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Effects of coupled sulphate and temperature on internal strain and strength evolution of cemented paste backfill at early age
Construction and Building Materials 230 (2020) 116937
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Effects of coupled sulphate and temperature on internal strain and strength evolution of cemented paste backfill at early age Baoxu Yan, Wancheng Zhu ⇑, Chen Hou, Yongjun Yu, Kai Guan Center for Rock Instability and Seismicity Research, School of Resource and Civil Engineering, Northeastern University, Shenyang 110819, China
h i g h l i g h t s Internal strain of sulphated CPB that related to strength development can be considered as a hydration process indicator. Sulpahte concentration and curing temperature can significantly affect the evolution of internal strain in CPB. Final internal strain is an important indicator for the barricades stability as well as mining cycle.
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Article history: Received 27 May 2019 Received in revised form 15 August 2019 Accepted 9 September 2019
Keywords: Cemented paste backfill Sulphate concentration Hydration Shrinkage Fiber Bragg Grating
a b s t r a c t Cement hydration is a volume reduction reaction which can result in negative pore water pressure while strengthening the structural stiffness of cemented paste backfill (CPB). Both sulphate and temperature significantly affect the hydration process and thus influence the internal volume change and mechanical properties of CPB. The expansive hydration products produced in sulphated CPB may create an increase in volume which subsequently weakens the CPB. This paper presents an experimental study conducted to evaluate the coupled effects of sulphate and curing temperature on the internal strain evolution of early aged CPB. CPB samples of various initial sulphate concentrations (0, 5000 and 25,000 ppm) were prepared and cured at various temperature (20 °C and 35 °C) at early ages (7 days). Mechanical tests were conducted on the samples as internal strain was monitored by Fiber Bragg Grating (FBG) sensors. The results indicate that the FBG sensor is sufficiently sensitive to detect the internal strain development of sulphated CPB mixtures during the hydration process. At sulphate concentration of 25,000 ppm, the expansive strain occurred at curing temperature of 35 °C due to the internal expansive force of the expansive materials. The final internal strain value corresponds closely to the strength of the sulphated CPB. As the internal strain of sulphated CPB decreased, the uniaxial compressive strength and tensile strength showed similar trends throughout the experiment. When expansive strain emerged after the initial thermal strain stage, however, there was a higher ratio of uniaxial compressive strength to tensile strength. This phenomenon merits careful consideration in the stability analysis of exposed CPB in underground mines. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Cemented paste backfill (CPB) has been increasingly widely used around the world in recent years [1–4]. The main advantages of CPB material are the economic and environmental benefits of its high recovery ratio, usage efficiency [5] and elimination of tailing dam failure risk [6–9]. In addition, the flotation method used in mineral processing can produce sulphide tailings, which can lead to the generation of acid mine drainage when they are disposed ⇑ Corresponding author. E-mail addresses: [email protected] (B. Yan), [email protected] (W. Zhu). https://doi.org/10.1016/j.conbuildmat.2019.116937 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
under atmospheric conditions. Relatively speaking, it is a safe and secure manner of storing tailings into underground stopes by using the CPB technology [10]. CPB is an engineered mixture of dewatered processing tailings, a hydraulic binder, and either fresh or mine-processed water which together form a high viscosity paste [11,12]. Its fine content is typically more than 15% by weight passing 20 lm; its solids content typically ranges from 78 to 85% [13]. Depending on the ultimate function of the backfill, CPB is prepared with the binder content varying from 3 wt% to 7 wt% [14,15]. To increase the overall performance of the prepared backfill, alternative binders and fibers are used [16,17]. CPB strength and deformation are markedly influenced by the material’s sulphate concentration. The sulphate contained in
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cement affects the quality and quantity of C-S-H gel during the hydration process [18,19] and thereby has potentially damaging effects on the quality of cement-based materials [20]. Sulphate ‘‘attacks” on concrete and mortar are often external [21], but is essentially internal for CPB materials. There are so many sulphate sources in the CPB production process including (i) the oxidation of the sulphate minerals, (ii) the sulphate dioxide or air method for gold mining [22], and (iii) the mine-processed water to the cement. Existing mechanical strain gauges have no gauge-points that can be established before the CPB is hardened, which makes experiments in the plastic stage infeasible. This is the primary issue at hand in assessing the evolution of internal strain and hydrated temperature of any type of CPB material [23]. The Fiber Bragg Grating (FBG) sensor, conversely, is quasi-feedback free and with very high resolution [24]. The device is capable of measuring signal not influenced by sulphate or water [25]. It can be embedded directly into the CPB specimen and is non-invasive due to its small size [26]. Investigations have been conducted on the early-age shrinkage and temperature evolution of cemented paste using FBG sensors [23–25,27,28]. However, there has been no previous study on early age CPB containing different initial sulphate concentrations and at various curing temperatures using FBG sensors. In this study, fiber optic sensors were adopted to evaluate the effects of coupled initial sulphate concentration and curing temperature on the strength and internal strain development of CPB. The internal strain and temperature changes were obtained instantaneously after casting until an age of 7 days over which time the contrast increased. The contents of hydrated products and other ingredients were also assessed by thermal gravity (TG) and differential thermal gravity (DTG) analyses. The proposed method can be used to predict the hydration process and eventually the trends and approximate values of UCS and tensile strength (BTS). 1.1. Background The mechanical stability of the CPB structure within a given amount of time is an important criterion for its quality control. CPB must satisfy static and dynamic load resistance requirements within a certain range to ensure its self-stability and to maintain safety in the underground working stope [6,29–31]. Uniaxial compressive strength (UCS) is the most commonly assessed geotechnical property in practice due to its inexpensiveness and convenience [32]. Generally, minimum target values of CPB strength are considered to be >150 kPa for minimizing the risk of liquefaction of underground disposal or fill voids at an early stage. When CPB is used as an artificial sill pillar, its strength should be more than 4 MPa [6]. It is of great practical significance to accurately predict the behavior of sulphated CPB at early ages, particularly in large underground stopes [33]. During the initial usage period, CPB maturing is a complex physicochemical and environmental phenomenon that influences the ultimate strength and any nonuniform deformation of the material. In addition, curing conditions (time and stress) as well as placement configurations are also important and play a significant role in reaching superior backfill performance [34–36]. The sulphate concentration effects are dependent on curing temperature, as temperature determines the dissolution speed of the anhydrous clinker phases as well as the quality of calciumsilicate-hydrate (C-S-H) and expansive minerals, and thus the binding force between the particles and the CPB’s mechanical properties [37–39]. Within the deep mining context, elevated curing temperatures due to deep rock temperature and the accumulated heat of larger stopes from the massive backfill body as well as the potential addition of heat to the CPB (The initial temperature in CPB and the generated by friction in the pipeline) [40,41]. Com-
pared to standard curing (A constant temperature 20 °C and humidity RH > 95%), more heat is generated by the precipitation of quantities of hydration products [42]. In addition to their impact on CPB strength, sulphate and temperature also affect the self-desiccation process; this is mainly due to the internal water ‘‘sink” and dissipation which occur in all types of cementitious materials [39]. When CPB does not yet contain sulphate, suction develops and the cement matrix continually shrinks. Although the shrinkage volume is small, it is recognized as the origin of the material’s strength and the indicator of phase transition (fluid, skeleton formation, and hardening) in cemented materials. This is important in terms of the early age mechanical properties of CPB [26]. Once sulphate is introduced to produce CPB, the sulphate ions affect the hydration process and thus influence selfdesiccation which drives an internal strain evolution within the material. In addition to inhabiting the hydration process, sulphate addition causes the participation of expansive minerals, such as gypsum and ettringite, which place pressure on the pore wall and stretch the pore structure. Many previous researchers have investigated the strength of CPB materials at different initial sulphate concentrations [1,5,6,22,33,43,44]. Extant studies have demonstrated that sulphate significantly impacts the mineralogical composition, and thus strength of CPB; various factors may increase or decrease the strength. The binder hydration of CPB inhibited by additional sulphate ions is the primary negative effect on its strength. Another negative factor is the absorption of sulphate by C-S-H resulting in lower quality