Slags of varying CaO content, having same SiO2/Al2O3 ratio were hydrated in presence of varying proportions of lime and mixtures of lime and gypsum for different periods. The hydrated materials were subjected to DTA. When slags were hydrated in presence of lime only there appeared tetracalcium aluminate hydrate in all cases. Tobermorite and hydrogarnets were also possibly formed. Absorption of lime was indicated in all cases. This decreased with increase of CaO1 in the slag and increased with time. When slags were hydrated in presence of lime and gypsum, the compounds identified were ettringite, crystalline solution of ‘low-sulfate’ aluminate sulfate double salt and tetracalcium aluminate hydrate. The former changed over to the latter when the proportion of gypsum in the mixture was low and lime in the mixture or CaO in the slag was high. Absorption of lime during hydration in presence of lime and gypsum was always more than in presence of lime only and this increased with acidity of slag and with time. An explanation of the relative behaviour of different slags towards absorption of lime has been offered. A possible mechanism of slag activation is postulated. © 1962, Taylor and Francis Group, LLC