Sulfation is a relatively slow reaction (several hours) compared with carbonation ( will come into contact with CaCO than CaO. However, both reactions are possible, and are shown below.
:Indirect sulfation: CaO + SO + 1/2 O → CaSO
:Direct sulfation: CaCO +  + 1/2 O → CaSO + CO
Because calcium sulfate has a greater molar volume than either CaO or CaCO a sulfated layer will form on the outside of the particle, which can prevent the uptake of CO by the CaO further inside the particle. Furthermore, the temperature at which calcium sulfate dissociates to CaO and SO is relatively high, precluding sulfation's reversibility at the conditions present in CaL.