Minne"snow"ta castable build....... I think?

[jakeanderton]

Hello, After lurking for several months I am finally posting. While I am not new to pizza ovens, I am new to undertaking something of this magnitude. Previously, I have built a vermicrete oven that was wonderful (see pics). However, it wasnt as durable as I wanted, and forming it over a gym ball only allowed me enough room for two pizzas at a go. I want something capable of three to four 10" pizzas at a time. So, I am starting a new build. I am planning on using KS-4 Plus as the castable, with 3" of thickness over a firebrick floor, mold would be of sand. Has anyone done a castable of an inside diameter as large as 38"?

[jakeanderton]

Work so far:

Base pad of 5" thickness with 4500PSI concrete, reinforced with remesh, which was left to cure for 10 days.
Built the block up and filled in the cores with concrete, reinforced with rebar
Built the form for 4" thick pad of 4000PSI concrete with remesh and rebar for reinforcement. Currently, on day 2 of a 10 day cure, watering each day and covered with a tarp. Plan to remove the side forms in 4 days. Leave the support forms in place for a total of 10 days, before removing.

[jakeanderton]

After removing the forms, the next part will be building the perlcrete base under the floor of the firebricks. Any recommendations on the thickness this pad should be? My previous build had it 1 1/2" thick with the firebricks on top.

david s seems like you know your stuff and would be interested in your thoughts.

[david s]

Some folk say you can’t have too much insulation, but there comes a point where more doesn’t really improve things as it’s subject to the law of diminishing returns. It also depends on how insulative is the material that you choose.
2” of calcium silicate board is considered the minimum. For the last big oven I built I used two layers (80mm) of 40mm thick calcium silicate board. If you cast your own slab with vermicrete to save money rather than paying outrageous price for cal sil, you need to make it strong enough (5:1) to hold the weight. This reduces its insulation value so to get the equivalent you need double the thickness of the cal sil.
For my ovens I use a combination of 25mm 5:1 vermicrete over 45 mm loose perlite.
Check the insulative value of the product you want on their data sheet. The insulation value is the reciprocal of the product’s thermal conductivity. This can be a tricky exercise because thermal conductivity is quoted in a number of different units which means you often have to go to conversions to make comparisons.

[david s]

For the castable refractory you are using, check to ensure it contains burn out fibres (most of them do). If you can’t find the answer on the data sheet take a small amount of the product and sieve some with a small fine sieve (a tea strainer will do) These fibres are essential to reduce the possibility of steam spalling that can ruin your casting, exploding from the middle of the cast wall. Also make sure you have dense castable not insulating castable, otherwise you’ll have the same problems you had with your perlcrete/gym ball oven. Do not mix any more than half a (25kg) bag at a time because the stuff goes off really fast. The stuff is very temperature dependant, so In summer I always use chilled water for this reason.
Interested to know how the perlcrete oven performed, it’s heat up time, ability to retain heat and how and where it failed.

[Americana]

Jake

We have used the KS-4V plus in both pizza ovens and masonry wood heaters with good results. I recently completed a 40” cast dome for a pizza oven.
This was a segmented oven hand cast over a mold.

I have cast over sand using a a square of masonry to retain the wet castable. Would have been nice to hand cast it and keep a consistent thickness. (Drier mix) Cracked a lot being a monolithic wetish cast but has worked well.

I will post my build in a bit.

Rod

[jakeanderton]

For my ovens I use a combination of 25mm 5:1 vermicrete over 45 mm loose perlite.

Good to know. I think I will do 50mm of 5:1 perlcrete for the insulating base. Then the fire bricks on top. Not sure I like the idea of a loose fill anywhere in this build.

[jakeanderton]

For the castable refractory you are using, check to ensure it contains burn out fibres (most of them do). If you can’t find the answer on the data sheet take a small amount of the product and sieve some with a small fine sieve (a tea strainer will do) These fibres are essential to reduce the possibility of steam spalling that can ruin your casting, exploding from the middle of the cast wall. Also make sure you have dense castable not insulating castable, otherwise you’ll have the same problems you had with your perlcrete/gym ball oven. Do not mix any more than half a (25kg) bag at a time because the stuff goes off really fast. The stuff is very temperature dependant, so In summer I always use chilled water for this reason.
Interested to know how the perlcrete oven performed, it’s heat up time, ability to retain heat and how and where it failed.

I will look for fibers. If it doesn't contain any, can you recommend some for mixing into the dense castable?

Agreed on utilization of the castable. I plan to go in stages. Making a single layer all the way around, at maybe 6-8”. Then coming back and doing another layer the next day, and repeating the process until done. I figure the layers will allow a sort of built in expansion joint and lead to less cracking?

