The effect of SiO
2 addition on the decomposition of Na
2CO
3 was investigated using the thermo-gravimetric and differential scanning calorimetric method (TG-DSC). Addition of SiO
2 greatly enhanced the decomposition of Na
2CO
3 . The main decomposition reaction began to take place at a eutectic temperature of the Na
2O–SiO
2 system (800°C). The initial decomposition product was identified as Na
2SiO
3, irrespective of the mixing ratio of Na
2CO
3 to SiO
2 . The governing reaction of the decomposition was concluded to be Na
2CO
+ SiO
2 = Na
2SiO
3(s)+CO
2(g). The decomposition rate was independent of the Na
2CO
3 /SiO
2 mixing ratio until either one has completely been exhausted and hence was not available any more for the above reaction. If surplus Na
2CO
3 exists after formation of Na
2SiO
3 by reacting with all SiO
2, the remaining Na
2CO
3 decomposes partly by reacting with Na
2SiO
3 to form Na
6Si
2O
7 and partly by thermal self-decomposition. A liquid layer that was formed at the carbonate/additive interface facilitated the decomposition of the carbonates. The melting behavior of a mold flux was greatly influenced by the lowest eutectic temperature that a mold flux system can exhibit. The apparent activation energy of the decomposition of Na
2CO
3 in existence with SiO
2 was estimated to be 426 kJ · mol
-1.
View full abstract