In the last few decades, there has been a significant increase in the accumulation of hazardous wastes across various areas of the world. Part of the problem is that these wastes are generated daily, and their chemical characteristics limit the application of some remediation methods. Solidification/Stabilization (S/S) represents a method for treating diverse contaminants, including hazardous wastes like trace metals. Notably, this technique has garnered attention as a promising alternative owing to its capacity to offer practical resolutions for both human and environmental health (Al Tabbaa & Stegemann, 2011). Cement-based solidification/stabilization techniques have gained extensive utilization in producing stable forms of contaminated soils and curtailing the movement of contaminants into the environment.
However, the solidified/stabilized matrix structures present at sites are continually exposed to various aggressive conditions, including fluctuating loads and atmospheric carbon dioxide. Moreover, there is a lack of information regarding the long-term performance of S/S techniques under these specific environmental conditions. The presence of external loads promotes crack growth, facilitating the penetration of aggressive agents such as water into the S/S matrix. This phenomenon accelerates the deterioration of the S/S structure. Conversely, carbonation leads to the precipitation of calcite (carbonation products) into these cracks, reducing porosity and enabling self-healing processes. Self-healing, in turn, has the potential to enhance the hydraulic properties of the S/S matrix. Therefore, investigating the impact of loading and carbonation on the chemical and mechanical properties of the S/S matrix is important for comprehending the long-term stability of the structure.
The primary objective of this project entailed the simulation of environmental conditions, such as loading and exposure to CO2, on solidified sand. The simulation of the solidified matrix was achieved by inducing physicochemical alterations through the implementation of various experimental scenarios, namely: no carbonation/no loading (NC/NL), carbonation only (C), mechanical loading only (L), carbonation followed by mechanical loading (C/L), and mechanical loading followed by carbonation (L/C). Solidified samples with a water-to-cement ratio (W/C) of 2 were prepared and subjected to leaching tests using a flow-through leaching setup. These simulations were conducted using a modified triaxial cell. The experimental findings presented in this thesis elucidated the deterioration resulting from the aforementioned scenarios, which encompassed mechanical stresses and carbonation. Moreover, an investigation into the physicochemical healing of micro-cracks within the S/S matrix was carried out through hydraulic conductivity testing and X-ray CT scanning. Furthermore, the effects of these experimental scenarios were examined in relation to compressive strength, electrical conductivity, pH, leaching of aluminum, silicon, calcium, and copper, as well as thermogravimetric analysis.
Upon subjecting the samples to various test scenarios, distinct results were observed. The control scenario exhibited a marginal decrease in hydraulic conductivity, indicating a negligible impact. Conversely, carbonation exhibited significant advantages by reducing hydraulic conductivity and lowering porosity, thereby facilitating self-healing processes. Mechanical stresses induced fractures and inflicted damage upon the solidified sand, which, albeit impossible to completely eliminate, were mitigated to some extent through carbonationinduced self-healing, thereby reducing crack formation. Furthermore, the order of operations, as revealed by the CT analysis, demonstrated perceptible differences (e.g., carbonation versus loading), primarily attributed to the self-healing effects that promote partial recovery of the physical properties of the samples.
The findings of this thesis also showed different behaviors in terms of calcium leaching. The non-carbonation scenarios exhibited stable calcium leaching over an extended period. However, the presence of carbonation in the C, C/L, and L/C scenarios resulted in a significant decrease in calcium leaching. In the case of copper leaching, it was modest and stable, as evidenced by the NC/NL scenario. However, upon exposure to carbonation, three distinct phases of copper leaching were observed, namely a notable increase, followed by a partial decrease, and ultimately reaching a steady state. Mechanical stresses tended to diminish copper leaching, with no detectable copper concentration in the leachate of the L scenario after a short duration. Furthermore, the leaching test revealed a slight increase in pore volume in the NC/NL scenario, while carbonation significantly reduced the overall porosity of the sample. Conversely, the loading effect increased the total porosity in the L, C/L, and L/C scenarios.
The results indicate that the influence of stressors on leaching, void size distribution, compressive strength and permeability is complex and characterized by interactions between the stressors.
| Date | 19 Dec 2023 |
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| Original language | American English |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | François Duhaime (Supervisor) & Jean-Sébastien Dubé (Co-supervisor) |
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