Application of a microwave impulse technique to the measurement of free water content in early hydration stages of cement paste

A microwave impulse method is designed and employed to the monitoring of residual moisture content in early hydration stages of cement paste. Complementary experiments consisting in determination of the times of beginning and end of setting and of the course of bending strength, compression strength, Young's modulus and hydration heat production during the early hydration period are performed in order to find possible correlation between the measured moisture changes, development of mechanical parameters and hydration heat release. It is observed that the fastest decrease of residual moisture content roughly agrees with the early stages of the hardening process, i.e., with the time period between the end of setting and the moment when first measurable values of mechanical parameters are obtained.