Abstract
Mechanism of moisture sorption hysteresis of hardened cement paste is discussed in this paper. Firstly, cement paste with three type of cement and two different water to cement ratioes are tested for sorption isotherm. There exist the hysteresis in the range lower than 30％Rh. This phenomena is not common for normal porous materials. It is experimentally found that the adsorption isotherm as well as desorption isotherm is on the B.E.T plot, and the surface area calculated B.E.T theory from adsorption process and those from desorption process has the linear relationship. This results implies that, in the process of sorption isotherm test, some reproductive chemical reaction occur. Secondly, hysteresis experiment in high relative humidity of hardened cement paste is conducted. The experimental data in re-adsorption process shows convex downwards. This decrease in water content with increase in relative humidity indicates that the surface area of hardened cement is decreased. The two experimental data showed the evidence that the chemical reaction of hydrate occurs behind the desorption or adsorption process. In order to confirm this reaction, IR spectra and NMR spectra of synthesized C-S-H under different relative humidities, i.e. 95％RH, 60％RH, 0％RH（＝105℃-dry for 24hours）were measured. According to the experiment, the Si network is increased even in the drying process from 95％ RH to 60％ RH. This reveals that the C-S-H or hardened cement paste is not the stable substance in desorption or adsorption process with regard to water.
