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

Abstract

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 α-tricalcium phosphate (α-TCP) nanoparticles obtained by calcining ATCP powders. For those purposes, five calcination temperatures were selected in the range of 500 to 800 °C. Calcination led to the formation of powders consisting of ATCP (500 °C), α-TCP (600, 650 and 700 °C) and β-tricalcium phosphate (β-TCP; 700 and 800 °C). The powder reactivity was assessed by mixing 0.2 g of powder and 1 mL Na₂HPO₄ 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 °C, it increased to several hours for ATCP powder calcined at 600, 650 and 700 °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 °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.