Effect of superplasticizers on the consistency and unconfined compressive strength of cemented paste backfills

Construction and Building Materials 181 (2018) 59–72

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Effect of superplasticizers on the consistency and unconfined compressive strength of cemented paste backfills Drissa Ouattara a, Tikou Belem a, Mamert Mbonimpa a,⇑, Ammar Yahia b a b

Research Institute on Mines and Environment, Université du Québec en Abitibi-Témiscamingue (UQAT), 445 Boul. de l’Université, Rouyn-Noranda, Québec J9X 5E4, Canada Department of Civil Engineering, Université de Sherbrooke, 2500 Boul. de l’Université, Sherbrooke, Québec J1K 2R1, Canada

h i g h l i g h t s
Use of high-range water reducer (HRWR) superplasticizers in cemented paste backfills (CPB).
Determination of the optimum HRWR addition mode.
Influence of HRWR type, binder content, curing time, and tailings characteristics.
Assessment of the contributions of solids content and HRWR on CPB strength development.
HRWR addition allows potential reduction in binder content in CPB.

a r t i c l e

i n f o

Article history: Received 29 December 2016 Received in revised form 30 May 2018 Accepted 31 May 2018 Available online 18 June 2018 Keywords: Cemented paste backfill (CPB) High-range water reducer (HRWR) Slump Unconfined compressive strength (UCS)

a b s t r a c t This paper assesses the effects of different types of high-range water reducer (HRWR) on the consistency (slump) and unconfined compressive strength (UCS) of cemented paste backfills (CPB) prepared mainly with tailings T1 at 80% solids mass content (a few tests were also conducted with tailings T2). A blended binder made of 80% ground granulated blast furnace slag and 20% GU (general use Portland cement) was prepared at three different proportions: 3.5%, 4.5%, and 6% (by dry mass of tailings). Six HRWR types were tested: four polycarboxylates (PC1 to PC4), a polymelamine sulfonate (PMS), and a polynaphtalene sulfonate (PNS). The slump values for fresh CPB and the UCS for the hardened CPB specimens were determined at different curing times. Test results showed that the addition of HRWR to CPB mixtures improved CPB consistency and UCS, regardless of HRWR type. For tailings T1, a minimal dosage of 0.121% (by dry mass of tailings + binder, for each PC type) was required to achieve a slump value of 152 (6 in., which is generally considered the lower limit in backfilling operations to achieve pumpable CPB). The PC-type HRWR more effectively increased CPB consistency compared to PNS- and PMS-type HRWR. However, UCS values at 28 days were comparable for CPB incorporating all HRWR types. The UCS increase after 28 days of curing for superplasticized CPB ranged from 21% to 29% compared to the control CPB mixture (without HRWR), depending on binder content. Results are discussed in terms of the impact of HRWR addition on the change in water-to-binder ratio, the contribution of solids content and HRWR to UCS improvement, and the potential reduction in binder consumption. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction In cut-and-fill and long-hole sublevel stoping, cemented paste backfill (CPB) provides a secondary ground support that improves ore recovery by reducing dilution and ensures miner safety. CPB is also used in underground mines for different functions, including [1]: 1) preventing spalling in highly stressed (either compressive or

⇑ Corresponding author. E-mail address: [email protected] (M. Mbonimpa). https://doi.org/10.1016/j.conbuildmat.2018.05.288 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

tensile) rock around an opening, 2) acting as a secondary pillar in artificially supported mining methods (e.g., cut-and-fill, longhole, and sub-level stopes) that enable complete ore recovery, 3) providing a working platform for miners and equipment in undercut and fill operations, 4) absorbing part of the excess stress in order to minimize damage due to rockburst, 5) depositing large quantities of tailings underground, 6) alleviating environmental hazards associated with surface tailings disposal, and 7) preventing surface subsidence in shallow or soft rock mines. Backfilling also

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provides environmental benefits by reducing the overall amount of tailings stored on the surface [2–6]. CPB is a complex composite material produced by mixing filtered mine tailings with relatively small amounts of binder (usually 2–8% by dry mass of tailings) and mixing water [2,7–9]. The coupled phenomenon of binder hydration and self-weight consolidation contributes to the CPB hardening and compressive strength development required to ensure stable backfilled stopes with an exposed wall face [1,7,10–12]. The unconfined compressive strength (UCS) of CPB at different ages is an important parameter for the stability analysis of backfilled stopes. CPB is usually transported through reticulated pipelines by gravity and/or by pumping and delivered into underground stopes. At backfill plants, the quality control method for assessing CPB consistency for transport purposes consists of the slump test according to ASTM C 143 standard [13]. Hence, backfill system design begins with transport and rheological characterization [5,14]. Conventionally, tailings slurry from the concentrator is first thickened and then filtered (solids mass content Cw > 85%) before CPB preparation. In order to minimize the volume of tailings in the tailings storage facility (TSF), the amount of tailings used for CPB preparation should be maximized (increasing Cw). Although higher CPB solids mass content results in higher compressive strength development, the maximum amount of tailings that can be used in practice for CPB proportioning is dictated by the desired consistency. Thus, higher Cw results in relatively lower slump (depending on the tailings density), which could negatively affect CPB transportability. In general, CPB slump values ranging from 152 mm (6 in.) to 254 mm (10 in.) are required to ensure adequate pipeline transport [8]. Extra water is generally added to the filtered tailings during CPB mixing to achieve the target slump and facilitate transport [1,4]. Unfortunately, added water during CPB preparation results in a significantly higher water-to-cement ratio (w/c > 3) than that required (w/c  0.5) for adequate binder hydration [7,15], which can negatively affect the mechanical properties of CPB. High-range water reducers (HRWR), also called superplasticizers (SP), are chemical compounds (admixtures) that can be adsorbed on the surface of cementitious material particles, thereby producing repulsive forces that disperse the particles and improve the rheological properties of the suspensions [16–18]. The mechanical properties are improved as well, due to better hydration of the cementitious materials. Adding HRWR to CPB mixtures may also allow reducing the mixing water content (i.e., lower water-to-binder ratio w/b), for the following potential advantages: 1) lower binder content required to obtain a given strength, 2) higher density of the final placed backfill (higher tailings content in the CPB mixture), and 3) superior CPB durability due to improved microstructure. Despite these potential advantages, only a few studies have considered HRWR addition to backfill formulations [16,17,19–22], yet the results have shown higher UCS for CPB containing HRWR. For example, Klein and Simon [22] reported up to 35% higher UCS for CPB incorporating 0.185% (by dry mass of solids) of polycarboxylate (PC) HRWR, whereas 0.8% dosage of polynaphtalene HRWR resulted in lower improvement compared to control specimens prepared without HRWR. However, these studies did not investigate the effects of the HRWR addition sequence (mode) on either CPB consistency or compressive strength. Due to the complex binder hydration process in CPB material compared to neat cement pastes, mortars, and concretes [7], the mechanical response of CPB incorporating HRWR must be assessed for quality control requirements. This paper investigates the effects of various types of HRWR on the consistency and UCS of different CPB formulations. The effects of HRWR type, addition mode, and dosage on the consistency and UCS of different CPB mixtures prepared with different binder

