Document Type

Thesis

Degree Name

Master of Science (MSc)

Department

Chemistry

Program Name/Specialization

Biological and Chemical Sciences

Faculty/School

Faculty of Science

First Advisor

Scott Smith

Advisor Role

Thesis supervisor

Second Advisor

Philippe Van Cappellen

Advisor Role

Thesis committee member

Third Advisor

Vladimir Kitaev

Advisor Role

Thesis committee member

Abstract

Stormwater ponds (SWPs) are engineered stormwater infrastructure known to be major greenhouse gas (GHG) emitters, including carbon dioxide (CO2). Concrete-based materials have great potential to capture CO2 as carbonate minerals (CaCO3), through aqueous carbonation, driven by their alkaline nature and high portlandite (Ca(OH)2) content. However, the mechanistic understanding of how specific parameters including mass, surface area, and degradation of calcium silicate hydrate (C-S-H) phases collectively control carbonation kinetics and CO2 uptake under dynamically evolving conditions remains underexplored. Laboratory-scale experiments were conducted to evaluate the effects of cement dosage, particle surface area, and water chemistry on CO2 consumption. Temporal changes in pH, dissolved Ca2⁺, dissolved silica, dissolved inorganic carbon (DIC), and gas-phase CO2 outflow were monitored. CO2 removal was quantified independently using gas-phase measurements and solid CaCO3 formation determined by thermogravimetric analysis (TGA), and the results were normalized to cement mass and extractable Ca. Total CO2 capture increased with cement dosage, while the fine-particle Portland cement samples exhibited relatively consistent mass-normalized capacities of approximately 0.22-0.24 g CO2 g-1 cement by TGA and 0.20-0.25 g CO2 g-1 cement by gas phase measurements. Portland cement mineralized approximately 0.63-0.71 mol CO2 per mole of extractable calcium ions (Ca2+). The large-particle sample exhibited lower CO2 uptake, particularly according to the gas-phase method, demonstrating that reduced surface area limited carbonation over the experimental period. Strong relationships between BET surface area and total CO2 capture (R2 = 0.997-0.980) identified exposed reactive surface area as a major control on carbonation performance. In pond water matrix, the TGA-derived capacity remained comparable to that measured in Milli-Q water, although the gas-phase estimate was lower. Concrete waste exhibited a substantially lower carbonation capacity than fresh Portland cement, reflecting its smaller inferred inventory of reactive Ca-bearing phases. A kinetic geochemical model was developed in PHREEQC by coupling aqueous speciation and mineral equilibria with rate expressions for portlandite dissolution, calcite precipitation, CO2 gas transfer, wollastonite dissolution as a proxy for C-S-H decalcification, and amorphous-silica (SiO2(am)) formation. The kinetic parameters were optimized simultaneously against five measured responses: pH, dissolved Ca2+, dissolved silica, DIC, and cumulative carbon mineralized as calcite. Two representations of SiO2(am), kinetic precipitation and thermodynamic equilibrium, were evaluated to determine their ability to reproduce the late-stage dissolved silica plateau. The model reproduced the principal stages of carbonation, including the initial rise in pH and dissolved Ca2+, sustained CO2 removal under alkaline conditions, progressive calcite formation, and the subsequent approach toward atmospheric CO2-calcite-controlled conditions. The simulations indicated that cement dosage and reactive surface area primarily controlled the duration of the alkaline CO2-removal period, while coupled portlandite dissolution, C-S-H decalcification, and calcite precipitation governed the temporal Ca2+ and silica responses. Although the simplified mineral representations limited direct extrapolation to heterogeneous field materials, the model provided a mechanistic framework for interpreting the laboratory results and evaluating concrete-based carbonation under environmentally relevant conditions. Overall, the model captured the principal temporal trends in most measured responses, including pH, dissolved Ca, DIC, and cumulative carbon mineralized as calcite. Both SiO2(am) configurations reproduced the general carbonation behaviour; however, treating SiO2(am) as an equilibrium phase provided better agreement with the measured dissolved-Si concentrations, particularly the late-stage concentration plateau. The equilibrium configuration also reduced the number of fitted rate constants and was therefore selected as the preferred representation of silica behaviour in the model. Overall, this study demonstrates that aqueous carbonation of concrete-based materials can provide a pathway for CO2 removal while identifying reactive Ca2+ availability and exposed surface area as key controls on carbonation capacity and kinetics. By integrating experimental carbon balances with a mechanistic geochemical model, this work provides a basis for predicting and optimizing the performance of concrete-based materials in water and supports further evaluation of their potential application as passive or low-energy carbon-removal media in stormwater ponds and other engineered aquatic systems.

Convocation Year

2027

Convocation Season

Spring

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