Effect of curing humidity on performance of cemented paste backfill

1.   Introduction

Exploitation and utilization of mineral resources can certainly provide people with valuable mineral products, but this also creates huge amounts of mine wastes. The type of mine waste produced is mainly tailings, which are traditionally discharged in impoundments behind dams [1]. The conventional management of mine tailings may have severe consequences, such as failure of tailings impoundments that can result in subsequent environmental contamination, property damage, or even loss of human life. Also, the tailings ponds occupy farmland, and the construction and maintenance of tailings dams are also expensive. Except for the tailings produced, underground mining operations also create large quantities of mined-out areas, which pose a potential threat to the safety of miners at work. It has become one of the mining industry’s major issues. As a practical and effective solution to eliminating these negative consequences induced by tailings and mined-out areas, cemented paste backfill (CPB) technology has been introduced and employed [2–9]. This technology is to prepare CPB materials by mixing tailings, binder, and water, and then transport them into underground openings via pipeline by gravity or pump pressure. The CPB technology can not only achieve the management of both tailings and mined-out areas but also improve the recovery rate of ore resources. Therefore, the CPB technology has been extensively and increasingly utilized throughout the world [10–16].

After being prepared, the fresh CPB is placed into an underground mined-out stope. The fresh CPB starts to harden gradually, and the hardened CPB structure needs to give support for the surrounding rock. Therefore, the CPB is needed to show satisfactory mechanical performance. Whether the CPB has sufficient mechanical stability is dependent on how much strength is developed in the CPB during the hardening process. Hence, the hardening process is important to the CPB strength gain. During the hardening of CPB, a chemical reaction between the binder and water takes place to produce hydration products. These products are adhesive and can provide bonding between tailings particles, thus increasing the strength of CPB. The underground circumstance provides a natural curing environment (mainly includes the ambient temperature and humidity) for the hardening of CPB (namely, the process of binder hydration). The CPB water content is important to the hardening of CPB because water takes part in the process of binder hydration. The water content is closely related to curing humidity. For instance, a dry environment with low curing humidity can lead to CPB water evaporation.

Several researchers carried out significant studies for understanding the influence of curing conditions on the performance of CPB. For instance, Walske et al. [17] carried out a laboratory experiment to evaluate the effect of curing temperature on the mechanical behaviors of CPB. They use a temperature control system to improve a hydration cell, which could mimic in situ curing conditions. The results of their study showed that high curing temperatures were able to facilitate the generation of effective stress. Fall and Pokharel [18] applied an experimental approach to indicate the influence of temperature (curing temperatures of 0, 25, 20, 35, and 50°C were selected) on the development of strength and microstructure of hardened CPBs. Furthermore, Fall et al. [19] conducted an experimental study to reveal the effect of curing temperature (2, 20, 35, and 50°C were used) and combined influence of temperature and CPB composition on the primary mechanical parameters (strength, modulus of elasticity, and stress–strain relation) of CPB. They demonstrated that the effect of temperature on the mechanical performance of CPB also depended on the tailings type, binder type, ratio of water to binder, and curing time. In consideration of extreme cases, Fall and Samb [20] conducted a range of experimental programs to investigate the impact of high curing temperatures (100, 200, 400, and 600°C were selected) on mechanical strength and microstructure of CPB. They found that elevated temperatures up to 200°C improved the strength of most types of the CPB studied, but when the curing temperature is above 200°C, the strength of CPB is reduced. The decrease of the CPB strength was associated with a remarkable change in the microstructure. Moreover, Fall and Samb [21] performed an experimental study to assess the influence of curing temperature and exceptional thermal loads on the pore structure of CPB. The obtained results of their research showed that when curing temperature was up to 50°C, the pore structure of CPB was refined, while a heating temperature of above 400°C led to the deterioration of the microstructure of CPB. Considering unusually low curing temperature situation, Jiang et al. [22] carried out an experimental study to investigate the influence of sub-zero environmental temperatures (–1, –6, and –12°C) on the yield stress of CPB and its evolution with time. They indicated that the sub-zero temperatures were to the detriment of the yield stress development in CPB.