contents as well as different tailings were assessed. The impact of HRWR type on the change in water-to-binder ratio during curing, the contributions of HRWR type and solids content to the UCS of CPB, and the potential for lowering the binder dosage are discussed. 2. Materials and testing methods The experimental program was designed to assess the compressive strength development of CPB mixtures made with two different tailings and different types of HRWR. The CPB mixtures were prepared using two different types of binder and a fixed solid mass content (Cw) of 80% (by total mass of CPB). The CPB mixtures were proportioned with different HRWR dosages to achieve targeted slump values ranging from 152 to 254 mm (6–10 in.) [23,24]. 2.1. Materials Two different tailing samples (T1 and T2) taken from two different hard rock mines located in the Abitibi-Témiscamingue region in the province of Québec, Canada, were used. Sample T1 is polymetallic ore tailings and sample T2 is gold ore tailings. 2.1.1. Chemical composition Representative and homogenized tailings (T1 and T2) samples were dissolved in concentrated acid solutions, including nitric (HNO3), bromide (Br2), fluoride (HF), and hydrochloric (HCl) acid, to ensure total dissolution of all solid phases. The resulting solution was then filtered, diluted, and analyzed using a Perkin Elmer ICPAES (Inductively Coupled Plasma-Atomic Emission Spectroscopy) Optima 3000 DV. The chemical composition of T1 and T2 tailings is summarized in Table 1. 2.1.2. Mineralogical composition The mineralogical composition of the tailings was determined by X-ray diffraction (XRD) analysis. Table 2 summarizes the types and amounts of mineral content. The predominant mineral found in both tailings samples is quartz at 42.7% and 86.6% content for tailings T1 and T2, respectively. In addition, albite, muscovite, and pyrite were found in both tailings. Paragonite (4.5%), chlorite (5.5%), and gypsum (0.5%) were found only in tailings T1, whereas ankerite (2.4%) was found only in tailings T2. The pyrite content differs considerably between the two tailings: 30.5% (polymetallic tailings) for T1 versus only 1.04% (gold tailings) for T2. 2.1.3. Grain-size distribution (GSD) The grain-size distribution of tailings T1 and T2 was obtained using a MalvernÒ Mastersizer S2000 laser particle size analyzer. The cumulative grain-size distribution curves for tailings T1 and T2 are shown in Fig. 1. The main gradation parameters, including the coefficients of uniformity (CU) and curvature (Cc), the diameter Dx corresponding to x percentage passing on the cumulative GSD curve, the percentage of particles passing 20 and 80 lm (P20lm and P80lm), the specific gravity Gs, and the specific surface area (Sm) obtained using a BET (Brunauer–Emmett–Teller) method are summarized in Table 3. The specific gravity (Gs) of the tailings was determined according to ASTM C128 [25] standard using a helium pycnometer (AccuPyc 1330, MicrometricsÒ). Tailings T1 and T2 showed a Gs of 3.42 and 2.80, respectively. Fig. 1 shows that both tailings contained about 83% particles less than 80 mm (P80mm), with 42% ultrafine particles less than 20 mm (P20mm) for tailings T1 and 52% for tailings T2. According to Landriault et al. [8], adequate tailings for CPB preparation should have at least P20lm = 15%. Based on the grain-size characteristics, both

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D. Ouattara et al. / Construction and Building Materials 181 (2018) 59–72 Table 1 Chemical composition of tailings T1 and T2. Chemical elements

Al

Ba

Na

K

Ca

Fe

Si

Mg

Mn

Stot

Ti

Zn

Pb

As

T1 (%) T2 (%)

3.98 1.70

0.02 –

– 0.34

– 0.56

0.63 0.75

24.10 1.80

50.95 93.24

0.19 0.54

0.02 0.02

19.80 0.84

0.08 –

0.14 –

0.05 –

0.01 0.21

are 900 m2/kg and 3.15, and 2750 m2/kg and 2.94 for slag, respectively.

Table 2 Mineralogical composition of tailings T1 and T2. Mineral

T1 (%)

T 2 (%)

Quartz Albite Chlorite Ankerite Paragonite Gypsum Muscovite Pyrite

42.7 13.6 5.5 – 4.9 0.5 2.5 30.5

86.6 5.4 – 2.4 – – 4.5 1.04

2.1.5. HRWR types Three types of polycarboxylate (PC)-based HRWR (superplasticizer – SP) were investigated (PC1, PC2, and PC3), provided by three different manufacturers. In addition, to compare SP efficiency, a polycarboxylate-based SP (PC4), a polynaphtalene sulfonate-based SP (PNS), and a polymelamine sulfonate-based SP (PMS) were tested. The characteristics of the six HRWR types, including pH, specific gravity (Gs), and solids content (Cw_SP), are summarized in Table 5. 2.2. CPB mix proportioning and specimen preparation

Fig. 1. Cumulative grain-size distribution curves for tailings T1 and T2.

