A simplified model for glass dissolution in water

Numerical simulations of the water dissolution of a random ternary solid ar e presented. The three elements represent silica, soluble oxides (alkalis a nd boron) and quasi-insoluble oxides (Al2O3, ZrO2, Fe2O3, ...). The soluble species are dissolved immediately when they are in contact with the soluti on. Their proportion is kept below the percolation threshold. For the other species, one introduces a model of dissolution-recondensation. It is shown that the dissolution rate constants should be dependent on the bonding env ironment in order to include surface tension. The condensation fluxes are p roportional to the concentration of each species in solution. In the dynami c regime (no recondensation), one observes the congruent dissolution of sil ica and soluble species, after a short initial phase of selective extractio n of the soluble species. The common rate of dissolution decreases with the proportion of insoluble species and increases sharply with that of soluble species. This is mainly due to the formation of a porous hydrated layer wh ose active surface area increases markedly with the proportion of soluble s pecies. In the static regime (finite solution volume), the equilibrium solu bility of silica decreases with the proportion of insoluble species and is practically independent of the proportion of soluble species. The porous hy drated layer is rearranged and almost free of soluble species. The ripening of the surface layer makes it protective and inhibits further extraction o f the soluble species. These results are in general agreement with the expe rimental observations on the dissolution of durable glasses. (C) 2001 Kluwe r Academic Publishers.