Perlite and gym ball oven question

[Jalla]

In videos online people mix perlite, Portland cement and water and add this to a gym ball. Can I, in stead of Portland cement use refractory mortar to the mix? Will it do the same job as ordinary Portland cement?

[david s]

Yes you could but I don’t know why you would want to.
A calcium aluminate cement will certainly help to handle the temperature (whilst increasing materials expense), but using a lightweight aggregate like perlite ior vermiculite n any mix reduces its strength (see attached table) resulting in a casting that will be very susceptible to knocks and abrasions. In addition, because it’s so lightweight the casting will lack thermal mass, a desirable quality for a pizza oven and essential if you want to do retained baking and roasting. The longevity of these lightweight cast ovens is still an unanswered question, but as I’ve not seen any reports on their long term success, despite invitations to provide reports, I’d assume they haven’t lasted too well.
A far better approach IMO is to make a dense casting then insulate over it well. There are many suitable castable refractories that contain calcium aluminate cement. Refractory mortar is a product designed to be used between brick units and should not be diluted with an added aggregate.

[Jalla]

david s, what if I make a solid concrete dome, then cover it with insulation. Will that be a better solution?

[UtahBeehiver]

I suggest you review some of the cast refractory ovens (some commercial refractory mixes and some home brew mixes) on the site to give you a better understanding. There are several that have been done recently.

[SableSprings]

Welcome to the Forum Jalla. A problem with using Portland Cement for your oven dome casting is that concrete does break down in the normal heat of firing. Even though the home brew mortar mix recommended here has one part cement, it is the lime addition that provides the long term strength and durability. In addition to the durability issues of perlite (and/or vermiculite) noted by David when used in as oven casting materials, they are also insulating materials...not heat retaining. So, the concept of using these materials in construction of a WFO dome oven for use in any longer term roasting or baking (without an active fire) is not realistic (IMHO ).

Although the gym ball/perlite-cement video has attracted a lot of attention, we have not had many positive responses on the results by members on the forum who have tried the method. As with so many things, "quick, simple, and dirt cheap" may simply provide you something else to waste your time and energy on.

We are always open to new ideas and methods that improve wood fired oven core construction/efficiency, but so far, using firebrick, refractory casting mixes, or home brew are still the best options based on many home builder and commercial experiences.

As Russell noted, there are many well documented builds on the Forum using casting or brick. Your best investment at this stage is to spend a lot of time reading and learning how others have faced and dealt with building a WFO that can be used and enjoyed for a long time.