Strength development and microstructural investigation of lead-zinc mill tailings based paste backfill with fly ash as alternative binder

Journal Pre-proof Strength development and microstructural investigation of lead-zinc mill tailings based paste backfill with fly ash as alternative binder S.K. Behera, C.N. Ghosh, D.P. Mishra, Prashant Singh, K. Mishra, J. Buragohain, Phanil K. Mandal PII:

S0958-9465(20)30045-7

DOI:

https://doi.org/10.1016/j.cemconcomp.2020.103553

Reference:

CECO 103553

To appear in:

Cement and Concrete Composites

Received Date: 22 August 2019 Revised Date:

15 December 2019

Accepted Date: 3 February 2020

Please cite this article as: S.K. Behera, C.N. Ghosh, D.P. Mishra, P. Singh, K. Mishra, J. Buragohain, P.K. Mandal, Strength development and microstructural investigation of lead-zinc mill tailings based paste backfill with fly ash as alternative binder, Cement and Concrete Composites, https:// doi.org/10.1016/j.cemconcomp.2020.103553. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.

Strength development and microstructural investigation of lead-zinc mill tailings based paste backfill with fly ash as alternative binder S.K. Beheraa1, C.N. Ghosha, D. P. Mishrab, Prashant Singha, K. Mishraa, J. Buragohaina, Phanil K. Mandala a

CSIR-Central Institute of Mining and Fuel Research, Dhanbad - 826015, Jharkhand, India b Department of Mining Engineering, Indian Institute of Technology (ISM), Dhanbad – 826004, Jharkhand, India

Abstract Paste backfill for underground mines is site specific, increasing demand of paste backfilling focus on alternative binder optimisation and bulk waste disposal. The main objectives of this study are to investigate the effect of fly ash addition on the key mechanical parameters (compressive strength and cohesion-friction angle) and correlate the strength development with microstructural evolution (SEM-EDS results). It was experimentally determined that mix PBF1 (8 wt% OPC), PBF2 (7 wt% OPC), PBF3 (6 wt% OPC), FA8a (7 wt% OPC + 1 wt% FA) and FA8b (6 wt% OPC + 2 wt% FA) could achieve the targeted 28 days’ uniaxial compressive strength (UCS) of 1.1 MPa for the backfilling stope. Thus fly ash (FA) is a suitable binder substitute and replacement is possible upto 25% of the total ordinary portland cement (OPC) content. This strength development is correlated with microstructural development, as calcium silicate hydrate C-S-H was not developed in OPC-FA binder based paste backfill at early days’ of curing and gypsum like microstructure was found only in OPC based paste backfill samples. Thus the obtained results would help in better understanding and design of paste backfill structures for the lead-zinc underground metal mines. Keywords: Mining; mill tailings; fly ash; paste backfill; uniaxial compressive strength; microstructure.

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Corresponding author: S. K. Behera Tel.: +918986760221, Email: [email protected]

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1. Introduction Bulk environment friendly disposal of solid wastes like mine overburden, fly ash (FA), mill tailings (MT), slag, jarosite and red mud generated due to mining and mineral processing industries is one of the most essential components of waste management [1-4]. Reutilisation of these wastes as backfilling material in underground mine voids is always beneficial to the industry [5-7]. Underground mining with caving or backfilling is essentially done to extract deep seated mineral deposits. In caving method, mine voids (stopes) are left unfilled and the overlying rock is allowed to collapse. This leads to surface subsidence and associated problems. Whereas, in mining with backfilling, the stopes are filled with backfill materials including mineral processing wastes [8-9]. Mine backfilling is a major means of ground control that eliminates surface subsidence, enhances mines safety, increases productivity, creates safe working environment and facilitates utilisation of industrial wastes [10-12].

The paste backfill technology was first executed in 1957 by Falconbridge Nickel Mines Ltd. at the Hardy mine in Sudbury and Bad Grund Mine in Germany in late 1970 [13]. Because of its techno-economic advantages and scope for bulk utilization of industrial wastes, application of paste backfill technology in many underground metalliferous mines narrates the success stories [14-15]. Presently, though there are several backfilling methods, paste backfill technology is widely applied in underground metal mines [16-21].

There is a good scope for application of paste backfill technology in a large scale in India [22-23]. This technology typically utilizes mill tailings up to a desired proportion added with precise quantity of binder maintaining the consistency for better flowability. The blend of paste backfill is so maintained that it is relatively of high density, non-segregating, and 2

satisfying the rheological and geotechnical requirements [24-27]. The paste backfill is transported from surface to the stopes through pipeline system for filling the underground voids.

