The kinetics of the direct CaCO3 particles is investigated. The experiments were carried out in a thermogravimetric analyzer under isothermal conditions between 740 and 890 degrees C. A pure calcium carbonate sample with a mean size of 16 mu m and a specific surface area of 0.6 m(2) g(-1) was used. The variation of reaction rate with temperature shows that the process is controlled by the chemical reaction. An activation energy of around 110-130 kJ mol(-1) was measured. The dependence between chemical rate and temperature is given by k(p) = 740 exp(-113000/RT) (mol m(-2)) s atm. The process was modeled according to a shrinking unreacted core model. Whereas the CaO sulfation stops due to the blockage of external pores, the CaCO3 sulfation does not exhibit abrupt changes in reactivity, probably due to the porosity of the production layer. The recarbonated samples show higher sulfation rates than uncalcinated CaCO3 samples.