of this work is to analyze the coupled effects of time and temperature on the rheological properties of CPB. A model was developed for calculation of the resistance with consideration of the effect of time and temperature on the rheology of fresh CPB. The transformation equation of time-temperature effect was established. The distribution of the flow parameters in the pipeline was obtained, and a numerical model for study of characteristics of the flow pattern was proposed. 2. Materials and methods 2.1. Tailings and binder sampling The materials used for preparation of the CPB samples were the unclassified-tailings (tailings of all particle sizes) taken from a nickel mine, the P.O 42.5 Portland cement according to China Common Portland Cement Standard (No. GB175-2007), and the tap water for mixing. The nickel mine, Jinchuan Nickel Deposit, is located in Jinchang city, Gansu province, China, which is the largest copper nickel sulfide deposit in China and the third in the world. In 1958, the amount of nickel metal reached 5.57 million tons, accounting for 79% of China’s proven reserves, The unclassified tailings used was dried first, and its density is 2.852 t/m3, bulk density 1.545 t/m3, and packing density 0.5417. The binder has a density of 3.03 t/m3, bulk density 1.424 t/m3, and packing density 0.4699. A Malvern Mastersizer was used for the determination of the particle size distribution (PSD), as shown in Fig. 1 the PSD of the tailings and the cement. The 20 lm and 75 lm particles account for 21.67% and 60.12% of tailings respectively, which are 70.13% and 99.9% for the cement. Tailings are well graded when the coefficient of uniformity Cu is 5–10, and have a high compaction rate when the curvature coefficient Cc is 1–3 [41]. The tailings are well graded as their Cu and Cc values are 8.87 and 1.58. Thus, these tailings could benefit the CPB preparation. The chemical composition was measured by the X-ray diffraction, as listed in Table 1. The sulfur content in the tailings is low, only 1.63%, and the overall performance is inert. 2.2. Testing procedures 2.2.1. Experiments on influence of time on rheology The thixotropic behavior of paste reflects the response of internal structure to shearing and time [42,43]. Joseph Assaad [44,45] pointed out that in the case of self-consolidating concrete, the degree of thixotropy increases with the decrease in binder content.

100

Unclassified tailings P.O42.5 cement 80

Cumulative volume (%)

2

60

40

20

0 1

10

100

1000

Particle size (μm) Fig. 1. Grain size distribution of unclassified tailings and P.O 42.5 Portland cement.

3

H. Cheng et al. / Construction and Building Materials 231 (2020) 117117 Table 1 Chemical composition of unclassified tailings and Portland cement (%). Materials

SiO2

Al2O3

Fe2O3

CaO

MgO

S

Ni

Cu

Others

Unclassified tailings Portland cement

36.41 21.5

7.77 4.5

9.9 2.0

3.09 63.5

27.79 4.0

1.63 2.5

0.28 –

0.2 –

12.93 2.00

This is attributed to the relative increase in coarse aggregate volume that can lead to a greater internal friction. The changes in shear rate and shear time induce the floc network structure to turn into more ordered arrangements and gradually tend to be stable. The movement pattern of floc net structure is more regular, which promotes the free flow of water. Some scholars believe that the thixotropic area can characterize the structural build-up of cemented paste [46], and the thixotropic reaction of slurries could be analyzed by the upward and downward cycle experiments of shear rate. The discrepancy between dynamic and static yield stress is tied to the thixotropy [3]. However, it is found that the thixotropic loop is particularly different when the shear rate peaks at different values, as shown in Fig. 2. During regression, the increases in the plastic viscosity after thixotropy is unreasonable. The thixotropic loop cannot reflect the effect of time on thixotropy. It is found that the curves of shear stress to time at fixed shear rate could be obtained, and two shear stresses before and after the thixotropy could be gained [47,48]. If the shear stress at multiple shear rates is under regression analysis, the yield stress is returned,

as shown in Fig. 3. The area within the upward curve and the downward curve in Fig. 3(b) could be used for quantitative evaluation of paste thixotropy. The upward curve shows the relationship between shear stress and shear rate prior to thixotropy, while the downward curve shows that after undergoing a thixotropic time. The effect of shear rates and the effect of time on thixotropy are shown by the different abscissa values (c_ 1  c_ 4 ) and the difference in shear stress (sn  sne ). Experiments were designed on slurry concentrations of 68%, 69%, 70% and 71%, and tailings-cement (t/c) ratios of 2:1, 4:1, 12:1 and 20:1. The proportioning is based on previous exploratory experiments and slump tests with slump values ranging from 23 to 26, which are of good fluidity and stability [49]. Rheological parameters of fresh CPB were measured by a rotational rheometer with a vane geometry in shear rate-controlled mode [3]. By controlling the shear rate, the changes in shear stress with time could be obtained [50]. Various Shear rates, 30 s1, 60 s1, 90 s1 and 120 s1, were used, and the shear time was set at 1200 s. In view of the important role of binder that performs hydration reactions

Fig. 2. Thixotropic loop experiment.

