The decomposition temperature of carbonates depends on the metal ion in the carbonate, For CaCO3 it is 840°C, for MgCO3 it is > 350°C.

Hi Govardhana,

Your problem is scaling and that represents insoluble carbonates [e.g., CaCO3, MgCO3, Ca,Mg(CO3)3 etc.] Or bicarbonates. If you have these alkaline earth metals in your system with pH more than 10, you can’t stop precipitation of these carbonatesa and bicarbonates. Simply keep your system‘s pH lower than 5 to avoid the scaling due to these insoluble compounds. If not, then keep it below 10 and make sure there is no Mg in your system as Mg(HCO3)2 is also insoluble compound like MgCO3. However, Ca(HCO3)2 is soluble several orders of magnitude than that of CaCO3 but you may still encounter some scaling in the form of Ca-bicarbonate depending on the amount of Ca in the system. You can checkout the solubility of it. Regarding temperature dependence of pH, here is the link that provides a good scenario for water as an example:

https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Acids_and_Bases/Acids_and_Bases_in_Aqueous_Solutions/The_pH_Scale/Temperature_Dependence_of_the_pH_of_pure_Water

Good luck,

Arshad