Sintered samples of calcined limestone were sulfated for extended times in a differential reactor to study the reaction kinetics after formation of a product layer. The reaction rate was controlled by product layer diffusion, except during the initial period. The product layer diffusivity increased with temperature, but decreased with increasing SO2 concentration. The diffusivity not only depends on current conditions, but also on previous conditions that led to the formation of the layer. The results support a crystallization and fracture model and a hypothesis that the rate-limiting mechanism changes from inward gas diffusion control in the early stages to outward ionic diffusion control after formation of a continuous product layer, a change attributed to the need for the reaction to do mechanical work to displace the product layer and make room for increased solid volume at the CaO/CaSO4 interface. A criterion is established to determine whether a product layer can be fractured. A gas containing 2250 ppm SO2 at 900 degreesC cannot fracture a product layer as thick as 233 nm, while such a layer can be fractured by steam at 250 degreesC and a partial pressure of 1 bar.