Table 3 Physical characteristics of tailings T1 and T2. Parameter

Tailings T1

Tailings T2

Cc (-) CU (-) D10 (mm) D30 (mm) D50 (mm) D60 (mm) D80 (mm) D90 (mm) Gs (-) P20mm (%) P80mm (%) Sm-BET (m2/kg)

1.1 9.8 3.5 11.3 24.2 34.2 70.0 113.0 3.42 42 83.9 1930

0.8 7.6 3.5 9.1 18.4 26.9 67.2 115 2.80 52 82.5 2445

tailings are comparable to common hard rock tailings, as described in the literature [26]. 2.1.4. Binder type The binder used in this study is a blend of 80% ground granulated blast furnace slag (GGBFS) and 20% general use Portland cement (GU), hereinafter referred to S-GU. GU acts as a hydration activator. This S-GU binder develops better strength performance in CPB and is the most commonly used for mine backfilling [7]. The main oxide compositions of the GU, slag, and S-GU binders are summarized in Table 4. The BET (Brunauer-Emmett-Teller) specific surface area (Sm-BET) and the specific gravity (Gs) for GU

The influence of HRWR type on CPB consistency and compressive strength was investigated in two stages (Phase I and Phase II). In Phase I, the paste backfill mixtures were prepared with tailings T1 at different solids contents Cw of 70, 75, and 80% and without added HRWR (Table 6). The reference CPB (75% solids content, 4.5% S-GU binder) is the CPB formulation without HRWR that achieved the target slump to allow CPB pipeline transport, and the control CPB is the CPB formulation at Cw of 80% and without HRWR. The effect of three types of PC-based HRWR (PC1, PC2, and PC3) added to the CPB mixtures at 80% solids content and 4.5% binder (by dry mass of tailings) was investigated. Dosages of 0.09, 0.121, 0.13, and 0.153% by dry mass of solids (tailings + binder) were used in CPB mixtures at Cw of 80% with a fixed 4.5% (by dry mass of tailings) of S-GU binder. These mixtures were prepared to determine: 1) the minimum HRWR dosage to achieve the targeted lower limit slump of 152 mm (6 in.), and 2) the effect of HRWR addition on CPB strength development. A total of 15 CPB formulations, as presented in Table 6, were prepared to assess the influence of solids content and HRWR dosage on CPB consistency and strength development. For each mixture, the UCS was determined on triplicate specimens at four different curing times of 7, 14, 28, and 56 days. In Phase II, the minimum HRWR dosage obtained from Phase I was used to assess the influence of binder content (3.5, 4.5, and 6%), HRWR type (PC1 to PC4, PNS, and PMS), and tailings type (T1 and T2) on the UCS of CPB prepared at 80% solids content. The additional CPB mixtures investigated in Phase II are summarized in Table 7. The CPB mixtures were prepared in both phases using a Hobart mixer equipped with a rotational paddle. Deionized water was used to obtain the targeted 80% solids mass content. The water contained in the liquid HRWR was taken into account to calculate the total mixing water. Two HRWR addition modes, immediate and delayed, were assessed for comparison purposes. In the immediate addition mode (IAM), the HRWR is first diluted in deionized mixing water and then mixed with the tailings and binder during CPB preparation (Step 1: one half at the beginning of mixing and the second half from 60 to 90 s after the beginning of mixing, as illustrated in Fig. 2). In the delayed addition mode (DAM), tailings, binder, and deionized mixing water are first mixed for 3 min and the HRWR is then added in Step 3 (see Fig. 2). In both addition modes (immediate and delayed), the mixing water (deionized water + liquid

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Table 4 Chemical composition of the GU, Slag, and S-GU binders. Binder type

GU Slag S-GU

Chemical composition (%) SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

TiO2

P2O5

MnO

Cr2O3

V2O5

19.3 35.6 30.5

4.99 10.5 8.6

2.97 0.65 0.77

64.3 37.5 40.9

2.1 11.9 6.2

0.27 0.46 0.40

0.50 0.51 0.5

0.27 0.98 0.64

0.26 0.93 0.61

0.12 0.50 0.31

0.01 <0.01 0.01

0.03 0.01 0.01

Table 5 Characteristics of the six HRWR types used (provided by four manufacturers). HRWR

Gs

pH

Cw_SP (%)

PC1 PC2 PC3 PC4 PNS PMS

1.07 1.10 1.10 1.08 1.20 1.12

5 5.7 3 – 7 8

31 46 40 40 40 40

HRWR for the IAM; deionized water only for the DAM) is added to the tailings and binder in Step 1 while the mixer rotates at 140 rpm for 3 min (180 s). The mixing is then halted for 30 s in order to test for mix homogeneity and clean the bowl. Step 2 is the same for both addition modes. For the delayed addition mode, liquid HRWR is added in Step 3 for 30 s while the mixer rotates at 140 rpm (Fig. 2). In Step 4, the mixing speed is ramped up to 285 rpm for an additional mixing time of 90 s. The total mixing time is 5 min. The delayed addition mode was used to prepare most of the CPB mixtures for assessment of the influence of the different factors.

curing age, the bleeding water accumulated on the top of the mold (due to self-weight consolidation of CPB) was removed before preparing the specimen for compression testing. A digital MTS 10 G/L mechanical press having a maximum load capacity of 50 kN was used for UCS determination. Compression tests were performed according to ASTM C39 standard [28]. A uniform vertical load was applied at a constant displacement rate of 1 mm/min until specimen failure. For each CPB formulation, triplicate specimens were tested at each curing time, and the average value was reported as the unconfined compressive strength (UCS). The mean standard for the tested samples was 5%. After each compression test, a piece of the CPB specimen was oven-dried at 50 °C for 48 h for determination of the gravimetric water content w(%). Because compressive strength development with curing time is influenced by the w/b ratio, the water content of hardened CPB can be used to calculate the w/b ratio, according to Eq. (1).



 
w wð%Þ 100 100 100 ¼ ¼ 1þ 1 1þ b 100 Bw% C w% Bw%

ð1Þ

where, w(%) is the gravimetric water content (in %), Bw% is the binder content (in %) calculated as the ratio of the mass of binder to the mass of dry tailings, and Cw% is the solids mass content (in %).

