Vermiculite insulation

[fox]

Hi guys, I have a question about vermiculite, can or should I use high temp cement in the mix?
i used it on my first oven under the fire bricks but it took an age to go hard, I wonder if quick setting high temperature cement would be better than useing standard cement?

[david s]

Because the vermicrete does not see high temperature there is no need to use a calcium aluminate cement. You could but as it is way more expensive and has limited working time it's not a good solution IMO. When you say it took ages to go hard, that kind of depends on how lean you make it. Normally, if mixed well with the correct proportion of water, it will go hard in 24 hrs. That is hard enough, but you can carve it back if needed. After around 3 days it should appear dry and white on the surface although it won't be as dry as it looks.
you can make it stronger by adding more cement and some sand, but this will reduce its insulating capacity. Recommended ratio is 5:1 for under floor insulation and 10:1 for over the dome.

[fox]

Thank you, it is pretty damp where I live at this time of year, I could compromise and use a fast setting cement. I also worry about the strength of the mix as even once set hard, it has to take considerable weight! The problem I have is the availability of high temperature board, I can get vermiculite or perlite but anything else would have to be ordered and imported at considerable expense!
However I notice that a lots of people sit their ovens on vermiculite so I guesse it works!

[david s]

Yes, many ovens have been made using vermicrete for underfloor insulation with great success. The water content required does vary with the grade used. The finer the grade the more water is required. Also the stuff can take up water from the humidity in the air which also can effect the amount required when making up the mix. As a general rule you need 3 parts water to every 10 parts vermiculite by volme. Also making the mix richer with more cement will demand an increase in water volume. I find the easiest way is to mix it in a barrow and if water pools in the bottom then that's a wee bit too much. ̃he addition of a handful of powdered clay for every litre of cement makes the mix more workable. The resulting strength of the 5 to 1 mix has proved adequate. Remember that the load is spread over a wide area.