Utilizing Waste Cement for Carbon Dioxide Sequestration and Capture: The Role of Water Content on the Growth of Calcium Carbonate

High-alkaline waste cement possesses significant potential for effectively capturing and sequestering carbon dioxide, transforming it into calcium-rich, highly reactive, supplementary cementitious materials. Water plays a pivotal role in facilitating this reaction, yet there is a conspicuous absence of extensive research in this domain. This study elucidates the mechanisms by which water affects the growth behavior (precipitation and crystallization) of calcium carbonate (Cc) in hydrated cement paste during carbonation. The findings indicated that the presence of water dramatically affected Cc's polymorphs, morphology, and crystal properties. With an increase in water content from RH = 55% to a liquid–solid ratio of 40, the calcite grain size underwent significant enlargement from less than 200 to over 400 nm. Moreover, as the water content increased, the different Cc polymorphs evolved into mainly calcite, and the polycrystalline Cc was transformed to monocrystalline calcite. Insufficient water elevated the local pH and ion concentration, resulting in a prolonged supersaturation duration. Meanwhile, the nanosized water film restricted the Cc growth and hindered Ostwald ripening, leading to the preservation of amorphous calcium carbonate (ACC) and poor crystallization of Cc.