anyone have experience with mixing vermiculite into concrete?

[Günter VögelBerg]

I want to make a large planter (24 inches square) out of concrete. I did this once and it did not work. The mix I used involved mostly guesswork mixing it up with what I had on hand--some leftover sand topping mix, some play sand and some portland cement. I tried to loosen the mold too soon and it began to crack. When I let it fully cure and tried to remove the mold the crack widened, at which point I smashed it with a sledge hammer and resigned to start over.

I was planning on using sand topping mix reinforced with nylon fibers. However, I am wondering if I could cut the concrete with vermiculite to make it lighter. When I google this I see widely different ratios to use, some saying it can be added to ready-mix and some saying it has to be mixed with another aggregate and portalnd cement, and some saying that the ration can be as high as 5 parts vermiculite to one part concrete. Does anyone ahve any experience with this?

[Bill Jobe]

If it were me, I would not use vermiculite. I think it can be dangerous to handle, particularly the dust, since it does contain asbestos.
If you do use it I recomend "going overboard" with breathing protection.

[Jerome Stanek]

We used that for swimming pool bottoms

[Günter VögelBerg]

We used that for swimming pool bottoms

So how could I determine what ratio to use?

[Paul F Franklin]

Asbestos free vermiculite is (at least around here) easy to find at garden centers. It will be clearly marked asbestos free. Precautions for the dust are certainly a good idea, and not just for the vermiculite but also the dust from the concrete, etc.

To the OP: you might want to read about casting concrete countertops as I would think many of the techniques used there might be helpful.

[Günter VögelBerg]

Asbestos free vermiculite is (at least around here) easy to find at garden centers. It will be clearly marked asbestos free. Precautions for the dust are certainly a good idea, and not just for the vermiculite but also the dust from the concrete, etc.

To the OP: you might want to read about casting concrete countertops as I would think many of the techniques used there might be helpful.

I actually have cast concrete countertops. When i did that I used a special-order mix and some nylon mesh. I have a roll of the mesh left over, but I am unsure I can place it in the mould vertically.

I'm used to working with silica-based materials, so I've got the lung protection covered.

[Stan Calow]

We used this in college to make lighter concrete canoes for competition. Its an engineering school tradition. I can't recall using any formula.

[Kev Williams]

What about ground walnut shells? Not the sandblasting media but what they grind for tumbling, it's fairly coarse, and pretty much non-hazardous...?

and there's always good ol' chicken wire...

[Jerome Stanek]

It's been many years ago and I can't remember the ratios

[Peter Rawlings]

You may get some insight by Googling hypertufa. Most recipes use vermiculite as I recall. You could skip the peat, I suppose. When we made a couple pots this way, I got anxious and broke mine taking out of mold too early. Wife waited and hers is still intact today. I imagine that was at least six years ago.

[roger wiegand]

Look at the Forno Bravo forum for lots of discussion on this— they are making insulating shells for pizza ovens, but have the same problems. There is a standard recommended formula, but I can’t access it from here. Lots of people prefer perlite to vermiculite.