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CaO needs to show high activity to be used as Ca-sorbent and slagging agent. Hydration activity is an important char-acteristic to evaluate the activity of CaO. In this study, carbide slag from polyvinyl chloride (PVC) industry was uti-lized as precursor for preparing high activity CaO. The roles of crystallite grain, average pore diameter (APD) and volume fraction of pore b 200 nm in diameter (VF200) in hydration activity of CaO from carbide slag (CS-CaO) were respectively investigated. The hydrolysis kinetics model of CaO shows a three-dimensional spherically sym-metric diffusion model (D4), which suggests that hydration activity was mainly associated with APD and VF200 of CS-CaO with limited correlation to the crystal size. Specifically, the hydration activity of CS-CaO is increased with increasing VF200, while decreased with increasing APD. Under the invariable calcination temperature, the core–shell structure formed by the addition of graphite or CaCO3 to CS effectively inhibits the sintering of CS-CaO and improves VF200. Consequently, the hydration activity of CS-CaO increased from 22.79 ℃·min?1 to 27.19 ℃·min?1 and to 29.27 ℃·min?1, with addition of 5％graphite or 5％CaCO3 into carbide slag, respectively.