The results of free lime as a function of firing temperatures are illustrated in Fig. 4. The free lime content of mix R1 decreases with the increasing temperature. This was due to calcium nitrate could be easily decomposed at low temperature [22], as given in Eq. 1, and consequently increased the rate of reaction with high reactivity of nano-silica. Meanwhile, free lime content of R2 increased with function of temperature, whereas decreased at 1100C. It is well known that the calcination process of calcium carbonate begins at above 800C [ref], which lead to slowly react with nano-silica. For over 1000C, the decomposition of calcium carbonate was complete, resulting in a decrease of free lime. Also, the slow rate of decomposition of calcium carbonate of R2 has an effect on high free lime content, compared with R1.