Enhancing performance and sustainability of ultra-high-performance concrete through solid calcium carbonate precipitation.

Ultra-high-performance concrete (UHPC) exhibits high compressive strength and good durability. However, owing to the dense microstructure of UHPC, carbonation curing cannot be performed to capture and sequester carbon dioxide (CO 2 ). In this study, CO 2 was added to UHPC indirectly. Gaseous CO 2 was first converted into solid calcium carbonate (CaCO 3 ) using calcium hydroxide, and the converted CaCO 3 was then added to UHPC at 2, 4, and 6 wt% based on the cementitious material. The performance and sustainability of UHPC with indirect CO 2 addition were investigated through macroscopic and microscopic experiments. The experimental results showed that the method used did not negatively affect the performance of UHPC. Compared with the control group, the early strength, ultrasonic velocity, and resistivity of UHPC containing solid CO 2 improved to varying degrees. Microscopic experiments, such as heat of hydration and thermogravimetric analysis (TGA), demonstrated that adding captured CO 2 accelerated the hydration rate of the paste. Finally, the CO 2 emissions were normalized according to the compressive strength and resistivity at 28 days. The results indicated that the CO 2 emissions per unit compressive strength and unit resistivity of UHPC with CO 2 were lower than those of the control group.