Freeze-Drying Conditions Non-Altering to Ettringite Contained in Hardened Cement and the Pore Size Distribution

Abstract

The freeze drying process for hardened bodies of cement consists of a freezing step by liquid nitrogen and a drying step by vacuum. The use of liquid nitrogen during the process involves the risk of cracking due to heat impact. The authors made improvement to existing freeze-drying equipment to minimize heat impact on specimens and tested various freezing conditions and drying temperatures in the current study. The specimens used in this experiment were hardened bodies of hauyne-containing cement with ettringite, which was susceptible to structural destruction by drying. As the result of comparative study, the authors determined optimum conditions for drying where peak area measured by the powder X-ray diffraction remained almost unreduced successfully. This revealed that the proposed conditions were almost non-altering to ettringite. The pore size distribution of the specimens prepared by this method was found to differ significantly in major pore diameter and total pore volume from those of specimens dried by other common methods : D-drying or drying at 105°C. The pore size distribution data for ettringite obtained by the freeze drying under the conditions proposed in the current study further allowed prediction that the data would represent the extremely fresh condition in which most of ettringite remained intact.