Studies to improve the hydration resistance of lime-based materials are reviewed. The methods described include densification of the lime aggregate by addition of sintering aids and surface treatment. Methods for assessing hydration resistance are also reviewed. Additions of ZrO 2 , TiO 2 (including Ti chelating compounds) and rare earth oxides can improve the hydration resistance of CaO aggregate by, it is believed, enhancing solid state sintering without sacrificing refractoriness via formation of hydration resistant solid solutions. Fe 2 O 3 , Al 2 O 3 (as well as Al chelating compounds and Al metal), CuO and V 2 O 5 , act as liquid forming aids. These additives improve the sintering of lime and subsequently the hydration-resistance via formation of liquids at high temperature which cool to produce hydration-resistant solids surrounding the lime polycrystals. However, the liquids formed may significantly reduce the refractoriness of lime materials, thus eroding their particular advantage. The same problem is thought to arise on addition of salts such as CaCl 2 and NaF. The hydration resistance of CaO aggregates can also be improved by surface carbonation and surface treatment using acids, phosphates, machine oils, silicones and fluxes. Surface carbonation of CaO aggregate is proving to be a promising experimental method, but practical problems currently limit its application on an industrial scale. A combination of the two methods mentioned above should therefore be even more effective in improving the hydration resistance of CaO refractory materials.