The perlcrete oven did quite well for 3yrs. Heat up time to 700F only took 20mins. It didn’t retain heat for very long. After 2hrs of cooking pizza I could put my hand inside and feel warmth, but not get singed. Ultimately, I wanted to be cooking pizzas faster because I ended up cooking pizzas and being the last to eat at a party. Not what I wanted to be doing at gatherings.

[jakeanderton]

Jake

We have used the KS-4V plus in both pizza ovens and masonry wood heaters with good results. I recently completed a 40” cast dome for a pizza oven.
This was a segmented oven hand cast over a mold.

I have cast over sand using a a square of masonry to retain the wet castable. Would have been nice to hand cast it and keep a consistent thickness. (Drier mix) Cracked a lot being a monolithic wetish cast but has worked well.

I will post my build in a bit.

Rod

I would be VERY curious to see your build. That’s what I am attempting as well. I couldn’t find anyone doing a castable over 32” and this give me hope that my large size could work!

[david s]

The burnout fibres if not already contained in your dense castable product (do not use insulating castable or you’ll have the same problems as the perlcrete domes encounter. Ie poor strength and poor thermal mass) can be easily sourced but using only the fine fibres contained in a two part pack designed for reinforcing concrete. I use Sika Radmix 48/19. It’s the fine 19 mm fibres you need to use. As they are both polypropylene the thicker longer fibres are unsuitable for heat applications, use them somewhere else. For strength enhancement use either stainless steel needles (search melt extract fibres) or AR (alkaline resistant) zirconium coated glass fibres. These are not suitable for a kiln, but are ok for an oven as their melting temperature is around 900C, well above our service temps. These fibres are commonly used by the concrete counter top guys so that’s the best source.

Layering is ok, but is best done immediately, the longer the time spent between layers, the greater the chance of bonding issues and possible delamination.

50 mm of cal sil is considered the minimum insulation thickness. To get the equivalent insulation value from a 5:1 vermicrete, you need to make it 100mm thick.
In my build the two piece, triple layered floor sits over a basin in the supporting slab which contains the loose perlite

[jakeanderton]

The burnout fibres if not already contained in your dense castable product (do not use insulating castable or you’ll have the same problems as the perlcrete domes encounter. Ie poor strength and poor thermal mass) can be easily sourced but using only the fine fibres contained in a two part pack designed for reinforcing concrete. I use Sika Radmix 48/19. It’s the fine 19 mm fibres you need to use. As they are both polypropylene the thicker longer fibres are unsuitable for heat applications, use them somewhere else. For strength enhancement use either stainless steel needles (search melt extract fibres) or AR (alkaline resistant) zirconium coated glass fibres. These are not suitable for a kiln, but are ok for an oven as their melting temperature is around 900C, well above our service temps. These fibres are commonly used by the concrete counter top guys so that’s the best source.

The KS-4 Plus is a dense castable. In reading the data sheet, I am not finding any fibers present.

Having a little difficulty in finding the fibers in my area. I can easily get these:
But unable to find something less than 1”. Unless it’s a fiberglass material. I’m guessing the fiberglass is not what you’d recommend.

I’ll adjust the thickness of the perlcrete floor or do the ceramic fiber board.

[jakeanderton]

Removed the outside forms today, and planning to remove the bottom supports in a week (13 days after the pour). Built the form for the perlcrete base layer separately, and pouring that in the garage so there is some overlapping cure time on both items.

[Americana]

The KS-4V Plus has fibers in it.

[fox]

I use a total of 4” ceramic fibre board under the floor on my oven builds, where I live CFB seems to be more reasonably priced than it is in Australia at least.
From my experience using 2 inches of ceramic fiber board is fine for many folk but is not quite enough for long heat storage, so I double up the board.

There is no doubt that perlite or vermiculite cement mix does work but it is just so much easier and faster to use board.

4” - 6” of precrete will take weeks to dry out (depending on weather and humidity) but you can be building on top of CFB as soon as it is in place.

Americana that looks like a fantastic hand cast job you have done there!

[jakeanderton]

I use a total of 4” ceramic fibre board under the floor on my oven builds, where I live CFB seems to be more reasonably priced than it is in Australia at least.
From my experience using 2 inches of ceramic fiber board is fine for many folk but is not quite enough for long heat storage, so I double up the board.

There is no doubt that perlite or vermiculite cement mix does work but it is just so much easier and faster to use board.

4” - 6” of precrete will take weeks to dry out (depending on weather and humidity) but you can be building on top of CFB as soon as it is in place.

Thank you for the response. The board ends up being quite expensive in my area; $19/sq ft. I am going to stick with the perlcrete for the cost. I am not worried about the drying time.