2.3. Testing methods 3. Test results 2.3.1. Slump tests Slump testing was conducted immediately after CPB preparation according to ASTM C143/C143 M-05a standard [13]. A small truncated cone (one half the size of the standard cone) with height 150 mm, base diameter 100 mm, and top diameter 50 mm was used. In this study, the proportionality factor of 2.28 was used to convert the measured slump values to the corresponding values for the standard Abrams cone. This factor was chosen based on available in-house laboratory investigations and data from the literature [23,27]. 2.3.2. Unconfined compression tests Immediately after CPB preparation, 12 cylindrical plastic molds (50.8 mm diameter and 101.6 mm height) were filled with CPB. The CBP was then subjected to 25 S with a 1 cm diameter steel rod to ensure proper filling and to expel air bubbles. The molds were then capped and placed in a humidity chamber at 23 ± 2 °C and a relative humidity (RH) higher than 90% for curing. Four curing times of 7, 14, 28, and 56 days were considered. At a given

3.1. Impact of solids content on the slump and UCS of CPB mixtures without HRWR Fig. 3a shows the slumps for CPB mixtures prepared without HRWR at different solids contents (Cw%) of 70, 75 (defined as the reference formulation; see Section 2.2), and 80% (defined as the control formulation) and with 4.5% S-GU binder, and Fig. 3b shows the variation in UCS with curing time. As mentioned above, the rheology (i.e., slump) of CPB is a key factor in the design of CPB pipeline transport systems. Based on several case studies, Landriault et al. [8] suggested that in order to obtain optimal flow performance combined with the required strength, CPB should have a standard slump height ranging from 152 mm or 6 in. (lower limit) to 254 mm or 10 in. (upper limit). These authors [8] also suggested that CPB mixtures with 254 mm slump can be delivered easily by gravity, whereas for 152 mm slump, positive displacement pumps are required to ensure successful transport.

Table 6 CPB mixture formulations prepared with tailings T1 and used to determine the required HRWR dosages and to assess the effect of the solids content Cw. HRWR type

Cw (%)

Binder type (–)

Binder Content, Bw (%)

Water-to-binder ratio (w/b) (–)

HRWR dosage v-PC (%)

Curing time (days)

– – – PC1 PC2 PC3

70 75 80 80 80 80

S-GU S-GU S-GU S-GU S-GU S-GU

4.5 4.5 4.5 4.5 4.5 4.5

9.95 7.74 5.81 5.81 5.81 5.81

0 0 (reference) 0 (control) 0.09, 0.121, 0.135, 0.153 0.09, 0.121, 0.135, 0.153 0.09, 0.121, 0.135, 0.153

7, 7, 7, 7, 7, 7,

14, 14, 14, 14, 14, 14,

28, 28, 28, 28, 28, 28,

56 56 56 56 56 56

Cw is the CPB solids mass content; Bw is the CPB binder content; w/b is the water-to-binder ratio; vPC is the PC dosage expressed as the ratio of the dry mass of PC to the dry mass of solids (tailings + binder).

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D. Ouattara et al. / Construction and Building Materials 181 (2018) 59–72 Table 7 CPB mixtures investigated in Phase II (using the minimum HRWR dosage from Phase I) to assess the effects of binder content, HRWR type, and tailings type. Dosage vHRWR (%)

Cw (%)

Tailings type (–)

Binder type (–)

Binder content (%)

HRWR type (–)

Curing time (days)

0 (Control) 0.121 0.121 0 (Control) 0.121

80 80 80 80 80

T1 T1 T1 T2 T2

S-GU S-GU S-GU S-GU S-GU

3.5, 4.5, 6 3.5, 4.5, 6 4.5 4.5 4.5

– PC1, PC2, PC3 PMS, PNS, PC4 – PC1, PC2, PC3

7, 7, 7, 7, 7,

14, 14, 14, 14, 14,

28, 28, 28, 28, 28,

56 56 56 56 56

Binder content Bw = dry mass of binder/dry mass of tailings; HRWR dosage vHRWR = dry mass of HRWR/dry masses of tailings and binder.

Fig. 2. CPB mixing sequence: immediate and delayed HRWR addition modes.

Fig. 3. Effects of solids content of CPB mixtures prepared Tailings T1 with 4.5% S-GU binder and without HRWR on (a) standard slump values and (b) variation in compressive strength with curing time.

Fig. 3a shows a slump of 274 mm for CPB at 70% solids content, which exceeds the upper limit of 254 mm, whereas the CPB at 75% Cw shows a slump 168 mm slump, which is within the recommended 152–254 mm range required for pipeline transport. However, the CPB at 80% Cw shows a 76 mm slump, which is too far

below the lower limit of 152 mm. CPB with a slump below 152 mm may require a powerful and expensive pump to ensure successful pumping. Fig. 3b shows that, for a given curing time, CPB with higher solids content obtained higher UCS. Moreover, UCS increased with

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age for all CPB mixtures. For example, after 7 days of curing, CPB at 75% and 80% Cw showed UCS values of 459 kPa and 736 kPa, respectively, corresponding to a relative UCS increase of 60% for only 7% increase in the solids content. This increase becomes 27% after 28 days of curing. Yin et al. [29] observed similar results for increased solid content (Cw) from 74% to 80%. 3.2. Effect of HRWR addition mode on CPB slump and UCS Various studies have reported that the HRWR addition mode can improve the rheological properties of cementitious materials [18,30]. Fig. 4a shows the slump values for the CPB mixture made with tailings T1 at 80% solids content, 4.5% S-GU binder, and 0.135% PC using both the immediate and delayed addition modes (see Fig. 2). Confirming previous findings by the authors [24], HRWR addition improved the CPB mixture consistency according to the addition mode. Thus, compared to the control CPB, the slump for CPB made with 0.135% PC1, PC2, and PC3 and using the immediate addition mode increased by 61%, 76%, and 113% compared to 182%, 187%, and 190%, respectively, using the delayed addition mode. Consequently, the slump obtained using the delayed addition mode shows the greatest improvement in consistency. The lowest difference between slump heights obtained with the immediate and delayed addition mode was observed for CPB prepared with PC3, indicating that the PC3-type HRWR was less sensitive to the addition mode compared to the PC2 and PC1 types. Fig. 4b shows the UCS values for the CPB incorporating PC-type HRWR using both the immediate and delayed addition modes. Higher UCS was obtained after 28 days of curing compared to the control CPB. When PC1, PC2, and PC3 were incorporated using the immediate addition mode, the increase in UCS is 15%, 13%, and 25%, respectively, compared to control. However, using the delayed addition mode, the increase in UCS is 29%, 27%, and 25% for PC1, PC2, and PC3, respectively (for a nearly 100% increase over the immediate addition mode for PC1 and PC2). Because adding PC using the delayed addition mode obtained greater improvements in both the consistency and compressive strength of CPB compared to the immediate addition mode, the delayed addition mode was adopted for further experiments in this investigation. 3.3. Effect of HRWR dosage on CPB consistency and compressive strength The variation in slump for CPB proportioned at 4.5% S-GU binder, 80% solids content, and incorporating various dosages of PC1,