C-S-H gel formation [33]. These factors reduce hydration production and thus decrease the binding force between the particles of mine tailings. Two important sulphate reactions exist in the cement system leading to the formation of the expansive minerals – ettringite and gypsum – which form the source of the aforementioned positive effects and are greatly influenced by the curing temperature [44]. If the precipitation of expanded minerals does not create an excess of empty pores, it can refine the pore structure and enhance the material’s strength. The amount of expansive minerals is a key factor in CPB strength; thus the sulphate concentration and curing temperature are crucial considerations in the design of CPB materials. In-situ strength values are usually 2–4 times higher than laboratory strength values for any given CPB recipe or curing time [45,46]. Field simulation experiments have shown that CPB insitu strength is characterized by gentle humps that appear in the backfilled stope height [47]. These differences in strength can be explained in part by the curing conditions [48,49]. Previous studies have centered on the effects of sulphate and curing temperature on the long-term strength of CPB (28 days) [43,44] while few have explored the effects of coupled sulphate and curing temperature on the early strength development of CPB (1, 3, and 7 days). To the authors’ knowledge, no previous researcher has investigated the coupled initial sulphate concentration and curing temperature effects on the internal strain and hydrated temperature development simultaneously at early ages; this process is directly associated with the strength and damage characteristics of the hydration process. The principal cause of failure in backfill under the impact of blast vibrations is tensile stress [50], and so the magnitude of tensile strength is also particularly important, as is its relation to internal strain development. 2. Materials and methods 2.1. Materials 2.1.1. Tailings Artificial silica tailings were used in the experiments (i) for eliminating the uncertainty of minerals and chemical elements that may exist in natural tailings, which may affect the process of hydration on CPB mixture, and thus the under-
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B. Yan et al. / Construction and Building Materials 230 (2020) 116937 standing of the experimental results; and (ii) to reflect the typical cumulative grain size distribution range of paste tailings in gold mines of Shandong Gold Group Co., ltd. The silica tailings consist of almost 99.8 wt% quartz and possess about 42 wt% fine particles (diameter less than 20 lm). The grain size distribution of the silica tailings is shown in Fig. 1 as-measured by MASTERSIZER 2000. The main physical characteristics and chemical composition of the silica tailings are listed in Tables 1 and 2. 2.1.2. Binder Portland Cement Type I (PCI) was used as the binder in all samples. Table 3 shows the main chemical composition and physical properties of PCI. 2.1.3. Mixing water Tap water of the chemical composition shown in Table 4 was used to mix the silica tailings and binder. To ensure a specific sulphate concentration (0, 5000, and 25,000 ppm, ppm is an abbreviation of parts per 2 in the CPB mixture, ferrous sulfate (FeSO47H2O) was added to the tap water [22]. 2.2. Sample preparation and curing conditions With a water to cement ratio of 5, a binder content of 5 wt%, and the required tailing quantity, the binders were first mixed for about 10 min before adding water with different sulphate concentrations and mixing for a further 10 min to ensure the homogeneity of CPB mixture. The slump of the mix does not exceed 18.6 cm. The CPB mixture was poured into the Plexiglas cylindrical mould 50 mm in diameter, 102 mm and 30 mm in height respectively. Entrapped air was removed by manual vibration and the mix was stirred with an iron bar to evenly distributed and fill it into the mould. Each CPB sample was sealed in the mould by a plastic cover and tape to prevent water evaporation and cure under undrained condition, as any reduction in internal relative humidity (RH) would influence the rate of cement hydration and final test result [49]. A cement slurry with a water to cement ratio of 5:1 is so diluted that segregation and bleeding would readily occur, so a constant water to cement ratio of 2 [5] was ensured for the microstructural analysis of samples prepared via the same procedure described above. A larger plexiglass square mould (200 mm width, 400 mm length, 100 mm height) equipped with a pair of FBG strain and temperature sensors was also used for monitoring tests. The prepared CPB mixtures were filled into the mould and stirred with a steel bar for compaction and removal of voids and to ensure effective bonding to the FBG sensors. Once the CPB mixtures were cast, they were cured and sealed in the mould by a plastic cover in a controlled room with a designated curing temperature (25 °C and 35 °C) and RH of 70% to model the environment of the backfilled stope [6,51]. The shrinkage and internal temperature changes were monitored immediately, simultaneously after casting till an age of 7 days. 2.3. Testing method 2.3.1. Mechanical tests At the age of 1, 3, and 7 days after casting, UCS and BTS tests were performed on the sulphated CPB specimens according to ASTM C 39 on an electronic testing machine (Humboldt HM-5030) to determine the effect of coupled curing temperature and initial sulphate contents. The loading capacity is 50 kN and the loading rate is 1 mm/min. Before the test, the ends of the sample were controlled for flatness and parallelism with a grinding machine; the parallelism between the surfaces was checked at less than 0.05 mm. The UCS value is considered here to be the max-
Table 1 Physical characteristics of silica tailings. Element
Value
Specific gravity/Gs Specific surface area/cm2/g D10/lm D30/lm D60/lm Coefficient of uniformity Cu = D60/D10 Coefficient of curvature Cc = D230/(D60 D10)
2.7 3593 2.23 10.6 39.4 17.67 1.28
imum stress value. The BTS value was calculated according to the peak load observed during the test. Each mechanical test was carried out at least three times to ensure its repeatability. 2.3.2. Thermo-gravimetric analysis TG and DTG analyses were conducted on the early age CPB samples to investigate the effects of sulphate on their microstructural characteristics. In an inert nitrogen atmosphere, about 25 mg of ground CP powder was placed in an alundum crucible and covered with an alundum lid. The weight loss of the sample was recorded simultaneously at a rate of 10 °C/min up to 1000 °C during thermal treatment. Prior to testing, as per the effects of initial sulphate content, the constant mass of a CP sample was determined by removing free water in a dying room at 50 °C. 2.3.3. FBG sensor installation and testing The FBG sensor is an effective means of monitoring the cement hydration process and tracking its internal structural changes during the hydration process [25]. The FBG has the characteristic to reflect a specific light with a certain wavelength. when the interrogator emits a broadband source of light into the optical fiber, the interaction between the external incident photons and the doped particles in the fiber core leads to periodic or permanent changes of fiber core refractive index along the fiber axis. This then leads to the formation of spatial phase grating in the fiber core. This grating center wavelength is related to the effective core index of refraction and the periodicity of the index modulation. When there is a strain on FBG sensor because of stretching due to hydration of the CPB, the periodicity of the index modulation is changed, and photoelastic effect can also lead to the change of the effective core index of refraction. Therefore, the strain evolution in CPB can be obtained through temperature compensation by adding an additional FBG in the same temperature field [28]. Other researchers [23] have demonstrated that the FBG sensor has high precision and sensitive components to detect the strain of early stage cemented mixture. Accounting that the CPB materials is a mixture of cement and tailings, thus these sensors are suitable for detecting the early aged internal strain evolution of sulphated CPB. A total of 12 FBG sensors were fabricated at the JEMETECH fabrication facility with Bragg wavelengths spanning the spectral region between 1510 nm and 1590 nm. The sensors have a reflectivity of 99%, range of about ±2000 le, and sensitivity coefficient of 1.15–1.53 pm/le for the strain sensor and about 30 to 120 °C with a sensitivity coefficient of 10.5–11.3 pm/ °C for the temperature sensor, respectively. The following assumptions were made as the FBG sensors were embedded into the CPB specimens [23,28]. (i) The interfacial slippage between the FBG sensor and CPB is negligible. (ii) The fiber grating region does not suffer damage or deterioration. As a precondition for obtaining an accurate internal strain measurement, in this study, the semicircular fixed ends were arranged on both sides of the strain grating (Fig. 2) so that no slippage occurred between the fiber grating and CPB mix. Before casting CPB mixtures into the mould, FBG strain and temperature sensor were installed horizontally at the center of, and parallel to, the mould (Fig. 2). The FBG strain sensor was directly contact with CPB mixture, whereas the temperature sensor was placed inside a rigid pipe to isolate them from contacting the CPB mix. In this way, the temperature sensors were not influenced by volume changes in the CPB and simply monitored the temperature inside the specimens. The final setup is shown in Fig. 2. The FBG system takes all FBG sensor measurements together at a rate of one reading every 1e 3 s. In the experiment, it was set for logging data every 30 s at early age (3 days) and afterward (7 days) for every 10 min. The data was transmitted to the interrogator and computer for processing and analysis according to a previously published method [23].
3. Results and discussion 3.1. Coupled effect of sulphate and temperature on early age CPB strength development
Fig. 1. Grain size distribution of silica tailings.
