Abstract: Mass transfer is a key factor affecting the energy storage and release in Ca(OH)₂/CaO thermochemical storage system. This study investigated the effects of particle size, specific surface area and porosity of three different particle sizes (100, 50, 25 μm) on mass transfer efficiency during hydration. During the hydration stage, Compared with the initial state, the particle size of CaO particle clusters with different particle sizes decreased, the specific surface area increased, and the porosity decreased. Due to the scale effect, the originally discrete CaO particles were more likely to aggregate into clusters, which hindered the mass transfer efficiency. Compared with the initial state(t=10 min), at the late stage (t=50 min), with the increase of hydration conversion rate, the agglomeration force of three CaO particles with different particle sizes (100, 50, 25 μm) decreased by 27.01%, 32.62% and 42.61%, the effective gas diffusivity decreased by 14.24%, 12.40% and 11.09%, and the hydration reaction front velocity decreased by 78.99%, 76.80% and 76.20%, respectively. Although the small particle size was more likely to aggregate into clusters, it had a high effective gas diffusivity, which enhanced the mass transfer efficiency in the reaction process and showed higher hydration capacity. Based on the modified random pore model, the synergistic effects of agglomeration behavior and effective gas diffusivity on the apparent reaction rate were quantified.