2025 Theses Doctoral

Integration of Solid Waste Upcycling and Carbon Sequestration for the Development of Sustainable Building Materials

Zhao, Diandian

This dissertation delves into the exploration of calcium carbonate polymorphs and silica-based materials upcycled from waste cement paste using a two-step extraction and carbonation process as supplementary cementitious materials (SCMs) to simultaneously achieve solid waste upcycling and carbon sequestration in the built environment. With the growing urgency to mitigate carbon emissions associated with the cement industry—one of the largest industrial contributors to anthropogenic CO₂ emissions globally—there is a pressing need to develop low-carbon alternatives that do not compromise the performance of concrete. This research is motivated by the potential of marrying carbon capture, utilization, and storage (CCUS) and alternative SCMs through CO₂ mineralization of industrial by-products and waste materials to lower the embodied carbon of cement and concrete. This approach addresses two critical issues: the reduction of construction and demolition (C&D) waste and the creation of highly reactive SCMs that can partially replace ordinary Portland cement (OPC). The dissertation is structured into two primary parts: the first focuses on the synthesis and potential applications of calcium carbonate polymorphs that can be derived from CO₂ utilization, and the second on the upcycling of waste cement paste into reactive silica-based materials.

The first part (Chapters 2, 3, and 4) of the dissertation centers on three anhydrous calcium carbonate polymorphs, calcite, aragonite, and vaterite, which were synthesized under controlled laboratory conditions. These polymorphs were first characterized comprehensively using analytical techniques including scanning electron microscopy (SEM) to observe their distinct morphologies, X-ray diffraction (XRD) to determine their crystalline structures, laser diffraction (LD) to analyze their particle size distribution, and Brunauer-Emmett-Teller (BET) analysis to measure their specific surface areas. The rheology, hydration, and stability of these polymorphs were then investigated after they were used as substitutes for Portland cement (OPC) in cement pastes at 10 wt% or 20% replacement level.

In Chapter 2, detailed rheological analyses were conducted, including rotational and oscillatory shear tests, to evaluate the influence of these polymorphs on the viscosity, yield stress, and structural buildup of cement pastes. Aragonite, with its needle-like crystals, was found to significantly enhance the static yield stress and structural build-up rates while having a minimal impact on dynamic yield stress, making it particularly suitable for applications requiring high thixotropy, such as 3D printing of concrete.

In Chapter 3, the polymorphs were found to affect the hydration kinetics of the cement pastes, with aragonite exhibiting the most pronounced accelerating effect, thereby contributing to faster early-age strength development. The differences in thermodynamic stability of the three polymorphs also resulted in slightly different phase assemblages as revealed via thermodynamic modeling, indicating potential beneficial effects of using metastable aragonite and vaterite in cement-based materials. The metastable vaterite was also found to be stabilized in cement systems despite its instability and tendency to convert to calcite in aqueous environments.

In Chapter 4, the mechanisms underlying the stabilization of vaterite in cement paste were explored with carefully designed experiments to construct model systems to decompose the complex cement-based systems and isolate dominating factors. The deposit and growth of cement-hydrated phases on the surface of vaterite and calcite seeds were found to be the dominant mechanisms preventing the transformation of vaterite to calcite, stabilizing metastable vaterite even in the presence of calcite seeding.

The second part (Chapter 5) of the dissertation investigates the reactivity of amorphous silica-based materials extracted from waste cement paste using a pH swing process as alternative SCMs. The upcycling process involves the leaching of calcium from waste cement paste, followed by a pH swing to precipitate undesired elements to isolate calcium for CO₂ mineralization. The resulting materials, referred to as "residue" and "precipitate," were thoroughly characterized and found to exhibit strong pozzolanic reactivity. When used as 10% replacements for OPC, these upcycled materials significantly improved the compressive strength of cement pastes, particularly at early ages. The study also explored the phase assemblages formed in these cement pastes after hydration via XRD and the chemical circularity of silicate structures during the upcycling and reincorporation processes. The results indicated that the incorporation of these upcycled SCMs can contribute to the hydration of cement pastes and enhance their mechanical properties, which proved the feasibility of using these upcycled materials as alternative SCMs.

Overall, this dissertation presents a comprehensive study on the potential of calcium carbonate polymorphs and upcycled silica-based materials as alternative SCMs to lower the embodied carbon of cement-based materials. Calcium carbonate polymorphs can be incorporated into cementitious materials to improve their rheological properties, hydration behavior, and mechanical performance, while amorphous silica-based materials exhibited high pozzolanic reactivity and contributed to the enhancement of compressive strength. This research paves a new way to decarbonize the built environment through the combination of solid waste upcycling and carbon sequestration, contributing to the global efforts to reduce the carbon footprint of the cement industry and promote a circular economy within the construction sector.