<b>Sustainable Proactive Cementitious Materials: Thermal Energy Storage, CO</b>_{<strong>2 </strong>}<b>Capture, and Depollution</b>

<p dir="ltr">The increasing threat of global warming and the need to reduce greenhouse gas emissions have triggered discussions and actions during the last decades. The construction and building sector, accounting for a significant 37-39% of energy-related CO2 emissions, is a primary concern, with 28% attributed to heating and cooling operations. Furthermore, the production of cement, one of the main compounds of concrete, is responsible for around 8% of the global CO2 emissions. Efforts must be made to increase the sustainability of our infrastructure, especially concrete, as it is the most used construction material in the world. However, the fact that concrete is omnipresent in our built environment also opens great opportunities. Providing cementitious composites with proactive effects can make them part of the solution rather than contributing to the problem.</p><p dir="ltr">This dissertation aims to study three different approaches to reducing CO2 emissions by providing cementitious materials with proactive properties: (i) the reduction of energy consumption through the increase of heat energy storage in building envelopes, (ii) CO2 capture or sequestration, and (iii) enhanced depollution abilities.</p><p dir="ltr">The first approach includes a novel method to incorporate Phase Change Materials (PCMs) into mortars with different water-to-cement ratios. PCMs can store thermal energy and rerelease it when the material undergoes a phase change. This way, PCMs can increase thermal inertia and lower the energy consumption of buildings by increasing temperature in the morning and decreasing it during the night while maintaining thermal comfort. However, adding them to construction materials reduces their mechanical properties with the current addition methods. The results presented in this study suggest that the novel method of adding PCM to cementitious systems improves thermal and mechanical properties. In addition, no leaching of the PCM was observed, which may be due to the PCM used being solid at room temperature and the effects of capillary forces. The findings from this study indicate that mixtures with low cement content (w/c = 0.65) can achieve the same or even higher compressive strength and, as well as higher thermal effusivity after treatment compared to untreated mixtures with higher cement content (w/c = 0.55). Thus, the novel PCM incorporation method can produce a cementitious composite with low cement content, which benefits sustainability, lowers costs, and provides high thermal inertia without compromising strength.</p><p dir="ltr">Another of the novel approaches explored in this dissertation is focused on elucidating the fundamental mechanisms that govern the CO2 uptake rate variations produced by using nano-modification of cement pastes under varying CO2 concentrations. Results showed that adding 1% of nano- TiO2 increased the CO2 capture rate and the overall CO2 sequestration of cement pastes. The results of this new study suggest that, besides reducing the Calcium Hydroxide (CH) crystal size, the refinement of the pore structure produced by the nano-additives is also responsible for the observed enhanced CO2 uptake rate. At the same time, the use of nano-TiO2 reduced the overall porosity and increased the total pore surface area. Results suggest that this increase in the pore surface area is one of the primary mechanisms responsible for the increased CO2 uptake rate.</p><p dir="ltr">Previous studies showed that accelerated CO2 exposure treatment can increase the CO2 uptake while reducing the porosity and enhancing the compressive strength of cementitious composites. In this dissertation a study focused on understanding the effect of CO₂ exposure age on flexural strength development of cement paste was conducted. Samples were exposed to an accelerated CO₂ treatment at early (6 days) and late (27 days) ages. The results showed that late exposure led to a 60% increase in 28-day flexural strength while the increase was only 7% for the same testing age but with early exposure. Thermogravimetric analysis (TGA) and X-Ray diffraction (XRD) analysis confirmed that early CO₂ exposure produces less crystalline calcite and lower total hydration products than late CO₂ exposure, which explains the different effects observed for early and late CO2 exposure.</p><p dir="ltr">The third approach to tackling the CO2 footprint of the built environment is focused on the proactive reduction of ambient CO2 by using a TiO2-based surface treatment in concrete pavements. The research showed that higher application rates of the TiO2 surface treatment result in increased CO2 reduction without an increase in the carbonation depth in the samples, thus suggesting that the enhancement of the CO2 reduction produced by the surface treatment is based on the CO2 photoreduction process rather than an acceleration of the natural CO2 uptake of the hydration products. These results suggest that the TiO2-based surface treatment may be a more suitable approach than incorporating TiO2 into the mixture when used in steel-reinforced concrete since the surface treatment can provide the material with CO2 decomposition ability while maintaining corrosion resistance.</p><p dir="ltr">The large-scale potential of the same TiO₂ surface treatment on concrete pavements was assessed using both experimental data and solar irradiance modeling. Estimates of daily and annual CO₂ reduction were calculated across different U.S. cities, revealing that environmental conditions such as daylight duration and cloud coverage significantly impact overall performance. For instance, the results showed that 100 miles of a four-lane treated roadway could remove over 12 million kg of CO₂ annually based on real daylight irradiance data of Phoenix (AZ) in 2023 using a conservative model, making this a highly scalable carbon mitigation strategy.</p><p dir="ltr">Finally, the use of the same TiO₂ surface treatment for NOx decomposition on different pavement types was addressed. Results suggested that both the pavement type and age significantly influence the efficiency of NOx removal, with the highest performance observed when the treatment was applied to new mortar and assessed after aging.</p>