PC2, and PC3 are shown in Fig. 5. For each PC type, the dosage vPC was varied from 0 to 0.153% by dry mass of total solids (tailings + binder). As can be seen, CPB mixtures incorporating 0.09% PC show slump values below the suggested lower limit of 152 mm, regardless of PC type [8]. Without powerful and expensive positive displacement pumps, such backfills would be difficult to pump. However, dosages higher than 0.121% resulted in slump values that exceeded the suggested lower limit for CPB transport. The 0.121% PC dosage was therefore retained as the minimum dosage (lower limit) for easy CPB pumpability in the assessment of the effects of the different mixture parameters. According to mining practices [8], CPB can be delivered by gravity when the slump ranges from 210 mm (8 ¼ inches) to 254 mm (10 in.). It is reasonable to assume that PC dosages higher than 0.135% would provide suitable consistency for gravity flow. Indeed, the CPB mixtures at 80% Cw and incorporating 0.135% and 0.153% of each PC type showed slumps of about 215 mm and 260 mm, respectively, regardless of PC type. Fig. 6 shows the effects of increasing dosages (from 0.09% to 0.153%) of different PC-type HRWRs (PC1, PC2, and PC3) on CPB compressive strength development across curing times (7, 14, 28, and 56 days). The UCS results indicate that increasing PC dosage provides greater increases in the UCS compared to control (without SP), regardless of PC type. Moreover, PC1, PC2, and PC3 at dosages up to 0.121% obtained significant UCS increases for all curing times (Fig. 6a, b). Considerable increases in UCS were obtained after 14, 28, and 56 days of curing for all PC dosages. The noticeable increase in CPB strength after 14 days of curing, regardless of HRWR type and dosage, may be due to the characteristics of the binder and tailings. Thus, the high CaO content in GU and silica content in slag combined with the high specific surface area of the S-GU binder and the possible presence of sulfates in the tailings may have contributed to pozzolanic reactions, leading to a relatively significant early strength development. However, further investigations are required to understand the exact mechanisms at play. Moreover, all dosages of the three PC types induced a similar trend in strength development. Table 8 summarizes the relative increase in UCS after 28 days of curing for all PC dosages. For each PC type, dosages of 0.09% and 0.121% obtain a significant strength increase. For dosages above 0.121%, UCS appears to reach a stationary plateau, with no significant increase thereafter. Based on the slump and UCS results, the 0.121% dosage was confirmed as the minimum dosage, and was used in CPB formulations for subsequent analyses of the effects of other factors.

Fig. 4. Comparison between immediate and delayed HRWR addition modes for different PC types (at 0.135% dosage) in CPB prepared with tailings T1 at 80% solids content with 4.5% S-GU binder; (a) slump values and (b) UCS values.

D. Ouattara et al. / Construction and Building Materials 181 (2018) 59–72

Fig. 5. Variation in slump values for CPB mixtures proportioned with tailings T1 at 80% solids content and 4.5% S-GU binder and incorporating different dosages of PC1-, PC2-, and PC3-type HRWR.

3.4. Coupled effect of HRWR and binder content on CPB consistency and compressive strength Fig. 7 shows the slump values for CPB prepared with tailings T1 at 80% solids content, 0.121% PC, and various S-GU binder contents Bw of 3.5%, 4.5%, and 6%. The slumps for all the CPB mixtures are

65

more than two-fold greater than those for control (at approximately 75 mm). Moreover, the slump for all CPBs exceeds the lower limit for pipeline transport. Furthermore, the slump appears to increase slightly with increasing binder content Bw. Fig. 8 a, c, and e show the variation in UCS for CPB mixtures prepared at 80% solids content and incorporating the three PC types for S-GU binder contents Bw of 3.5%, 4.5%, and 6%, respectively. Fig. 8b, d, and f show comparisons between the UCS for CPB mixtures containing HRWR and control mixtures after 28 days of curing. For each binder content, the results show higher UCS for CPB containing HRWR compared to control, regardless of PC type. For example, the CPB mixture containing 3.5% S-GU binder and 0.121% PC1 developed higher UCS compared to control by 152%, 46%, 29%, and 15% after 7, 14, 28, and 56 days of curing, respectively. The UCS improvement was 50%, 36%, 29%, and 24% for 4.5% binder content, and 69%, 12%, 16%, and 33% for 6% binder content. The overall trend is that PC1 appears to produce higher UCS at early age (7 days of curing) compared to control, whereas PC2 and PC3 showed a certain retardation effect on early-age UCS development. As expected, the highest UCS was obtained with 6% binder content and the lowest with 3.5% binder content, regardless of PC type. 3.5. Effect of HRWR type on CPB consistency and compressive strength The performance of a fourth polycarboxylate (PC4), one polymelamine sulfonate (PMS), and one polynaphtalene sulfonate

Fig. 6. Variation in UCS for CPB mixtures proportioned with tailings T1 at 80% solids content and 4.5% S-GU binder and incorporating different dosages of a) PC1, b) PC2, and c) PC3 at curing times of 7, 14, 28, and 56 days.

