Alkaline durability of some silicate glasses containing CaO. FeO and MnO

In the process of developing an alkali-resistant glass composition to use as reinforcing fibres with cement, seven glasses in the system Na2O-CaO-FeO-Fe2O3-MnO-SiO2 with a constant 70 mol % SiO2 and at least one divalent oxide, were prepared and their intrinsic chemical durability (by the grain test) in aqueous solutions, buffered at different pH values, was examined at 75° C. The durability in this glass system decreased with increasing soda content. The glass composition having the minimum alkali content in this series was 5Na2 O, 12.5 FeO, 12.5 MnO, 70 SiO2 and this glass showed the maximum alkaline durability of this series. Molar substitution of FeO, and particularly of MnO, for CaO in all these glasses increased the durability in the pH range 9 to 12; however, durability in the acid range (pH=0 to 2) was found to deteriorate when this substitution was made. The thermodynamic stability of the different oxides under present investigation (FeO, Fe2O3, MnO, Mn2O3 and SiO2), in aqueous solutions at different pH values has been discussed with standard thermo-chemical data, and it has been shown that the acid and alkaline durability of all these glasses can be satisfactorily correlated with the thermodynamic predictions.