In this paper; results of the degree of hydration of the pastes and the compressive strength of the 50 mm cubes prepared with the 100 cement and fly ash blended cement were determined. 24 hours after casting mortar cubes and the paste samples were cured for 28 days in the fog room. After 28 days curing, a set of three cubes and a paste sample was dried in the 100, 75, 65, 40 and 12 ambient relative humidity at the constant temperature of 27°C. Drying conditions showed significant effects on the compressive strength and the degree of hydration. Highest compressive strength of 70 MPa was measured of mortar cubes dried in 100 RH; similarly, 97 degree of hydration was obtained for 100 cement samples dried in the 100 RH. For mortar cubes dried in 12 RH, the compressive strength was measured between 47 and 53 MPa. The similar paste samples showed the maximum degree of hydration as 81. Statistically valid correlations were obtained between the degree of hydration and the compressive strength of normal and blended cement system. © 2011 Korean Society of Civil Engineers and Springer-Verlag Berlin Heidelberg.