Effect of thermal treatments on the reactivity of nanosized tricalcium phosphate powders

This study had two aims: (i) to determine the effects of calcination temperature on the hydraulic reactivity of three nanosized X-ray amorphous tricalcium phosphate (ATCP) powders, and (ii) to try to bring a better understanding of the poor reactivity of a-tricalcium phosphate (alpha-TCP) nanoparticles obtained by calcining ATCP powders. For those purposes, five calcination temperatures were selected in the range of 500 to 800 degrees C. Calcination led to the formation of powders consisting of ATCP (500 degrees C), alpha-TCP (600, 650 and 700 degrees C) and beta-tricalcium phosphate (beta-TCP; 700 and 800 degrees C). The powder reactivity was assessed by mixing 0.2 g of powder and 1 mL Na(2)HPO(4) 0.2 M solution in an isothermal calorimeter. The total reaction time was a function of the calcination temperature: from a few minutes for ATCP powders and ATCP powder calcined at 500 degrees C, it increased to several hours for ATCP powder calcined at 600, 650 and 700 degrees C. Interestingly, this change of reaction time was mostly due to an increase of the so-called induction time, i.e. time to start the reaction. It increased from a few seconds to a few hours. Strikingly, the induction time and the total reaction time were reduced by an increase of the calcination temperature from 650 to 700 degrees C. The results were attributed to the formation of a defect-free surface upon calcination and the concomitant difficulty in creating surface defects to allow dissolution and hence reaction.