Here we investigate the mechanism by which shrinkage reducing admixture (SRA) affects hardened cement paste (hcp). The first desorption process for hcp is always accompanied by irreversible shrinkage. Initially we demonstrate the well-known mechanism of SRA acting on capillary force using Vycor glass. Additionally, sorption isotherms and length-change isotherms are measured for both saturated hcp as well as hcp aged at 11% RH for two years. SRA was concluded to be present on the surface of the concave meniscus of the pore solution in Vycor glass, and that the inclusion of SRA reduces the surface tension of the pore solution, the equilibrium Kelvin radius, and the shrinkage due to capillary force. However, a comparison of long-term and short-term length-change isotherms and water vapor sorption isotherms of hcp suggests the possibility of partial evaporation of SRA molecules during the 2-year drying process, as well as the presence of immobile SRA in cement paste. Moreover, the immobile SRA is still active, and is found to reduce the amount of water sorption and shrinkage strain. It is thought that the secondary role of the SRA, which is related to immobile SRA in the cement paste, becomes active at room temperature, at below 80％ RH, and only occurs in the irreversible shrinkage component of hcp produced by the initial desorption process.
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