The binder type and dosage largely affect the mechanical properties and economy of paste backfill. The binder selection depends upon the availability of material, disposal scenario, method of working, stope geometry and economy [28]. Ordinary portland cement (OPC) is extensively used as binder in paste backfill as it enhances the mechanical properties [29-32]. Despite of the advantages of using OPC as a binder in paste backfill, the economy is a major concern for the mine operators. It is noteworthy to mention that backfilling bears 25% of the total mining cost, of which 75% is the binder cost considering OPC as a sole binder [33]. . Recent advancements in the use of alternate binding materials have not only reduced the backfilling cost but also improved the mechanical properties and sulphate attack resistance of the paste backfill [34-36]. Various alternative binders typically used in paste backfilling are fly ash, lime, limestone powder, granulated blast furnace slag, granulated marble waste and jarosite [37-39]. Partial replacement of OPC with fly ash has been studied by several researchers globally [40], however limited studies has been reported about leadzinc mill tailings paste backfill with fly ash as a binder replacement in Indian context. Also, Microstructural analysis of cemented paste backfill is important for understanding the mechanism of strength development with curing time [41-43]. Strength development is the outcome of hydration reaction with development of hydration products, such as calcium silicate hydrate gel (C-S-H) [44-45], portlandite [31], gypsum, ettringite [46], alite [47] and

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calcium aluminates. There are limited studies available in literature which correlates the microstructural results (SEM-EDS) with the strength development. Thus the novelty lies in this articles are the studies conducted to link the scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS) results with the uniaxial compressive strength (UCS) development, the lead-zinc tailings material used and site (Indian context). The present study investigates the physico-chemical properties of leadzinc mill tailings and fly ash, strength development characteristics and microstructural properties of paste backfill. Also, it explores the applicability of fly ash as a partial binder replacement for paste backfilling in a lead-zinc underground mine in western India.

2. Experimental programme Initially Physical, chemical properties of mill tailings and fly ash were determined. Further paste backfill mix was prepared in the lab and studies were conducted for consistency, strength development and microstructural analysis with cement hydration products identification (Fig. 1).

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Fig. 1. Experimental process: materials to methods. 2.1 Materials Lead-zinc mill tailings and fly ash are used as paste backfill materials for this study. The disc filters of paste fill plant and a captive power plant of the mining company were used as the source of sampling for tailings and fly ash respectively. In general, sublevel stoping with backfilling is adopted in the mine, whereas in some part of the mine, blast-hole stoping with paste backfilling is practiced. The mine uses the mill tailings produced as by-product of ore beneficiation for filling the stopes. Ordinary portland cement (OPC) Grade-43 (India) and OPC-fly ash (FA) mix were used as binder in this study with an aim to partially replace OPC with fly ash. 2.1.1 Physical characterization

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The physical properties of mill tailings and fly ash samples, such as; specific gravity, bulk density and particle size distribution were studied. The bulk density (ρ) of the samples were determined and later specific gravity (G) of the samples were determined using a water pycnometer as per IS: 2386-III (1963) [48]. The particle size distribution of lead-zinc mill tailings and fly ash were determined with Malvern MASTERSIZER – 3000 (Worcester, United Kingdom), it works on wet dispersion method using laser diffraction technique. The scanning electron microscopy (SEM) technique was used to investigate the morphology of the mill tailings and fly ash samples. For this SEM study, ZEISS-MERLIN VP COMPACT (Jena, Germany) instrument was used. 2.1.2 Chemical characterization The cement hydration process after the backfill is placed inside the stopes is largely affected by the chemical composition of the backfill materials. The elemental composition and microstructure of mill tailings and fly ash samples were examined using scanning electron microscopy (SEM) [Model: ZEISS-MERLIN VP compact with energy dispersive X-ray spectroscopy (EDS) analysis with TEAM™ EDS System using Apollo X silicon drift detector (SDD)]. The samples are non-conducting in nature, so these were coated with gold using Quorum gold coater. The chemical compositions (element oxides) of the samples were determined by X-Ray Fluorescence (XRF) Technique using Rigaku, ZSX Primus (Tokyo, Japan) instrument, which is a wavelength dispersive X-ray fluorescence spectrometer. 2.2 Methods 2.2.1 Paste preparation Currently 8 wt% (dry weight of binder/weight of solid) OPC is used as a binder in paste backfilling for the study site with a uniaxial compressive strength (UCS) of 1.1 MPa after 28

