Suitable sample preparation and measuring conditions are studied for BET specific surface area (BET SSA) evaluation by one-point nitrogen gas adsorption of the solid phase of early hydrated cement paste from just after mixing to 2 hours after mixing. Then, it is analyzed what phase of early hydrates is affected by pre-heating temperature for BET SSA measurement. It became clear that the suitable condition for BET SSA measurement is as follow : 1) Stopping the hydration of early hydrated cement paste by immersing into acetone, 2) Evaporating by exposing under ambient room environment for two hours, 3) Drying the sample under a relative humidity of 11% more than one day, 4) Pre-heating under a temperature of 60°C. Pre-heating temperature does not affect the BET SSA of unhydrated cement. Pre-heating over 80°C increases the BET SSA of early hydrates by four times because of the effects of ettringite decomposition.