Fig. 3. Finding parameters of thixotropy through stress relaxation curve: (a) Stress relaxation curve and (b) Yield stress regression.

4

H. Cheng et al. / Construction and Building Materials 231 (2020) 117117

Oulow

Temperature controller

Inflow (b)

(a)

Fig. 4. Experimental instrument and temperature control: (a) TC-550 Refrigeration/heating cycle bath and (b) Constant temperature water cycle.

in CPB slurries and exerts effect on the yield stress in static conditions, a number of exploratory tests were conducted to measure the rheological influence of hydration. It is noticed that in continuous stirring, the effect of hydration on the yield stress within the initial 1 h is almost negligible. Dynamic stirring affects the formation of chemical structure, and the yield stress has little change which has been verified in experiments [51,52]. A satisfied homogeneity of the slurry is necessary prior to the measurement, as a quick settlement of the solids would lead to a nonuniform distribution of rake torque and thus the rheological parameters could not be determined accurately. Typically, the vibration segregation rate of fresh CPB is less than 15%, and the rotational rheometers are not applicable for those above this rate. The paste should be newly prepared for each test as rheological curves corresponding to different shear rates are required for this method. Thus, errors may be caused and affect the repeatability of the experiments. Generally, this method is practical under the conditions of consistent sampled materials, testing time, segregation rate and stable operation of the instrument.

from the software, the relationship between rheological parameters and flow resistance can be established through Buckingham equation, and the rheological characteristics could be effectively reflected in resistance, flow rate and pressure. The numerical simulation factors are comprehensively determined according to exploratory experiments, software features and engineering transportation parameters. In the model, the horizontal pipe was 5 m in length and 200 mm in diameter, and it was divided by free tetrahedral mesh and the mesh elements were 240,416 in total. The weight concentration of paste was 70%, the tailings-cement (t/c) ratio 2:1, the slurry density 1.837 t/m3, and the packing density 0.6006. The laminar flow model is adopted and inlet velocity was 1 m/s. As the gravity force was added in the pipe model, setting outlet pressure at zero leads to nonconvergence or result error, thus the distribution of outlet pressure under gravity conditions was introduced into the model. 3. Results and discussion 3.1. Time effect on paste rheology

2.2.2. Experiments on influence of temperature on rheology In the experiment, the temperature was set at 5 °C, 20 °C, 35 °C and 50 °C, with a constant paste concentration of 70% and tailingscement (t/c) ratio of 4:1. A TC-550 refrigeration/heating cycle bath as shown in Fig. 4 was used. At the start, the water and materials at preset temperature were mixed into paste and 400 ml was put in the thermostat cup covered by a plastic wrap to prevent volatilization and air condensation. The shear rate was controlled from 0 linear up to 120 s1 with shear time being 120 s, in comparison to a shear rate constant at 60 s1. This is aimed at investigation and development of a better understanding of the combined effects of time and temperature on key rheological properties of CPB [53]. 2.2.3. Numerical simulation of pipeline flow In order to describe paste distribution in the pipeline, rheological characteristics under time and temperature effects were embedded into the multi-field coupling numerical simulation software, COMSOL Multiphysics, and a numerical model was developed for prediction of the flow or rheological behavior of CPB under coupled influence of time and temperature. Di wu [23,39,54] analyzed the effect of temperature and cement hydration on the rheological properties of a backfill material using the COMSOL Multiphysics, and believed that the two factors have significant impact on the rheological behaviour and flowability. Although the rheological parameters cannot be directly obtained

