Problem with CalSil Board

[Pete C]

Hi Everyone,

I am thinking of starting a build thread, but haven't yet since my build is nothing unusual- a 36" Pompeii built very close to the FB instructions. However, I have just had two days hard work on the oven just got erased for me and I want to share what happened.

My floor insulation is a 4.5" layer of #4 Perlite and Portland cement mixed in the ration of 6:1. I mixed and applied it carefully and it set up and hardened very well. I should have built my dome directly on that. But I wanted to go the extra mile, so I purchased a 1" layer of ceramic fiber insulation board and placed it on top of the set Perlcrete. I sure wish I hadn't. I had about 3 courses set so far, and wanted to build the arch all the way across before I continued the dome. But as I wet the bricks to keep the mortar from losing its moisture to quickly, the ceramic board began to get wet, spongy and soft. So much so that when I lightly tapped my arch keystone into place, the entire arch separated from the dome and failed in multiple places due to the increasingly soft surface that it now sits on.

I spent alot of time on that arch.

So, my question is: has this happened to anyone? Will this material regain its compressive strength once dry? (I don't know, since even after 24 hours on 100F California heat, it's still wet and soft). Or is it best to demo what I have so far and build directly on the Perlcrete or try another insulating board?

Thanks in advance to anyone who can help.

[UtahBeehiver]

How long did you let the pcrete dry? Even though is appears dry on the surface there is a lot of water in pcrete and it can take weeks to dry out. Once the fire brick floor is down with the CaSi sandwiched in between the pcrete and the brick the moisture has no place to go but towards the path of least resistance (the moisture absorbant CaSi)

CaSi does get soft and spongy when it gets wet but it will dry out and maintain it K value and compressive strength. Did you apply mortar directly to the CaSi? The first course typically is not mortared to the insulation. What is the specs on the CaSi?

[Pete C]

Hello Russell,

Ok, all is well now. This morning I wetted down a small section of the board and let it dry. I took all day, but once it was dry it did indeed regain the desired properties. Thank for calming me down! I was afraid my WFO was going to be sitting on a permanently soft sponge.

In regards to your questions:

-The Pcrete was only about a week in the sun before I set down the board. Also, I have been continually wetting down my mortar joints since it is hot and dry here (Sacramento CA). It will dry eventually, but the soft surface makes construction difficult. I re-set my arch, carefully this time, and it should be OK once I get a few brick courses high.

-I did not mortar anything to the CaSi. I used a 50/50 mix of sand and fireclay to bed the bricks.

-I will post specs once I get them from Harbison Walker.

Overall, I think the lesson learned here is to use aluminum foil or something similar on top and bottom of the boards when installing them.