Stabilization/solidification (S/S), an effective and economical method of treating contaminated soils, involves mixing the soil with a hydraulic binder, often Portland cement. This contaminant immobilization technique reduces the solubility and mobility of inorganic contaminants (stabilization) and forms a monolith that physically encapsulates the contaminants (solidification). This method is economical and can be used to treat large, contaminated sites. The impact of carbonation on copper leaching, mineralogy and microstructure of S/S cement pastes is the subject of our study. The partial replacement of cement with 30 % montmorillonite-rich bentonite will also be discussed. The materials were subjected to two carbonation methods. Leaching tests with nitric acid, sodium hydroxide and water were used to assess the solubility of copper, aluminum, silicon and calcium as a function of pH.
For samples without bentonite, copper is released more as a result of induced carbonation. Accelerated carbonation acts positively on the solubility of copper by promoting its immobilization down to neutral pH. Bentonite rich in montmorillonite plays an important role on copper adsorption for non-carbonate and carbonate materials. However, it weakens the buffering capacity of S/S cement pastes due to the depletion of portlandite, which has an important buffering effect to resist acid attacks. X-ray microtomography (CT scan) results show that porosity decreases after accelerated and induced carbonation for a material composed of 100 % cement. On the other hand, when the cement paste is mixed with bentonite (30 % in our case), the porosity increases after the accelerated carbonation unlike the induced carbonation which lowers the porosity and connectivity of the pores. Thermogravimetric analyzes (TGA) show that accelerated carbonation causes almost total and partial dissolution of portlandite and C-S-H, respectively, to form calcium carbonate for a bentonite-free material (SB-CA). For clay materials depleted in portlandite (B-CA), this phenomenon causes the decalcification of the C-S-H in part to form calcite. Observations using a scanning electron microscope (SEM) and a simulation of chemical equilibria using the geochemical code PHREEQC were also carried out in order to confirm the hypotheses put forward concerning the effect of carbonation and the addition of bentonite on the solubility of copper.
El Bedoui, S. (Author),
Duhaime (Supervisor) &
Dubé (Co-supervisor),
18 Nov 2024Student thesis: Doctoral thesis › Doctorate in Engineering: Engineering