Influence of CaCO ₃ on the Hydration of 3CaO·Al ₂ O ₃

Abstract

The influence of CaCO 3 on the hydration of 3CaO Al 2 O 3 containing about 90% C 3 A is studied with compacts of their mixtures proportioned to 0, 2.5, 10, 20, and 50 wt% CaCO 3 . The compacts are exposed to H 2 O in liquid and in vapor phase, the attendant expansion being measured as a function of time. Differential thermal analysis, infrared absorption, and X–ray diffraction are used in conjunction with this technique. It is shown that the hydration reaction of 3 CaO · Al 2 O 3 is suppressed by CaCO 3 additions and that this is due primarily to the formation of the low form of calcium carboaluminate on the surface of the C 3 A grains. Expansion measurements of the compacts during the reactions show that this technique indicates the progress of hydration and detects the formation of the carboaluminate at an early stage of the reaction. Using the same technique, a study at 50% relative humidity provides the basis for the conclusion that C 3 A reacts with CaCO 3 under these conditions by a direct mechanism.