Multiscale and Multiphysics Investigation of the Geotechnical Behaviour of Nano-Cemented Paste Backfill Plug

Abstract

Cemented paste backfill (CPB) technology is well-known as an excellent alternative to other, older techniques of mine waste management (e.g., surface slurry tailings dams). This technology tackles the geotechnical and environmental challenges that are associated with the traditional methods of mine waste (tailings) disposal. CPB consists of tailings, binder, water, and extra additives; the latter are used to improve the performance of CPB in various ways, such as improving mechanical or rheological properties. CPB is primarily employed for backfilling mine stopes. The process involves conveying CPB, which is produced in a ground-based mixing facility, to the underground voids, aiming to achieve three main objectives. Firstly, preventing the stopes from failing by providing sufficient mechanical strength ensuring the safety of both personnel and equipment. Secondly, increasing the mine's overall productivity by maximizing the extraction of the most ore bodies from the ground. And last, preventing hazardous elements that are present in the mine tailings from being disposed of on the ground surface which are entangled with detrimental impacts on surrounding environment.

The backfilling of a stope with CPB generally involves the following critical steps/phases. First, a retaining structure, called a barricade, is built at the draw-point base of the stope to hold the fresh CPB in place during curing and early ages. Next, a "plug" of CPB is poured a few meters above the height of the barricade to seal it. Once the CPB plug is sufficiently hardened, the main or residual pour (usually with low binder content) is continuously backfilled into the stope.

To properly fulfill the roles described above in underground mining, the CPB plug must have satisfactory geotechnical properties (e.g., mechanical strength, low pore water pressure, suction development). Furthermore, the rate of development of these geotechnical properties is of paramount importance in mine backfill operations. A faster rate of development (e.g., a faster rate of strength gain, a faster rate of pore water pressure dissipation, a faster rate of suction development) is associated with increased mine productivity, which is obviously linked to significant financial gain. The primary measure to increase the development rate of these geotechnical properties is to increase the portion of binder in CPB mixture. However, due to the high contribution of binder content on the final cost of backfilling (up to 75%), and also environmental considerations due to carbon footprint of cement (cement production is responsible for about 7% of the global green gas generation), this method would no longer be considered as a sustainable or environmental-friendly approach. Rather than increasing the binder content, which has a substantial environmental impact due to its high cement consumption, there is an alternative approach to enhance the rate at which paste backfill of the plug gains strength. This involves incorporating different additives, such as nanoparticles. However, the geotechnical properties and behaviour of CPB plug with nanoparticles (nano-CPB) are not well understood. Therefore, in this thesis, the geotechnical properties and behavior of nano-CPB were studied experimentally at different scales (small sample and high column). In addition, the geotechnical response of nano-CPB plug subjected to multiphysical hardening conditions close to those encountered in the field was evaluated. To examine the influence of nanoparticles on the geotechnical performance of CPB, different sets of samples were prepared, each varying in nanoparticle type and dosage. This allowed the study of key geotechnical parameters that determine CPB's geotechnical behavior, including compressive strength, negative pore water pressure, and physical properties such as void ratio, porosity, and dry density. A detailed monitoring program was also implemented to investigate the evolution of these parameters and explain the geotechnical behavior of CPB containing nanoparticles. This program included monitoring of suction, electrical conductivity, and volumetric water content, along with various microstructural analyses such as thermogravimetric analysis, X-Ray diffraction, and mercury intrusion porosimetry.

The results of the initial phase of this study indicated that if nanoparticles were properly dispersed, their inclusion could enhance the geotechnical performance of CPB. The observation was that nanoparticle addition, with a dispersing agent, could enhance CPB's mechanical properties through different mechanisms such as accelerating binder hydration and the filler effect of nanoparticles. This was consistent with monitoring and microstructural analysis results, which showed that nanoparticle addition improved properties such as increased negative pore water pressure development, higher generation of hydration products, and microstructural refinement.

A pioneering aspect of this study was the formulation of a high-rise framework to simulate the curing process of a CPB column under field adapted loading condition. This area of inquiry had been largely uncharted in preceding studies, thereby leaving an evident knowledge deficit concerning CPB behavior under multiscale and multiphysics simulation. Utilizing this novel framework, it was recognized that the inclusion of nanoparticles in a CPB column, subjected to simulated overburden loadings, augmented various aspects of CPB's mechanical performance. A key observation was the expedited dissipation of pore water pressure at the bottom of the nanoparticle-augmented CPB column compared to the control column. Pertaining to the effect of curing stress, it was determined that CPB samples subjected to elevated levels of curing stress exhibited an increase in compressive strength. This was ascribed to several factors, notably microstructure refinement due to the effect of curing stress on void ratio, coupled with its impact on increasing the hydration reaction. This culmination led to an increase in quantity of hydration products in samples that were subjected to elevated levels of curing stress. Nonetheless, the most profound impact of curing stress was observed at the early stages of CPB, when it embodies more fluid characteristics, with the effect diminishing as the transition from a fluid to a solid state emerges within the CPB column. These findings are significantly important for optimizing the costs of backfilling while maintaining the essential geotechnical properties that guarantee the safety and stability of the backfill structures.

In the subsequent phase of this study, we extended our analysis to include additional field curing conditions, specifically non-isothermal curing temperatures, which have a profound impact on the geotechnical performance of nano modified CPB. This dissertation elucidates the comprehensive effects of field-adapted mechanical loading and non-isothermal curing temperatures on nano-CPB characteristics. Key performance indicators such as evolution of strength gain, pore water pressure, total and effective stress, and physical properties including void ratio and dry density were thoroughly examined. Findings from this study underscore the critical role of elevated field curing temperatures in enhancing CPB behavior, notably by accelerating binder hydration, which in turn significantly increases the rate of strength development, promotes pore pressure dissipation, and leads to microstructural refinement by producing more hydration products that are mainly responsible for filling the voids and provide bonding for solid particles within CPB matrix. Moreover, it was observed that CPB columns subjected to field-adapted curing stress in conjunction with non-isothermal curing temperatures outperformed all other columns subjected to alternative curing conditions, demonstrating superior strength and stress development characteristics.