In addition to curing temperature, some other curing conditions were also discussed in some previous studies, such as curing time and curing stress. For instance, Huang et al. [23] used an experimental study to analyze the compressive strength of CPB under dynamic loading. They found that the dynamic strength of CPB increased with the curing time when the same cement content was used in CPB. By taking advantage of an improved lab apparatus called CUAPS (curing under applied pressure system), Yilmaz et al. [24] experimentally investigated the curing time effect of various binder types and contents on the one-dimensional consolidation behavior of CPB samples. Yilmaz et al. [25] also used CUAPS to analyze the effect of curing stress on the geotechnical and hydromechanical characteristics of CPB specimens. Their study revealed that the compressive strength of unconsolidated and undrained CPB sample was lower than that of consolidated and drained one. Moreover, CUAPS was further used to investigate the influences of curing and stress conditions on geochemical characteristics of CPB [26]. In their study, they found that the addition of stress during curing could positively promote the hardening of CPB and thus the mechanical performance of CPB due to the removing of water by consolidation.

Some previous studies have been reported to analyze the effect of curing humidity on the mechanical behavior of concrete. For instance, the test results of Cebeci [27] revealed that the compressive strength of concrete declined as curing humidity is reduced. The elastic modulus of concrete structures is also reduced when they were subjected to decreased curing humidity [28–29]. Several researchers concluded that concrete was sensitive to curing humidity since the drying shrinkage of concrete varied significantly when concrete structures were exposed to different curing humidity [30–31].

Although several previous studies, as discussed above, were conducted to investigate and describe the effect of curing conditions on performance of CPB, relevant studies are still very limited. Although CPB and concrete are similar since they are both cement-based materials, there are still some differences between them, such as compositions and application conditions. Furthermore, no studies have been carried out to investigate the response of CPB to (curing) humidity so far. Taken these limitations into account, the authors feel it significant to commence this study. It aims to assess the influence of curing humidity on the performances of CPB, including thermal, hydraulic, and mechanical behaviors, by conducting a series of laboratory programs incorporating the tests of temperature development, internal relative humidity evolution, stress–strain relationship, and microstructure of CPB.

2.   Experimental

2.1   Materials

The tailings used in the current study were obtained from an iron mine located in Western China’s high-altitude area. This iron mine’s surface ambient temperature was approximately 10°C, and the lowest humidity was about 45%. Ordinary Portland cement 425# bought from the market is used as the binder, and water from a nearby tap was used.

Since the physical and chemical characteristics of the tailings exert significant influence on the CPB properties, a series of laboratory tests were conducted on the tailings. The bulk density of the tailings is 2.77 g/cm³. Fig. 1 shows the particle size composition of the tailings used. It can be found from Fig. 1 that the tailings used can be regarded as medium tailings. The X-ray diffraction (XRD) results are presented in Fig. 2, which shows that the main mineral constituents of the tailings are quartz, dolomite, and hematite.

Fig. 1.  Particle size distribution of the tailings used.

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Fig. 2.  XRD profiles of the tailings used.

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2.2   Testing methods

According to the actual situation of the mine, four groups of CPB samples with various solid contents and binder-to-tailings mass ratios (b/t) were selected to conduct relevant tests. Table 1 shows the CPB samples subjected to different curing conditions. It should be stated that all the CPB samples were cured at the temperature of 10°C, which is to mimic in situ thermal conditions. The humidity of 45% (chosen for providing extreme low humidity condition that applies to the mine) and 95% (a standard humidity condition according to GB/T50081—2002 [32]) were selected for the curing of the CPB samples while aiming to discuss the influence of curing humidity on the behavior of CPB.

Table  1.  Tested CPB samples and curing conditions

Group	Solid content / wt%	b/t	Curing age / d	Curing temperature / °C	Curing humidity / %
1	72	1/8	28	10	45
2	72	1/8	28	10	95
3	74	1/10	28	10	45
4	74	1/10	28	10	95

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The tailings, binder, and water were mixed homogeneously in a mixer, afterwards, they were poured into curing molds with a dimension of 10 cm × 10 cm × 10 cm in length × width × height to form cubic specimens. It should be pointed out that, the curing molds had been brushed with lubricating oil, to facilitate the removal of CPB samples from the molds when the curing age (28 d) was completed. The CPB samples tested (along with the molds holding them) were placed in a standard curing chamber (type: YH-40B).

