Homebrew Castable vs Firebrick pros/cons

[icrackracquets]

Apologies if this has been posted before but I'm having trouble finding the answers to a few of my questions

I'm relatively new to the forum. Got a strong desire to build a pizza oven in the past month but there's not enough time before winter to fully plan/measure/build anything. So i get to spend the winter researching and collecting materials so i've got everything as ready as possible.

From base research i've determined it's going to be a 36" pompeii style oven. Ideal usage would be about once a month for pizza and then some bread (either with the thermal inertia or assisting the hold in temperature) I will build an enclosure filled with insulation so that the dome is completely protected from rain and snow (i live in middle Sweden). What i'm having trouble with now is choosing between a homebrew castable and a firebrick dome. Price is a concern which is why i'm seriously looking at castable. Plus trying to find equivalent materials in Sweden is just a nightmare. I know the language rather well but this is advanced language stuff we're talking about here, not to mention general availability (i can't even find standard CMU here).

Some important questions I haven't quite been able to figure out yet:

What's a rough estimate of service life of a firebrick oven?
What's a rough estimate of service life of a homebrew castable?
I know from reading that a pro of the firebrick is that they are fully fire cured so I wouldn't have to worry about high temperatures in the oven at all and that with a castable I shouldn't go above 500C. My main question regarding this is, is this a realistic concern? Is going over 500C something that i'd have to actively try to do or could it happen accidentally.

I also just want some help with a general pro/con list for each of the oven types. I have never done WFO nor have I any other experience with WFO. Here's what i have so far

Castable
Pros: Cheaper, faster construction, not as advanced construction technique, thinner walls therefore smaller footprint
Cons: lifespan (rebuild after X years), parts availability (SS needles, etc)

Firebrick
Pros: lifespan (assumed to be quite long), measured and tested performance from countless builds
Cons: more expensive, steeper learning curve, more tools required (wet saw), longer construction time


Thanks for any feedback and sorry again if this is a 'repost'

[UtahBeehiver]

See post just made to Old Bricklayer.

[icrackracquets]

I don't understand what that means
I've looked at the forums and still haven't found the answers to my questions above or general pros and cons from people who have completed ovens

[david s]

Impressed with the performance of the homebrew mortar compared to that of commercial refractory mortars with their difficulty in application, expense and limitations regarding recommended maximum thickness, I could see no logical reason why the homebrew mix could not be used as a castable mix, given its performance as a suitable mortar for the service temperature range of our ovens as well as its highly workable consistency.
Back in around 2009 I think, I recommended to two locals that they try to build a cast oven using the 3:1:1:1 mix over a sand mould, because of the ease of construction as well as the minimal cost., not knowing how successful it would be in terms of longevity. Recently one of those individuals reported to me that his oven is still performing well. Obviously the life of any oven depends of frequency of use and the temperatures is is subjected to as well as how quickly those temperatures are reached. I wouldn’t recommend a homebrew cast for a commercialm oven that is fired daily, however they appear to perform as well as commercial castable.
You have outlined the pros and cons of brick vs cast builds well, but also the method of construction using a single one piece, cast in situ method over a sand mould has distinct advantages compared to a multi piece dome or one that requires relocation. After the cast is completed a separate flue gallery cast in front is advisable.
One disadvantage of using a homebrew castable s commercial castable refractory is its requirement to be damp cured which takes a minimum of a week. If this is omitted, full strength will not be attained. Calcium aluminate vented as used in commercial refractories achieve full strength far earlier and therefore do not require the extended damp curing.For the home builder this is a minor consideration given the speed of construction, but for manufacturers delays production.
Although the homebrew proves adequate as a castable for the dome and flue gallery, it will not withstand the higher temperatures and uneven temperature differences experienced for the floor. Firebrick, preferably loose laid proves to be a far more durable solution here.
For all ovens using unfired materials, the 500-650C range is the most destructive, but as this is generally out of our range it is not a concern.
In short the extremely low cost and far reduced labour of the homebrew cast oven are its main advantages. There are plenty of homebrew cast builds that are well documented in the Other Ovens section and should provide you with the appropriate information and first time builders experience.

[icrackracquets]

Right so above 500c is difficult to attain? It's not possible to accidentally achieve this, I'd have to be actively pushing for it?
What about the service life estimates?

[david s]

The surface temperature at the top of the dome will probably exceed 500C as indicated by a laser IR temp gun, but be below that deeper in. For this reason an air temperature thermometer in the centre of the oven is a more reliable indicator. Regarding oven service life, my previous comment is the best I can give you. We have not had any adverse reports from homebrew castable oven owners, but it’s hard to tell what usage they give their ovens.

[icrackracquets]

Ok, so it's unlikely that the oven would routinely get hot enough to chemically change the homebrew and therefore decrease the service life?

[david s]

Yes.
Portland cement begins to degrade north of 300C (do your own F conversion if that’s what you’re more familiar with). Lime begins to break down north of 500C and clay begins to sinter at 573C and becomes harder and denser the higher it goes. The idea is that if the Portland fails then the lime takes over. If the lime fails then the clay takes over. This explains why the proportion of cementious material in the homebrew is so rich. In the 500-650C range some serious chemical changes occur and for this reason the temperature must be ramped up very slowly and controlled, which is not possible in a wood fired oven using a very variable fuel input and a large air intake. Attempts to do so result in damage.
Additionally, apart from chemical changes in the 500-650C range there is also a big increase in thermal expansion of most materials, but that rate also varies considerably from one material to another which can cause micro cracking causing a reduction in strength. For these reasons it is best to avoid over firing any wood fired oven, but particularly those made from un-sintered material. Commercial ovens made from castable refractory (which use calcium aluminate cement also fall into this category.