Structure properties relationship in calcium sodium metaphosphate and polyphosphate glasses

Two series of glasses, the first with the formula (50-x/2)Na2O-xCaO-(50-x/2)P2O5 and an oxygen over phosphorus O/P ratio between 3 and 3.33 and the second with the composition (50-x)Na2O-xCaO-50P2O5 with an O/P = 3, were obtained by melt quenching and studied in order to understand the effects of the CaO addition on the structure, physical properties and thermochemistry of polyphosphate and metaphosphate glasses. The spectroscopic investigations by Fourier Transform Infrared spectroscopy and 31P Magic Angle Spinning Nuclear Magnetic Resonance indicate in the first series, the shortening of the polyphosphate chains as CaO content increases. The spectra of the second series indicate the conservation of the metaphosphate chains. The density and thermal analysis measurement show the shrinking and the reticulation of the phosphate network as CaO is added in both series and the chemical durability of the glasses is enhanced by the substitution of the PONa and POP bonds by the POCa- bonds. The thermochemical study shows the decrease of the dissolution heat of the glasses when the average chain length decreases and when the chemical durability of the glass increases. The molar enthalpy of mixing indicates the presence of two ranges of interaction in the metaphosphate compositions around 0-20% and 25-50% of CaO.