At a constant shear rate, a long reaction time is required for shear stress to reach stable, shown in Fig. 5(a), and the thixotropic time is similar under different shear rates. The shear stress decreased from initial 60.03 Pa to 38.84 Pa after 280 s of shearing when at shear rate 30 s1. Regression results of shear stress before and after thixotropy at different shear rates were shown in Fig. 5(b), implying a good linear relationship. The resulted rheograms were modelled by the Bingham model [47,55] which was used to evaluate the Bingham yield stress and plastic viscosity of the slurries. A regression analysis was carried out every 15 s, 26 analyses in total, to obtain the curves of shear stress and plastic viscosity during 400 s, as shown in Fig. 6. The yield stress and plastic viscosity reached a stable state at about 280 s, that is, the curve could be divided into two stages, thixotropic stage and stable stage. The regression analysis of thixotropic phase shows that the change of yield stress conforms to a negative exponential function, as shown in Fig. 6 (a). From 20 s to 280 s, the reduction rate of plastic viscosity is 23.59%, and the unit time decreases by 0.9‰. The change of plastic viscosity can be described by a linear equation, as shown in Fig. 6 (b). Functional formula (1) and formula (2) were constructed to characterize the changing of yield stress and plastic viscosity under thixotropic conditions.

s0 ðtÞ ¼ s00  expðktÞ

ð1Þ

5

H. Cheng et al. / Construction and Building Materials 231 (2020) 117117 75

75

30s-1 60s-1 90s-1 120s-1

Shear stress (Pa)

65 60

y=55.26+0.13663x R2=0.96689

65

55

t=280s

50

Before thixotropy After thixotropy

70

Shear stress (Pa)

70

60 55 50

y=34.83965+0.11271x R2=0.9184

45 45

40 40

35 0

100

200

300

400

20

40

60

Time (s)

80

100

120

Shear rate (s-1)

(a)

(b)

Fig. 5. Stress relaxation curve and parameter regression (68 wt%, t/c ratio 2:1): (a) Stress relaxation curve and (b) Thixotropic parameter regression.

60

0.15

Shear stress Regression curve

Plastic viscosity Regression curve

50

Model

Asymptotic1

Equation

y = a-b*c^x

Reduced Chi-Sqr

0.05961

Adj. R-Square

0.99866

0.14

Value a ?$OP:F=1

45

Plastic viscosity (Pa.s)

Shear stress (Pa)

55

Standard Error

29.76613

0.51974

b

-29.11184

0.40408

c

0.99379

2.52345E-4

40

Equation

y = a + b*x

Weight

No Weighting

Residual Sum of Squares Pearson's r Adj. R-Square

0.13

8.85466E-6 -0.99602 0.99158 Value

?$OP:A=1

Intercept Slope

Standard Error 0.13665

3.44664E-4

-9.28292E-5

2.01527E-6

0.12

0.11

35

Phase I

Phase II

Phase I 30

Phase II

0.10 0

100

200

300

400

0

Time(s)

50

100

150

200

250

300

350

400

Time (s)

(a)

(b)

Fig. 6. The curve of yield stress and plastic viscosity with time (wt68%, t/c ratio 2:1): (a) Yield stress with time and (b) Plastic viscosity with time.

g0 ðtÞ ¼ g00  mt

ð2Þ

where s0(t) (Pa) is yield stress changing with time; g0(t) (Pas) is plastic viscosity varying with time; s00 (Pa) is pre thixotropic yield stress; g00 (Pas) is thixotropic yield stress; k and m are thixotropic parameters; t (s) is thixotropic time. The variation curves of each rheological parameter with time were obtained under a fixed shear rate, and through a number of stress relaxation experiments at different shear rates, the curves of yield stress and plastic viscosity with time were finally fitted. Initially, a large number of disordered and mechanically stable floc net structures formed under the action of friction, electrostatic and flocculation force, at which time the yield stress and plastic viscosity are the largest. Under the action of constant shear disturbance, with the increase of time, the arrangement of the floc network structure gradually changes from disorder to order, and tends to a stable form under the new mechanical equilibrium condition, with the yield stress and plastic viscosity gradually stabilized.

exist in the slurry and are stable in some degree. With increase in temperature, the stable structures are destroyed and release a certain amount of free water, thus enhancing the fluidity of the slurry and reducing the yield stress and plastic viscosity. An

3.2. Temperature effect on paste rheology Temperature caused a great difference in the rheological curve, as shown in Fig. 7. With increase in temperature, the yield stress generally decreases. A large number of floc network structures

Fig. 7. Rheological curves at different temperatures.

