This study investigated the carbonation behaviour of a calcium-based sorbent in the calcium-looping CO2 capture process as a non-catalytic gas-solid reaction using a modified grain size distributed (GSD) model. In GSD model, the calcium-based particle consisted of numerous spherical grains with variable size distribution. The GSD model was modified by considering a variable diffusion coefficient of the gaseous reactant through the development of the product layer. The overall conversion over the reaction time was calculated by a mathematical modelling. The overall sorbent conversion obtained by using the modified GSD model showed better agreement with the experimental data with the average absolute deviation of 4.2 % than the results obtained using the grain model with the average absolute deviation 9.4 %. In the modified GSD model, the physical microstructure of the sorbent particle changes due to the molar volume expansion during the carbonation reaction. The model also predicted accurately the change of particle structural properties such as porosity, molar density, and specific surface area during the carbonation reaction.