Dear Nisar Ahmed there are many reasons, that could have happened.

Would be easier to understand which mix design you followed because it’s not clear.

However what was your target ? And what was the temperature you cooked your recipe?

About raw materials selection, Fly ash is too general term, class F or class C, because the first are more refractory coming from Anthracite coal than the second that comes from lignite coal too much rich on carbonates.

Why calcining the fly ash? it is waste from power plant so it has been already burned. But if you want to increase its amorphous content, you should have cooked them at least till 1500°C. Have you done something like that treatment?

Then you added kaolin, if I have red correctly , you have used an unburned kaolinite that is totally crystalline and not reactive in its natural form. This is a mistake because during the heating test kaolin loses its crystal water changing its structure from crystalline to amorphous (Metakaolin), with lost of ignition of about 12% of water, that is huge amount of gas that will break your recipe. If you continue to cook ,increasing the temperature above 1100°C ,will start the crystallization of mullite phase that has the same chemical formula of Metakaolin but is totally not reactive because is stable crystalline refractory mineral .

I can see you used sodium and sodium hydroxide, maybe thinking to obtain a Geopolymer material?

Not knowing what type of fly ash you have used and considering that kaolin isn’t reactive to obtain a Geopolymer reaction you have instead obtained an Alkali Activated Materials (it isn’t a mineral polymer, but a hydraulic materials with CSH like precipitations) that contains an unbalanced amount of alkali salts, not used from the system described as precursor, that generates NASH (where H means hydrate so water ) and this kind of product is not stable when calcined because it generates gas emissions (the crystal water will boil) and it reduces the melting point of the recipe.

Other problem for a material you want test in temperature is the use of boric acid (It is normally used ,in ceramic tiles manufacturing, to reduce the melting point 🤷🏻‍♂️, so it works against the refractory behavior of the product you wanted tp achieve ) that it has also acid pH so reduces the effect of the alkalis you selected. Last issue the adding of calcium carbonate has no sense because at 850°C it becomes CaO emitting CO2 that is other gas that will leave you material creating cracks during the fire test. So it is already a miracle that the sample didn’t explode completely during the heating or at the cooling because inertial internal temperature and pressure, due the phase changes and gas emissions.

The picture shows laterally the swelling due the expansion because the exit of the gasses.

Best regards

Dr. Alex Reggiani