6

H. Cheng et al. / Construction and Building Materials 231 (2020) 117117

translating the points on other temperature lines, which could be described by the Eqs. (3)–(5). The time-temperature equivalent principle could be used to explain the change process, as shown in Fig. 10. As it can be seen, a same stress could be obtained through temperature change compared to that in the stress relaxation phenomenon caused by the time effect. For example, the yield stress at a certain time at T1 temperature can be gained at T2 temperature only by changing the thixotropic time.

Fig. 8. Stress relaxation curves of paste slurry at different temperatures.

experimental temperature range of 5–50 °C was used, as the slurry may be frozen if the temperature is too low and if the temperature is too high, evaporation may affect water distribution and the real rheological parameters cannot be obtained. Temperature also exerts great influence on the thixotropy of fresh paste. In Fig. 8, the thixotropic curves at different temperatures at a shear rate of 60 s1 were shown. The thixotropic time increased when the corresponding temperature decreased. It was 990 s when at 5 °C, declining to 830 s at temperature of 50 °C. On average, the thixotropic time decreases by 3.56 s with each 1 °C rise in temperature, and this change is also nonlinear. The yield stress and plastic viscosity gradually decreased with increase in temperature, shown in Fig. 9. When the temperature was 5 °C, the yield stress was 145.63 Pa, and the plastic viscosity was 0.328 Pas. As the temperature rose to 50 °C, the yield stress decreased to 96.8 Pa exponentially and the plastic viscosity decreased to 0.328 Pas linearly. 3.3. Coupling effect of time and temperature on rheological behavior The time effect is equivalent to the temperature effect of paste slurry to some extent. It means that the yield stress and plastic viscosity of a certain time on a temperature line could be obtained by

s0 ðt; T Þ ¼ s00 ðt0 ; T 0 Þ  expfk½t þ c1  ðT  30Þg

ð3Þ

g0 ðt; TÞ ¼ g00 ðt0 ; T 0 Þ  m½t þ c2  ðT  30Þ

ð4Þ

t n ¼ t 0 þ c 1  ðT n  T 0 Þ

ð5Þ

where s00(t0,T0) (Pa) refers to the yield stress at initial time and temperature T0; s00(t,T) (Pa) is the yield stress at temperature T and thixotropic time t; g00(t0,T0) (Pas) is the plastic viscosity at initial time and temperature T0; g00(t,T) (Pas) is the plastic viscosity at temperature T and thixotropic time t; c1 and c2 are regression coefficients. 3.4. Coupling effect of time and temperature on pipeline flow 3.4.1. Control equation The Buckingham equation shows the relationship between rheological parameters and pipeline resistance [56,57]. The flow resistance could be calculated using Formula (6) on condition that the yield stress and plastic viscosity are known [58].

i¼

DP 2sw 16 32v ¼ s0 þ 2  g ¼ L 3D R D

ð6Þ

The thixotropic and temperature effects of paste slurry are not considered in Formula (6). As the yield stress and plastic viscosity are not invariable, Formula (3)–(5) were taken into Formula (6), thus the paste resistance with time-temperature effect in consideration was obtained, as shown in Formula (7). 8 16 iðt; T Þ ¼ 3D s00  expfk½t þ c1  ðT  30Þg þ 32D2v  fg00  m½t þ c2  ðT  30Þg t  tt > > < iðt; T Þ ¼ iðt t ; T Þ t > tt
 > > : tt ¼ a  ðCV Þlnðb  C V Þ  exp  T30 u

c3

ð7Þ

The Navier Stokes equation (N-S equation) is the momentum conservation equation describing viscous incompressible fluid, and the laminar flow model is shown in Formula (8).

Fig. 9. Curves of rheological parameters with temperature: (a) Yield stress curve with temperature and (b) Plastic viscosity curve with temperature.

H. Cheng et al. / Construction and Building Materials 231 (2020) 117117

7

Fig. 10. Coupling effect model of time-temperature on rheology: (a) Ideal model of temperature effect on yield stress and (b) Ideal model of temperature effect on plastic viscosity.

The plug flow zone shrinks and the plug velocity increases.

The shear flow zone expands.

Fig. 11. Paste velocity distribution slice.

