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Cemented rockfill (CRF) has more superior mechanical properties (e.g., compressive strength, stiffness, cohesion and friction angle) over other types of backfill material, such as cemented paste backfill (CPB) and hydraulic fill, with the same amount of binder. A comprehensive understanding of the mechanical performance of CRF is critical for effectively applying CRF technology to underground mines. In this paper, the effects of aggregate gradation, the addition of sand, and binder content and type on the uniaxial compressive strength (UCS) and stress-strain behaviour of CRF are experimentally investigated for up to 90 days of curing. The results show that CRF with coarser particles and the best grain size distribution has the highest strength over that with uniform-sized rock aggregate, and the strength of the CRF increases with increased maximum particle size of the rock aggregate. The strength of the CRF increases in proportion with the amount of ordinary Portland cement (OPC). The addition of sand enhances the strength of the CRF samples; the extent of the increase in strength depends on the sand/aggregate replacement ratio. A partial replacement (20–50%) of OPC with fly ash (FA) appears to improve the strength of the CRF samples due to the micro-filler effect, whereas a negative effect is observed when blast furnace slag (Slag) with relatively low strength activity index is used instead. Moreover, the stress-strain response of CRF is strongly influenced by the curing age, aggregate gradation, addition of sand, and binder content and type. The results of this investigation provide technical information for designing cost-effective, safe and durable CRF structures in underground mines.

The feasibility of using plastic waste as a binding material to develop a new construction material (mortar with plastic waste binder (MPB)) is presented in this paper. Two types of plastic: high density polyethylene (HDPE) and low density polyethylene (LDPE), are combined in three blend (weight) ratios (HDPE/ LDPE of 40/60, 50/50, 60/40) and mixed with sand in four plastic contents (45%, 50%, 60%, and 65%). First, the plastic material is melted in an oven. Then, sand and the melted plastic are thoroughly mixed under a controlled temperature into an homogenous mixture. The mixture is then poured into standardized molds and cured for different periods of time. Mechanical, physical and water absorption tests are performed on the MPB samples after different curing times to assess the influence of the change in the plastic content and the blend ratio (HDPE/LDPE or H/L) on their mechanical and physical properties. Moreover, microstructural analyses and tests are performed on some of the samples for a better understanding of the behavior of this MPB at its microstructure level. The results show that samples with plastic contents of 50% and 60% and a blend ratio of 50/50 have the highest unconfined compressive strength. The same range of plastic content is found to provide the MPB with the highest splitting tensile strength when the H/L blend ratio is equal to 40/60 or 60/40. A relationship between the splitting tensile strength (ft) and the compressive strength is proposed. It is also found that the MPB is more ductile with greater plasticity than conventional mortar materials that use Portland cement. The plastic content and H/L ratio have significant effects on the water absorption, density and microstructure of the mortar. The microstructure of the MPB plays an important role in the development of its strength. This new material has good potential for use as a construction material to recycle plastic waste and reduce the impact of plastic on the environment.

In the field, cemented paste backfill (CPB) structures are subjected to non-isothermal curing conditions. In this study, the non-isothermal strength, pore structure development, and self-desiccation of CPB were investigated. CPB specimens with different types of binders and tailings were prepared and cured under isothermal (room temperature: 20 °C) and field non-isothermal conditions. Unconfined compressive strength (UCS) tests, microstructural tests and analyses as well as monitoring experiments, were conducted on CPB specimens cured over various ages. The obtained results show that the non-isothermal curing condition significantly increases the strength as well as the strength increase rate of the CPB regardless of the type of binder and tailings used. Moreover, the non-isothermal condition leads to more intense self-desiccation of CPB as well as the refinement of the pore structure of CPB due to the generation of more hydration products, such as calcium–silicate–hydrate (C–S–H), in the non-isothermal CPB. It is also found that a mine cavity (stope) backfilled with CPB material made with Portland cement type I (PCI) has a significant advantage over stope backfilled with CPB made with slag/PCI (50%/50%) in terms of strength formation and enhancement of self-desiccation due the difference in the non-isothermal curing conditions. The findings of this study provide insight into the development of the mechanical and microstructure properties as well as self-desiccation of CPB structures when exposed to field thermal curing condtions, thereby contributing to the improvement of mechanical stability and economic performance of future CPB designs. The impact of non-isothermal curing conditions on CPB properties need to be considered in mine backfill design and operations.

