I just tried casting a ribbon burner out of kastolite30 and it totally came apart. I have made several before but with different refractories, and this kastolite is all thats available to me right now. I mixed in 18% water by weight just like the instructions said. I greased the mold and sprayed the straws with WD40. After pouring and inserting the block, I let it set at ~20C for 36 hours before demolding. After removing the sides of the mold, I tried to gently pull the block off the faceplate (that holds the staws). The plenum just separated instantly from the block. Pictures attached.

Is there anything I'm doing wrong? Should I give it more time next time? I've never used the kastolite before and I'd like to have a good idea of what to change before attempting again.

I haven't used it myself, but lots of people on here have talked about curing it in a high humidity environment. Easiest way is to put in a big garbage bag or a large tub (sealed) that also has a wet towel in it. The water doesn't have to be touching the refractory, but the humidity helps its curing process.

Was the failure within the kastolite itself or the result of poor adhesion between the kastolite and the steel? If the refractory itself is nice and solid, you might try reattaching it with a high-temperature adhesive. For example, Grainger carries a "high heat furnace cement" that's rated to 2,500° F.

I know that we are always railing against people using "refractory cement" in their forges, but that's a caution against using such products to form surfaces that would be directly exposed to flame. That would not be the case here.

It's kind of hard to tell from the photo, but it looks to me like part of the problem could be that your design doesn't provide much resistance to the block pulling off the plenum. Am I reading this correctly, that the stepped tabs are supposed to key into the cured kastolite? In that case, it may be that since they appear to be parallel with each other, any weakness in the refractory would allow the block to pull straight off. If you were simply to flare those tabs outwards, that would give you a lot more mechanical strength.