8
 h i < q @u þ qðu  rÞu ¼ r  pl þ l ru þ ðruÞT  2 lðr  uÞl þ F

With the volume force F in Formula (8) converted and Formula (7) taken into the N-S equation, the equation of paste pipeline motion with time and temperature was established. The mathematical model was developed to predict and assess the evolution of the rheological properties of CPB under the coupled effects of temperature and time. And it was also implemented in the numerical software, COMSOL Multiphysics.

time goes on and the distance increases, the boundary layer between the shear flow zone and the plug flow zone shrinks and tends to be stable. The paste evolves from Bingham fluid to the ideal state of Newton fluid which is impossible to achieve. Along the pipeline, 20 tangents were monitored, and the radial distribution of velocity was obtained, as shown in Fig. 13. At the beginning, the velocity close to the pipe wall is large, indicating a high yield stress when the movement starts and the paste tries to move forward as a whole. As the thixotropic action manifests itself, the yield stress and the velocity near the wall decrease, the speed at the center of the pipe, however, increases. The fluid velocity tends to be stable as the transportation time extends.

3.4.2. Velocity characteristics under time influence in pipeline The velocity distribution of paste in the pipeline was sliced and the three-dimensional distribution of velocity along the radial direction was obtained, as shown in Fig. 11. In the initial stage, the shear flow zone is mainly close to the wall of the pipeline. With increase in the transport distance, the shear flow zone gradually expands, the plug flow zone gradually shrinks, and the plug velocity increases. The velocity vector was deformed in proportion to obtain the velocity distribution along the pipeline, as shown in Fig. 12. As

3.4.3. Velocity characteristics under temperature influence in pipeline The transverse section was cut at 2.5 m from the inlet of the pipeline to analyze the flow characteristics at different temperatures (Fig. 14). When the temperature is 5 °C, the plug flow zone, deep red area at the center, is large and the maximum flow velocity is 1.17 m/s. The velocity gradient becomes more obvious and the maximum velocity goes to 1.25 m/s when the temperature is 20 °C. The velocity climbs to 1.31 m/s when the temperature rises to 35 °C and reaches 1.36 m/s when the temperature is 50 °C.

@t

3

: @ q þ r  ðq  uÞ ¼ 0 @t

ð8Þ

8

H. Cheng et al. / Construction and Building Materials 231 (2020) 117117

Shear flow zone Plug flow zone Shear flow zone Initial state

boundary layer

Stable state

Fig. 12. Paste velocity vector in the pipeline.

From 1 m/s to 1.35 m/s. Center velocity increases. Boundary velocity decreases. Velocity transfer process.

Fig. 13. Radial distribution of paste velocity along different points of pipeline.

The velocity distribution on the crosscutting line in the above four pictures were monitored, as shown in Fig. 15. In the temperature range of 5–50 °C, the shear flow zone increases with temperature, and the flow velocity decreases. It is quite the contrary for the plug flow. Typically, fresh CPB has a high solids concentration and considerable internal network structures that having certain

stability. The increase in temperature can render the break of the stable structure and the release of some free water, thus enhancing the slurry fluidity with a lower yield stress and plastic viscosity. The increase of temperature promotes the transformation of the internal structure from floc network structure to liquid network structure. Under the action of fluid force, the particles and floc structures are in a more regular and ordered

Fig. 14. Velocity distribution at cross section 2.5 m distant to the entrance: (a) 5°C (b) 20 °C (c) 35 °C and (d) 50 °C.

H. Cheng et al. / Construction and Building Materials 231 (2020) 117117

9

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51934003), the Key Laboratory of Ministry of Education of China for Efficient Mining and Safety of Metal Mines (No. ustbmslab201801), the Program for Innovative Research Team (in Science and Technology) in University of Yunnan Province, and the Research Start-up Fund for Introduced Talent of Kunming University of Science and Technology (KKSY201821024). References

Fig. 15. Velocity distribution on the crosscutting line.

