AbstractExperiments by isothermal calorimetry, indicate that the hydration of 3CaO·SiO2(C3S) is influenced very little by gibbsite; it is influenced by bayerite to a somewhat larger extent.In the presence of amorphous Al(OH)3the reaction of C3S with water shows a very complicated course and gives four heat peaks. If CaO is added in addition to this Al(OH)3, the third and the fourth heat peaks are more pronounced. From qualitative d.t.a., infra‐red, electron‐microscope andX‐ray investigations, as well as from quantitativeX‐ray analysis, a reaction mechanism is proposed.The quantity of C3S reacted, determined by means of quantitativeX‐ray analysis, is greater during the reaction of 2·00 g C3S with 0·40 g amorphous Al(OH)3, 0·08 g CaO and 2·00 ml water, than during the reaction of 2·00 g C3S wit