It has been shown that calcination of limestone is endothermic reaction and takes place on a definite boundary between the undecomposed limestone and the layer of porous lime formed outside it. This boundary moves towards the center of the limestone but remains spherical in shape. The rates of reaction occurring during the calcination of limestone are involved the following four processes.
The first two processes are heat transfer between gas and limestone by radiation and convection and conduction of heat in the product layer （lime）. The other two processes are chemical reaction in the reaction interface and CO₂ gas diffusion in lime. Especially the rate of reaction was controlled by the conduction of heat in lime.
Applying to the unreacted-core model, rigorous expressions based on heat transfer rate have been derived for the prediction of relations between time required for complete calcination, temperature, and sizes of limestone.
When the rate of reaction of limestone was given by ignition loss weights of CO₂ at any time, it was proportional to the difference between the temperature supplied from the surroundings and the decomposition temperature of limestone. The experimental values of the thermal conductivities of limestone and lime were determined by the unsteady state method in the temperature ranges below 600 ℃.