arrangement, and the yield stress and plastic viscosity are gradually reduced. 4. Conclusions The characterization of paste rheology and flow performance was put forward through experiments and numerical simulation, and it is proposed that the coupled effect of time and temperature plays a vital role in the evolvement of fresh cemented paste rheology in pipeline. The main conclusions are as follows: (1) The thixotropic behavior of paste could not be accurately described by the thixotropic ring. A method of thixotropy analysis was proposed according to the stress relaxation of paste slurry. With increase in shear time, the yield stress decreases negative exponentially and the plastic viscosity decreases linearly. The rheological parameters gradually become stable after a certain time. (2) The yield stress changes exponentially with temperature, and the plastic viscosity decreases linearly with temperature. As the temperature rises, the floc structure of paste develops into a liquid network structure, the movement of particles and flocs tends to be orderly under the flow force, and the yield stress and the plastic viscosity decline. (3) By embedding the time-temperature equivalence principle into the COMSOL Multiphysics, the visualization analysis of transportation of paste is realized. As time goes on and the distance increases, the boundary layer between the shear flow zone and the plug flow zone shrinks and tends to be stable. In the temperature range of 5–50 °C, the shear flow zone increases with temperature, and the flow velocity decreases. It is quite the contrary for the plug flow. Despite the results obtained, further studies are necessary in better understanding of the coupled effect of time and temperature on the rheology of paste with the following topics recommended for further work: (i) Rheological properties in a wider temperature range. (ii) Microscopic mechanism of rheological evolution with time and temperature. (iii) Numerical model of fluid-particle coupling of CPB. 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.

[1] E. Yilmaz, M. Fall, Paste Tailings Management, Springer International Publishing, 2017. [2] S. Cao, E. Yilmaz, W. Song, Evaluation of viscosity, strength and microstructural properties of cemented tailings backfill, Minerals 8 (8) (2018) 352. [3] Y. Qian, S. Kawashima, Distinguishing dynamic and static yield stress of fresh cement mortars through thixotropy, Cem. Concr. Compos. 86 (2018) 288–296. [4] C. Qi, Q. Chen, A. Fourie, Q. Zhang, An intelligent modelling framework for mechanical properties of cemented paste backfill, Miner. Eng. 123 (2018) 16– 27. [5] M. Becker, G. Yorath, B. Ndlovu, M. Harris, D. Deglon, J.P. Franzidis, A rheological investigation of the behaviour of two Southern African platinum ores, Miner. Eng. 49 (2013) 92–97. [6] L. Pullum, L. Graham, M. Rudman, R. Hamilton, High concentration suspension pumping, Miner. Eng. 19 (5) (2006) 471–477. [7] J.J. Assaad, J. Harb, Y. Maalouf, Measurement of yield stress of cement pastes using the direct shear test, J. Nonnewton. Fluid Mech. 214 (2014) 18–27. [8] W.U. Ai-Xiang, Y. Yang, H.Y. Cheng, S.M. Chen, Y. Han, Status and prospects of paste technology in China, Chin. J. Eng. (2018). [9] A. Kashani, R. San Nicolas, G.G. Qiao, J.S.J. van Deventer, J.L. Provis, Modelling the yield stress of ternary cement–slag–fly ash pastes based on particle size distribution, Powder Technol. 266 (2014) 203–209. [10] M. Fall, M. Benzaazoua, S. Ouellet, Experimental characterization of the influence of tailings fineness and density on the quality of cemented paste backfill, Miner. Eng. 18 (1) (2005) 41–44. [11] C.F. Ferraris, K.H. Obla, R. Hill, The influence of mineral admixtures on the rheology of cement paste and concrete, Cem. Concr. Res. 31 (2) (2001) 245–255. [12] R.d. Kretser, P.J. Scales, D.V. Boger, Improving clay-based tailings disposal: case study on coal tailings, AIChE J. 43 (7) (2010) 1894–1903. [13] J. Merrill, L. Voisin, V. Montenegro, C.F. Ihle, A. McFarlane, Slurry rheology prediction based on hyperspectral characterization models for minerals quantification, Miner. Eng. 109 (2017) 126–134. [14] E.R. Bobicki, Q. Liu, Z. Xu, Effect of microwave pre-treatment on ultramafic nickel ore slurry rheology, Miner. Eng. 61 (2014) 97–104. [15] G. Xue, E. Yilmaz, W. Song, E. Yilmaz, Influence of fiber reinforcement on mechanical behavior and microstructural properties of cemented tailings backfill, Constr. Build. Mater. 213 (2019) 275–285. [16] H. Jiang, Z. Qi, E. Yilmaz, J. Han, J. Qiu, C. Dong, Effectiveness of alkali-activated slag as alternative binder on workability and early age compressive strength of cemented paste backfills, Constr. Build. Mater. 218 (2019) 689–700. [17] S. Cao, E. Yilmaz, W. Song, E. Yilmaz, G. Xue, Loading rate effect on uniaxial compressive strength behavior and acoustic emission properties of cemented tailings backfill, Constr. Build. Mater. 213 (2019) 313–324. [18] L. Yang, E. Yilmaz, J. Li, H. Liu, H. Jiang, Effect of superplasticizer type and dosage on fluidity and strength behavior of cemented tailings backfill with different solid contents, Constr. Build. Mater. 187 (2018) 290–298. [19] G. Xue, E. Yilmaz, W. Song, S. Cao, Compressive strength characteristics of cemented tailings backfill with alkali-activated slag, Appl. Sci. 8 (9) (2018) 1537. [20] E. Yilmaz, T. Belem, M. Benzaazoua, Study of physico-chemical and mechanical characteristics of consolidated and unconsolidated cemented paste backfills, Gospodarka Surowcami Mineralnymi – Min. Resour. Manage. 29 (1) (2013) 81–100. [21] Y. Guo, T. Zhang, J. Wei, Q. Yu, S. Ouyang, Evaluating the distance between particles in fresh cement paste based on the yield stress and particle size, Constr. Build. Mater. 142 (2017) 109–116. [22] S. Yin, A. Wu, K. Hu, Y. Wang, Y. Zhang, The effect of solid components on the rheological and mechanical properties of cemented paste backfill, Miner. Eng. 35 (2012) 61–66. [23] D. Wu, S.-J. Cai, G. Huang, Coupled effect of cement hydration and temperature on rheological properties of fresh cemented tailings backfill slurry, Trans. Nonferrous Met. Soc. China 24 (9) (2014) 2954–2963. [24] J. Mewis, Thixotropy – a general review, J. Nonnewton. Fluid Mech. 6 (1) (1979) 1–20. [25] J. Mewis, N.J. Wagner, Thixotropy, Adv. Colloid Interface Sci. 147 (2009) 214– 227. [26] A. Mujumdar, A.N. Beris, A.B. Metzner, Transient phenomena in thixotropic systems, J. Nonnewton. Fluid Mech. 102 (2) (2002) 157–178. [27] S. Jarny, N. Roussel, S. Rodts, F. Bertrand, R. Le Roy, P. Coussot, Rheological behavior of cement pastes from MRI velocimetry, Cem. Concr. Res. 35 (10) (2005) 1873–1881.