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Table 8 Relative UCS increase (in %) at 28 days of curing for different PC dosages. PC type

PC 1 PC 2 PC 3

Dosage (%) 0.09

0.121

0.135

0.153

19.4 10.7 20.1

29.1 23.9 27.9

29.4 27.0 25.0

29.4 24.2 25.7

terms of slump and UCS. Fig. 10 shows the slump results for CPB prepared with tailings T2 at 80% solids content, 4.5% S-GU binder content, and incorporating 0% (control) and 0.121% PC1, PC2, and PC3. The results indicate that for tailings T2, the use of 0.121% PC was insufficient to achieve the minimum targeted slump of 152 mm (6 in.). In contrast to tailings T1, higher PC dosages are needed to achieve a slump that allows easy pumpability. This is probably partly due to the difference in volumetric content (Cv-CPB) between the resulting CPB mixtures (Cv-CPB = volume of solids (binder and tailings)/total volume of CPB) in relation to the difference in specific gravity between tailings T1 (Gs = 3.42) and tailings T2 (Gs = 2.80). Thus, for a given mass of CPB, the use of tailings T2 will produce a relatively higher volumetric content compared to tailings T1, resulting in higher interparticle friction and higher HRWR demand to achieve good consistency. This results in higher consistency for CPB prepared with tailings T1 compared to CPB prepared with tailings T2. For the same CPB solids mass content Cw%-CPB of 80%, the difference in slump between CPB mixtures prepared with tailings T1 and T2 can be explained by the volumetric solids content (Cv-CPB), as follows (Eq. (2)).

C wCPB

C v CPB ¼

Fig. 7. Variation in slump values for CPB mixtures prepared with tailings T1 at 80% solids content, different binder contents, and with different PC types (single dosage of 0.121%).

(PNS) HRWR was assessed, and the results were compared to those for CPB incorporating PC1, PC2, and PC3 HRWR. Fig. 9 shows the initial slump and UCS values for CPB prepared at 80% solids content and 4.5% S-GU binder and incorporating 0% (control) and 0.121% of the five HRWR types after 7, 14, 28, and 56 days of curing. The results show that the CPB mixtures prepared with PNS and PMS HRWR at 0.121% dosage showed a slight increase (around 5%) in slump compared to control. In contrast, the use of PC HRWR resulted in at least a 115% slump increase, regardless of PC type. CPB mixtures incorporating PC HRWR showed lower UCS values (due to a slight retardation effect) at early age (7 days) compared to both control and CPB incorporating PMS and PNS HRWR, except for PC1. After 14 days of curing, the CPB incorporating PC HRWR showed higher UCS than both control and CPB incorporating PMS and PNS HRWR. The compressive strength development is similar for all HRWR types (Fig. 9). Furthermore, CPB prepared with HRWR showed 20% higher UCS than control after 28 days of curing. After 56 days of curing, the CPBs incorporating PC4, PMS, and PNS HRWR showed higher UCS than CPBs incorporating PC1, PC2, and PC3 HRWR. The strength improvement for CPB incorporating PNS and PMS HRWR is probably due to the progressive dispersion effect, which contributes to improve the hydration kinetics. Furthermore, the sulfate content in the PNS and PMS HRWR can contribute to the hardening process (precipitate in the form of gypsum), for a beneficial effect on CPB strength development. Fan et al. [31] reported some improvement in compressive strength for backfill at 72% solids content and incorporating 0.3% by dry mass of PNS HRWR binder. In this case, the 0.3% dosage allowed gravity flow transport and placement of the backfill. 3.6. Effect of PC-type HRWR on the compressive strength of CPB made with tailings T2 Tailings T1 (Gs = 3.42) was used to obtain the results presented in the previous sections. Tailings T2 (Gs = 2.80) was used to assess the impact of the tailing characteristics on HRWR performance in

h

i ¼ ð1 þ wCPB  GsCPB Þ1

C wCPB þ qsCPB qw
1   1 ¼ 1þ  1  GsCPB C wCPB 1C wCPB

ð2Þ

and

qsCPB ¼



qsb  qst  ð1 þ Bw Þ 1 Bw ¼ ð1 þ Bw Þ þ qsb þ qst  Bw qst qsb

1 ð3Þ

where qs-CPB (g/cm3) is the specific density of the backfill solids (tailings and binder), wCPB is the gravimetric water content of the backfill [note that C w ¼ ð1 þ wÞ1 ], Gs-CPB is the specific gravity of the backfill, Bw is the binder content (=mass of binder/dry mass of tailings), qs-t is the specific density of the tailings (g/cm3), and qs-b is the specific density of the binder (g/cm3), calculated using Eq. (4).

qsb ¼



0:2

qsGU

þ

0:8

qsSl

1 ð4Þ

where, qs-GU is the specific density of the GU binder (g/cm3) and qs-Sl is the specific density of the slag binder (g/cm3). The calculated volumetric solids content Cv-CPB is 0.54 and 0.59 (or 54% and 59%) for CPB prepared with tailings T1 and T2, respectively. This means that for the same solids mass content Cw%-CPB of 80%, CPB prepared with tailings T2 has a higher solids volume than the CPB prepared with tailings T1. This suggests that the HRWR dosage should be considered exclusively in volume rather than mass. Fig. 11 shows a comparison of the strength development for CPB prepared with tailings T1 and T2. The results indicate almost similar UCS values for the control mixtures prepared with tailings T1 and T2. This could be explained in part by the fact that the two tailings types have similar grain size distribution (see Fig. 1 and Table 3). The UCS for CPB mixtures prepared with tailings T2 and incorporating PC-type HRWR increased compared to the UCS for the control CPB, for all curing times, except when using PC3 (see Fig. 11c) after 7 and 14 days of curing. Comparing CPB prepared with tailings T1 and T2, the results show about 15% higher UCS values for T1 incorporating PC1, for all curing times, whereas PC2 obtains lower UCS values. Using PC3, UCS values are higher for T1 up to 28 days of curing. However, this trend reverses after 56 days. In all cases, the UCS differences for a given PC type for tailings T1 and T2 are relatively small. These

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67

Fig. 8. Compressive strength development with curing time (a, c, and e) and comparison of UCS values after 28 days of curing (b, d, and f) for different CPBs proportioned with tailings T1 at 80% solids and 3.5% S-GU (a and b), 4.5% S-GU (c and d), and 6% S-GU (e and f), and incorporating 0% (control) and 0.121% PC1, PC 2, and PC3 dosage.

Fig. 9. Variation in a) initial slump and b) UCS for CPB mixtures prepared with tailings T1 at 80% solids mass content and 4.5% S-GU-binder content and incorporating 0% (control) and 0.121% dosage of different HRWR types after 7, 14, 28, and 56 days of curing.

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300 Standard slump (mm)

type and solids content to the UCS of CPB and the potential for reduced binder dosage.

