Growth and ripening of porous layers in water altered glasses

Cesium borosilicate glasses with various Ca and Zr contents have been synthesized and altered by water during several months. The relation between the degree of alteration of a glass and the morphology of the altered material has been studied by chemical analysis, thermoporometry and small angle X-ray scattering which do not require a drying of the samples. The invasion of water in the glass is described as a percolation mechanism due to the leaching of the most soluble elements in the vitreous matrix (alkalis, boron). The durability of the glasses strongly depends on the amount of zirconium present in the initial composition. The altered layers are very porous (porosity >40%) and the pore radii are in the 2-4 nm range. A rapid nucleation of nanometric pores is followed by a slow and limited growth. X-ray scattering indicates that the pores are compact. The pore volume is directly related to the partial volume of the elements leached from the glass. The case of the long durability nuclear waste confinement French glass R7T7 has also been studied in contrast. (C) 2000 Elsevier Science B.V. All rights reserved.