10

H. Cheng et al. / Construction and Building Materials 231 (2020) 117117

[28] M. Fall, S.S. Samb, Effect of high temperature on strength and microstructural properties of cemented paste backfill, Fire Saf. J. 44 (4) (2009) 642–651. [29] M. Nehdi, S. Al Martini, Estimating time and temperature dependent yield stress of cement paste using oscillatory rheology and genetic algorithms, Cem. Concr. Res. 39 (11) (2009) 1007–1016. [30] Y. Wang, A. Wu, Z. Ruan, H. Wang, Y. Wang, F. Jin, Temperature effects on rheological properties of fresh thickened copper tailings that contain cement, J. Chem. 2018 (2018) 1–8. [31] J.-Y. Petit, K.H. Khayat, E. Wirquin, Coupled effect of time and temperature on variations of plastic viscosity of highly flowable mortar, Cem. Concr. Res. 39 (3) (2009) 165–170. [32] J.-Y. Petit, E. Wirquin, K.H. Khayat, Effect of temperature on the rheology of flowable mortars, Cem. Concr. Compos. 32 (1) (2010) 43–53. [33] E. Drouet, S. Poyet, J.-M. Torrenti, Temperature influence on water transport in hardened cement pastes, Cem. Concr. Res. 76 (2015) 37–50. [34] F. Brue, C.A. Davy, F. Skoczylas, N. Burlion, X. Bourbon, Effect of temperature on the water retention properties of two high performance concretes, Cem. Concr. Res. 42 (2) (2012) 384–396. [35] J. Haiqiang, M. Fall, L. Cui, Yield stress of cemented paste backfill in sub-zero environments: experimental results, Miner. Eng. 92 (2016) 141–150. [36] C.W. Bakker, C.J. Meyer, D.A. Deglon, Numerical modelling of non-Newtonian slurry in a mechanical flotation cell, Miner. Eng. 22 (11) (2009) 944–950. [37] W.A. Xiang, R.Z. En, W.Y. Ming, Y.S. Hua, W.S. Yong, W. Yong, W.J. Dong, T. Beijing, Simulation of long-distance pipeline transportation properties of whole-tailings paste with high sliming, J. Central South Univ. 25 (2018) 141– 150. [38] X.X. Zhang, D.P. Qiao, Simulation and experiment of pipeline transportation of high density filling slurry with coarse aggregates, Chin. J. Nonferrous Met. 25 (1) (2015) 258–267. [39] D. Wu, M. Fall, S.J. Cai, Coupling temperature, cement hydration and rheological behaviour of fresh cemented paste backfill, Miner. Eng. 42 (2013) 76–87. [40] Q. Yuan, D. Zhou, K.H. Khayat, D. Feys, C. Shi, On the measurement of evolution of structural build-up of cement paste with time by static yield stress test vs. small amplitude oscillatory shear test, Cem. Concr. Res. 99 (2017) 183–189. [41] W. Sun, A. Wu, K. Hou, Y. Yang, L. Liu, Y. Wen, Experimental study on the microstructure evolution of mixed disposal paste in surface subsidence areas, Minerals 6 (2) (2016) 43. [42] R. Neelakantan, F.G. Vaezi, R.S. Sanders, Effect of shear on the yield stress and aggregate structure of flocculant-dosed, concentrated kaolinite suspensions, Miner. Eng. 123 (2018) 95–103.

