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Hearth slab: castable refractory cement?; DIY - coverage

Gadjowheaty's picture

Hearth slab: castable refractory cement?; DIY - coverage

Hello all, just pricing out a possible brick oven project.  I'm weighing how much more costly it would be to go with castables v. my terrible back, and PITA to do my own mixes.

Having some trouble finding castable refractory concrete.  Plenty of cement, mortar.  I do see Heatcast 40, but I can't tell if that's a concrete ready to pour for a slab, or I need to add in components like below.  Also, hoping to come in much less costly (just a hearth slab using this alone comes to $1800!).  Any recommendations for the pre-mixed refractory concrete?

If I do go the DIY route, I was using the recipe for refractory concrete of:

Mixture: (parts ratio is 3 x 2 x 2 x 0.5, plus water)
3 shovels of the gravel or grog/crushed firebricks
2 shovels of sand (Portland concrete type only)
2 shovels of the cement
1/2 shovel of lime (for Portland concrete type)
This amount will require approximately 6-7 liters of water to mix the concrete.

My slab is 8.7 c.f.  How do I go about calculating the volume of the above recipe?  Meaning, I can buy the cement with known coverage, but obviously once I add in the other things I will have much more than needed for the slab (I think.  I'm a space cadet).  Anyone have any guidance on figuring out how much cement (and therefore, all the rest of the components) to buy?

Thanks, all.

Beth's picture

To determine the yield of your mix, you

1) Assume the volume of each shovelful (say, 6 shovels = 1 cubic foot) to convert quantities from shovels to cubic feet
2) Multiply the volume by the bulk density (i.e., the weight of the individual material that will fit in a 1 cubic foot mold - this is much less than the material density because there are spaces between individual rocks) to determine the weight of each constituent of a batch. The sum of these will give you the total weight of the batch your recipe makes. (Don't forget to include the water! - note that if you are being precise, you also need to add the water that is on your aggregates when you scoop them.)
3) Divide the weight of each constituent by its material density (or specific gravity times unit weight of water) to determine the amount of space it will take up in the mixture. Sum these, and add 3-5 percent for air. This will give you the total volume of the batch your recipe makes.

Just like when you make bread, it is more precise to measure by weight than by volume. Even worse, there is more variation in aggregates than in flours. Here's a spreadsheet with some reasonable starter assumptions:

Concrete Spreadsheet

Based on these assumptions (with 7 liters of water) and 3% air, each recipe produces 0.97 cubic feet. I would allow for 10-15% loss to spillage/sticking to the mixer when ordering materials, which will put you at about 10 batches.

Gadjowheaty's picture

Wow, fantastic, thank you Beth!  Extremely helpful.

idaveindy's picture

I'm not up on all the terminology, but DIY ovens intrigue me.

Is this castable slab you're talking about... 

a) to go under the cooking surface of refractory brick, or...

b) to be the cooking surface?


Gadjowheaty's picture

It's the hearth slab, which goes under the firebrick, the cooking surface.  Do you have a copy of The Bread Builders?  Fantastic book in my opinion - my introduction to all of this, so I'm completely new to this myself (the forum on brick ovens here is really insightful, I've found.  Beth's above is really helpful!).  Most immediately want to play with building a mud oven, but once we settle over the next couple of years, I really hope to build a true brick oven.

All:  I know this has been asked a million times, but I actually tried to get ahold of Nick Scott, the late Alan Scott's son, via his FB page.  I DM'ed a couple times, never connected though I see he posts every now and then.  Does anyone know of a source for TBB's plan, as used to be sold by Ovencrafters?

idaveindy's picture

Have the book, but only skimmed-read it, and looked at the pictures.  I was confused because of my previous exposure to the word "refractory", which in my previous context, "I thought" specifically referred to surfaces exposed to flame.

I realized from the book's photos, and online photos, that some  kind of slab goes under the refractory brick cooking surface.  But when you called that slab "refractory", I got confuzzled.

So does "refractory" refer to any material that has to undergo significant heat expansion/contraction, even if not exposed to flame?

I got enough out of my skim-read to pick up that the aggregate in the concrete needs to be such that it allows thermal expansion and contraction over the heating/cooling cycles.   I think there was also a part about using some kind of loose insulation on the dome so that it can fill in any cracks that develop on the upper concrete cladding that covers the upper refractory bricks.

Lots of choices for DIY and make-shift ovens.  Even using large clay flower pots, two of differing sizes and using vermiculite to insulate.  One problem is finding pots/tiles/whatever made of material that won't outgas poisonous fumes.

I don't mind baking in my kitchen oven during the 7 months of the year when I don't run the air-conditioning.  But I finally realized that the extra cost of air-conitioning, in the other  5 months, is still a lot cheaper than building and operating an outdoor oven.  And I don't have a convenient and cheap source of firewood, so I would likely have to use propane to heat it anyway.

Gadjowheaty's picture

Hey Dave,


Keep in mind I'm sure you know a heck of a lot more than I do - the book was my first introduction to these materials and methods, so take it for what it's worth.

Refractory - yes, that's my understanding too in that it refers to the material's ability to withstand heat (not necessarily direct flame).  I think, though I'm not certain, it specifically indicates the material not only withstands the rated heat, but will not be chemically altered such that it begins to decompose or slough off (e.g., like red brick can begin to break down and start dropping material on top of the loaves.  The process actually has a name, though I've forgotten).  For the hearth slab, I'm price comparing using castable refractory concrete v. DIY, which I got from, of 3:2:2:.5 fireclay grog, sand (portland concrete sand only), refractory cement, lime.

I'm in a cold climate so I'm doing a good deal of thinking on hyper-insulation.  His underlaying insulation for the hearth slab is a 6:1 cement:vermiculite mix.  His insulation on top of the concrete cladding coating the dome is loose vermiculite, which he indicates is useful to fill in any minor cracking with heat cycles over time.

If you haven't checked out "Build your own earth oven," it's a pretty cool book.  Mud ovens, with a lot of connection to the Quebec ovens, themselves based on traditional French peasant (and I'm sure, traditional everywhere throughout a good part of the globe) ovens.  FYI also, the book is fascinating, and is available online (.pdf):  The Bread Ovens of Quebec.