Understanding the atomistic origin of hydration effects in single and mixed bulk alkali-silicate glasses

Silica-water interaction plays an essential role for the mechanical strength and chemical durability of alkali-doped-silicate glasses. A comprehensive study of single and mixed alkali-silicate glasses with 30% molar content of Li2O, Na2O, and K2O, and half-half mixture of Li2O-Na2O, Li2O-K2O, and Na2O-K2O in hydrated models is carried out using density functional theory methods. Information on atomic geometry, electronic structure, interatomic bonding, partial charge distribution, mechanical, and optical properties are obtained and compared. It confirms that water in the solvated and confined bulk models can be either dissociated or remains as H2O molecule depending on the distribution and specific alkali elements. A quantum mechanical metric, the total bond order density is used to unravel the atomistic origin of the internal cohesion and strength of glasses in different environments. In particular, we show that the mechanical strength of bulk alkali-silicate glasses is enhanced by hydration with some evidence that mixing of alkali ions tends to degrade the strength of the hydrated glasses. These results are discussed in the context of experimental observations and a few existing simulations using classical molecular dynamics.