T1-CPB T2-CPB Lower slump limit Upper slump limit

4.1. Relationship between HRWR addition and the water-to-binder ratio (w/b) of cured CPB

250 200

Fig. 12 summarizes the gravimetric water content (w) measured on CPB specimens after compression testing and the corre-

150 100 50 0 Control

PC1

PC2

PC3

Fig. 10. Comparison of slumps for CPB prepared with tailings T1 and T2 at 80% solids content, 4.5% S-GU binder, and incorporating different PC types at 0.121% dosage.

preliminary results indicate that the tailings characteristics have a paramount influence on the HRWR contribution to CPB strength development. Further investigations will be conducted to explore the mechanisms involved in the effects of PC type. 4. Discussion The impacts of HRWR on the water-to-binder ratio (w/b) of CPB are discussed, along with the individual contributions of HRWR

sponding calculated solids mass content (C w% ¼ 100  ð1 þ wÞ1 ) for fresh and hardened CPB mixtures investigated in Phase II (Table 7). The results show that for all CPB formulations, the hardened specimens exhibit reduced water content compared to the corresponding fresh CPB specimens (initial state), whereas the solids content of hardened CPB increases by 3% on average. Fig. 13 summarizes the w/b ratios for the CPB mixtures investigated in Phase II, calculated using Eq. (1). The results show that the w/b decreases with curing time for all CPB formulations, resulting in higher UCS. This reduced water content can be explained by the binder hydration and self-weight consolidation, which led to the accumulation of some bleed water on the top of the mold (this water was removed before compression testing). The effect of self-weight consolidation on CPB strength development is well documented in the literature [32,33]. The use of PC1, PC2, and PC3 resulted in comparable reductions in water content, except for the CPB mixture prepared with 6% binder, where PC1 appears to achieve the highest reduction.

(b)

(a) 4000 3500

4000 T1-Control

T2-Control

T1- PC1

T2-PC1

3500

T2-Control

T1- PC2

T2-PC2

3000 UCS (kPa)

UCS (kPa)

3000

T1-Control

2500 2000 1500

2500 2000 1500

1000

1000

500

500

0

0 7

14 28 56 Curing time (Days)

7

14 28 56 Curing time (Days)

(c) 4000 3500

T1-Control

T2-Control

T1- PC3

T2-PC3

UCS (kPa)

3000 2500 2000 1500 1000 500 0 7

14 28 56 Curing time (Days)

Fig. 11. Comparison of UCS values for CPB prepared with tailings T1 and T2 at 80% solids mass content, 4.5% S-GU binder, and incorporating different PC types at 0.121% dosage: a) PC1, b) PC2, and c) PC3.

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69

Fig. 12. Gravimetric water content w (%) and corresponding solids content Cw% for CPB mixtures prepared with tailings T1 at 80% solids content and incorporating 0.121% HRWR (Phase II, Table 7).

4.2. Contributions of solids content and HRWR to the UCS of CPB The CPB mixture prepared at 75% solids content 4.5% S-GU binder and without HRWR achieved an adequate slump (168 mm). This CPB formulation was defined and considered as a reference CPB (RE). The contribution of higher solids content (Cw) to UCS development was determined for CPB mixtures prepared at 80% solids content and 4.5% S-GU binder without HRWR (control mixture) compared to the reference mixture (RE). For a given CPB age, the contribution of solids content (D%UCSCw ) can be calculated using Eq. (5):

D%UCSCw ¼

UCSControl  UCSreference  100 UCSreference

ð5Þ

where UCScontrol is the measured UCS for the control CPB specimen prepared at 80% Cw and 4.5% Bw and without PC, and UCSreference is the UCS for the reference CPB specimen RE (75% Cw, 4.5% Bw, and without PC). The contribution of HRWR addition to UCS increase was determined by comparing the UCS for the control CPB (80% Cw, 4.5.% Bw, and without PC) with the UCS for CPB specimens incorporating 0.121% dosage of PC (PC1, PC2, and PC3) at different curing times

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Fig. 13. Water-to-binder ratio (w/b) for CPB mixtures prepared with tailings T1 with 80% solids content and 0.121% HRWR dosage (Phase II, Table 7).

of 7, 14, 28, and 56 days. The percentage increase in UCS due to HRWR addition (D%UCSPCi ) at a given curing age was calculated using Eq. (6):

D%UCSPCi ¼

UCSPCi  UCScontrol  100 UCScontrol

ð6Þ

where UCSPCi is the UCS determined for the CPB specimens incorporating 0.121% PCi, and UCScontrol is the UCS value determined for the control CPB. Fig. 14 shows the results in terms of UCS improvement, indicating that increased solids content and HRWR addition improved the strength development. For example, the increase in solids content from 75% to 80% (a 7% increase) resulted in 60%, 55.4%, 27%, and 16% increases in UCS after 7, 14, 28, and 56 days of curing, respectively (Fig. 14). Furthermore, when incorporating 0.121% of PC, UCS rose further, for additional increases of 50%, 36%, 29%, and 25% after 7, 14, 28 and 56 days of curing, respectively. In general, PC2 and PC3 HRWR obtained lower additional increases in UCS. However, PC1 appears to induce the highest additional UCS increase at the early age of 7 days, whereas PC2 and PC3 are more effective beyond 7 days of curing. 4.3. HRWR and the potential for reduced binder content Fig. 15 shows that the CPB mixtures prepared at 80% solids content and 3.5% S-GU binder, and incorporating 0.121% dosage of PC1, PC2, and PC3 developed comparable UCS values to the reference CPB (75% solids content, 4.5% S-GU binder, without HRWR) at

