THE EFFECT OF TIME ON THE HYDRATION REACTIONS OF A REFRACTORY CALCIUM ALUMINATE CEMENT INVESTIGATED BY SEM

摘要

Calcium aluminate cements are the most pure form of refractory high alumina cements. Calcium aluminate (CA) are the most important hydraulically setting cements used for preparing refractory castables, because they develop high strength at very early ages after placements. The anhydrous phases of this kind of cements are CA, CA_2 and traces of C_(12)A_7 and alpha-alumina. The major hydraulic phase in all the CA cements including ciment fondue is CA. Once it hydrated, starts to form the hexagonal crystals of CAH_(10) and C_2AH_8 that depending on the time and temperature of hydration converts to the cubic crystals of C_3AH_6 and AH_3. During firing these hydrates break down, leaving very reactive products, which in turn re-crystallize to anhydrous calcium aluminates. This re-crystallization produces a frame work of sintered material at a temperature lower than that which would have been required to sinter the anhydrous calcium aluminate cement powder. The nature of the calcium aluminate hydrate, Their morphology and the way in which they are broken down, plays an important role in determining the properties of a fired cement. For this reason the hydration of the cement at various ages such as 10 minutes up to 50 days has been investigated by SEM and is discussed here in depth.