Na2CO3 start to decompose before melting(~800 C). Decomposition undergoes according to equation:
Na2CO3 = Na2O + CO2.
Only, when CO2 is added in close system, you can observe melting. Pure Na2CO3 can only exist in CO2 atmosphere at the temperature of melting point (860 C).
And, of cource, Na2O with water forms NaOH, but no NaOH is formed during sintering.