DIY Pizza Oven Cure time / Insulation Questions

[curiousgeorge]

Hi all,

I'm making a DIY Pizza oven (see attached picture) using maximizer concrete and finishing the inside (for heat) and outside (for aesthetics) with mortar mixed with fire clay. The walls are about 3.5" thick, diameter 29", and I have not applied the mortar yet. I have a few questions:

1. How long do I need to wait before applying the mortar? Technically it says 28 days for a full cure of the concrete, but I've also ready that mortar is more readily binds to wet concrete. Any thoughts? Hoping to not have to wait that long...
2. Any thoughts on how thick of mortar I need on the inside? I was thinking about 1" at the top of the dome down to .5" down the sides. Hoping to keep it under 250 lbs.
3. For the floor I was going to lay 1.5" of concrete and then 1" of mortar/fireclay. Is there any reason to use fire bricks instead?

Thanks for your help in advance!

-George

[wotavidone]

I had to look up maximizer concrete. I don't think I've ever seen it in Australia.
Anyway, the SDS I found for SAKRETE brand says it is based on Portland Cement.
I don't think it right for the job.
Most people who cast/mould domes with Portland cement for binder include an equal amount of slaked lime (calcium hydroxide) so, when the heat inevitably weakens the Portland cement, the lime, which will have begun converting to limestone (calcium carbonate) will offer some strength.

Anyway, it's done now, so may as well carry on and see what happens.
I would put any extra layers on wet to wet.
An important consideration is that lots of cement based things shrink when they dry, so it would be best if it all shrank together.
I don't think the floor is going to work well with mortar mixed with fireclay. I suggest firebrick splits.

[Jon in Albany]

I think cement paste in general bonds better to wet, existing concrete. Some rehabilitation have a soaking procedure before putting new concrete up against existing. Never seen or heard of this material. This is the first oven I've seen with it.

"Cure" is a word that can get confusing because it is used a few different ways. When they are saying it takes 28 days to cure concrete, they are talking about strength gain. When we talk about curing and oven, that is more about slowly driving excess water out in a manner that will minimize any cracks. Take a wet oven up to 800 degrees on day 1, all that extra water turns to steam, has to get out of the 800 degree oven wall and causes a big crack...and that can still happen on Day 28.

Is there any insulation plan?

[curiousgeorge]

Thanks for your replies wotavidone and Jon.

For clarity, Maximizer concrete is essentially your basic concrete with an aggregate in it to reduce weight. The mortar I'm using is EZ Mix Type S, which is composed of cement, hydrated lime and dried masonry sand. I then added fire clay to the mix.

I ended up applying the mortar on top 2 days after molding the concrete portion. I wet the cement before applying the fire mortar mix. So far it's held up but I have not fired it yet.

Jon - noted about the curing process. Will definitely follow a 6-day curing procedure before reaching full temperature. No insulation plan...I've seen some other use actual insulating fabric between layers of concrete but I skipped that to save costs, size, and weight. Hoping the 4-5" walls will be sufficient

Here's a picture of it with the mortar applied. I'll send an update when I first fire it!

[Tscarborough]

Maximizer has expanded clay shale aggregate, so it may be alright.  It is going to crack, no question, but with the plaster inside, it should not crumble too quickly.  Note however, that Maximizer will take much longer to dry out.  Go slow with the curing.