Step 2 Casting and cutting concrete. Working with refractory concrete

Casting concrete

Making the moulds

It's practical to use plastic coated chipboards as a bottom for the moulds, as they are already waterproof and don't need to be painted. Saw the laths that form the mould and fix them to the bottom with screws (not with glue).

Paint the laths (2 layers). When the paint has dried, fill the inside brims with a bit of acrylic sealant.

If you use the mould again, first clean it well and fill the brims again.

Casts

There are two moulds to be cast: the hatch that closes the bottom of the stove, and the exhaust piece: the block with the opening for the flue pipe. Mixing ratio of the concrete is 1 portland cement : 2 sand : 3 fine gravel.

Hatch

550x200x24 mm

Exhaust block

550x200x50 mm with a Ø 122 mm opening for the flue pipe

Saw concrete

Cutting the blocks and tiles to size can be done with an angle grinder, but it’s much better and more precise with a water-cooled diamond circular saw.

The hatch is only 24 mm thick, so wait at least 5 days before demoulding.

To be sure the flue pipe will comfortably fit in, the exhaust piece has a cut-out of 122 mm: 2 mm larger than the flue pipe.

You can use the flue pipe as a shape for the opening. Cut a strip of plastic and wrap it around the end a few times. To hold the tube in place you tighten 4 screws into the mould. See the pictures.

Mark clearly and use a guide bar. With a 125 mm diamond blade you saw through a concrete tile in six passes: approximately 9 mm depth per pass.

The first two tiles, for the base layer, are 48 x 20 x 5 cm. First saw 6 cm from the short sides (see photo above), and then saw the tile in half.

The bottom slab of the stove, which will be laid on top of the base layer, consists of a whole and a half tile. There's a cut-out of 25 x 19.5 cm, 9 cm from the short side.

The blocks for the bell are 60 x 20 x 5 cm (a tile in half lengthwise) and 50 x 20 x 5 cm. For the latter, first saw off 5 cm from the short sides, and then cut the tile in half lengthwise.

Before casting, grease the mould well with oil to prevent the cured casting from sticking to the mould.

After pouring in the concrete, vibrate it by tapping the mould with a hammer for a few minutes. While curing, cover the mould with a piece of plastic. Even after demoulding, keep the castings in plastic to prevent evaporation. Concrete hardens through the reaction with water, and that's only completed after 28 days.

Handling fire concrete

Working with fire concrete is very different from working with normal concrete. Things like temperature, amount of water to be added and the open time for handling come very exact. Mixing and vibrating are skills to be learned by experience; if you are doing this for the first time it's wise to try it out with a small amount first.

Temperature

To process the material well, the concrete as well as the water to be added need to have a temperature between 10 - 20º Celsius. The very minimum is 7ºC. Exceeding 20ºC the curing process will be substantially faster. There may be too little time then to process the mixture well.

Water

Use good quality drinking water of 10º - 20º C temperature. From the product information sheet, calculate the exact amount of water to be added. Note that this can be very different per product! Do not exceed the maximum amount as indicated on the product sheet.

Mould

Before putting the concrete into the mould, the inside of the mould needs to be thoroughly greased with oil, otherwise the concrete would be fixed to the mould after curing.

Mixing

Definitely use a dust mask to prevent breathing cement dust. Generally the concrete is shipped in sacs as a dry mix, needing only water to be added. For the bell of the Roquetinho you can very well use home-mixed fire concrete: one part aluminum cement to three parts chamotte granules.

Weigh the amount of concrete you need and put it in a bowl. Add the measured amount of water and mix thoroughly with a trowel. Mixing time shouldn't exceed 3 minutes. You now have an earth-humid material that immediately needs to be shoveled into the mould.

Applying

Fill up the mould and start vibrating. After some minutes of shaking the concrete will get more fluid and air bubbles will move out. With a putty knife, push the concrete into the corners. If needed, add more concrete into the mould. Vibrating time shouldn't be too short, but certainly not too long: that would cause demixing. Water on top of the mixture is an indication of demixing. This should be prevented.

Hardening

The concrete hardens by a chemical reaction between the aluminium cement and the water. This may produce heat. Directly after vibrating, cover the mould with a piece of plastic to prevent evaporation of the water. At 15 - 20ºC, hardening will take about

6 -8 hours. 80% of its strength is then attained (after ± 2 weeks the concrete has fully completed hardening). At lower temperatures of working space and/or material hardening will take longer. Before demoulding check if the concrete has hardened well. To demould, take off one side of the mould and tap the form out.

As mentioned, it is definitely wise to first do a test with a small amount of fire concrete. Also because of the high purchase price of fire concrete, it would be a shame if a large form failed.

It’s a matter of trying it out to find the right quantities and proportions. Too much water results in reduced quality of the fire concrete, too little makes vibration very difficult. As an indication: for the longer parts 10 kg (2.5 kg aluminum cement on 7.5 kg chamotte) with 1.7 liters of water, and for the shorter parts 8.3 kg (2 kg aluminum cement on 6.3 kg chamotte) with 1.4 liters of water.

too much airbubbles

this is okay

Two moulds for fire concrete parts

Inside dimensions: 600 x 200 x 50 mm (lxwxh). Short parts:

500 x 200 x 50.