Human activities since the beginning of the industrial revolution have led to vast amounts of CO₂ being emitted into the atmosphere (May 2023, 424 ppm; NOAA), which is principally responsible for anthropogenic climate change; the effects of which are expected to be globally devastating. In order to combat the unbalanced carbon cycle and reduce the effects of climate change, it is widely accepted that the adoption and use of carbon capture utilization and storage (CCUS) technologies will be necessary to limit warming to 2.0 degrees C (IPCC 2022). Among the industrial sectors to decarbonize, the built environment will be notably challenging, principally due to the challenges associated with reducing the carbon impact of structural materials, such as cement and steel. These construction industries currently account for roughly 8% of global emissions, with projections that this number will increase with a growing global population and more rapid urbanization, especially in developing areas.
The abysmal state of U.S. infrastructure decay (ASCE 2021 Grade: C-minus) is also concerning as the replacement of concrete, cement, steel, and asphalt will require and release numerous amounts of carbon; however, this also presents a unique opportunity to deploy CCUS technologies to capture and utilize carbon in the creation of next-generation built-environment materials. These materials include fillers, pozzolans, supplementary cementitious materials (SCMs), and geopolymers such as carbonates, amorphous silica/silicates, bio-chars, and structural allotropes of carbon, like crystalline carbon nanotubes (CNTs) or graphene. The production of these carbon neutral or even carbon-negative materials can be achieved through advanced ex-situ carbon mineralization reaction pathways and novel biomass-to-carbonate conversion technologies, such as alkaline thermal treatment (ATT). In the former, alkaline waste, such as landfilled concrete rich in Ca, can be processed to produce calcium carbonates. This captures atmospheric CO2 and also creates new filler materials which can be incorporated into construction materials. The latter reaction pathway, ATT, takes waste biomass and alkaline wastes as feedstocks and directly converts them into H2 gas and carbonates in a single reaction pathway conducted at moderate conditions (1 atm, 300 degrees C). Although advantageous, both of these synthesis pathways contain unique challenges related to kinetic barriers, conversion issues, mass transfer limitations, and the degree of carbon capture and utilization which can be achieved. Thus, the objective of this study is directed towards overcoming these limitations and coupling these two relatively understudied and novel reaction pathways as a tandem method for carbon sequestration and the creation of new building materials and clean energy.
First, the hydrometallurgical processing of waste concrete in a two-step aqueous dissolution and carbonation reactor system is explored. This system is developed and designed to process hydrated waste cement paste, which has extremely high Ca and Si contents. The kinetics of the dissolution process are detailed and the properties of the unreacted residue is examined, revealing a high surface area, amorphous Si-rich material which was shown to be an excellent clinker replacement in new cement mixtures. The dissolution kinetics were fit to a diffusion controlled reactive model, and methods to further increase the elemental extraction of Ca, Al, Fe, and Si, such as internal abrasive media, was also studied. The Ca-rich mother liquor was then carbonated at various conditions using CO₂ gas and the properties, uses, and potential CO₂ capture metrics of these Si-rich residues and calcium carbonates was detailed.
In an effort to explore alternative carbon mineralization processes, the use of novel leaching agents, such as regenerable ammoniacal salts was also studied. Typical dissolution and carbonation processes require copious amounts of commodity chemicals, such as acid and base; however, cycling of ammonia/ammonium, similar to the chemistries of the Solvay process, is a promising alternative. The use of ammonium chloride and ammonium bisulfate was studied in the context of material extraction from waste cement paste and subsequent carbonate formation. The process was assessed from a carbon circularity standpoint, revealing significant cost and emissions reductions when compared to conventional carbon mineralization for the treatment of alkaline waste. Additionally, the profiling of carboxylic acid ligands, such as formate, glutamate, acetate, and citrate, to further enhance kinetics during leaching were studied. These agents were found to significantly enhance both of these parameters, resulting in almost 100% of Ca material extraction in a single pass with a sodium citrate ligand.
Producing the metastable forms of anhydrous calcium carbonate, such as vaterite and aragonite, was heavily examined in the context of new built-environment building blocks as alternatives to conventional limestone. The production and use of these polymorphs has not been well studied or documented; yet, both vaterite and aragonite are expected to have niche market uses in a carbon-constrained world. The favorable conditions to synthesize vaterite was explored, revealing that a high carbonate to calcium molar ratio is necessary to stabilize this polymorph at ambient reaction conditions. Surface active salt species, such as NH₄+ and sulfate, also had a profound effect on vaterite morphology and stability. Aragonite was successfully stabilized at high carbonation reaction temperatures (~70 degrees C); however, the use of alternative crystallization systems, such as a semi-continuous system vs. traditional batch process was able to produce higher purity aragonite at lower temperatures (40-50 degrees C). The use of crystal seeding also showed remarkable templating abilities and allowed for aragonite production at room temperature (25 degrees C). Both polymorphs were shown to be exceptional fillers for cement through isothermal calorimetry, with aragonite also showing high yield stress development via rheometric testing adding to its potential use in advanced 3-D printed cements, plastics, and papers.
The alkaline thermal treatment of waste biomass derived from coastal marine sources was studied to convert biomass into carbonate materials in a carbon-negative manner, while producing H₂ as a valuable product. Various reaction conditions were profiled, including temperature, steam load, hydroxide utilized, and biomass source. A strong correlation was found between carbon conversion/H₂ yield and the basicity of the hydroxide salt, which matched well with thermodynamic calculations performed. These findings were utilized to inform more advanced and refined ATT reaction pathways, coupled with the use of novel regeneration schemes. One potential reactant regeneration pathway is through the use of molten salt electrolysis, in which carbonates are electrosplit into solid carbon (e.g., CNTs) and O₂. Eutectic regenerable alkaline hydroxide mixtures of Li, Na, and K were studied in their ability to convert biomass into electrochemically active carbonate salts mixtures. Interestingly, the content of LiOH in the salt greatly poisoned the biomass conversion potential; however, lithium is the most electroactive carbonate salt for downstream electrolysis. Finally, the electrolytic processing of these biomass-derived carbonate salts was shown to yield high amounts of CNTs, a valuable allotrope of carbon and significant strength enhancer of cementitious materials.
Lastly, a brief discussion of the combination of these two reactive pathways in the context of producing low-carbon or even carbon-negative next-generation built environment building materials is performed with respect to a circular material economy and scalability. From the results of these studies, recommendations are made for future research advancements to continue to accelerate CCUS activities and reaction pathways in the realm of the most difficult-to-decarbonize industries, such as the built environment.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/16gp-e026 |
| Date | January 2024 |
| Creators | Williams, Jonah Martin |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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