[43] X. Li, Z.C. Grasley, E.J. Garboczi, J.W. Bullard, Simulation of the influence of intrinsic C-S-H aging on time-dependent relaxation of hydrating cement paste, Constr. Build. Mater. 157 (2017) 1024–1031. [44] J.J. Assaad, K.H. Khayat, Effect of viscosity-enhancing admixtures on formwork pressure and thixotropy of self-consolidating concrete, ACI Mater. J. 103 (4) (2006) 280–287. [45] J. Assaad, K.H. Khayat, Assessment of thixotropy of self-consolidating concrete and concrete-equivalent-mortar—effect of binder composition and content, ACI Mater. J. 101 (5) (2004) 400–408. [46] Q. Yuan, D. Zhou, B. Li, H. Huang, C. Shi, Effect of mineral admixtures on the structural build-up of cement paste, Constr. Build. Mater. 160 (2018) 117–126. [47] A. Yahia, S. Mantellato, R.J. Flatt, Concrete rheology: a basis for understanding chemical admixtures, Sci. Technol. Concr. Admix. (2016). [48] Y. Qian, S. Kawashima, Use of creep recovery protocol to measure static yield stress and structural rebuilding of fresh cement pastes, Cem. Concr. Res. 90 (2016) 73–79. [49] A. Wu, H. Cheng, Y. Wang, H. Wang, X. Liu, G. Li, Transport resistance characteristic of paste pipeline considering effect of wall slip, Chin. J. Nonferrous Met. 26 (1) (2016) 180–187. [50] L. Zhang, A. Wu, H. Wang, H. Cheng, Y. Wang, Evolution law of yield stress in paste tailings, Chin. J. Nonferrous Met. 28 (8) (2018) 1631–1636. [51] H. Cheng, S. Wu, A. Wu, W. Cheng, Grading characterization and yield stress prediction based on paste stability coefficient, Chin. J. Eng. 40 (10) (2018) 1168–1176. [52] A. Wu, Y. Yang, H. Cheng, S. Chen, Y. Han, Status and prospects of paste technology in China, Chin. J. Eng. 40 (5) (2018) 517–525. [53] Z. Aldhafeeri, M. Fall, M. Pokharel, Z. Pouramini, Temperature dependence of the reactivity of cemented paste backfill, Appl. Geochem. 72 (2016) 10–19. [54] D. Wu, G. Sun, G. Huang, Experimental and simulation study on seepage characteristics of cemented tailings backfill, J. Central South Univ. Sci. Technol. 46 (3) (2015) 1050–1057. [55] M. He, Y. Wang, E. Forssberg, Slurry rheology in wet ultrafine grinding of industrial minerals: a review, Powder Technol. 147 (1) (2004) 94–112. [56] C. Qi, Q. Chen, A. Fourie, J. Zhao, Q. Zhang, Pressure drop in pipe flow of cemented paste backfill: experimental and modeling study, Powder Technol. 333 (2018) 9–18. [57] M.Z. Li, Y.P. He, Y.D. Liu, C. Huang, Effect of interaction of particles with different sizes on particle kinetics in multi-sized slurry transport by pipeline, Powder Technol. 338 (2018) 915–930. [58] T. Belem, M. Benzaazoua, Design and application of underground mine paste backfill technology, Geotech. Geol. Eng. 26 (2) (2007) 147–174.