Perlite Mixed with Refractory Cement as Insulation Help!

[geraldevans]

Hello

I'm English but live in Italy but building my oven. 2 level pizza oven! Or at least that's the plan. I'm looking at mixing a 50/50 by volume for the outside of the oven with perlite and refractory cement I've read online that they use this for forges! I've a lot a refractory cement to use and perlite and I need to know if this is going to be ok and if I'm going to need foil on the outside of my oven before cementing? I'm hoping to do this tomorrow so please give me your hard earned advice.

Thanks

Gerry

[david s]

When you say refractory cement do you mean a premixed refractory mortar or are you using just the cement component (calcium aluminate cement)? The addition of any sand or high temperature fine aggregates in a premixed mortar will add strength to the brew, but reduce insulation (see attached table)
If you want that layer to be insulating then you need to add as little cement as you can get away with, usually around 10:1. Any richer than this will give you more strength, but enormously reduces the insulating capacity of that layer.. A 5:1 vermiculite or perlite, cement ratio (by volume) is approximately double the strength but half the insulating capacity of a 10:1 ratio.

Regarding the two level plan, I wish you luck. The only experience I have with this idea was when I tried baking bread rolls in my small 21" diam mobile oven. Because I had a fair crowd I made up a steel mesh rack which sat in the oven giving me a second layer. Unfortunately it was unsuccessful in that the bottom layer of bread did not get baked on top properly, i assumed this was because it was shielded by the loaves on top. I think because a WFO cooks largely by the heat radiated from the walls and roof rather than by convection in a conventional oven, that this was the cause. I've never bothered with the rack since.

[geraldevans]

Hello!

Thanks for the response so quickly. Yes it's early stages. The cement is a premixed refractory mortar made by unistar and so yes I plan to use it with perlite. I'm more interested in strengthening the whole thing first as well as a little insulation. I intend to add either ceramic fiber or rock wool afterwards as well! Do you know if I need a layer of foil over the firebricks? Some say online the cement shouldn't come into contact with the firebricks? If so how will it strengthen it. I also intend to have a large refractory stone halfway up the oven with 2 doors to have the 2 cooking areas. So the heat in theory should heat the shelf and pass up the back of the oven where there will be a space between he shelf and the back wall.

Any info is greatly appreciated as you know the work that goes into these passions!

Gerry

[david s]

If you are adding your 50/50 mix in an effort to gain strength then a foil layer between it and the oven will not allow it to bond. In addition a foil layer acts as a vapour barrier unless it's perforated. This can result in locking moisture in and making drying the inner oven more difficult.
As refractory mortar is designed to bond firebrick I can't see why you can't use it directly against the brickwork.

[geraldevans]

Hello

I think I've made a mistake. Thank God I didn't start yesterday! The refractory cement is a mortar and I just read it says maximum thickness 4cm so I'm guessing I'm mistaken in wanting to use this it is just for holding bricks together. Now I have sacks of the stuff! No problem I'll return it. What is the correct cement to use with perlite. For example we have a Portland 42,5R which has no aggregate in it which is very hard when set. We use it here in Italy for cement blocks or foundations. Would that be any good with the perlite. Man this stage is taking much too long! But I'm not a builder but love this project.

How do the pros do it?

Thanks for all your help

Gerry