A physical approach to modify the hydraulic reactivity of α-tricalcium phosphate powder

A microsized alpha-tricalcium phosphate (alpha-TCP) powder was calcined at various temperatures (350 degrees C < T < 800 degrees C) for various durations (1-24 h) and the resulting physico-chemical and reactivity changes were measured. Without calcination, the alpha-TCP powder started reacting within minutes after contacting a 0.2 M Na2HPO4 solution as measured by isothermal calorimetry. The overall reaction was finished within a few days. After calcination at 350 degrees C <= T <= 550 degrees C for 24 h, no significant changes in the crystalline composition, crystallite size, particle size or specific surface area were noticed. However, the powder reactivity was progressively changed. More specifically, the hydraulic reaction of the powders calcined at 500 and 550 degrees C only started after 2-3 h whereas the overall hydraulic reaction was only slightly postponed, suggesting that physical or chemical changes had occurred at the particle surface. As mainly physical changes were detected at the particle surface during calcination at 500 degrees C, it was speculated that the appearance of this reaction delay (= induction time) was due to the disappearance of surface defects during the calcination step, i.e. to the need to create surface defects to induce dissolution and hence reaction. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.