Effect of Mixed Alkaline-Earth Oxides in Silicate Glass
Matthana Khangkhamano, University of Sheffield
The effects of mixed alkaline-earth oxides on density, chemical durability and thermal properties of sodium silicate glasses were investigated by using Archimedes’ method, Product Consistency Test B (PCT-B) and Differential Thermal Analysis (DTA), respectively. Vapour Phase Hydration Test (VHT) was also employed to promote the formation of alteration products. Optical microscopy (OM) was used for visual observation and Scanning Electron Microscopy (SEM) was applied to determine the chemical homogeneity and observe the appearance of VHT glass surfaces. Three different pairs of mixed alkaline earth oxides were separately added to a glass matrix (60 mol% SiO2 - 20 mol% Na2O) by varying their mixing ratio but retaining the total amount at 20 mol%. The investigated glasses are in the general formula of XMO × (20- X)RO × 20Na2O × 60SiO2, where X equals to 0, 4, 8,12, 16 and 20 mole%, respectively, MO and RO are different types of alkaline earth oxides; MO = MgO, RO = CaO for series A, MO = MgO, RO = BaO for series B, MO = CaO, RO = BaO for series C. The addition of mixed MgO-CaO has relatively little effect on density but considerably decreases glass transition temperature, Tg, by approximately 30 °C lower than that of single alkaline-earth base glasses. The introductions of (MgO-BaO), and (CaO-BaO) lead to significantly increase glass density, but decreases Tg , proportionately to BaO content. Furthermore, mixed alkaline-earth glasses exhibited an intermediate crystallisation behavior of the single alkaline-earth base glasses. This effect strongly depends on the mixing ratio in which, i.e. glass series A; the higher MgO : CaO ratio the lower crystallisability. Therefore, the nonlinear deviation in Tg and the change in crystallisation behavior of mixed alkaline-earth glasses can be considered as being evidence of the mixed alkaline earth effect. The durability of mixed alkaline-earth glasses in series B and C appeared to considerably higher than single alkaline-earth base glasses due to providing a minimum value of total normalized elemental release rates (NRi) at a particular composition of X = 8 for series B, and X=12 and 16 for series C. This phenomenon can be regarded as mixed alkaline-earth effect, although less pronounced than mixed alkali effect. Moreover, the results from VHT experiment, which has been applied to glass series A and C, suggested the different structure and appearance of the hydrated layers and distinct formation of alteration phases which is highly dependent upon glass composition. Calcium silicate and magnesium hydroxide are produced in glass series A, whereas barium, calcium and sodium silicates are generated in glass series C.