3.1.1. UCS of sulphated CPB at early age As shown in Fig. 3a, b, the strength of the sulphate and sulphated free CPB samples are less than 200 kPa at curing tempera-
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Table 2 Main chemical composition of silica tailings. Composition
SiO2 (wt.%)
CaO (wt.%)
Fe2O3 (wt.%)
Al2O3 (wt.%)
MgO (wt.%)
K2O (wt.%)
LOI (m2/g)
PH
Silica tailings
99.8
0.01
0.035
0.05
< 0:01
0.02
0.1
7
Table 3 Main chemical composition and physical properties of PCI Composition
SiO2 (wt.%)
CaO (wt.%)
Fe2O3 (wt.%)
Al2O3 (wt.%)
MgO (wt.%)
SO3 (wt.%)
K2O (wt.%)
SSA (m2/g)
Relative density
PH
PCI
21.43
62.24
2.57
3.56
1.36
2.62
0.34
1.14
3.12
7
Table 4 Primary chemical composition of the tap water used Element
Fe
Ca
Al
Mg
Na
SO24
PH
Value (ppm)
0.06
40.8
0.09
1.4
6.2
43
7.26
Fig. 2. The detailed experimental setup of the FBG sensor installation and mornitoring.
ture of 20 °C before curing time of 1 day, but less than 500 kPa at curing temperature of 35 °C. The higher curing temperature can accelerate the rate of hydration process at the initial stage. At the same curing temperature, there exists the effect of retardation due to the sulphate concentration at the initial stage. For given a 20 °C curing temperature, the UCS value of sulphated CPB is lower than sulphate-free CPB at early ages. This because the solubility of the tricalcium aluminate (C3A), which is the major contribution to the early strength, is suppressed by the presence of sulfate ions [5], which can retard and inhibit cement hydration. This mechanism is also observable in electrical conductivity tests [22]. Interestingly, in comparison to the 20 °C curing temperature, the UCS of samples with sulphate concentrations of 5000 ppm and 25,000 ppm show higher rate and values of strength increase at curing temperature of 35 °C after 3 days (Fig. 3b). The UCS value of the 25,000 ppm sample was 52% and 36% higher than that of 0 ppm and 5000 ppm samples at curing temperature of 35 °C cured at 7 days, respectively. This shows that sulphate concentration can retard the cement hydration, and the process provide the cohesion force between particles (As Fig. 4b shows). However, at the curing temperature of 35 °C after 3 days, an appropriate amount of expansive minerals emerged within this CPB sample, which likely led to infilling and refinement of the pore structure and thus counteracting the negative effect of the large amount of voids and micro pores in the cement matrix as well as the retardation of sulphate. The expansive pressure of components with high initial sulphate contents exert negative effects on the material, but at a certain level,
their refinement effects on the CPB structure can outweigh the negative effects. This is in accordance with the TG and DTG analysis results at curing ages of 7 days (Fig. 9) which are discussed in Section 3.3. 3.1.2. BTS of sulphated CPB at early ages The tensile strength of CPB is an important indicator in the underground mining context, especially in terms of the dynamic disturbance and stress wave refraction area. Two step mining as well as cut and backfill mining are affected by the stress wave reflection at the surface of backfill forming a tensile stress concentration area. If the backfill has low tensile strength, tensile damage and flaking failure may readily occur. The BTS of sulphated CPB under two curing temperature conditions were assessed to investigate the tensile characteristics under various initial sulphate concentrations as shown in Fig. 4a, b. At the 20 °C curing temperature, strength increases with curing age and shows similar trends to those of UCS development. When given a higher curing temperature (35 °C, Fig. 4b), CPB specimen with the sulphate concentration of 25,000 ppm shows the lowest earlyage tensile strength compared to sulphate concentrations of 0 ppm and 5000 ppm. BTS value variations at curing temperature of 35 °C differ markedly from those of UCS development as curing age increases. As shown in Fig. 5, higher sulphated CPB at relative high temperature can produce more expansive materials. The lateral strain of the mould is limited, so deformation of the CPB matrix produced an upward expansive trend induced by the internal expansive
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Fig. 4. Tensile strength of CPB for various initial sulphate concentrations (0 ppm, 5000 ppm, 25,000 ppm) and curing temperatures (20 °C and 35 °C) at early ages (7 days). Fig. 3. UCS of CPB for various initial sulphate concentrations (0 ppm, 5000 ppm, 25,000 ppm) at curing temperature of (a) 20 °C and (b) 35 °C at early ages (7 days).
pressure during the experiment. This process initiated tensile cracks around the pores of the material. To this effect, minimal tensile stress can cause failure in highly sulphated CPB. Despite the fact that the expansive material produced micro cracks, the compaction characteristics become obvious at sulphate concentration of 25,000 ppm. The UCS curves of the highly sulphated CPB sample cured at 35 °C (Fig. 6) evidence this phenomenon as well. The high BTS value at sulphate concentration of 5000 ppm can be attributed to the abundance of hydrated C-S-H gel products, which ‘‘glue” the tailing particles together; this interpretation is indirectly supported by the results shown in Fig. 9d. The expansive material has little impact on the tensile strength of sulphated CPB. It can be concluded that when high initial sulphate content is incorporated into CPB, the tensile strength and ductility can be significantly deteriorated. The results presented above indicate that when high initial sulphate content is incorporated into CPB, the tensile strength and ductility of the material significantly deteriorate. To further investigate the effect of coupled sulphate concentration and curing temperature on strength development, the internal strain must be accurately monitored. The real cohesive strength of the CPB
material is mainly attributable to the hydration process, which affects the volumetric shrinkage. Chemical shrinkage occurs soon after the cement reacts with water accompanied by the net volume loss [52]. Autogenous shrinkage (or self-desiccation effect via the internal water ‘‘sink”) [53] is also induced by the suction development in CPB. The bulk stiffness of the CPB skeleton increases significantly during the hydration process. More suction force is necessary in response to the incremental volume strain. The sulphated CPB volume can expand due to the internal thermal changes as well as the formation of swelling materials (e.g., gypsum and ettringite) which drive further volume expansion. Detailed volume changes in terms of the internal strain in sulphated CPB samples were monitored in this study as discussed below.
3.2. Effect of initial sulphate content and curing temperature on internal strain development 3.2.1. Effect of sulphate content on internal strain development The evolution of internal strain in sulphated CPB (0 ppm, 5000 ppm and 25,000 ppm) at different curing temperatures is shown in Fig. 7. All curves are similar except the 25,000 ppm at curing temperature of 35 °C. The internal strain evolution can be
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Fig. 5. Schematic diagram of sulphated CPB sample under internal sulphate attacks in the cylindrical mould.
Fig. 6. Typical UCS test curves of sulphated CPB samples at curing temperature of 35 °C
divided into four stages: (i) a rest stage wherein the strain field of CPB mix cannot transfer to the FBG sensors; (ii) an expansion stage due to hydration of cement and an increase in temperature with thermal strain; (iii) a rapid shrinkage stage mainly caused by chemical shrinkage and autogenous shrinkage; and (iv) a stable stage characterized by large bulk stiffness through a progressive hydration process where more suction was needed for deformation. Similar phenomena have been observed in concrete and mortors [23]. As the sulphate concentration increased in the experiment, the rate of shrinkage decelerated and little thermal strain occurred at curing temperature of 20 °C, which may be attributed to the sulphate icons inhibiting the hydration process. The DTG results shown in Fig. 9c support this observation, where the peak value of DTG at about 350 °C is significantly lower than at other temperatures; only a small amount of C-S-H gel was produced thus more time was necessary for setting. The shrinkage strain at the stable stage is better correlated with the final strength of all samples except for the sulphated concentration of the 25000 ppm sample at curing temperature of 35 °C. Samples with a high sulphate content (25,000 ppm) show the lowest strength and samples without sulphate content are strongest because an increase in sulphate content makes the CPB pores denser, which gradually increased the capillary water pressure over the course of the experiment. Higher sulphate content also makes the CPB bulk stiffness lower in the same period of time; only a small suction force can produce sufficient deformation [52]. The results also show that when the initial sulphate content is constant and only shrinkage strain appears, a lower final shrinkage
Fig. 7. Evolution of internal strain in CPB for various initial sulphate concentrations (0 ppm, 5000 ppm, 25,000 ppm) at curing temperature of (a)20 °C; (b) 35 °C at early ages ( Note: red circle indicates point at which sensors malfunctioned).
