Cement sinks to bottom of vermiculite-cement mix, what did I do wrong

[HenkWestbroek]

To get the hang of making a vermiculite/cement/water-mix, I tried pouring a batch in paper coffee cups. The ratios were 3 vermiculite : 1 concrete with enough water to produce the consistency of clay in the mixture. But after I let it set for a weekend, with plastic foil over the cup to avoid drying out of the concrete, the concrete didn't set correctly at all. The top of the coffee cup consists of very crumbly vermiculite, I gently brushed that all off for the picture. The underside of the cup consists of a grey and very solid layer of probably 100% cement. So what seemed to have happened is that the dissolved cement sank to the bottom. What am I doing wrong?

[UtahBeehiver]

Sounds like too much water. The consistency should be relatively dry, just enough water so cement can coat vermiculite, don't over work the mix. 3 to 1 ration is not a very good ratio in terms of insulation. For support under the firebrick floor and dome walls it should be around 5 to 1. For insulating the dome walls 8 to 1. Attached is pic showing thermal ratings for different ratios.

[david s]

I agree with Russell, it sounds like too much water. When mixing, add just enough water so that the water won’t pool in the bottom of your barrow. If it does then add a little more dry material. The finer the grade the more water is required. I use medium grade and find that for every 10 parts vermiculite you need 3 parts water. A little powdered clay in the mix helps coat the vermiculite grains with the cement slurry, rather than it washing off.

Just a note that there is an error on the table Russell provided. The strength of the 5:1 mix should read 175-225 (not 175- 125)
Remember also that even a 10:1 mix has more mass of cement than of vermiculite. Doubling the cement content to 5:1 increases the density enormously, so as the cement reduces insulation value it needs to be kept to a minimum. As Russell said the under floor ratio is best at 5:1 which is strong enough to support the weight of everything on top of it, but over the dome you should go way leaner. I find 10:1 is about as lean as you can go whilst still having a workable mix that places ok.