Kinetics of Tricalcium Aluminate Hydration in the Presence of Boric Acid and Calcium Hydroxide

Isothermal calorimetry has been used to study the hydration behavior of tricalcium aluminate (3CaO·Al2O3) in the presence of calcium hydroxide (Ca(OH)2) and boric acid (H3BO3). Hydration has been initiated with water and boric acid solutions of 0.18M‐0.89Mand at constant temperatures of 25°‐55°C. The resulting hydration products have been characterized via X‐ray diffractometry. Hydration of a 1:1 molar mixture of 3CaO·Al2O3 and Ca(OH)2 in water forms 3CaO·Al2O3·6H2O and produces ∼241 kJ/mol of heat. In boric acid solutions with concentrations of >0.27M, well‐crystallized 4CaO·Al2O3·½B2O3·12H2O forms. The amount of heat that is generated increases as the H3BO3 concentration increases. At H3BO3 concentrations of >0.70M, the total amount of heat that is generated remains constant at ∼330 kJ/mol. The rate curves that are associated with hydration in boric acid solutions of >0.27Mare characterized by two reaction peaks, which are separated by an induction period. The first peak indicates initial and rapid hydration, which occurs during the first hour. The second thermal event reflects the remainder of the 3CaO·Al2O3 that is being consumed. The length of the induction period increases as the H3BO3 concentration increases and decreases as the temperature increases. Such behavior is indicative of diffusion‐controlled processes. An amorphous calcium borate layer is believed to precipitate around the active 3CaO·Al2O3 particles, which retards hydration. Higher temperatures and/or solutions of lower H3BO3 content favor the formation of highly crystalline reaction products. These conditions are associated with a short induction period. The rapid precipitation of 4CaO·Al2O3·½B2O3·12H2O under such conditions may cause “flash set.”