strain remits greater UCS value. The particularities of samples with sulphate content of 25,000 ppm are discussed in detail below. 3.2.2. Effect of curing temperature on internal strain development Regardless of the sulphate concentration conditions, as shown in Fig. 8, each sample underwent internal strain evolution in the first rest stage. The internal strain was subsequently no longer equal to zero during the so-called initial setting time (which is generally found in concrete, cement paste, and mortars) [54]. The ini-
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tial setting time of the sample with 25,000 ppm sulphate concentration and 20 °C curing temperature was relatively long. For 20 °C curing temperature, the initial setting time for 0 ppm, 5000 ppm, and 25,000 ppm was 102 min, 115 min, and 324 min, respectively; for the 35 °C curing temperature it was
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90 min, 79 min, and 224 min, respectively. A higher curing temperature can accelerate the hydration process, and thus facilitate the formation of more cementitious minerals. In the sample cured at 35 °C, the hydration intensified and the shrinkage strain became steady in less time than the sample cured at 20 °C. This is because high temperature accelerates the cement hydration process along with the internal water ‘‘sink”, i.e., enhances the self-desiccation effect thus accelerating the internal water consumption. Due to the high solid content of the CPB mixture, the material’s internal water was quickly depleted under high curing temperature accompanied by slow and steady autogenous shrinkage. Internal water consumption represents the production of hydration products which increase the bulk stiffness of the CPB mixture. In the presence of these products, more negative pore water pressure is required to shrink the solid skeleton, so its value is small in relation to the curing temperature of 20 °C. The rate of hydration is relatively at this curing temperature, however, which slows down the increase in the bulk stiffness of the solid skeleton. To accommodate this, for a given volume strain, only a small corresponding negative pore pressure is needed. When CPB sulphate content was 25,000 ppm in this experiment, after the initial thermal strain stage, the internal strain transformed from shrinkage to expansion at curing temperature of 35 °C. This is because a large amount of ettringite and gypsum were produced in the pores of sulphated CPB under high curing temperature. The TG and DTG results shown in Fig. 9 support this assertion. Excessive internal expansive pressure expands the CPB to produce macro-cracks which were indeed observed in the surface of this sample. As the expansion strain appears, the BTS value decreases and the refinement effect of the expansive material is subject to the pores of the material itself. The internal strain development is in accordance with the final UCS value of sulphated CPB and the UCS/BTS ratio. The quick rate of internal strain variations thus represents an intense hydration process. The proposed method facilitated accurate preliminary predictions of the trends of sulphated CPB strength as per stable and final internal strain values, as well as the intensity of internal cement hydration through the early development of shrinkage strain. The internal strain evolution is directly related to the environment and material composition, so the application of this method in an underground stope may prevent erroneous mechanical parameters caused by the difference between laboratory and in-situ characteristics. 3.3. Results of Thermo-gravimetric analysis
Fig. 8. Comparison of internal strain in CPB for various initial sulphate concentrations (a)0 ppm, (b)5000 ppm, (c) 25,000 ppm at curing temperatures of 20 °C, 35 °C at early ages ( Note: red circle indicates point at which sensors malfunctioned).
The CP and hydration product components can be decomposed over certain temperature ranges during thermal treatment. Three primary peaks signifying weight loss are observable in Fig. 9 at temperature ranges of 50–200 °C, 400–450 °C, and 600–750 °C, respectively. Gypsum and ettringite crystal water usually dehydrate at 100–160 °C, while calcium hydroxide dehydrates at 400– 500 °C. The decomposition of calcium carbonate as well as CH occur at 600–750 °C [1,55]. At the age of 1 day and curing temperature of 20 °C, there is almost no peak in the 25,000 ppm sample at 450 °C but an obvious peak at 650 °C, which indicates that a small amount of C-S-H gel was produced relative to the sulphate-free sample. A small amount of expansive materials, such as gypsum or ettringite, was dehydrated at 100–120 °C which cannot produce excessive internal expansive force and therefore no obvious expansive strain occurred (Fig. 8). The peak value of DTG at 450 °C and 650 °C without sulphate is the highest and 120 °C is the lowest, which suggests that these samples contained the least amount of ettringite and gypsum but the most Ca(OH)2 and CaCO3 among the samples and thus the maximum UCS value. At 35 °C curing temperature,
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Fig. 9. CP samples with sulphate concentrations of 0, 5000, and 25,000 ppm at curing temperatures of 20 °C (a) TG (c) DTG and 35 °C (b) TG (d) DTG at early ages (7 days).
the 5000 ppm sample shows peak values at about 450 °C and 750 °C, i.e., in accordance with the peak values of the sulphatefree sample. In other words, its strength is comparable to that of the sulphate-free CPB sample. At a curing age of 7 days, the weight loss peak of 25,000 ppm at about 120 °C is largest at curing temperature of 35 °C, indicating that a large amount of ettringite and gypsum materials were produced in this sample. These components fill and refine the pore structure. The excessive crystallization or expansive pressure attributed to the ettringite can lead to physical damage, however, and thus decrease the cohesion between particles. This observation is supported by UCS test results and the evolution of the internal strain as shown in Figs. 3 and 7. 3.4. Further discussion Previous researchers have found that high sulphate concentration in CPB can produce a large amount of swelling. It is important to understand how these swellings affect the internal strain or structure of CPB, as this process is directly associated with the solidification and strength development of the material. Li et al. [5] obtained the suction development of slag-CPB samples with various initial sulphate concentrations at early ages under room temperature using a matrix water potential sensor. Doherty et al. [52,56] found that an incremental volume strain in CPB generates a corresponding suction change in numerical simulations. The pre-
sent study was conducted to investigate how the internal strain evolution and is affected by curing temperature and sulphate concentration as sulphated CPB solidifies. The results presented and discussed above suggest that internal strain evolution in sulphate-free CPB are similar to the previous work in terms of the four stages at early ages [26]. However, the internal strain can be transformed from shrinkage to expansion as a large ratio of UCS/BTS emerges when sulphate concentration and curing temperature are high. This is mainly attributed to the competitive effects of expansive materials mentioned above. Comprehensive understanding of this phenomenon may represent workable technical guidance as well as practical implications for CPB stability assessment and design. The early opening of a barricade can decrease mining cycle time, hence increasing mining production and efficiency. This is obviously associated with the time at which CPB begins to solidify, that is, when the internal strain begins to change. The complexity of underground environments make the UCS values obtained in the laboratory generally inconsistent with in-situ values. The internal strain in CPB can be determined to predict the intensity of the hydration process as per rates of shrinkage or expansion. This process is associated with the CPB strength development regardless of the differences between laboratory and in-situ environmental conditions. Further research is yet necessary to determine the correlation of the internal strain of CPB with the deformation of the sur-
B. Yan et al. / Construction and Building Materials 230 (2020) 116937
rounding rock mass, so as to predict indirectly the behavior of the surrounding rock mass through the deformation of the CPB materials. As for FBG sensor type, its working principle is the same. The sensor size selected in this study is relatively small, which can minimize the influence on the CPB’s hardening process in the mould, so as to ensure its high sensitivity. The evolution of the internal strain in CPB is also related to the placement conditions, and this process involves many aspects, such as filling rate, sedimentation, consolidation and curing conditions, therefore, further study should be made to investigate on these aspects.
4. Conclusions CPB samples from different mould sizes containing different initial sulphate concentrations (0 ppm, 5000 ppm, and 25,000 ppm) were prepared in this study and cured in a controlled temperature and RH room at 20 °C and 35 °C, respectively, to investigate the coupled effect of sulphate concentration and curing temperature on internal strain behavior and strength at early ages (7 days). Mechanical tests including UCS and Brazil disk tests were performed to explore the effect of coupled initial sulphate concentration and temperature on CPB behavior. TG analysis was conducted to provide additional information regarding the hydration process. FBG strain and temperature sensors were also embedded into the CPB matrix to monitor the evolution of the internal strain and temperature. The conclusions can be summarized as follows. - The FBG sensor, in comparison to the strain gauge, is sufficiently sensitive to the internal strain evolution of sulphated CPB mixtures during the hydration process regardless of curing conditions. The internal strain in CPB can be a hydration process indicator. - Sulpahte concentration and curing temperature affect the evolution of internal strain in CPB to a great extent. At curing temperature of 35 °C, the sulphate concentration of 25,000 ppm has a large UCS/BTS ratio due to competitive effects in the expansive materials: (i) refining and infilling the pore structures and (ii) micro-cracks induced by the internal expansive force. - The final internal strain value corresponds closely to the strength of the sulphated CPB. When the internal strain of the sulphated CPB shrinks simultaneously, the evolutions of UCS and BTS are similar. When expansive strain occurs, the UCS and BTS may show different trends depending on the internal expansive materials which create micro-cracks due to the internal expansive force. This phenomenon must be carefully considered in analyzing the stability of exposed CPB, as failure is mainly attributable to the material’s tensile strength. The results presented here may provide a new perspective on sulphated CPB materials at early ages to the benefit of both CPB experimenters and mining engineers. It is necessary to monitor the evolution of internal strain and suction at the same time for further work, so as to investigate the internal stiffness in CPB along with hydration reaction process, which will provide an in-depth understanding of backfill hydration process and an important data support for numerical simulation analysis.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements This work is funded by the National Key Research and Development Program of China (Grant No. 2016YFC0801607), National Science Foundation of China (Grant Nos. 51525402, 51874069 and 51761135102), and the Fundamental Research Funds for the Central Universities of China (Grant Nos. N170108028, N170106003, and N180115009). These supports are gratefully acknowledged. Moreover, we thank Mr. Haiqiang Jiang and Jing Han for providing laboratory resources. Also, the authors would like to thank Mr. Maurice Sunkpal and Hailong Wang, Miss Cui Li for their help.
