“…The high initial permeability value was obtained for 0.4 wt% HfO 2 . With the addition of both impurities, CaO/SiO 2 the initial permeability improves in comparison with that of samples containing CaO due to the formation SiO 2 -CaO-HfO 2 and HfO 2 -CaO [30,33,34]. However, its value is less than those other two sets of composition.…”