Evaluation of drying methods by nitrogen adsorption

When cementitious materials are subjected to drying, the microstructure is significantly altered by high capillary forces established in the fine pores. To avoid drying stresses when preparing samples for microstructural characterization, our previous studies developed the supercritical drying method (SCD) (Zhang and Scherer, 2017) which showed rather good preservation for the microstructure of hydration products compared with the other conventional methods (Zhang et al., 2018). In this study, various drying methods, including SCD, are evaluated by using the nitrogen adsorption (NAD) technique in terms of BET surface area, sorption isotherms, pore size distribution and pore volume. Results show that the isopropanol (IPA) replacement method can better preserve the microstructure than the other methods with a slight chemical effect of IPA on the calcium silicate hydrate (C-S-H). Contrary to the previous studies, IPA was not found to have a greater influence on calcium hydroxide (CH) than the other drying methods. SCD shows slightly lower microstructural preservation than IPA replacement due to pore blockage. The fluid used for SCD (trifluoromethane) also chemically interacts with both CH and C-S-H, which may result from the pressurization during drying. Freeze-drying is suggested if one wants to avoid chemical effects on the hydration products, at the expense of damage to the microstructure.