For each group of CPB samples, four CPBs were prepared and placed in the chamber with a curing temperature of 10°C and curing humidity of 45% and 95% for 28 d. During this period, one of the four CPBs was subjected to the investigation of the evolutions of temperature and internal relative humidity versus time. A temperature and humidity measurement system (Fig. 3) was used for monitoring and recording the data. As shown in Fig. 3, the temperature and humidity probe (with a dimension of 3 cm × ϕ1.1 cm in height × diameter) was inserted into the CPB to monitor its temperature variation and internal relative humidity evolution with curing age. A PVC pipe was used to fitly wrap the probe to protect the probe from being destroyed during CPB’s hardening. Twelve holes in the PVC pipe were punched to obtain CPB specimen temperature and humidity data conveniently, as shown in the Fig. 4. The probe was placed in the center of the CPB specimen together with the PVC pipe to collect the data in the CPB experiment, and the test method can be seen in Fig. 5.

Fig. 3.  Temperature and humidity measurement system used for tests.

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Fig. 4.  PVC pipe with 12 holes.

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Fig. 5.  Schematic diagram of the method for monitoring temperature and humidity of CPB.

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After the process of 28 d, the other three of the four CPBs were subjected to mechanical tests for obtaining the stress–strain relationships, and the average value was used. The mechanical tests were conducted by a piece of testing equipment (type: YAW-600), which could provide a maximum load of 600 kN and output the stress–strain relations for the CPB specimens tested. The compressive load rate of the testing apparatus for rock mechanics was set as constant (2 mm/min).

To further analyze the microstructural properties of the CPB, scanning electron microscope (SEM) analyses were carried out on the CPB specimens after the mechanical tests. Fig. 6 shows the complete experimental method.

Fig. 6.  Schematic diagram of the complete experimental procedure.

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3.   Results and discussion

3.1   Evolution of internal relative humidity of CPB

Fig. 7 illustrates the evolution of the internal relative humidity of the CPB samples (groups 1–4) versus curing time (28 d). As shown in Fig. 7, for example, group 1 denotes that the solid content of CPB is 72wt%, the binder-to-tailings ratio (b/t) is 1/8, and the curing humidity is 45%. Fig. 7 reveals that curing humidity significantly affects the relative humidity inside the CPB. As predicted, the composition of CPB’s solids (or water content) also clearly affects its internal relative humidity.

As shown in Fig. 8, the evolution of internal relative humidity of CPB versus curing time can be broadly divided into three stages, which are the saturation stage, sharp decline stage, and stable stage.

In the saturation stage, there is sufficient water involved in the binder hydration cycle (OA in Fig. 8), and the effect of curing humidity on CPB’s internal relative humidity is quite minimal during this point. However, when it comes to the sharp decline stage (AB in Fig. 8), massive water is consumed because water acts as a reactant, takes part in the binder hydration process. Large consumption of water results in a distinct moisture gradient between the CPB and its curing ambient, thereby causing a moisture exchange in between. Therefore, the CPB cured with a higher curing humidity can acquire more moisture from the curing ambient, leading to a higher internal relative humidity in the CPB. This is why a higher CPB’s internal relative humidity is associated with a higher curing humidity.

Fig. 7.  Evolution of internal relative humidity of CPB with curing time.

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Fig. 8.  Evolutional stages of internal relative humidity of CPB with curing time.

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Afterward, the content of water stays roughly the same in the stable stage (BC in Fig. 8). Given that the process of binder hydration has nearly ceased, and the pores within CPB have been filled with hydration products in the later stage of reaction. As a result, the blockage of water flow passage prevents the exchange of moisture between the CPB and its surrounding ambient.

The starting and ending moment of the three stages are also closely related to the curing humidity. It’s visible from Fig. 7 increasing the curing humidity can lead to delays at the beginning of a sharp decline. This finding can provide practical information that is very significant. Due to the production of hydration products and their bonding effect, CPB’s strength develops in the rapid decline stage. A rapid strength gain in the CPB structure in situ is commonly required since it is directly relevant to the efficiency of production, and thus operational cost. Therefore, some solutions should be adopted if the on-site ambient humidity is unsatisfactorily high (such as the field conditions of the mine mentioned above who provides the tailings tested), for instance, local ventilation can be deployed near the CPB to lower the ambient humidity.

3.2   Evolution of temperature development in CPB

Fig. 9 demonstrates the development of temperature within the CPB specimens versus curing time. From this figure, it can be found that the curing humidity certainly poses an impact on the temperature development of CPB. However, this influence is not as significant as one of the curing humidity on the internal relative humidity of CPB. This is due to the fact that the curing humidity directly affects the humidity of CPB by moisture exchange, while the curing humidity indirectly influences the temperature of CPB by exerting an effect on the binder hydration, which is an exothermic reaction.

