On the hydration of MgO in cement pastes hydrated up to eight years

The present work extends the knowledge on the behaviour of high magnesia cements, pure or with pozzolanic additives cured for 8 years in potable water at 18+2°C. For this purpose, clinker containing 10% MgO was prepared in an appropriate furnace. The raw materials used were all of industrial origin. Four cements were prepared by cogrinding clinker with gypsum and the following pozzolanic materials: Santorin Earth (S.E.), Fly Ash I (F.A.I.), Fly Ash II (F.A.II) and Fly Ash III (F.A.III), in a proportion of 30%.The Mg(OH)₂ produced during the hydration of the above cements was determined quantitatively by thermogravimetric method, while the hydration phenomena were studied by X.R.D. It is concluded that when clinker containing ca. 10% MgO is coground with a good pozzolan, the hydration of MgO reaches a limited value at the age of ca. 6 years. The percentage of hydrated MgO permits the safe use of these cements, as for the long term unsoundness due to MgO hydration.