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days’ curing duration. Accordingly, the mix compositions and uniaxial compressive strength tests were conducted. In order to reduce the OPC consumption, fly ash was tried as a replacement of OPC. Initially two reference paste backfill mix were prepared at 8 wt% OPC and 5 wt% OPC. Thereafter, OPC was gradually replaced with fly ash. The paste backfill mix was prepared by mixing dry mill tailings, OPC and fly ash in a mixer for about 3 min. After dry mixing, the mixture was thoroughly mixed by adding water for about 15 min. Immediately after preparation of the paste backfill, slump test was conducted and the mix was poured into cylindrical plastic moulds of 54 mm diameter and 108 mm height maintaining a length to diameter ratio (L/D) of 2 for evaluating the strength development with 7, 14, 28 and 56 days’ of curing time. 2.2.2 Study of workability and setting time The workability of the paste backfill is measured by slump test as per ASTM C 143 (2015) [49]. Slump is the difference in height between the slump cone and the height of the slumped material [50]. Standard conical frustum of 300 mm height with top and bottom diameters of 100 and 200 mm respectively with a metal thickness of 1.60 mm was used for the study. Setting time is one of the important parameter in paste backfilling as it could affect the stoping (extraction of ore) and backfilling operation. Vicat needle apparatus was used to determine initial setting time and final setting time. Paste density (g/cm3) was also determined in the lab simultaneously so as to eliminate the error due to time lagging. 2.2.3 Compressive and shear strength properties The importance of strength properties of paste backfill is unavoidable as the backfill has to be self-standing, after extraction of secondary stopes. The mechanical properties of paste backfill such as UCS, cohesion (c) and friction angle (φ) were determined after 7, 14, 28 and 56 days’ of curing time (Fig. 2). A constant humidity and temperature similar to the 7

underground metal mines environment was maintained in the lab. Also, Drained samples were used for strength development evaluation. A constant loading rate of 0.5 MPa/Sec (IS: 9143, 1979) [51] was maintained throughout the loading process for determining UCS (Fig. 2c). These tests were conducted in triplicate to maintain accuracy, reliability of the results and the mean of the results were considered. The cohesion (c) and friction angle (φ) of the backfill samples were determined in accordance with IS: 13047 (1991) [52] as shown in Fig. 2b. The triaxial setup consist of testing machine for applying compressive load, control unit for controlling axial load, triaxial pressure cell and an assembly for generating and controlling confining pressure. Normal compressive load was applied gradually at a rate of 0.5 MPa/Sec until shearing takes place in the sample. The amount of confining pressure was selected based upon the UCS development for each binder category and curing days’. For all of these tests cylindrical specimens were maintained a length to diameter (L/D) ratio of 2 and plane surfaces were maintained at the two end surfaces by smooth polishing, if required (Fig. 2a). Under uniaxial loading condition, most of the samples followed an hourglass type failure pattern (Fig. 2d).

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Fig. 2. Experiments for studying the mechanical properties of paste backfill samples: (a) Cylindrical paste backfill samples, (b) Triaxial test, (c) UCS test, and (d) sample failure under uniaxial loading. 2.2.4 Microstructural analysis of paste backfill samples Paste backfill samples after mechanical testing, were investigated under the SEM to visualise the microstructure and development of various cement hydration products. The microstructural analysis was carried out in two parts. In the first part, micrographs of sole OPC based paste backfill samples were investigated and in the second part, OPC-FA binder based paste backfill samples were studied. EDS technique was also used to identify various elements within the paste backfill specimen. 3. Results and discussion 3.1 Physical properties

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Physical properties reveal mill tailings with specific gravity 2.91 is comparatively heavier than the fly ash with specific gravity 2.13 (Table 1). The particle size ranges of lead-zinc mill tailings and fly ash particles falls within 0.1-420 and 0.1-120 (Fig. 3) respectively. Table 1 Physical characterisation of lead-zinc mill tailings and fly ash. Parameters Mill tailings Colour Gray Specific Gravity (G) 2.91 3 Bulk density (ρ), g/cm 1.49 Particle size distribution D (4,3), µm 39.54 D (3,2), µm 9.972 D90, µm 89.13 D50, µm 37.92 11.75 D10, µm D30, µm 22.48 D60, µm 50.73 Coefficient of uniformity, Cu 4.32 Coefficient of curvature, Cc 0.85 Specific surface area (m2/kg) 481.4 *(Cu) = D60/ D10 *(Cc) = (D30 × D30) / (D60 × D10) *D (4,3) is volume moment mean *D(3,2) is surface area moment mean

Fly ash Light Gray 2.13 0.82 53.49 11.19 74.26 29.1 6.16 18.1 40.28 6.54 1.32 654.2

The D50 (29.10 µm) of fly ash is lesser than the mill tailings (D50 = 37.92 µm). Hence, it is evident that the fly ash samples are relatively finer than the tailings and mixing these materials together for backfilling makes the mix much finer. In the case of mill tailings since Cu < 6 and Cc < 1, it is established that tailings is poorly-graded for this study samples. Whereas, in the case of fly ash, Cu > 6 and Cc > 1, so fly ash is well-graded in accordance with the standard for classification and gradation of soils, ASTM D2487–11 (2011) [53].

