The kinetics of ultrafast calcination and sintering of Ca(OH)(2) powder is studied in the 900-1050 degrees C temperature range in an entrained-flow reactor. Time-resolved kinetic data are obtained for 0-300 ms time scale with the residence time of the solids being measured on-line. The reaction exhibits very high initial calcination rates with sharp attenuation and subsequent virtual "die-off" at higher residence times. The interplay of calcination kinetics and sintering kinetics and the net effect on the overall surface area evolution is clearly elucidated. A modified grain model incorporating the sintering of the product CaO layer and the role of diffusion of evolved H2O is proposed to explain the overall phenomena. The model predictions are in good agreement with the observed experimental results.