Cryo-FIB–SEM and MIP study of porosity and pore size distribution of bentonite and kaolin at different moisture contents

Clays often constitute the main component of poultices used for salt extraction from porous materials in conservation intervention. Knowledge of the evolution in porosity and pore size of clay based poultices, due to shrinkage during drying, is of crucial importance for the selection of the most suitable poultice. We have studied the porosity and pore size distribution of kaolin and bentonite based poultices at different moisture contents. Both Mercury Intrusion Porosimetry (MIP) measurements on freeze-dried samples and cryo-FIB–SEM observations on wet samples are employed. The results show that these complementary techniques provide complete information on the porosity, pore size and pore structure of clay materials at different moisture contents. Both kaolin and bentonite poultices show a change of their total porosity and pore size distribution during drying: the changes are moderate in the case of kaolin, whereas the changes are very significant in the case of bentonite. These findings underline the necessity, when selecting a desalination poultice, of taking into account possible changes in its pore size distribution during drying, since these changes may affect the effectiveness of the salt extraction. Our results indicate that the good desalination efficiency of kaolin on substrate of pore size between 1 and 10 μm observed in practice is related to the presence in the poultice of pores that are very effective in capillary transport (0.2–2 μm) and to the relatively constant pore size distribution of the poultice during drying.