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110 100

% Passing (Cumulative)

90 80 70

Fly ash

60

Mill tailings

50 40 30 20 10 0

0.1

1.0

10.0

100.0

1000.0

Particle size (µm)

Fig. 3. Particle size distribution curves for mill tailings and fly ash. The SEM micrographs reveals mill tailings are angular, uneven in shape with majority of particles having sharp edges (Fig. 4a). In contradictory, fly ash particle are evenly shaped, rounded to sub-rounded. The spherical shaped microstructures (Fig. 4b) are termed as ‘cenospheres’. These spherical particles within fly ash, mix properly with tailings and fill the void spaces in between tailings and makes the mix a suitable backfill material.

Fig. 4. Scanning electron micrographs of (a) lead-zinc mill tailings and (b) fly ash.

3.2 Chemical properties The chemical compositions of the mill tailings and fly ash illustrates that the lead-zinc mill tailings are abundance of in SiO2 (maximum 33.34%), CaO (17.42%) and MgO (6.43%) whereas fly ash contains 60.71% of SiO2, 2.08% of CaO and 0.56% of MgO (Table 2a). 11

From the chemical composition results, presence of large amount of SiO2 may offer greater strength and better load bearing capacity to the paste backfill after being poured into the stope. Chemically this particular fly ash can be compared with OPC grade 43 [54] by using the following relation:

.

.

(1)

.

This lime to percentages of silica, alumina and iron oxide is fund to be 0.0071 which is far lower than the desired value of 0.66-1.02. Table 2a Chemical composition of lead-zinc mill tailings and fly ash. SiO2 Al2O3 Fe2O3 TiO2 CaO MgO Material (%) (%) (%) (%) (%) (%) Mill 33.34 12.13 16.58 0.37 17.42 6.43 tailings Fly ash 60.71 27.27 4.61 1.67 2.08 0.56

P2O5 (%) 1.08

SO3 (%) 10.43

Na2O (%) 0.49

K2O (%) 1.73

0.79

0.86

0.39

1.05

The results of elemental analysis (Table 2b) show that the mill tailings is mostly consisting of O, Si, Mg, Ca, S, Fe and Al with minor concentrations of Y, K, Mn, Ho, Na and Eu. Also, the fly ash sample contains the major elements such as O, Si and Al, and the minor concentrations of Mg, K, Ca, Fe, Br and Nb [55]. 3.3 Slump and setting time In practice the mines uses a paste backfill of 77 wt% solid with a slump value of 195 mm + 2 mm variation in slump height. It was required to determine the optimum solid percentage (wt%) of paste with different binder composition (Table 3). After varying the water content by 1 wt% at each trail, the final solid to water ratio for each binder composition was determined by maintaining a slump of 195 mm (+ 2 mm variation). Amaratunga and

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Yaschyshyn, (1997) [25] had also used a constant slump of 200 mm for the paste backfill mix using fine tailings. A solid percentage of 77 wt% and 78 wt% was found to be optimum. The initial setting time, determined by Vicat Needle Apparatus varied between 10 to 13 hours for the entire paste backfill and the final setting time was within the range of 20 to 24 hours. Also, a maximum paste backfill density of 2.13 g/cm3 was found with PBF1 (OPC 8 wt%) whereas the minimum value was found with FB5c (2 wt% OPC + 3 wt% FA).

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Table 2b Elemental composition of lead-zinc mill tailings and fly ash. Material O Mg Al Si Y S Mill 45.13 7.87 2.13 26.20 0.67 4.97 tailings Fly ash

42.7

0.2

14.9

25.1

ND

ND

K 0.67

Ca 5.50

Mn 0.37

Fe 4.97

Ho 0.17

C ND

Na 0.80

Br ND

Eu 0.63

Nb ND

0.7

1.15

ND

1.75

ND

2.95

ND

7.2

ND

3.45

*ND-Not detectable Table 3 Paste mix with primary workability parameters and setting time. Mix Binder (wt%) Binder Slump (mm) replacement (wt% of OPC) PBF1 (Control) PBF2 PBF3 PBF4 (Control) PBF5 FA8a FA8b FA8c FA8d FB5a FB5b FB5c

8% OPC 7% OPC 6% OPC 5% OPC 4% OPC 7% OPC + 1% FA 6% OPC + 2% FA 5% OPC + 3% FA 4% OPC + 4% FA 4% OPC + 1% FA 3% OPC + 2% FA 2% OPC + 3% FA

0 0 0 0 0 12.5 25 37.5 50 20 40 60

Spread

195 193 195 197 197 195 195 197 197 193 195 197

340 335 337 339 339 340 340 339 339 335 337 339

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Solid to water ratio(wt%)

