Refractory Concrete Tutorial - Recipe and Instructions for Mixing it on Site Without Using a Concrete Mixer,

Heat resistant refractory concrete

Ingredients:

River gravel or crushed fire bricks
Sand
Calcium Aluminate cement. (if available in different grades choose the better one)
Lime (lime is usually cheaper, about 80% of the cement price)
Water
You can add into this mix a little standard Fireclay if you like.

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

What sort of lime is it? Hydrated Lime?

Yes it's hydrated lime, Calcium hydroxide, commonly sold in shops in bags. Calcium hydroxide is chemical compound with the chemical formula Ca(OH)₂ or traditionally called; Slaked lime, pickling lime but most often by builders as the Hydrated lime.

Put dry gravel and sand on a thin metal or plastic sheet, or on a clean concrete surface in an area you can make a bit dirty. (Be sure to clean the area and tools with water after working with cement.)
Add the cement and lime on top of the gravel and sand.
Calcium Aluminate cement and Lime will work the best as the cement in hot cooking conditions- This applies to refractory concrete as well as to heat resistant mortars.
(Lime is sometimes mixed into fire clays. This is mixed like a normal cement based mortar, but with half the cement replaced with the lime. The cement holds it together when it is drying but when the heat gets into the cement and burns it out, the lime holds it all together. Lime used in building - calcium hydroxide Ca(OH)₂ [oxide of calcium CaO of limestone] is used for making mortar and cement.)

Ingredients in the now days Portland cement are:
60% to 67% CaO
17% to 25% SiO₂
3% to 8% Al₂O₃
up to 6% Fe₂O₃
plus can contain also mini amount of MgO, MgSO₄, Na₂O, and K₂O

Before adding water, mix all dry ingredients together well with a shovel by taking from the bottom and placing every shovel on the top until the mixture turns to a uniform color.

Create a hole in the middle and pour in about 2 liters of water. Continue mixing with a shovel as you did with the dry ingredients. The mix will become semi dry.

Add another couple of liters of water and mix it again the same way.

It's good exercise, but it doesn't take too long before you start to notice the concrete becoming workable. If it's still too dry, add just a bit of water, but not too much at this stage because the consistency can change suddenly. If you add too much, don't worry; add more dry ingredients in the same ratio and just mix it in. The mixing could be done by anyone. (read about correct water addition down the page)

Mixing the refractory concrete this way, without a concrete mixer machine, is not a very hard job. It's good fun and I find it easier than mixing the pizza dough. A single dome for a wood burning oven takes about 0.2 cubic meter of gravel- it's a small job, and it's interesting.

The right consistency is achieved when you stop seeing dry patches in the mixture, and when not too much water is pooling or preventing the concrete from keeping its shape on the ground as if it was kind of standing rather up than flowing. When you pile it up with the shovel it should sit, stay high and hold together. For example, the correct consistency is achieved when a handful of concrete can be formed into a ball the size of a tennis ball and be tossed half a meter up into the air and then caught as a mass that stays together. You can easily test it for your selves. If it's too wet or too dry you will be able to tell.

Complete Wood burning oven building tutorial with 370+ photo series

A bit about 'refractory concrete' drying.

The following information is important to ensure correct concrete curing. After an initial concrete set, cover your work with plastic sheeting and spray it with a fine spray of water to prevent rapid loss of water on edges and the surface.* For the first 24 hours, drying should be slow. After 24 hours, remove coverings and let air dry without strong sun for 48 hours.

It depends on the weather, but complete drying can take up to 3 weeks if it's cold outside. Start with short, small fires in the oven for the first time. Increase temperature gradually not quickly. If there is any remaining water in the dome walls, heat could change it to a steam and crack the wall. With a small fire you will help to drag that remaining water out.

Enjoy.

Rado

* When concreting do not apply water on top at the end to smooth the surface with trowel or leveling tool. Your concrete block would lack the hard finish on the outside and you could experience little stones on the bottom of your pizza and between your teeth for a while until you pick them all up. Compared to fire bricks, refractory concrete hearths may develop peeling problems and cracks down the track. Like with our Earth Globe and it's tectonic plates- They are actually a large segments, if some part can not freely move a crack gets naturally created. In regards to the movement in dome, we are talking only about a couple of millimeters or less. Brick domes consists of many small segments-bricks, they create nice and natural movement in the structure.