The release of Ca2+ and OH” from tricalcium silicate into the aqueous phase was monitored over the first few hours. The solution becomes supersaturated with respect to pure calcium hydroxide with the maximum activity product being about 3.5 times greater than the solubility product. Evidence is presented in support of “silicate poisoning” of calcium hydroxide nuclei. The addition of soluble salts changes the time at which maximum supersaturation is attained but not its magnitude. The analytical data are compared with the rate of heat released from companion pastes.