Study on the Development of Low-Carbon Cement Materials Using CO<sub>2</sub> Nanobubbles and the Evaluation of Their CO<sub>2</sub> Sequestration and Material Properties

The cement industry accounts for approximately 7% of global CO2 emissions. Considering the anticipated increase in global cement demand, it is essential to implement measures to reduce CO2 emissions from the perspective of decarbonization and achieving carbon neutrality. In recent years, the development of blast furnace cement, in which a portion of ordinary Portland cement (OPC) is replaced with blast furnace slag (BFS), a by-product of ironmaking in blast furnaces, has progressed. However, it has been concerned that increasing the mixing ratio of BFS may decrease the early strength of the cement. Therefore, this study aimed to investigate the effects of applying CO2 nanobubbles (CO2NB), which have been reported to promote the carbonation reactions in cement concrete, to blast furnace cement, in order to improve its strength and evaluate the influence of the carbonation reactions. In the experiments, mixtures of OPC and finely ground blast furnace slag (BFS4000) were prepared at cement replacement ratios of 100%, 70%, 50%, 30%, and 0%. Each mixture was blended with either tap water or CO2NB water at a water-to-cement ratio (w/c ratio) of 50% to create the test samples. Flow tests, bleeding tests, uniaxial compressive strength tests, and X-ray diffraction (XRD) analyses were conducted on these samples. The results showed that the use of CO2NB water not only promoted the carbonation reaction in blast furnace cement but also improved the compressive strength of the samples by approximately 10-20% after 28 days, in addition to enhancing early strength. Consequently, the application of CO2NB to blast furnace cement enables the fixation of CO2 within the samples while allowing for a higher BFS replacement ratio without compromising the strength of the samples. This approach is expected to significantly reduce CO2 emissions and promote the environmentally sustainable use of cement-based materials.