The reaction of limestone in the slaking process has been studied under different conditions of parameters such as calcination times, soaking times and particle sizes. The reactivity of quicklime was determined by recording temperature rise and the rate of temperature rise during the slaking process. The obtained ‘milk of lime’ is characterized using instrumental measurements such as X-ray fluorescence (XRF) to identify the chemical composition that exists in the sample. In this paper, the quicklime used during slaking test indicates the differential of temperature pattern which influences by quicklime particle sizes, calcination temperatures and soaking times. Results indicate that the optimum and efficient distribution of heat transfer and thermal decomposition onto quicklime can be obtained by calcining at 1100 °C with 60 minutes of soaking time samples which produced a highly porous structure towards higher reactivity of quicklime.