Configurational heat capacity of Na2O–CaO–Al2O3–SiO2 melts

The determination of the glassy and liquid heat capacity of a series of Na2O–Al2O3–SiO2 and CaO–Al2O3–SiO2 melts using differential scanning calorimetry shows a scattered trend to higher Cp values with increasing amounts of Al2O3 for each series of melts. The determination of Cpconf as a function of composition, however, reveals a minimum value as a function of composition when Na2O = Al2O3, and a maximum value when CaO = Al2O3. This is the condition at which the melt structure must change as, with increasing amounts of Al2O3, there are not enough cations to charge balance the Al3+ in tetrahedral co-ordination. Furthermore, there is a minimum in Cpconf for the CaO–Al2O3–SiO2 melts at the condition Caatoms = Alatoms. This is the composition at which the Al3+ in tetrahedral co-ordination must begin to share their charge-balancers as the number of Alatoms is increased.As Cpconf is a measure of the energy required for the melt structure to rearrange to be in equilibrium with temperature, this data shows that less energy is required as the number of non-bridging oxygens in peralkaline Na2O–Al2O3–SiO2 melts decreases. There is then a minimum in Cpconf and the value begins to increase as the number of non-charge-balanced Al3+ in tetrahedral co-ordination increases in the peraluminous composition melts upon the further reduction of the amount of Na+ in the melt.Similar behaviour occurs in the metaluminous CaO–Al2O3–SiO2 melts. The Cpconf decreases as the number of non-bridging oxygens is reduced by the removal of CaO. However, when Caatoms < Alatoms (i.e. when there is no longer enough Ca2+ available that each Al3+ can have its own charge charge-balancer, and they must begin sharing Ca2+) the Cpconf begins to increase. There is another change in the Cpconf trend as the melts become peraluminous and the Al3+ no longer have enough Ca2+ to be charge balanced; and Cpconf begins to decrease. As the two melt series approach the end-member 33Al2O3·67SiO2 composition, the Cpconf values converge.The measurement of Cpconf values for these melts together with viscosity data allows the calculation of the Sconf(Tg) term in the Adam–Gibbs equation as a function of composition. The configurational entropy at Tg increases as the composition changes from the end-member peralkaline/metaluminous to the peraluminous compositions; indicating that the range of structures occurring in the melts increases as the (Na2O + CaO)/Al2O3 ratio decreases.