The behaviour of cemented paste backfill (CPB), a cementitious tailings-based construction material, when subjected to freezing in sub-zero environments is studied by means of columns experiments. A total of three columns are built and cured at specific curing temperatures of 14 °C (control), −1 °C and −6 °C for a period of 28 days. The top of the columns is left open, while the sides and the bottom are covered with a layer of thermal insulation material (fiberglass). The evolution of temperature, volumetric water content (VWC) and electrical conductivity at different heights of the column during curing are monitored. At the end of the curing, samples are taken from different heights of the columns, and then laboratory tests are conducted to evaluate their physical properties, uniaxial compressive strength (UCS) and saturated hydraulic conductivity. The obtained results show that the void ratio, water content and saturated hydraulic conductivity increase with decreasing temperature. Plotting the saturated hydraulic conductivity versus the void ratio shows that they are exponentially correlated. The UCS values of the CPB samples vary with temperature depending on the phase composition and content of the CPB material. Moreover, the results obtained indicate that the filling sequence and surface evaporation also affect the changes in the properties of CPB depending on the curing temperature. Finally, the effects of a temperature gradient found in both the longitudinal and cross sections of the column on the behaviour of CPB structures need to be carefully and comprehensively evaluated. The results presented in this study will contribute to a better understanding of the behaviour of CPB structures subjected to freezing in sub-zero environments.

The cementitious construction material, cemented paste backfill (CPB), is extensively used in underground mining operations worldwide. After the CPB is placed in the field, the evolution of its mechanical and thermal properties strongly affects its stability and the heat transfer within it. An experimental setup is designed to enable the curing of CPB samples under different stresses, filling rates, initial temperatures and drainage conditions as well as the monitoring of the changes in capillary pressure, temperature, amount of drained water and electrical conductivity in the CPB samples during curing. Furthermore, extensive testing is carried to investigate the combined effects of initial temperature, curing stress and drainage on the mechanical, microstructural and thermal properties of CPB. The obtained results reveal that these properties are strongly influenced by the initial backfill temperature, curing stress and drainage conditions, and their interactions. These can significantly affect the cement hydration, capillary pressure development and pore structure of CPB.

This paper presents the results of an experimental study on the sulphate effect on the early age (1, 3, 7 and 28 days) strength and self-desiccation of cemented paste backfill (CPB). The CPB specimens have an initial sulphate concentration of 0, 5000, 15,000 and 25,000 ppm and are cured at room temperature (20 °C). Tests on the mechanical properties were conducted on the CPB, while microstructural analyses were performed on the CPB and cement paste samples. Suction and electrical conductivity monitoring were also performed on the CPB specimens with different sulphate contents. The results show that sulphate has a significant effect on the early age strength and self-desiccation of CPB. In the early ages, sulphate can have negative effects, i.e., leads to a decrease in the CPB strength and reduction in the intensity and rate of self-desiccation within the CPB. The magnitude of these effects depends on the initial sulphate concentration. The inhibition of cement hydration by sulphate ions is a key reason for the observed decrease in CPB strength and self-desiccation intensity or rate. Ettringite formation, changes in the pore structure and sulphate absorption by C-S-H are found as additional negative factors that affect CPB strength. This study has demonstrated that the effect of sulphate on the early strength and self-desiccation of CPB is an important factor for consideration in the designing of cost-effective, safe and durable CPB structures as well as for reducing the mining cycle time; in other words, increasing mining productivity.

Fracture toughness is a key parameter in determining the stability of interlaminar structures. The interface between the cemented paste backfill (CPB) and the adjacent rock mass, which is one of the key controlling factors of the mechanical stability of a CPB structure, is regarded as an interlaminar structure. Fractures (mode-I, mode-II) can occur and propagate along this interface, thereby jeopardizing the stability of the CPB structure. In other words, assessing the fracture toughness of CPB-rock interface is essential for the stability analysis of CPB structures. Although many studies have been conducted on the fracture toughness of brittle materials, the evolution of the modes I & II fracture toughness of the interface between the cemented paste backfill (CPB) material and rock still remains unknown. Given the significant effect of curing time, temperature and sulphate ions on the binder hydration, it is necessary to study the evolution of the modes I & II fracture toughness of the CPB/rock interface under different thermal and chemical conditions. Therefore, many semi-circular samples (with or without sulphate ions) were produced and cured at 2 °C, 20 °C, and 35 °C for 3 days, 7, 14 and 28 days in this study. Thereafter, all specimens were subjected to semi-circular bend (SCB) tests. The obtained results show that both temperature and sulphate ions have considerable effects on the evolution of the modes I & II fracture toughness. Generally, the modes I & II fracture toughness rises as the temperature is increased (with an exception of the samples cured at 35 °C for 28 days). Nevertheless, the influences of sulphate ions on the growth of the modes I & II fracture toughness vary with the age of samples and the initial contents of sulphate ions.