2025 Volume 78 Issue 1 Pages 2-9
Small-angle neutron scattering（SANS）of autoclaved aerated concrete（AAC）which is mainly composed of 1.1nm tobermorite（5CaO・6SiO2・5H2O）, one of the calcium silicate hydrates, were performed to clarify the location and behavior of water molecules in tobermorite. The solvent contrast variation with a mixture of heavy water（D2O）and light water（H2O）showed the disappearing of the（002）diffraction peak of tobermorite at a certain D2O-H2O mixture ratio with 50wt％ and 6wt％ water content, respectively. Tobermorite crystal has a stacking structure consisting of CaO-SiO2 layers connected by SiO4 pillars in the c-axis direction, with interlayer water molecules. The disappearance of（002）diffraction peak indicates that the D2O-H2O mixture replaced periodically into the interlayer structure of tobermorite, i.e., a matching contrast between the CaO-SiO2 layers and the D2O-H2O mixture replaced the interlayer spaces. Considering the samples with 6wt％ moisture content were prepared in D2O-H2O mixture vapor atmosphere, the content water should exist as adsorbed D2O-H2O molecules. The significant increase in adsorption at low relative pressure in the water vapor adsorption isotherm of tobermorite previously reported also should be due to the penetration of water molecules into the interlayer. This method can be applied to the other calcium silicate hydrates such as cement hydrates（C-S-H）and is to be a powerful tool for analyzing the microstructure including water molecules.
