Sodium in coal is one of the major species that cause fouling in combustion chamber. Low melting point sodium silicate or sodium sulfate evaporate then condense forming sticky layer of fouling deposit. It might be hindered by introducing of coal ash for sodium binding additive to the combustion chamber. However the possible interaction of sodium with other component in coal ash has to be studied. Blairathol coal ash was heat treated under reduction and oxidation condition in the presence of iron oxide, calcium oxide and sodium carbonate as additives. Iron oxide promotes the formation of cristobalite (SiO2) on the contrary sodium oxide hinders the formation of cristobalite and in some degree hinders the formation of high melting point mullite. The phase appear resulted from reaction among ash components with calcium oxide depend on the reaction temperature and quantity of the additive. Low quantity calcium oxide yielded phase namely anorthite and the high quantity yielded phase namely gehlenite. Sodium oxide reacted with alumina silicate in coal ash at temperature as low as 800℃ yielded phase namely nepheline. No phase was identified as a result of reaction between sodium and mineral other than alumina silicate suggesting coal ash could be used as sodium binding additive effectively.