Fig. 9.  Temperature development in CPB with curing time.

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From Fig. 9 it is noticed that there is a remarkable temperature elevation in the CPB during the early age (0–3 d) because a large amount of heat is released by binder hydration in this period. Thereafter, the temperature of CPB begins to decrease owing to the fact that the progress of binder hydration retards and the heat accumulated in CPB diffuses to the surroundings (because of the temperature difference) until a thermal balance is attained between the CPB and ambient. Then the temperature of CPB almost remains unchanged and close to the ambient temperature. Generally, a higher curing humidity can slightly increase the temperature of CPB, except for some incongruence that may be ascribed to varied curing conditions and the water-to-binder ratio of CPB used. As discussed above, raising the ambient humidity can also increase the moisture within CPB. Therefore, more content of water participates in the binder hydration process, and thus more heat can be generated to motivate the development of temperature in CPB.

3.3   Stress–strain relationship of CPB

The stress–strain relationships of the checked CPB specimens and uniaxial compressive strengths are shown in Figs. 10 and 11, respectively. It can be seen from these two figures that a higher curing humidity can lead to a higher peak value of the total stress developing in CPB. This is because, based on the above discussions, increasing the curing humidity will increase both the water content and temperature of CPB. As a result, more water reacts with the binder to form hydration products, which are crucial to the strength development in CPB. Besides, the temperature rise in the CPB can also accelerate the binder hydration process to generate hydration products.

Fig. 10.  Stress–strain relationships of the CPB specimens tested.

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From Fig. 10, it may also be found that, in groups 3 and 4 CPB samples, where the strain is less than 0.01, the stress in group 3 CPB (with lower curing humidity) is higher than in group 4. The reason may be because, in this stage of compressive deformation, the CPB skeleton bears the compression. When the strain reaches 0.01, the stress in group 4’s CPB steadily exceeds group 3’s stress. It is because the compressive load is caused by pore water.

Fig. 11.  Effect of curing humidity on uniaxial compressive strength of the CPB specimens.

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3.4   Microstructural properties of CPB

As shown in Fig. 12, an X-ray energy dispersion spectrum (EDS) analysis is conducted on the hydration products of CPB. Furthermore, Fig. 13 displays the microstructure of the CPB samples according to SEM tests.

Fig. 13.  SEM micrograph of the CPB samples: (a) group 1; (b) group 2; (c) group 3; (d) group 4.

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The results of EDS analysis demonstrate the components of binder hydration products, and the SEM testing outcomes can present the micromorphology of hydration products. It can be found that a large amount of mCaO·nSiO₂·xH₂O (or C–S–H in abbreviation), which shows clusters like distribution, forms and compactly wraps around Ca(OH)₂ (it is distributed flakily) to form a relatively dense structure, contributing to the strength development in CPB. It is noticed from Fig. 13 that, the CPB samples cured with a higher ambient humidity can have a denser microstructure. The reason has been explained above, that a higher curing humidity signifies more content of water reacted, and thus more heat can be produced to increase the temperature of CPB.

Fig. 12.  X-ray EDS analysis result of CPB.

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4.   Conclusions

Comprehensive laboratory experiments are conducted to assess the influence of curing humidity on the internal relative humidity, temperature, and stress–strain relationship of CPB. Furthermore, the cause for the responding behavior of CPB to different ambient humidity is attempted to interpret from a microscopic viewpoint. Based on the obtained results, the following conclusions can be drawn.

(1) A higher curing humidity is associated with a higher internal relative humidity in the CPB. However, an improperly high ambient humidity poses an adverse effect on the quick gain of strength in the CPB.

(2) Increasing curing humidity helps the binder hydration cycle and thus increases temperature and improves peak stress in the CPB.

(3) Raising the curing humidity can lead to the generation of more hydration products, which thus contribute to a more compact microstructure of the CPB.

It should be noted that the structure of in-situ CPB is a large mass, so the impact of air humidity can be confined to a thin outer part of the mass of CPB. More studies are therefore required to be carried out to understand how the level of field humidity will affect the performance of in-situ CPB in the future.

Acknowledgements

The financial support from Yue Qi Young Scholar Project, China University of Mining and Technology-Beijing is highly acknowledged. The authors would also like to thank China Scholarship Council and BGRIMM Technology Group.