Burnout fibers in refractory (concrete) casting

I understand that burnout fibers are used to provide, once burnt out, channels for steam to escape. Yet some fibers don't melt until above water boiling temperature. For example I have included polypropylene in refractory test blocks cast recently. Most polypropylene melts at 184 degrees Celcius.

I wonder if my test blocks will explode when fired, due to steam formation.

Maybe wheat or rice etc. noodles would degrade below boiling, or wax threads.

Brian
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The best fibres to use are counter top reinforcing fibres but some may be made from  fibergalss and some from nylon or other low melting point material.
They are incredibly thin, much thinner than a human hair, the ones I buy melt around 160c but you can easily test them with a flame gun (or just a match).
All refractory should be slowly heated up over several fires, depending on the size and thickness this could he 2-3 fires or 10-20 fires.
Once the refractory has been cured and complexly dry, all the fibre will have melted out, any further moisture that might be absorbed from the air or directly from water ingress  will be dispersed without cracking the component.