A variety of clay containing alkalies gives up water at 360oC and 600oC on heating and at the same time preserves its micaceous habit. Muskovite is completely destroyed at 1100oC to 1200oC. Montmorrillinite does not change essentially in properties below 600oC. Later it loses its water of crystallization and undergoes conversion at 800oC to 850oC.
After heating the dehydrated clay is allowed to stand in the air it takes up water energetically and at the same time the alumina in part again becomes insoluble in HCl acid. It is more likely that we have to do here with gels of oxides Al2O3 and SiO2. In the form in which they exists in the dehydration product, their reactively was known to the ancient Romans, whose mortar consisted of brick dust and slaked lime, i.e. of calcium hydrate with silica and alumina to which it became bound chemically.
To be contd….
