Using the helium stereopycnometer the porosity was measured equal to 0.583. The difference of the measurement between the porosity measured by the mercury porosimeter and the porosity obtained from the helium stereopycnometer data was recorded for all the materials examined. This trend is attributed to the fact that very large pores cannot be measured by mercury porosimeter, since mercury easily penetrates into them before any pressure is applied. Furthermore, mercury cannot penetrate into pores smaller than 2 nm, while helium gas is much more permeable to small pores.