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Effects of coupled sulphate and temperature on internal strain and strength evolution of cemented paste backfill at early age Baoxu Yan, Wancheng Zhu ⇑, Chen Hou, Yongjun Yu, Kai Guan Center for Rock Instability and Seismicity Research, School of Resource and Civil Engineering, Northeastern University, Shenyang 110819, China
h i g h l i g h t s Internal strain of sulphated CPB that related to strength development can be considered as a hydration process indicator. Sulpahte concentration and curing temperature can significantly affect the evolution of internal strain in CPB. Final internal strain is an important indicator for the barricades stability as well as mining cycle.
a r t i c l e
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Article history: Received 27 May 2019 Received in revised form 15 August 2019 Accepted 9 September 2019
Keywords: Cemented paste backfill Sulphate concentration Hydration Shrinkage Fiber Bragg Grating
a b s t r a c t Cement hydration is a volume reduction reaction which can result in negative pore water pressure while strengthening the structural stiffness of cemented paste backfill (CPB). Both sulphate and temperature significantly affect the hydration process and thus influence the internal volume change and mechanical properties of CPB. The expansive hydration products produced in sulphated CPB may create an increase in volume which subsequently weakens the CPB. This paper presents an experimental study conducted to evaluate the coupled effects of sulphate and curing temperature on the internal strain evolution of early aged CPB. CPB samples of various initial sulphate concentrations (0, 5000 and 25,000 ppm) were prepared and cured at various temperature (20 °C and 35 °C) at early ages (7 days). Mechanical tests were conducted on the samples as internal strain was monitored by Fiber Bragg Grating (FBG) sensors. The results indicate that the FBG sensor is sufficiently sensitive to detect the internal strain development of sulphated CPB mixtures during the hydration process. At sulphate concentration of 25,000 ppm, the expansive strain occurred at curing temperature of 35 °C due to the internal expansive force of the expansive materials. The final internal strain value corresponds closely to the strength of the sulphated CPB. As the internal strain of sulphated CPB decreased, the uniaxial compressive strength and tensile strength showed similar trends throughout the experiment. When expansive strain emerged after the initial thermal strain stage, however, there was a higher ratio of uniaxial compressive strength to tensile strength. This phenomenon merits careful consideration in the stability analysis of exposed CPB in underground mines. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Cemented paste backfill (CPB) has been increasingly widely used around the world in recent years [1–4]. The main advantages of CPB material are the economic and environmental benefits of its high recovery ratio, usage efficiency [5] and elimination of tailing dam failure risk [6–9]. In addition, the flotation method used in mineral processing can produce sulphide tailings, which can lead to the generation of acid mine drainage when they are disposed ⇑ Corresponding author. E-mail addresses: [email protected] (B. Yan), [email protected] (W. Zhu). https://doi.org/10.1016/j.conbuildmat.2019.116937 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
under atmospheric conditions. Relatively speaking, it is a safe and secure manner of storing tailings into underground stopes by using the CPB technology [10]. CPB is an engineered mixture of dewatered processing tailings, a hydraulic binder, and either fresh or mine-processed water which together form a high viscosity paste [11,12]. Its fine content is typically more than 15% by weight passing 20 lm; its solids content typically ranges from 78 to 85% [13]. Depending on the ultimate function of the backfill, CPB is prepared with the binder content varying from 3 wt% to 7 wt% [14,15]. To increase the overall performance of the prepared backfill, alternative binders and fibers are used [16,17]. CPB strength and deformation are markedly influenced by the material’s sulphate concentration. The sulphate contained in
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cement affects the quality and quantity of C-S-H gel during the hydration process [18,19] and thereby has potentially damaging effects on the quality of cement-based materials [20]. Sulphate ‘‘attacks” on concrete and mortar are often external [21], but is essentially internal for CPB materials. There are so many sulphate sources in the CPB production process including (i) the oxidation of the sulphate minerals, (ii) the sulphate dioxide or air method for gold mining [22], and (iii) the mine-processed water to the cement. Existing mechanical strain gauges have no gauge-points that can be established before the CPB is hardened, which makes experiments in the plastic stage infeasible. This is the primary issue at hand in assessing the evolution of internal strain and hydrated temperature of any type of CPB material [23]. The Fiber Bragg Grating (FBG) sensor, conversely, is quasi-feedback free and with very high resolution [24]. The device is capable of measuring signal not influenced by sulphate or water [25]. It can be embedded directly into the CPB specimen and is non-invasive due to its small size [26]. Investigations have been conducted on the early-age shrinkage and temperature evolution of cemented paste using FBG sensors [23–25,27,28]. However, there has been no previous study on early age CPB containing different initial sulphate concentrations and at various curing temperatures using FBG sensors. In this study, fiber optic sensors were adopted to evaluate the effects of coupled initial sulphate concentration and curing temperature on the strength and internal strain development of CPB. The internal strain and temperature changes were obtained instantaneously after casting until an age of 7 days over which time the contrast increased. The contents of hydrated products and other ingredients were also assessed by thermal gravity (TG) and differential thermal gravity (DTG) analyses. The proposed method can be used to predict the hydration process and eventually the trends and approximate values of UCS and tensile strength (BTS). 1.1. Background The mechanical stability of the CPB structure within a given amount of time is an important criterion for its quality control. CPB must satisfy static and dynamic load resistance requirements within a certain range to ensure its self-stability and to maintain safety in the underground working stope [6,29–31]. Uniaxial compressive strength (UCS) is the most commonly assessed geotechnical property in practice due to its inexpensiveness and convenience [32]. Generally, minimum target values of CPB strength are considered to be >150 kPa for minimizing the risk of liquefaction of underground disposal or fill voids at an early stage. When CPB is used as an artificial sill pillar, its strength should be more than 4 MPa [6]. It is of great practical significance to accurately predict the behavior of sulphated CPB at early ages, particularly in large underground stopes [33]. During the initial usage period, CPB maturing is a complex physicochemical and environmental phenomenon that influences the ultimate strength and any nonuniform deformation of the material. In addition, curing conditions (time and stress) as well as placement configurations are also important and play a significant role in reaching superior backfill performance [34–36]. The sulphate concentration effects are dependent on curing temperature, as temperature determines the dissolution speed of the anhydrous clinker phases as well as the quality of calciumsilicate-hydrate (C-S-H) and expansive minerals, and thus the binding force between the particles and the CPB’s mechanical properties [37–39]. Within the deep mining context, elevated curing temperatures due to deep rock temperature and the accumulated heat of larger stopes from the massive backfill body as well as the potential addition of heat to the CPB (The initial temperature in CPB and the generated by friction in the pipeline) [40,41]. Com-
pared to standard curing (A constant temperature 20 °C and humidity RH > 95%), more heat is generated by the precipitation of quantities of hydration products [42]. In addition to their impact on CPB strength, sulphate and temperature also affect the self-desiccation process; this is mainly due to the internal water ‘‘sink” and dissipation which occur in all types of cementitious materials [39]. When CPB does not yet contain sulphate, suction develops and the cement matrix continually shrinks. Although the shrinkage volume is small, it is recognized as the origin of the material’s strength and the indicator of phase transition (fluid, skeleton formation, and hardening) in cemented materials. This is important in terms of the early age mechanical properties of CPB [26]. Once sulphate is introduced to produce CPB, the sulphate ions affect the hydration process and thus influence selfdesiccation which drives an internal strain evolution within the material. In addition to inhabiting the hydration process, sulphate addition causes the participation of expansive minerals, such as gypsum and ettringite, which place pressure on the pore wall and stretch the pore structure. Many previous researchers have investigated the strength of CPB materials at different initial sulphate concentrations [1,5,6,22,33,43,44]. Extant studies have demonstrated that sulphate significantly impacts the mineralogical composition, and thus strength of CPB; various factors may increase or decrease the strength. The binder hydration of CPB inhibited by additional sulphate ions is the primary negative effect on its strength. Another negative factor is the absorption of sulphate by C-S-H resulting in lower quality C-S-H gel formation [33]. These factors reduce hydration production and thus decrease the binding force between