Hydrothermal Interactions of Cement or Mortar with Zeolites or Montmorillonites

Abstract

ABSTRACTConcretes, grouts, clays and/or zeolites are candidate borehole, shaft or tunnel plugging materials for any nuclear waste repository. Interactions between these plugging materials may take place under mild hydrothermal conditions during the life of a repository. Class H cement or mortar (PSU/WES mixture) was reacted with one of two montmorillonites, clinoptilolite or mordenite at 100° and 200°C for different periods under a confining pressure of 30 MPa. The solid reaction products were characterized by x-ray powder diffraction and scanning electron microscopy after the hydrothermal treatments. When zeolites were in contact (not intimate mixture) with class H cement, they did not seem to alter but clinoptilolite altered to analcime, and mordenite became poorly crystalline in the presence of mortar (containing NaCl) at both 100° and 200°C. When cement or mortar was intimately mixed with zeolites or montmorillonites and reacted hydrothermally, the reaction resulted in the formation of Al substituted tobermorite (11Å type) in all cases (this type of reaction is expected at the interface) at both 100° and 200°C. The mechanism of tobermorite formation includes the decomposition of zeolites or montmorillonites in the presence of alkaline (pH ≃ 12) cement or mortar and recrystallization to form Al substituted tobermorite. Cesium sorption measurements in 0.01N CaCl2on the reaction products revealed that selective Cs sōrption increased in most cases, even though little or none of the original zeolites and montmorillonites remained in the products. For example, Cs sorption Kd(mL/g) increased from 80 in the untreated mortar + Ca montmorillonite mixture to 1700 in the interaction product which is Al substituted tobermorite. Thus, we discover here that Al substituted tobermorite has good selectivity for Cs.