Hearth heat expansion cracks/lifting on brick surround

[Vespasian]

The oven build is a concrete base with a dense concrete block plinth topped by pavers. On top of the pavers I used an ordinary cement base about 2 cm thick, followed by insulated aerated blocks surrounded by facing bricks. On top of this I did a 2-3cm sand base with the firebricks on top. I then used refractory cement to fix the firebricks on which the vermicrete dome will stand, with the hearth bricks (where the fire sits) on the loose sand base to allow for expansion. The dome is vermiculite/ciment fondu mix at 1:4/1:5 approx, about between 2-3in, followed by a 2-3in vermicrete/portland insulation layer topped by a fire blanket then a render on top. The dome seems fine. However when the oven is at full heat there is a crack down the centre of the hearth that reaches down to pavers and about a 5--7mm crack around the bricks that rim the firebrick hearth. I suspect that the firebricks are expanding with the heat and pushing outwards that widens the crack to the front and lifts the hearth a few mm.

I have used a thermometer to check the heat around the cracks and its as cold as the surrounding brick so its unlikely to be steam etc, besides which if it was it wouldnt open and close with each firing, so its down to expansion of the hearth.

The question then is should I fire it then fill it with a heat resistant expanding mix, leave it as just a mechanism of the oven finding its own space when heated, or otherwise?

[fox]

Fire brick base must be allowed to expand, the joints should not be mortared and an expansion gap of around 2-6 mm (depending on oven size ) should be allowed around the perimeter .
I guess you realise that now... can I ask how well your oven holds heat? I would imagine with a low mass dome it won’t be very warm the next day?
The most successful designs use high mass refectory cement or high mass fire brick for the dome, as this absorbs the heat and stores it for a long time.
unfortunately vermiculite domes seen to have a short life span and they most certainly don’t offer the same long term performance as a high mass dense dome.

[Vespasian]

I'm fine with the dome. It heats well, it cooks pizza well and seems to hold the heat for a day or so, I've cooked bread in it several hours later. I didn't measure the vermiculite dome as I went so it's probably about 7 to 8 inches dome and the exterior dome render gets to 40 degrees with the oven at high temp so its insulating quite well. Not as good as brick but I didn't want a generation long build as I retire in 4 years and we'll downsize the house then so it would be a waste of good money.

I think you're spot on with the hearth. I followed several builds for the pattern and should have been advised by my own wisdom and left the expansion gap. The question now is whether to fill the gap when expanded. Or cold. And what to fill it with

[fox]

I am not sure if filling the gap will help as the structure needs room to both expand and contract, it has just made its own room to do so.
Good luck with your oven, I hope it will last for you.