At high temperatures (almost) everything burns to ash. Is ash 'fireproof'?
Is further combustion of ash possible for example by combining with hydrogen, oxygen, etc?
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Short answer no, ash can be burnt. It may contain unburned particles that did not receive enough air (oxygen) to burn completely.
Although one may consider ash to be product of complete burning. In which case it won't burn easily. But given the right conditions it can burn.
In fact even water can burn, there is quite a bit of research in burning aluminum with water (especially for under water applications like submarines), this produces aluminum oxide and hydrogen.