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I've been experimenting further with an aerated cement (hebel) displacer and had good success with the following.

I rough out an oversized piston from the aerated cement. I then bore a hole down the centre at about half the size of the final required diameter finishing about 10mm away from the hot end. I then form a small aluminium plug that has been tapped with whatever the connecting rod requires and superglue that in the cold end. I then turn a threaded abour that remains chucked up until the end. Screw on the roughed outed aerated cement and turn to about 2ish mm below the required final OD. I then apply a light layer of automotive muffler putty and cure it with a butane torch. Screw the displacer on the arbour and turn to roughly smooth. Repeat the putty/turn procedure until you end up at final dimensions with a nice smooth finish.

I've found the putty seals the cement and stops it powdering when heated. So far they've held up to glowing red displacer cylinders with no issue. However I haven't tried running one for hours on end.

I like the fact that they are light, reasonably thermally inert and quite easy to make.

Can you share your process of making the cement? Do you use air to foam it?

I tried with just mixing the cement to a froth and had only moderate weight reduction.

I simply buy an aerated cement building block from my local hardware store. In Australia these are marketed under the brand name "Hebel". Once you have the brick, simply hack off a chunk (hacksaw or handsaw makes light work of it) and start shaping. One brick will supply many many pistons. I just use standard cutting tools on the lathe but cover the ways etc to protect from grit. I've found 330rpm seems to be a reasonable speed. To slow and it tends to tear, too fast and it shatters. Once the putty has been applied and cured they seem fairly robust.

I need to experiment more with just how much you can hollow them out before they become too fragile.

VincentG, there are plenty of web sites promoting home aircrete methods.

https://www.google.com/search?q=diy+foa ... -serp#ip=1

The one I've enjoyed is the Honey Do Carpenter on YouTube:

https://www.google.com/url?sa=t&source= ... N-QwA5GDUQ

I would stay away from Drexel. Apparently some of the heavier hair shampoo's work very well and aren't as toxic as Drexel.

They all seem to start from using a foam machine to make a big bucket of foam. My thoughts are wondering if a person could use a second hand beater from a mixer and a drill, or just the mixer for small batches, and add air injection to it from an air compressor. The Honey Do Carpenter welded a beater onto a long shaft so he could use it to mix to the bottom of a 5 gallon bucket of premade foam, using Suave shampoo. His YouTube channel has a lot of home workshop ideas and tests involving aircrete.

The mixer attachment might be easy to make from an autoparts steel brake line, bent into an 'L', (or '7' LOL), or hook.
It would be chucked in a hand drill. Cut a small hole in the side close to the drill for connecting an air compressor. Use a plumbing Tee slightly larger than the tube's OD, and plenty of stuffing-string and Silicone plumbing grease. Some duct tape and a couple of washers should keep the Tee centered over the hole. Could also just attach brake line tube to a beater implement with zip ties, or hose clamps.

The ingredients, Portland cement, sand, water and shampoo, could be mixed in a bucket adding air from a compressor at intervals to get the desired 'fluff'. The sand could be silica, aluminum oxide sand blast grit, magnesium oxide, or even chopped up fiber glass.


Anyway, just a thought or brainstorm.

Also, if an even more fire resistant ceramic is desired, you could use Tim Tinker's formula of sodium silicate and aluminum oxide, and fire it in a very hot kiln or fire:

https://timtinker.com/diy-refractory/

That is a very interesting site for DIY, heat resistant ceramic parts, wood fired stoves, camping, and other things. His aluminum oxide formula is described further down. He says it is strong and hard enough to sharpen knives on.

I also wonder if it could be used to cast ones own bench, Dremel, or angle grinder, stones? Maybe add, sawdust, pearlite, or shredded Styrofoam, to make even lighter and more insulation, or porous? Add magnesium oxide to make it more heat resistant? Or chopped up fiberglass to make it tougher? Aluminum oxide fiber? It might be very difficult to shape? Might have to use diamond grinding for tools?

Very interesting home made materials out of crystal cat litter box filler, lye, and aluminum oxide sand blast grit.

Thanks Fool that's a lot of good info. Time to do some studying.