Comparison of shrinkage and mass change of hardened cement paste under gradual drying and rapid drying

To identify the impact of drying rate on mechanisms of drying shrinkage, two hardened cement paste (hcp) samples were prepared. Mature samples were dried directly at the target relative humidity (RH), “rapid drying”, or at RH decreasing from 95% step by step till 11%, “gradual drying”. When comparing the relation of mass change versus drying shrinkage, at high RH range over 80%, gradually dried samples showed less mass change for same amount of shrinkage comparing to rapid drying samples. For the range of RH of 80%-40%, the incremental values of both mass change and drying shrinkage were same for two drying methods. The specimens were characterized by XRD, TG-DTA and water vapor sorption isotherms. By combining the results with findings in the literature, we postulated that an additional part of drying shrinkage is activated when dried at high relative humidity for a longer time and we attributed this additional part to gel pores of calcium silicate hydrates.