Calcium looping is an emerging technology for CO2 capture that uses a regenerable CaO-based sorbent. Here, a novel hydration-based reactivation strategy for spent sorbent, proposed by Industrial Research Limited of New Zealand, is investigated. They have called the process Ca(OH)(2) "superheating" and suggested that Ca(OH)(2) becomes more chemically stable under CO2, allowing release of steam at an elevated temperature ("superheated dehydration"). To investigate this, Ca(OH)(2) powder and pellets and hydrated calcined limestone and dolomite have been heated in various different atmospheres in a thermogravimetric analyzer with a mass spectrometer performing online gas analysis of the off-gas. The "superheated dehydration" effect was observed for Ca(OH)(2) pellets and hydrated calcined limestone, but not for Ca(OH)(2) powder or hydrated calcined dolomite. These findings are Consistent with a mechanism involving formation of an impermeable carbonate layer, which prevents H2O diffusion until rupture. The carbonate layer has a critical thickness that is not reached in the case of the Ca(OH)(2) powder, but is in the case of the Ca(OH)(2) pellets and hydrated calcined limestone. The network of MgO in the dolomitic particles results in CaO grains that are not large enough to accommodate the impermeable carbonate layer.