Growth and ripening of porous layers in water altered glasses

Cesium borosilicate glasses with various Ca and Zr contents have been synth esized 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-r ay 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 leac hing of the most soluble elements in the vitreous matrix (alkalis, boron). The durability of the glasses strongly depends on the amount of zirconium p resent in the initial composition. The altered layers are very porous (poro sity >40%) and the pore radii are in the 2-4 nm range. A rapid nucleation o f nanometric pores is followed by a slow and limited growth. X-ray scatteri ng indicates that the pores are compact. The pore volume is directly relate d 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 b een studied in contrast. (C) 2000 Elsevier Science B.V. All rights reserved .