Your problem is not the binding agent, but the formation of gas during sintering. This is the only way to explain the decomposition of the material. I assume that your ash was not calcined beforehand and therefore contains gas-forming substances (carbonates, chlorides and others). Only when you beforehand calcine the ash at temperatures above 800°C can you use it as an additive to ceramic mixtures with clay.

Exactly, that's what we have not covered. As the calcination also helps in greater binding strength later on along with impurities removal. Thank you Sir for providing valuable answer & filling on my gap..