I would like to insulate and fireproof the back of my fireplace insert in my living room. As precast vermiculite boards are rather expensive compared to loose vermiculite granules I would like to cast them myself. I've bought a bag (100l) of vermiculite grains and on the bag it say that 100l of vermiculite should be mixed with 50l of water and 45kg cement.
I would like to add PP reinforcement fibers (image) in it but I'm not sure whether that's ok due to fireplace temperatures. Should I rather reinforce these boards using metal mesh or are PP fibers ok? I've checked these fibers and they seem not to burn but rather just melt when in touch with direct fire. I expect those at the surface (maybe sticking out) to do the same but those inside would likely be preserved regardless of temperatures.
I'd like to cast boards 70cm × 70cm × 6.5cm in size and fix them on the wall behind my fireplace insert. I would also like to know how much do these boards expand due to temperature differences so I can account for it where these boards touch the outside shell made of grog/chamotte bricks to store the fireplace energy that are then cladded with natural stone.
What's your experience with vermiculite?