FACTORS ACCELERATING TRANSFORMATION OF HIGH ALUMINA CEMENTS

Abstract

During hydration of alumina cements, the initially forming unstable hexagonal hydrates (CAH10, CAH8, C4AH13) are transformed sooner or later into stable C3AH6 belonging to the regular system. Factors accelerating transformation are: - temperature - water content (high w/c ratio) - CO2 content in the system - presence of alkalines. Transformation has a significant strength loss as concomitant. This results directly from the fact that unstable compounds of higher mole volume change into stable products of less mole volume. Strength loss is due to porosity decrease in transformation. Presence of alkalines is a special risk, since - in final account - alkaline reactions may be accompanied by decomposition of aluminate hydrates. By chance, extreme effects are seldom to coincide. Such a careful technology may help to utilize to a certain degree valuable properties of alumina cements (high strength, corrosion resistance, fire resistance). Under normal conditions, no further strength loss of constructions already transformed, subject to strength loss, needs to be reckoned with.