Exploring the Aqueous Dissolution-Precipitation Mechanism of Ca-Si-Na-O Containing Reactants To Form Crystalline Calcium Silicate Hydrates in Carbonated Environments

The current growth trajectory of greenhouse gas emissions will lead to major environmental consequences. The production of Ordinary Portland Cement (OPC) accounts for up to 7% of CO2 emissions, and, if the cement industry is to decarbonize, new processes must be developed. One research approach is to extend cement longevity by taking inspiration from ancient Roman concretes. Roman concrete structures have stood for millennia in a region of notable seismic activity, partially attributed to the continuous precipitation of high strength, crystalline calcium silicate hydrate (CCSH) phases, like tobermorite. CCSH phases are not seen in conventional concretes because they from hydrothermally from 80-250 °C, but they are expected to improve strength and durability of current cementitious materials.
Calcium and magnesium silicates have been studied for the purpose of mineralizing CO2. The Ca3Si3O9 polymorphs have a high conversion rate to calcium carbonates when cured in CO2 and elevated temperatures, while also forming high strength cementitious materials. One polymorph, pseudowollastonite (pwol), precipitates CCSH phases when cured in high temperatures and in the presence of CO2 and NaOH. This dissertation dissects the aqueous reaction mechanisms necessary to precipitate CCSH phases through the following objectives:
1. Detail the precipitation mechanism of tobermorite from combinations of CaO and either amorphous or aqueous silica sources under varying partial pressures of CO2
2. Explore the relationship between colloidal silica and varied NaOH concentrations and their combined effect on precipitating tobermorite and other CCSH phases
3. Identify the conditions that allow the precipitation of CCSH phase and calcium carbonates from pwol by varying NaOH concentrations and the partial pressure of CO2
4. Investigate the potential of combinations of common industrial waste products, like calcium rich slags or silica rich ashes, to produce CCSH phases
A novel precipitation mechanism of CCSH phases was hypothesized, where crystals precipitated directly from solution when Ca2+ ions react with polymerized, aqueous silica. Direct precipitation was found to be dependent on the dissolution rate of the reactants, the concentration of NaOH, the starting phase of CO2 (gas vs carbonate salt), and time.