Castable and firebrick grog mix

[colinm]

Can anyone tell me if it is OK to mix castable and firebrick grog? And if so, what ratio should I use? I would like to cast a keystone for the top of the dome and it would be nice if it sort of matched the bricks - I know that it will not be seen, but what can I say - i’m an engineer!

[david s]

A proprietary castable already has the CAC cement and high temperature aggregates added in the correct proportions. When fired to service temperature it becomes essentially the same as a firebrick. Normally the castable is unfired ie it just relies on the CAC cement, but for a small piece like a dome plug you could place the castable in a thin plastic bag, place it in position, then remove it after a few days, dry it and take it to a potter who can biscuit fire it to 1000 C.If you add more aggregate by cutting the mix with crushed firebrick then you are weakening the mix. When adding firebrick aggregate the dust should be discarded and only larger pieces up to 5 mm should be added. Make sure you get dense castable not insulating castable.

[colinm]

Many thanks - I’ll stick to plain castable