Reactive Vaterite Cement Derived from Recycled Cement Paste: Phase Transformation and Hardening Mechanism

Recycled cement paste powder (RCP) is a byproduct of crushing concrete waste, which is mostly disposed of at landfills due to its low reactivity. In this work, a vaterite cement (VC) was produced by the carbonation of RCP, and utilized as a low-carbon binder. The hardening behaviors, mechanical properties, phase evolution, and microstructural development of VC were investigated, and the hardening mechanism was revealed. Experimental results showed that VC can harden upon contact with water. The setting and strength development of the VC resulted from the phase transformation from spherical vaterite to prismatic calcite. A robust microstructure was developed during the process, connecting all of the particles by ionic bonding and contributing to the high mechanical performance of VC paste. The entire process could be divided into three stages: dissolution of the vaterite particle, the precipitation and agglomeration of nanocrystals, and the growth and maturation of calcite. Generally, VC is composed entirely of vaterite that could directly capture 0.44 kg CO₂ per kg of the cement. The production of VC not only is a promising approach to sequestering massive CO₂ in traditional cement production but also facilitates the recycling of concrete waste.