Step 3  Refractory concrete castings

 
Handling the 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 do this for the first time consider your first castings to be try-outs.


    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 hardening 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! E.g. for CaldeCast StrongLite you need ±225 ml/kg, but for Calderys F50 it's only 115 ml/kg. 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 hardening.


    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. 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 switch on the vibration table's motor. After some minutes of shaking the concrete will get more fluid and air bubbles will move out. With a filling-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 de-moulding check if the concrete has hardened well. To de-mould, take off one side of the mould and tap the form out.

 

too much airbubbles

this is okay

Mould dimensions


Riser bricks (insulating refractory concrete). Height of mould: 15 mm. You can make a seperate mould for the 330 mm bricks, or use only this mould and grind off 20 mm from 7 castings.

Heel (insulating refractory concrete). Height of mould: 24 mm. After hardening, the heel is adjusted with a grinder to make it fit in the riser.

220 x 160 mm

Top of firebox (solid refractory concrete). Height of mould: 30 mm

90 x 159 mm

Port (solid refractory concrete). Height of moulds: 30 mm

90 x 159 mm

220 x 91 mm