77:23 78:22 78:22 78:22 78:22 77:23 77:23 78:22 78:22 78:22 78:22 78:22

Setting time (Hour) Paste Density

Initial 10 9 9 8 9 11 10 10 10 12 13 13

Final 20 21 21 21 21 21 20 20 19 23 23 24

(g/cm3) 2.13 2.12 2.11 2.1 2.09 2.08 2.05 2 1.94 2 1.98 1.96

3.4 Strength development Evaluation of strength development within paste backfill is highly essential for estimating the stability. UCS, c and φ of paste backfill samples as determined in the lab are explained herein. 3.4.1 UCS test results Mechanical properties play a significant role when evaluating the stability of paste backfill. The UCS of the samples were determined with 95% accuracy. Table 4 demonstrates the basic statistical parameters of UCS results. It shows the variation of UCS from the mean (standard deviation) is less than 0.15 for all the tested samples. Most of the samples failed following an hour glass type failure pattern with few samples failed with multiple shearing. All the paste backfill samples shows increase in strength with increase in curing period which is a clear indication of active cement hydration (Fig. 5a). During the hydration process, hydration products develop which fills the void spaces within the paste backfill and provide the binding mechanism. Thus with curing strength increases. It can be observed that, for most of the samples the maximum increase in strength is observed from 14 to 28 days’ and this rate decreases between 28 to 56 days’ of curing. This phenomenon is well explained with the help of micro structural analysis of paste backfill. The UCS is relatively low for 4 wt%, 5 wt% and 6 wt% OPC dosage whereas it significantly increases for 7 wt% and 8 wt% OPC (Table 4). As expected, the samples prepared with a higher OPC proportion develop higher UCS. This is because higher binder content is connected with the existence of more cement reagents (C3S, C2S, and C3A) in the mix and thus, formation of more cement hydration products [32]. The maximum UCS is achieved after 56 days of curing for the 8 wt% binder. However, the mines required a 28 days strength to be 1.1 MPa, which is achieved by, PBF1 (8 wt% OPC), PBF2 (7 wt% OPC) and PBF3 (6 wt% OPC). Sun et al. (2018) also reported strength 15

development of 1.5 MPa with waste rock and 0.95 MPa without waste rock after 28 days of curing using OPC as a sole binder. Table 4 Basic statistical analysis of UCS test results. UCS (MPa) 7 days’

14 days’

28 days’

56 days’

No. of samples (n): 3

No. of samples (n): 3

No. of samples (n): 3

No. of samples (n): 3

Mean

SD

CV

Mean

SD

CV

Mean

SD

CV

Mean

SD

CV

8% OPC

1.26

0.12

10.93

1.37

0.08

18.05

2.25

0.06

35.05

2.31

0.09

24.40

7% OPC

1.10

0.06

20.03

1.13

0.10

10.82

2.18

0.09

25.59

2.21

0.05

46.69

PBF3 PBF4 (Control) PBF5

6% OPC

0.82

0.10

7.85

1.11

0.02

47.92

1.16

0.05

21.92

1.43

0.05

30.33

5% OPC

0.71

0.07

10.83

0.98

0.10

9.45

1.08

0.05

21.83

1.18

0.03

46.76

4% OPC

0.51

0.03

17.78

0.81

0.06

14.55

0.98

0.12

8.24

1.15

0.06

17.94

FA8a

7%OPC + 1%FA

1.11

0.11

9.92

1.22

0.05

25.89

2.22

0.03

83.91

2.24

0.04

55.51

FA8b

6%OPC + 2%FA

0.95

0.05

19.33

1.13

0.03

35.05

1.63

0.03

50.60

1.65

0.04

39.55

FA8c

5%OPC + 3%FA

0.73

0.05

14.60

1.01

0.05

20.54

1.08

0.02

51.72

1.22

0.03

46.11

FA8d

4%OPC + 4%FA

0.64

0.09

10.48

1.07

0.04

25.78

1.18

0.05

25.75

4% OPC + 1% FA

0.64

9.75 11.56

0.90

FB5a

0.06 0.06

0.86

0.09

10.15

0.99

0.13

7.84

1.18

0.08

15.49

16.71

0.76

0.02

49.97

0.78

0.07

11.66

0.90

0.08

11.26

10.65

0.53

0.03

20.03

0.59

0.04

16.71

0.68

0.04

19.46

Mix

PBF1 (Control) PBF2

Binder (wt%)

FB5b

3% OPC + 2% FA

0.59

0.04

FB5a

2% OPC + 3% FA

0.50

0.05

SD: Standard deviation, CV: Coefficient of variance

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Fig. 5. Surface plot of interaction of binder dosage and curing time on strength (a) UCS and cohesion variation with OPC % and curing time, (b) UCS and cohesion variation with FA replacement-8 wt% group and curing time, and (c) UCS and cohesion variation with FA replacement-5 wt% group and curing time.

