Refractory Castable - repair holes?

[daniel.sessions]

Firstly I'd like to start by saying a big thank you for all of the knowledge shared on this forum regarding refractory castable, it's been a big help with my oven build!

I recently cast my oven and I was left with some small air holes on the inside on the dome (2-5mm). I followed the advice on this forum and sieved out all of the aggregates from some spare castable until I had a fine grey power. I mixed this into a thick paste and filled the tiny holes, making sure the pieces were well saturated before applying. I then wrapped in cling film and left overnight. When I removed the plastic wrap the castable I'd filed the holes with hadn't set and had gone back to a powdered state, the exact consistency of how it was before I mixed in the water. Has anyone ever come across this before and would you be able to share some advice/tips if I've done something wrong?

I was thinking of buying a premixed fire cement/mortar to fill the holes if I can't get the sieved castable to work but I'm unsure if the cement would stick or fall out upon the first firing?

Any advice or help would be most appreciated.

Thanks

Dan

[david s]

Hi Dan,
You don’t say whether your castable is homebrew or a proprietary castable refractory although I don’t see that it would make much difference. The principle is that the stuff needs time to remain moist to allow the hydration to take place. I find that if done one or two days after casting then there is sufficient moisture left in the casting so that any mixture applied to it will set adequately. It sounds to me like your casting was too dry. The procedure is really only cosmetic. I think you should move on. I’ve seen commercially made cast domes that have lots of holes on the surface caused by insufficient vibration. Unfortunately if applying a mix over a sand dome, it is not possible to vibrate the mix.