the particles of mine tailings. Two important sulphate reactions exist in the cement system leading to the formation of the expansive minerals – ettringite and gypsum – which form the source of the aforementioned positive effects and are greatly influenced by the curing temperature [44]. If the precipitation of expanded minerals does not create an excess of empty pores, it can refine the pore structure and enhance the material’s strength. The amount of expansive minerals is a key factor in CPB strength; thus the sulphate concentration and curing temperature are crucial considerations in the design of CPB materials. In-situ strength values are usually 2–4 times higher than laboratory strength values for any given CPB recipe or curing time [45,46]. Field simulation experiments have shown that CPB insitu strength is characterized by gentle humps that appear in the backfilled stope height [47]. These differences in strength can be explained in part by the curing conditions [48,49]. Previous studies have centered on the effects of sulphate and curing temperature on the long-term strength of CPB (28 days) [43,44] while few have explored the effects of coupled sulphate and curing temperature on the early strength development of CPB (1, 3, and 7 days). To the authors’ knowledge, no previous researcher has investigated the coupled initial sulphate concentration and curing temperature effects on the internal strain and hydrated temperature development simultaneously at early ages; this process is directly associated with the strength and damage characteristics of the hydration process. The principal cause of failure in backfill under the impact of blast vibrations is tensile stress [50], and so the magnitude of tensile strength is also particularly important, as is its relation to internal strain development. 2. Materials and methods 2.1. Materials 2.1.1. Tailings Artificial silica tailings were used in the experiments (i) for eliminating the uncertainty of minerals and chemical elements that may exist in natural tailings, which may affect the process of hydration on CPB mixture, and thus the under-
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B. Yan et al. / Construction and Building Materials 230 (2020) 116937 standing of the experimental results; and (ii) to reflect the typical cumulative grain size distribution range of paste tailings in gold mines of Shandong Gold Group Co., ltd. The silica tailings consist of almost 99.8 wt% quartz and possess about 42 wt% fine particles (diameter less than 20 lm). The grain size distribution of the silica tailings is shown in Fig. 1 as-measured by MASTERSIZER 2000. The main physical characteristics and chemical composition of the silica tailings are listed in Tables 1 and 2. 2.1.2. Binder Portland Cement Type I (PCI) was used as the binder in all samples. Table 3 shows the main chemical composition and physical properties of PCI. 2.1.3. Mixing water Tap water of the chemical composition shown in Table 4 was used to mix the silica tailings and binder. To ensure a specific sulphate concentration (0, 5000, and 25,000 ppm, ppm is an abbreviation of parts per 2 in the CPB mixture, ferrous sulfate (FeSO47H2O) was added to the tap water [22]. 2.2. Sample preparation and curing conditions With a water to cement ratio of 5, a binder content of 5 wt%, and the required tailing quantity, the binders were first mixed for about 10 min before adding water with different sulphate concentrations and mixing for a further 10 min to ensure the homogeneity of CPB mixture. The slump of the mix does not exceed 18.6 cm. The CPB mixture was poured into the Plexiglas cylindrical mould 50 mm in diameter, 102 mm and 30 mm in height respectively. Entrapped air was removed by manual vibration and the mix was stirred with an iron bar to evenly distributed and fill it into the mould. Each CPB sample was sealed in the mould by a plastic cover and tape to prevent water evaporation and cure under undrained condition, as any reduction in internal relative humidity (RH) would influence the rate of cement hydration and final test result [49]. A cement slurry with a water to cement ratio of 5:1 is so diluted that segregation and bleeding would readily occur, so a constant water to cement ratio of 2 [5] was ensured for the microstructural analysis of samples prepared via the same procedure described above. A larger plexiglass square mould (200 mm width, 400 mm length, 100 mm height) equipped with a pair of FBG strain and temperature sensors was also used for monitoring tests. The prepared CPB mixtures were filled into the mould and stirred with a steel bar for compaction and removal of voids and to ensure effective bonding to the FBG sensors. Once the CPB mixtures were cast, they were cured and sealed in the mould by a plastic cover in a controlled room with a designated curing temperature (25 °C and 35 °C) and RH of 70% to model the environment of the backfilled stope [6,51]. The shrinkage and internal temperature changes were monitored immediately, simultaneously after casting till an age of 7 days. 2.3. Testing method 2.3.1. Mechanical tests At the age of 1, 3, and 7 days after casting, UCS and BTS tests were performed on the sulphated CPB specimens according to ASTM C 39 on an electronic testing machine (Humboldt HM-5030) to determine the effect of coupled curing temperature and initial sulphate contents. The loading capacity is 50 kN and the loading rate is 1 mm/min. Before the test, the ends of the sample were controlled for flatness and parallelism with a grinding machine; the parallelism between the surfaces was checked at less than 0.05 mm. The UCS value is considered here to be the max-
Table 1 Physical characteristics of silica tailings. Element
Value
Specific gravity/Gs Specific surface area/cm2/g D10/lm D30/lm D60/lm Coefficient of uniformity Cu = D60/D10 Coefficient of curvature Cc = D230/(D60 D10)
2.7 3593 2.23 10.6 39.4 17.67 1.28
imum stress value. The BTS value was calculated according to the peak load observed during the test. Each mechanical test was carried out at least three times to ensure its repeatability. 2.3.2. Thermo-gravimetric analysis TG and DTG analyses were conducted on the early age CPB samples to investigate the effects of sulphate on their microstructural characteristics. In an inert nitrogen atmosphere, about 25 mg of ground CP powder was placed in an alundum crucible and covered with an alundum lid. The weight loss of the sample was recorded simultaneously at a rate of 10 °C/min up to 1000 °C during thermal treatment. Prior to testing, as per the effects of initial sulphate content, the constant mass of a CP sample was determined by removing free water in a dying room at 50 °C. 2.3.3. FBG sensor installation and testing The FBG sensor is an effective means of monitoring the cement hydration process and tracking its internal structural changes during the hydration process [25]. The FBG has the characteristic to reflect a specific light with a certain wavelength. when the interrogator emits a broadband source of light into the optical fiber, the interaction between the external incident photons and the doped particles in the fiber core leads to periodic or permanent changes of fiber core refractive index along the fiber axis. This then leads to the formation of spatial phase grating in the fiber core. This grating center wavelength is related to the effective core index of refraction and the periodicity of the index modulation. When there is a strain on FBG sensor because of stretching due to hydration of the CPB, the periodicity of the index modulation is changed, and photoelastic effect can also lead to the change of the effective core index of refraction. Therefore, the strain evolution in CPB can be obtained through temperature compensation by adding an additional FBG in the same temperature field [28]. Other researchers [23] have demonstrated that the FBG sensor has high precision and sensitive components to detect the strain of early stage cemented mixture. Accounting that the CPB materials is a mixture of cement and tailings, thus these sensors are suitable for detecting the early aged internal strain evolution of sulphated CPB. A total of 12 FBG sensors were fabricated at the JEMETECH fabrication facility with Bragg wavelengths spanning the spectral region between 1510 nm and 1590 nm. The sensors have a reflectivity of 99%, range of about ±2000 le, and sensitivity coefficient of 1.15–1.53 pm/le for the strain sensor and about 30 to 120 °C with a sensitivity coefficient of 10.5–11.3 pm/ °C for the temperature sensor, respectively. The following assumptions were made as the FBG sensors were embedded into the CPB specimens [23,28]. (i) The interfacial slippage between the FBG sensor and CPB is negligible. (ii) The fiber grating region does not suffer damage or deterioration. As a precondition for obtaining an accurate internal strain measurement, in this study, the semicircular fixed ends were arranged on both sides of the strain grating (Fig. 2) so that no slippage occurred between the fiber grating and CPB mix. Before casting CPB mixtures into the mould, FBG strain and temperature sensor were installed horizontally at the center of, and parallel to, the mould (Fig. 2). The FBG strain sensor was directly contact with CPB mixture, whereas the temperature sensor was placed inside a rigid pipe to isolate them from contacting the CPB mix. In this way, the temperature sensors were not influenced by volume changes in the CPB and simply monitored the temperature inside the specimens. The final setup is shown in Fig. 2. The FBG system takes all FBG sensor measurements together at a rate of one reading every 1e 3 s. In the experiment, it was set for logging data every 30 s at early age (3 days) and afterward (7 days) for every 10 min. The data was transmitted to the interrogator and computer for processing and analysis according to a previously published method [23].
3. Results and discussion 3.1. Coupled effect of sulphate and temperature on early age CPB strength development
Fig. 1. Grain size distribution of silica tailings.
