The enhancement effect of Na2CO3 additive on the reaction of CaCO3 with SO2 and O-2 at high temperature is a widely observed phenomenon; however, its mechanism remains unclear. From the solid-state physics point of view, this study proposed that the defect formation caused by the additive may explain the enhanced sulfation. The single-crystal CaCO3 with a smooth surface was used as a sample for direct sulfation, and the morphology and Na element distribution was investigated with scanning electron microscopy and energy-dispersive spectroscopy. It is observed that the Na ion will diffuse into CaSO4 during sulfation, and it does not diffuse into the lattice of the CaCO3 crystal by itself. The relationship of the diffusion coefficient in the solid state with the additive fraction was established and integrated into a new sulfation model. This new model included a simplified rate equation model to describe the product island formation and an ionic diffusion model that describes the product-layer growth. The experimental data on the direct sulfation of limestone, with and without additive, were used to validate the newly developed models.