Binder consumption has a direct influence in techno-economic evaluation of paste backfill. Fig. 5b and 5c shows the effect of partial replacement of OPC with fly ash (FA) for 8wt% OPC and 5wt% OPC group. The rate of strength development in FA substituted binder is comparatively slower than that of the control sample 1 (8 wt% OPC). The 28 days’ strength is found to be reduced by 24 % with 1 wt% (12.5% of the total OPC) binder replacement for 8 wt% OPC group. Further the strength is reduced by 52% with 4 wt% (50% of the total OPC) binder replacement. So the addition of more than 2 wt% (28% of the total OPC) of FA did not produce the desired result as the UCS development is lower than the threshold value of 1.1 MPa. Similar kind of results is also visible for FA replacement with 17

control sample 2 (5 wt% OPC group). This reduction in strength is may be due to the fact that, lime to percentages of silica, alumina and iron oxide is fund to be 0.0071 in FA which is far below than the value of 0.66-1.02 for OPC grade-43, India. Thus limited involvement of FA towards strength development of paste backfill samples. The complete reduction of OPC may not be feasible because of increase in reduction of strength with reduction of OPC. However the mines required threshold value of 1.1 MPa was achieved by mix FA8a (7 wt% OPC + 1 wt% FA) and FA8b (6 wt% OPC + 2 wt% FA). Benzaazoua et al. (2002) [40] also explained a 28 days’ of strength development of 1 MPa by using FA as a binder replacement. 3.4.2 Regression analysis The strength development in paste backfill is a function of multiple variables and their coupled interactions. To upgrade the understanding of binder type and dosage on the strength development, multiple regression was conducted using the experimental results. The regression model was produced with parameter estimation for every input variable. The Beta (standardised regression coefficients) value is a measure of how strongly each predictor variable influences the dependent variable [56]. The major input variables for this regression are OPC, FA replacement percentage for 8 wt%, 5 wt% binder group and curing time (days). Tailings percentage is not considered in the regression analysis as the backfill mix is prepared with a fixed and predetermined (from slump test) solid percentage. Therefore to evaluate the alteration in strength with respect to OPC, FA replacement percentage for 8 wt%, 5 wt% binder group and curing time (days), the multiple regression analysis of up to 2nd order interactions were carried out. The results of statistical analysis are shown in Table 5.

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Table 5 Interaction of independent variables and dependent variable (UCS). Independent Dependent Interaction Beta variable variable OPC OPC*Curing 0.690299 UCS Curing time time: UCS 0.558084 FA replacement FA replacement % (8 wt% % (8 wt% -0.665961 UCS group) group)*Curing time: UCS Curing time 0.583635 FA replacement FA replacement % (5 wt% % (5 wt% -0.725367 UCS group) group)*Curing Curing time time: UCS 0.581358

p-value 0.000010< 0.05 0.000110< 0.05 0.000017< 0.05 0.000075< 0.05 0.000008< 0.05 0.000075< 0.05

From Table 5 it is observed that the interaction effects of OPC and Curing time have positive Beta (regression coefficients), hence with increase in OPC and curing time strength increases due to formation of more cement hydration products which further consolidate the paste backfill structure. Whereas, FA replacement % (8 wt% group) and FA replacement % (5 wt% group) have negative Beta (regression coefficients) on the strength, most likely due to variation in chemical compositions of FA and OPC. Among all of the second order interactions, the interactions OPC*Curing time (Beta: 0.690299; P- value: 0.000010), FA replacement % (8 wt% group)*Curing time (Beta: 0.665961; P- value: 0.000017) and FA replacement % (5 wt% group)*Curing time (Beta: 0.725367; P- value: 0.000008) possess statistically significant impact on the strength. The interaction OPC*Curing time shows that the paste backfill prepared with OPC as a sole binding material would acquire higher strength significantly with increasing curing period. Whereas, if the OPC is partially replaced with the FA then the strength would reduce significantly as it can be seen from the interaction of FA replacement % (8 wt% group)*Curing time and FA replacement % (5 wt% group)*Curing time. However, the coefficient (Beta: -0.665961) of the interaction of FA replacement % (8 wt% group)*Curing

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time is higher than the coefficient (Beta: -0.725367) FA replacement % (5wt% group)*Curing time, so fly ash replacement in 8 wt% binder group is more sensitive towards higher strength than that of fly ash replacement in 5 wt% binder group. 3.4.3 Cohesion and friction angle The backfill is to be placed inside the stope in a loose condition, with time the material gain strength and start taking load. During which the fill mass may fail along a slip plane if it has a very low shear strength. To predict the lateral support backfill mass provides to the hang wall, shear strength parameters can be used. The triaxial test was used to determine the shear strength parameters (cohesion and friction angle).

Fig. 6. Samples with shear failure during triaxial loading.