3.1.1. UCS of sulphated CPB at early age As shown in Fig. 3a, b, the strength of the sulphate and sulphated free CPB samples are less than 200 kPa at curing tempera-
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Table 2 Main chemical composition of silica tailings. Composition
SiO2 (wt.%)
CaO (wt.%)
Fe2O3 (wt.%)
Al2O3 (wt.%)
MgO (wt.%)
K2O (wt.%)
LOI (m2/g)
PH
Silica tailings
99.8
0.01
0.035
0.05
< 0:01
0.02
0.1
7
Table 3 Main chemical composition and physical properties of PCI Composition
SiO2 (wt.%)
CaO (wt.%)
Fe2O3 (wt.%)
Al2O3 (wt.%)
MgO (wt.%)
SO3 (wt.%)
K2O (wt.%)
SSA (m2/g)
Relative density
PH
PCI
21.43
62.24
2.57
3.56
1.36
2.62
0.34
1.14
3.12
7
Table 4 Primary chemical composition of the tap water used Element
Fe
Ca
Al
Mg
Na
SO24
PH
Value (ppm)
0.06
40.8
0.09
1.4
6.2
43
7.26
Fig. 2. The detailed experimental setup of the FBG sensor installation and mornitoring.
ture of 20 °C before curing time of 1 day, but less than 500 kPa at curing temperature of 35 °C. The higher curing temperature can accelerate the rate of hydration process at the initial stage. At the same curing temperature, there exists the effect of retardation due to the sulphate concentration at the initial stage. For given a 20 °C curing temperature, the UCS value of sulphated CPB is lower than sulphate-free CPB at early ages. This because the solubility of the tricalcium aluminate (C3A), which is the major contribution to the early strength, is suppressed by the presence of sulfate ions [5], which can retard and inhibit cement hydration. This mechanism is also observable in electrical conductivity tests [22]. Interestingly, in comparison to the 20 °C curing temperature, the UCS of samples with sulphate concentrations of 5000 ppm and 25,000 ppm show higher rate and values of strength increase at curing temperature of 35 °C after 3 days (Fig. 3b). The UCS value of the 25,000 ppm sample was 52% and 36% higher than that of 0 ppm and 5000 ppm samples at curing temperature of 35 °C cured at 7 days, respectively. This shows that sulphate concentration can retard the cement hydration, and the process provide the cohesion force between particles (As Fig. 4b shows). However, at the curing temperature of 35 °C after 3 days, an appropriate amount of expansive minerals emerged within this CPB sample, which likely led to infilling and refinement of the pore structure and thus counteracting the negative effect of the large amount of voids and micro pores in the cement matrix as well as the retardation of sulphate. The expansive pressure of components with high initial sulphate contents exert negative effects on the material, but at a certain level,
their refinement effects on the CPB structure can outweigh the negative effects. This is in accordance with the TG and DTG analysis results at curing ages of 7 days (Fig. 9) which are discussed in Section 3.3. 3.1.2. BTS of sulphated CPB at early ages The tensile strength of CPB is an important indicator in the underground mining context, especially in terms of the dynamic disturbance and stress wave refraction area. Two step mining as well as cut and backfill mining are affected by the stress wave reflection at the surface of backfill forming a tensile stress concentration area. If the backfill has low tensile strength, tensile damage and flaking failure may readily occur. The BTS of sulphated CPB under two curing temperature conditions were assessed to investigate the tensile characteristics under various initial sulphate concentrations as shown in Fig. 4a, b. At the 20 °C curing temperature, strength increases with curing age and shows similar trends to those of UCS development. When given a higher curing temperature (35 °C, Fig. 4b), CPB specimen with the sulphate concentration of 25,000 ppm shows the lowest earlyage tensile strength compared to sulphate concentrations of 0 ppm and 5000 ppm. BTS value variations at curing temperature of 35 °C differ markedly from those of UCS development as curing age increases. As shown in Fig. 5, higher sulphated CPB at relative high temperature can produce more expansive materials. The lateral strain of the mould is limited, so deformation of the CPB matrix produced an upward expansive trend induced by the internal expansive
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Fig. 4. Tensile strength of CPB for various initial sulphate concentrations (0 ppm, 5000 ppm, 25,000 ppm) and curing temperatures (20 °C and 35 °C) at early ages (7 days). Fig. 3. UCS of CPB for various initial sulphate concentrations (0 ppm, 5000 ppm, 25,000 ppm) at curing temperature of (a) 20 °C and (b) 35 °C at early ages (7 days).
pressure during the experiment. This process initiated tensile cracks around the pores of the material. To this effect, minimal tensile stress can cause failure in highly sulphated CPB. Despite the fact that the expansive material produced micro cracks, the compaction characteristics become obvious at sulphate concentration of 25,000 ppm. The UCS curves of the highly sulphated CPB sample cured at 35 °C (Fig. 6) evidence this phenomenon as well. The high BTS value at sulphate concentration of 5000 ppm can be attributed to the abundance of hydrated C-S-H gel products, which ‘‘glue” the tailing particles together; this interpretation is indirectly supported by the results shown in Fig. 9d. The expansive material has little impact on the tensile strength of sulphated CPB. It can be concluded that when high initial sulphate content is incorporated into CPB, the tensile strength and ductility can be significantly deteriorated. The results presented above indicate that when high initial sulphate content is incorporated into CPB, the tensile strength and ductility of the material significantly deteriorate. To further investigate the effect of coupled sulphate concentration and curing temperature on strength development, the internal strain must be accurately monitored. The real cohesive strength of the CPB
material is mainly attributable to the hydration process, which affects the volumetric shrinkage. Chemical shrinkage occurs soon after the cement reacts with water accompanied by the net volume loss [52]. Autogenous shrinkage (or self-desiccation effect via the internal water ‘‘sink”) [53] is also induced by the suction development in CPB. The bulk stiffness of the CPB skeleton increases significantly during the hydration process. More suction force is necessary in response to the incremental volume strain. The sulphated CPB volume can expand due to the internal thermal changes as well as the formation of swelling materials (e.g., gypsum and ettringite) which drive further volume expansion. Detailed volume changes in terms of the internal strain in sulphated CPB samples were monitored in this study as discussed below.
3.2. Effect of initial sulphate content and curing temperature on internal strain development 3.2.1. Effect of sulphate content on internal strain development The evolution of internal strain in sulphated CPB (0 ppm, 5000 ppm and 25,000 ppm) at different curing temperatures is shown in Fig. 7. All curves are similar except the 25,000 ppm at curing temperature of 35 °C. The internal strain evolution can be
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Fig. 5. Schematic diagram of sulphated CPB sample under internal sulphate attacks in the cylindrical mould.
Fig. 6. Typical UCS test curves of sulphated CPB samples at curing temperature of 35 °C
divided into four stages: (i) a rest stage wherein the strain field of CPB mix cannot transfer to the FBG sensors; (ii) an expansion stage due to hydration of cement and an increase in temperature with thermal strain; (iii) a rapid shrinkage stage mainly caused by chemical shrinkage and autogenous shrinkage; and (iv) a stable stage characterized by large bulk stiffness through a progressive hydration process where more suction was needed for deformation. Similar phenomena have been observed in concrete and mortors [23]. As the sulphate concentration increased in the experiment, the rate of shrinkage decelerated and little thermal strain occurred at curing temperature of 20 °C, which may be attributed to the sulphate icons inhibiting the hydration process. The DTG results shown in Fig. 9c support this observation, where the peak value of DTG at about 350 °C is significantly lower than at other temperatures; only a small amount of C-S-H gel was produced thus more time was necessary for setting. The shrinkage strain at the stable stage is better correlated with the final strength of all samples except for the sulphated concentration of the 25000 ppm sample at curing temperature of 35 °C. Samples with a high sulphate content (25,000 ppm) show the lowest strength and samples without sulphate content are strongest because an increase in sulphate content makes the CPB pores denser, which gradually increased the capillary water pressure over the course of the experiment. Higher sulphate content also makes the CPB bulk stiffness lower in the same period of time; only a small suction force can produce sufficient deformation [52]. The results also show that when the initial sulphate content is constant and only shrinkage strain appears, a lower final shrinkage
Fig. 7. Evolution of internal strain in CPB for various initial sulphate concentrations (0 ppm, 5000 ppm, 25,000 ppm) at curing temperature of (a)20 °C; (b) 35 °C at early ages ( Note: red circle indicates point at which sensors malfunctioned).
