CaO aggregate was sintered from reagent-grade lightweight CaCO3 powder by the addition of 0–20% (molar ratio) MgO and ZrO2, respectively. The results showed that the CaO derived from lightweight CaCO3 was highly sinterable and compact CaO aggregate with relative density above 96% was obtained after sintering at 1400 °C for 2 h, but further increase of compactness was restrained due to the occurrence of abnormal grain growth. The densification of the aggregate was promoted due to the behavior of oxide addition on restraining the grain growth of CaO. With increasing the amount of oxide addition, the microstructure of CaO aggregate underwent a restructuration process. Homogeneous microstructure, with well growing MgO grains occupying most of the boundary triple points of CaO grain, formed by the addition of 20% MgO. Especially when 20% ZrO2 was added, CaZrO3 layer formed around CaO grains. The slaking resistance of the aggregate was appreciably improved due to the promotion of densification, the formation of CaO solid solution (while MgO added) and the modification of microstructure.