20

6

5

7 Days σ1 = 1.1381 + 2.853σ3, r2 = 0.9376 14 Days σ1 = 1.4995 + 3.0973σ3, r2 = 0.8735 28 Days σ1 = 1.7235 + 3.7952σ3, r2 = 0.9799 56 Days σ1 = 1.747 + 4.0281σ3, r2 = 0.9583

σ1 (MPa)

4

3

2

1

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

σ3 (MPa)

Fig. 7. Typical triaxial test results of paste backfill samples. For this particular study, loading at each confining pressure continued until the confining pressure starts rising due to the outward movement of the specimen inside the triaxial cell. The samples failed due to shear, Fig. 6 shows the shear failure pattern of the samples tested under triaxial loading. The normal pressure (σ1) is plotted against confining pressure (σ2) (Fig. 7) and the cohesion and friction angle were determined. Table 6 illustrates the results of cohesion and angle of internal friction with curing time at different binder composition. It can be envisaged with increase in curing time cohesion increases (Fig. 5a-c). The increase in compressive strength with curing due to cement hydration and bond strengthening is reflected in terms of increase in cohesion parameter. Based on the triaxial results friction angle values are estimated to be in between 280 to 380. Similarly, Rankine and Sivakugan, (2007) [29] reported φ values between 31.70 to 44.10 for cemented paste backfilling (CPB) samples cured in plastic moulds.

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Table 6 Development of cohesion and friction angle in paste backfill samples with curing. Mix PBF1 (Control) PBF2 PBF3 PBF4 (Control) PBF5 FA8a FA8b FA8c FA8d FB5a FB5b FB5a

Binder (wt%)

Cohesion (MPa)

Friction angle (Deg.)

7 days’

14 days’

28 days’

56 days’

7days’

14 days’

28 days’

56 days’

8% OPC

0.34

0.42

0.65

0.69

34.04

28.25

33.58

31.47

7% OPC 6% OPC 5% OPC

0.34 0.31 0.29

0.4 0.37 0.33

0.61 0.58 0.4

0.67 0.6 0.42

30.28 28.91 29.09

34.54 28.87 30.19

33.16 30.28 27.38

30.95 32.15 30.11

4% OPC 7%OPC + 1%FA 6%OPC + 2%FA 5%OPC + 3%FA 4%OPC + 4%FA 4% OPC + 1% FA 3% OPC + 2% FA 2% OPC + 3% FA

0.28 0.34

0.29 0.4

0.39 0.62

0.4 0.67

28.1 34.04

31.3 28.25

27.12 33.58

31.56 31.47

0.32

0.39

0.58

0.61

28.75

30.79

35.66

37.03

0.29

0.34

0.42

0.42

29.75

30.81

34.4

36.27

0.28

0.32

0.4

0.4

31.78

33.28

34.83

35.27

0.28

0.3

0.33

0.35

31.56

33.88

35.12

35.92

0.26

0.29

0.33

0.35

28.98

30.77

28.04

30.7

0.2

0.21

0.21

0.22

30.02

31.03

31.73

30.97

3.5 Microstructural evolution Microstructure analysis is important to visualise and investigate the growth of cement hydration products with curing. The paste backfill specimens were investigated under SEM after 7, 14, 28 and 56 days’ of curing to correlate with the results of strength development tests. The SEM micrograph indicates mixing of finer OPC and fly ash particles with coarser tailings helps in filling the intergranular particles and forms a well distributed cementitious matrix of OPC and fly ash (Fig. 8).

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Fig. 8. SEM micrograph showing the texture of paste backfill. The loose microstructure with porous texture of the micrograph justifies less strength development at early days’ of curing (Fig. 9a). Inversely a compact and dense cementitious matrix resembles higher strength which the paste backfill gains with curing time. However, with dissipation of water these voids are reduced (Fig. 9b and 10b). After 7 days’ of curing in OPC based paste backfill sample, development of C-S-H was found. Whereas, at the early days’ there was hardly any development of C-S-H in OPC-FA based paste backfill (Fig. 10a). With curing plate like microstructures were developed within the cementitious matrix, which resembles to development of mainly portlandite. Also, Needle like crystals of calcium aluminium sulphate mineral (ettringite) was also detected after 14 days’ of curing (Fig. 9b-d). However, detection of sulphur in the EDS spectra indicates the vulnerability of sulphate attack with time. At the same time in OPC-FA based paste backfill only ettringite was formed and portlandite was not visible (Fig. 10b).