strain remits greater UCS value. The particularities of samples with sulphate content of 25,000 ppm are discussed in detail below. 3.2.2. Effect of curing temperature on internal strain development Regardless of the sulphate concentration conditions, as shown in Fig. 8, each sample underwent internal strain evolution in the first rest stage. The internal strain was subsequently no longer equal to zero during the so-called initial setting time (which is generally found in concrete, cement paste, and mortars) [54]. The ini-
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tial setting time of the sample with 25,000 ppm sulphate concentration and 20 °C curing temperature was relatively long. For 20 °C curing temperature, the initial setting time for 0 ppm, 5000 ppm, and 25,000 ppm was 102 min, 115 min, and 324 min, respectively; for the 35 °C curing temperature it was
7
90 min, 79 min, and 224 min, respectively. A higher curing temperature can accelerate the hydration process, and thus facilitate the formation of more cementitious minerals. In the sample cured at 35 °C, the hydration intensified and the shrinkage strain became steady in less time than the sample cured at 20 °C. This is because high temperature accelerates the cement hydration process along with the internal water ‘‘sink”, i.e., enhances the self-desiccation effect thus accelerating the internal water consumption. Due to the high solid content of the CPB mixture, the material’s internal water was quickly depleted under high curing temperature accompanied by slow and steady autogenous shrinkage. Internal water consumption represents the production of hydration products which increase the bulk stiffness of the CPB mixture. In the presence of these products, more negative pore water pressure is required to shrink the solid skeleton, so its value is small in relation to the curing temperature of 20 °C. The rate of hydration is relatively at this curing temperature, however, which slows down the increase in the bulk stiffness of the solid skeleton. To accommodate this, for a given volume strain, only a small corresponding negative pore pressure is needed. When CPB sulphate content was 25,000 ppm in this experiment, after the initial thermal strain stage, the internal strain transformed from shrinkage to expansion at curing temperature of 35 °C. This is because a large amount of ettringite and gypsum were produced in the pores of sulphated CPB under high curing temperature. The TG and DTG results shown in Fig. 9 support this assertion. Excessive internal expansive pressure expands the CPB to produce macro-cracks which were indeed observed in the surface of this sample. As the expansion strain appears, the BTS value decreases and the refinement effect of the expansive material is subject to the pores of the material itself. The internal strain development is in accordance with the final UCS value of sulphated CPB and the UCS/BTS ratio. The quick rate of internal strain variations thus represents an intense hydration process. The proposed method facilitated accurate preliminary predictions of the trends of sulphated CPB strength as per stable and final internal strain values, as well as the intensity of internal cement hydration through the early development of shrinkage strain. The internal strain evolution is directly related to the environment and material composition, so the application of this method in an underground stope may prevent erroneous mechanical parameters caused by the difference between laboratory and in-situ characteristics. 3.3. Results of Thermo-gravimetric analysis
Fig. 8. Comparison of internal strain in CPB for various initial sulphate concentrations (a)0 ppm, (b)5000 ppm, (c) 25,000 ppm at curing temperatures of 20 °C, 35 °C at early ages ( Note: red circle indicates point at which sensors malfunctioned).
The CP and hydration product components can be decomposed over certain temperature ranges during thermal treatment. Three primary peaks signifying weight loss are observable in Fig. 9 at temperature ranges of 50–200 °C, 400–450 °C, and 600–750 °C, respectively. Gypsum and ettringite crystal water usually dehydrate at 100–160 °C, while calcium hydroxide dehydrates at 400– 500 °C. The decomposition of calcium carbonate as well as CH occur at 600–750 °C [1,55]. At the age of 1 day and curing temperature of 20 °C, there is almost no peak in the 25,000 ppm sample at 450 °C but an obvious peak at 650 °C, which indicates that a small amount of C-S-H gel was produced relative to the sulphate-free sample. A small amount of expansive materials, such as gypsum or ettringite, was dehydrated at 100–120 °C which cannot produce excessive internal expansive force and therefore no obvious expansive strain occurred (Fig. 8). The peak value of DTG at 450 °C and 650 °C without sulphate is the highest and 120 °C is the lowest, which suggests that these samples contained the least amount of ettringite and gypsum but the most Ca(OH)2 and CaCO3 among the samples and thus the maximum UCS value. At 35 °C curing temperature,
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Fig. 9. CP samples with sulphate concentrations of 0, 5000, and 25,000 ppm at curing temperatures of 20 °C (a) TG (c) DTG and 35 °C (b) TG (d) DTG at early ages (7 days).
the 5000 ppm sample shows peak values at about 450 °C and 750 °C, i.e., in accordance with the peak values of the sulphatefree sample. In other words, its strength is comparable to that of the sulphate-free CPB sample. At a curing age of 7 days, the weight loss peak of 25,000 ppm at about 120 °C is largest at curing temperature of 35 °C, indicating that a large amount of ettringite and gypsum materials were produced in this sample. These components fill and refine the pore structure. The excessive crystallization or expansive pressure attributed to the ettringite can lead to physical damage, however, and thus decrease the cohesion between particles. This observation is supported by UCS test results and the evolution of the internal strain as shown in Figs. 3 and 7. 3.4. Further discussion Previous researchers have found that high sulphate concentration in CPB can produce a large amount of swelling. It is important to understand how these swellings affect the internal strain or structure of CPB, as this process is directly associated with the solidification and strength development of the material. Li et al. [5] obtained the suction development of slag-CPB samples with various initial sulphate concentrations at early ages under room temperature using a matrix water potential sensor. Doherty et al. [52,56] found that an incremental volume strain in CPB generates a corresponding suction change in numerical simulations. The pre-
sent study was conducted to investigate how the internal strain evolution and is affected by curing temperature and sulphate concentration as sulphated CPB solidifies. The results presented and discussed above suggest that internal strain evolution in sulphate-free CPB are similar to the previous work in terms of the four stages at early ages [26]. However, the internal strain can be transformed from shrinkage to expansion as a large ratio of UCS/BTS emerges when sulphate concentration and curing temperature are high. This is mainly attributed to the competitive effects of expansive materials mentioned above. Comprehensive understanding of this phenomenon may represent workable technical guidance as well as practical implications for CPB stability assessment and design. The early opening of a barricade can decrease mining cycle time, hence increasing mining production and efficiency. This is obviously associated with the time at which CPB begins to solidify, that is, when the internal strain begins to change. The complexity of underground environments make the UCS values obtained in the laboratory generally inconsistent with in-situ values. The internal strain in CPB can be determined to predict the intensity of the hydration process as per rates of shrinkage or expansion. This process is associated with the CPB strength development regardless of the differences between laboratory and in-situ environmental conditions. Further research is yet necessary to determine the correlation of the internal strain of CPB with the deformation of the sur-
B. Yan et al. / Construction and Building Materials 230 (2020) 116937
rounding rock mass, so as to predict indirectly the behavior of the surrounding rock mass through the deformation of the CPB materials. As for FBG sensor type, its working principle is the same. The sensor size selected in this study is relatively small, which can minimize the influence on the CPB’s hardening process in the mould, so as to ensure its high sensitivity. The evolution of the internal strain in CPB is also related to the placement conditions, and this process involves many aspects, such as filling rate, sedimentation, consolidation and curing conditions, therefore, further study should be made to investigate on these aspects.
4. Conclusions CPB samples from different mould sizes containing different initial sulphate concentrations (0 ppm, 5000 ppm, and 25,000 ppm) were prepared in this study and cured in a controlled temperature and RH room at 20 °C and 35 °C, respectively, to investigate the coupled effect of sulphate concentration and curing temperature on internal strain behavior and strength at early ages (7 days). Mechanical tests including UCS and Brazil disk tests were performed to explore the effect of coupled initial sulphate concentration and temperature on CPB behavior. TG analysis was conducted to provide additional information regarding the hydration process. FBG strain and temperature sensors were also embedded into the CPB matrix to monitor the evolution of the internal strain and temperature. The conclusions can be summarized as follows. - The FBG sensor, in comparison to the strain gauge, is sufficiently sensitive to the internal strain evolution of sulphated CPB mixtures during the hydration process regardless of curing conditions. The internal strain in CPB can be a hydration process indicator. - Sulpahte concentration and curing temperature affect the evolution of internal strain in CPB to a great extent. At curing temperature of 35 °C, the sulphate concentration of 25,000 ppm has a large UCS/BTS ratio due to competitive effects in the expansive materials: (i) refining and infilling the pore structures and (ii) micro-cracks induced by the internal expansive force. - The final internal strain value corresponds closely to the strength of the sulphated CPB. When the internal strain of the sulphated CPB shrinks simultaneously, the evolutions of UCS and BTS are similar. When expansive strain occurs, the UCS and BTS may show different trends depending on the internal expansive materials which create micro-cracks due to the internal expansive force. This phenomenon must be carefully considered in analyzing the stability of exposed CPB, as failure is mainly attributable to the material’s tensile strength. The results presented here may provide a new perspective on sulphated CPB materials at early ages to the benefit of both CPB experimenters and mining engineers. It is necessary to monitor the evolution of internal strain and suction at the same time for further work, so as to investigate the internal stiffness in CPB along with hydration reaction process, which will provide an in-depth understanding of backfill hydration process and an important data support for numerical simulation analysis.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements This work is funded by the National Key Research and Development Program of China (Grant No. 2016YFC0801607), National Science Foundation of China (Grant Nos. 51525402, 51874069 and 51761135102), and the Fundamental Research Funds for the Central Universities of China (Grant Nos. N170108028, N170106003, and N180115009). These supports are gratefully acknowledged. Moreover, we thank Mr. Haiqiang Jiang and Jing Han for providing laboratory resources. Also, the authors would like to thank Mr. Maurice Sunkpal and Hailong Wang, Miss Cui Li for their help.
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