28 days of curing. Thus, at 28 days, the UCS for the reference CPB is 1569 kPa, whereas the UCS values for CPB prepared at 80% solids content and 3.5% S-GU binder and incorporating 0.121% dosage of PC1, PC2, and PC 3 HRWR are 1648 kPa, 1535 kPa, and 1542 kPa, respectively. Based on the slump and UCS results presented in Figs. 3, 7, and 8, a 0.121% dosage of any PC should allow reducing the binder content by about 1% for CPB prepared with tailings T1. In a typical underground backfill operation, the required 28-day UCS is in the range of 700–2000 kPa [2]. Considering a UCS of 1000 kPa as the commonly required strength after 28 days of curing, the UCS values for the reference CPB and the CPB mixtures incorporating HRWR and prepared with 3.5% S-GU binder have exceeded this target (Fig. 15). From an economic standpoint, these results indicate that the addition of HRWR would allow reducing the binder content by about 1% for CPB prepared with tailings T1. If the cost savings for the unused binder exceeds the costs of the HRWR that is used, it would be economically advantageous to incorporate HRWR in the CPB formulation. In terms of environmental concerns, the CPB mixtures proportioned at 80% solids content would allow storing slightly more tailings underground compared to the reference formulation (for the same stope volume). This could decrease the amount of waste in the tailings storage facility and reduce the environmental impacts. This would then defer future capital expenditures associated with tailings facility management and remediation [34,35]. Moreover, the smaller amount of interstitial water in the CPB having 80% solids content compared to the reference CPB (75% solids content) could provide an additional

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71

Fig. 14. Decoupled contributions of solids content and HRWR to UCS improvement for CPB mixtures prepared with tailings T1 at 80% solids content, 4.5% S-GU binder, and 0.121% dosage of PC: a) PC1 b) PC2, and c) PC3.

strength (UCS) of CPB mixtures prepared at 80% solids mass content (Cw) were investigated. Based on the results presented in this paper, the following conclusions can be drawn:

Fig. 15. 28-day UCS values for the reference CPB (75% solids content, 4.5% S-GU binder, without HRWR) and CPB prepared with tailings T1 at 80% solids content (3.5% S-GU binder; 0.121% dosage of PC1, PC2, and PC3 HRWR).

environmental benefit: the savings on water could be highly beneficial in certain situations, especially in arid regions with extremely limited water resources [36].

5. Concluding remarks The effects of different types of high-range water reducer (HRWR) on the consistency (slump) and unconfined compressive


Although incorporating HRWR using the delayed addition mode induces relatively higher slump compared to the immediate addition mode, the UCS for CPB prepared using the delayed addition mode is comparable to that for CPB prepared using the immediate addition mode.
Increased HRWR dosage improved the CPB consistency, regardless of the solids and binder contents. Furthermore, the slump and UCS for CPB incorporating HRWR improved when the binder content was increased from 3.5 to 6%. The maximum UCS was obtained with a PC dosage of 0.121%, regardless of curing age. This dosage appears to produce optimal binder hydration for CPB mixtures prepared at 4.5% S-GU binder.
PC-type HRWR resulted in relatively higher slump compared to PNS- and PMS-type HRWR. However, comparable UCS values were obtained, regardless of HRWR type.
During CPB curing, the water content (w) and water-to-binder ratio (w/b) decreased, whereas the solids content (Cw) increased from fresh to hardened state. These changes in Cw, w, and w/b can be partly explained by the strength development of the CPB.
The use of HRWR in CPB formulations can potentially allow a reduction in binder content of about 1% (from 4.5% to 3.5%) when tailings T1 are used. This can be advantageous if the cost of the saved binder offsets the cost of the HRWR. In addition,

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environmental benefits associated with water savings and a relatively increased volume of tailings returned underground could justify the extra cost of the HRWR. 6. Conflict of interest None. Acknowledgements The authors would like to acknowledge the helpful financial support of the NSERC, the Centre de recherche sur les infrastructures en béton (CRIB), and the Fonds institutionnel de recherche et de création (FIRC) of the Université du Québec en AbitibiTémiscamingue (UQAT). The authors would also like to thank Lafarge-Holcim Inc. (formerly, Lafarge North America Inc.) for kindly providing the Portland cement and the supplementary cementitious materials used in this study, and Euclid Canada and Sika Canada Inc. for kindly providing the HRWR. References [1] T. Belem, M. Benzaazoua, Design and application of underground mine paste backfill technology, Geotech. Geol. Eng. 26 (2008) 147–174. [2] F.W. Brackebusch, Basics of paste backfill systems, Min. Eng. 46 (1994) 1175– 1178. [3] R.J. Mitchell, R.S. Olsen, J.D. Smith, Model studies on cemented tailing used in mine backfill, Can. Geotech. J. 19 (1982) 14–28. [4] M. Benzaazoua, J. Quellet, S. Servant, P. Newman, R. Verburg, Cementitious backfill with high sulfur content physical, chemical, and mineralogical characterization, Cem. Concr. Res. 29 (1999) 719–725. [5] E. Yilmaz, M. Benzaazoua, T. Belem, B. Bussière, Effect of curing under pressure on compressive strength development of cemented paste backfill, Miner. Eng. 22 (2009) 772–785. [6] S.J. Jung, K. Biswas, Review of current high density paste fill and its technology, Miner. Resour. Eng. 11 (2002) 165–182. [7] M. Benzaazoua, M. Fall, T. Belem, A contribution to understanding the hardening process of cemented pastefill, Miner. Eng. 17 (2004) 141–152. [8] D.A. Landriault, R. Verburg, W. Cincilla, D. Welch, Paste technology for underground backfill and surface tailings disposal applications, Technical workshop, Vancouver, British Columbia, Canada, 1997, pp. 120. [9] A. Kesimal, E. Yilmaz, B. Ercikdi, Evaluation of paste backfill mixtures consisting of sulphide-rich mill tailings and varying cement contents, Cem. Concr. Res. 34 (2004) 1817–1822. [10] F.P. Hassani, J.F. Archibald, Mine backfill handbook, CIM (CD-ROM), Montreal, Quebec, Canada, 1998. [11] E. Yilmaz, T. Belem, B. Bussière, M. Benzaazoua, Relationships between microstructural properties and compressive strength of consolidated and unconsolidated cemented paste backfills, Cem. Concr. Compos. 33 (2011) 702– 715. [12] T. Belem, O. El Aatar, B. Bussière, M. Benzaazoua, Gravity-driven 1-D consolidation of cemented paste backfill in 3-m-high columns, Innov. Infrastruct. Solut. 1 (2016). [13] ASTM C143/C143M – 15a, Standard test method for slump of hydrauliccement concrete, 2015. [14] D.A. Landriault, Paste backfill mix design for Canadian underground hard rock mining, Proc. CIM Underground Operators Conf. Timmins, 1995.

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