23

24

Fig. 9. SEM micrographs and EDS spectra of paste backfill samples mixed with OPC as binder (a) 7 days’ (b) 14 days’ (c) 28 days’ (d) 56 days’. Similar kind of absence of portlandite in OPC-FA paste backfill was observed by Benzaazoua et al. (2002) [40]. The reaction of tri-calcium aluminate with tri-calcium sulphate present in OPC resulted in formation of Ettringite [46]. C3A + C3S → Ettringite Moreover, after 28 days’ of curing in OPC-FA based paste backfill traces of development of portlandite was found (Fig. 10c-d). After 56 days’ of curing in OPC based paste backfill sample gypsum like structure was observed. Optimum quantity of gypsum helps in filling the voids within the paste backfill, which further reduces the porosity and helps in maintaining the strength of backfill [41]. Analogous kind of development of gypsum in mill tailings based cemented paste backfill has also been reported in the literature [40]. C-S-H gel is combined with portlandite, ettringite and all these are interconnected with each other so as to provide a binding mechanism for the tailings particles and thus develop strength [45, 47]. Hence lack of formation of C-S-H (7 curing days’), portlandite (14 curing days’) and gypsum (56 curing days’) like structure in OPC-FA based paste backfill with different curing period results in comparatively less strength development as that of OPC based paste backfill. The micrographs of paste backfill samples are in accordance with the strength development with curing time as explained in the previous section.

25

26

Fig. 10. SEM micrographs and EDS spectra of paste backfill samples mixed with OPC-FA as binder at different curing times: (a) 7 days’ (b) 14 days’ (c) 28 days’ (d) 56 days’.

4. Conclusions In this study fly ash as a fractional OPC replacement was investigated for paste backfilling in underground metal mines. Following are the conclusions drawn based on the studies. a) The coarser angular tailings, finer spherical fly ash are forming a suitable paste backfill mix with optimum fines fraction and chemically both the materials are abundance in SiO2. However, tailings contain higher percentage of CaO and MgO than fly ash. b) The slump test study revealed, slump more than 195 mm resembles thick paste, thus an optimum solid percentage of 77 wt% and 78 wt% need to be maintained with a slump of 195 mm (+ 2 mm variation in slump height). c) The setting time study reflects this paste backfill have an optimum setting time so that it can be pumped within the required time and it would start taking load after curing. d) Paste backfill mix PBF1 (8 wt% OPC), PBF2 (7 wt% OPC), PBF3 (6 wt% OPC), FA8a (7 wt% OPC + 1wt% FA) and FA8b (6 wt% OPC + 2 wt% FA) achieved the desired UCS of 1.1 MPa. Thus FA replacement is possible upto 25% of the total OPC content. e) Most of the paste backfill specimens show hour glass type failure under uniaxial loading whereas under triaxial loading backfill samples show shear failure pattern. f) The loose microstructures observed during microstructural analysis of paste backfill samples justify the low strength development at early days. C-S-H was not developed at early days’ in OPC-FA binder based paste backfill. Also, gypsum like microstructures was found only in OPC based paste backfill samples. Lack of formation of ample amount of C-S-H, portlandite and gypsum in OPC-FA based paste

27

backfill with different curing period results in comparatively less strength development as that of OPC based paste backfill. It can be concluded that the microstructural evolution, with development of cement hydration products reflects in terms of strength gain with curing and both are related to each other. Thus the paste backfill explained in this study can be used in paste backfilling system for enhancing production, productivity with safety. Acknowledgements This study has been carried out by financial support from Hindustan Zinc Limited (HZL). HZL management also supplied lead-zinc mill tailings and fly ash samples for this study. The authors gratefully acknowledge the support received from HZL management. The authors would also like to acknowledge Director, CSIR-CIMFR, Dhanbad for his guidance and supports.

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Highlights: •

Investigated role of microstructural evolution on strength characteristics of paste backfill.

•

Hour glass type failure under uniaxial loading whereas shear failure pattern under triaxial loading.

•

Strength development is largely controlled by binder type and dosage.

•

Development of C-S-H, portlandite, ettringite visualised from SEM study.

Conflicts of Interest Statement Manuscript title: Strength development and microstructural investigation of lead-zinc mill tailings based paste backfill with fly ash as alternative binder. The authors whose names are listed immediately below report the following details of affiliation or involvement in an organization or entity with a financial or non-financial interest in the subject matter or materials discussed in this manuscript. Funding: This study was funded by Hindustan Zinc Limited (SSP/139/2016-17). Conflict of Interest: Author C.N. Ghosh has received research grant from Hindustan Zinc Limited. Author names: S.K. Behera, C.N. Ghosh, Prashant Singh, K. Mishra, J. Buragohain, P. K. Mandal Employed in CSIR-Central Institute of Mining and Fuel Research, Dhanbad - 826015, Jharkhand, India. D. P. Mishra - Employed in Department of Mining Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad – 826004, Jharkhand, India.

(Santosh Kumar Behera) Scientist Mine Back Filling Department CSIR- Central Institute of Mining and Fuel Research Barwa Road, Dhanbad, Jharkhand- 826015 India Mob: +918986760221 Email Id: [email protected]