Character of Hydration of 3CaO.Al ₂ O ₃

Abstract

The C 3 A compacts were hydrated and the reaction was studied by DTA, X‐ray diffraction, mercury porosimetry, and volume change analysis. The hexagonal hydroaluminates C 2 AH 8 and C 4 AH 19 formed at 2°, 12°, and 23°C by a direct mechanism between C 3 A and H 2 O. The hydration reaction at 52° and 80°C was stopped by formation of C 3 AH 6 around the C 3 A grains. The rate of conversion of the hexagonal hydrates to cubic C 3 AH 6 increased with temperature. Volume change analysis confirmed that C 3 AH 6 grows epitaxially on the surface of the C 3 A grain. The reaction at this surface and the passage of water through the layer of hexagonal hydroaluminates control the overall reaction rate. The conversion of the hexagonal hydrates to C 3 AH 6 accelerates the reaction by removing the layer of products from around the C 3 A grain by a solution mechanism. At 52° and 80°C, C 3 AH 6 may form without the intermediate formation of the hexagonal hydrate.