Questions about the chemistry of building with alker

Alker is a stabilized soil method of building. A small percentage of calcined gypsum (plaster of Paris) and hydrated lime is mixed into a soil containing some clay. Within twenty minutes, the block has set up hard enough to remove forms and keep building. It is supposedly hard, durable, has more tensile strength the adobe, and is more forgiving of clay to sand ratio then cob, and takes less work then rammed earth.

I'm interested in the chemistry. Is the Plaster of Paris rehydrating into gypsum the complete cause of the strength? The lime retards the set of the plaster; is that the only thing it does? Is there a long term pozzolinic reaction? Is the soil just filler, or does it participate in the reaction somehow?

Most information on Alker is either sketchy as to details and silent as to the chemistry, or is in Turkish.

I am interested in the lime being in the mix as a way to slow the set time.
I have considered using drywall mud,the dry mixes that set a 5,20,45,90,and120 minutes, in an alker style brick.

I had read some material regarding a 'modernized' (red 'proprietary'?) version of this a while back.  Wikipedia has some information on "Cast Earth" https://en.wikipedia.org/wiki/Cast_Earth

This is a topic of some interest to me, I hope more will chime in with additional information.

Yes, it looks like Cast Earth is a version of Alker with a proprietary setting retarder added.

Unlike lime stabilized soil, for which there is lots of information and detailed explanations of the chemistry, there is virtually no information on Alker.

I mixed my first batch today. It was almost impossible to mix, using a corded drill to drive a mixing paddle. So I had to add twice as much water as the recipe called for. This is probably because the soils they are using in Turkey have 10% clay (as far as I can find) and my soil has 30 percent clay, which will absorb much more water. So far it seems to be setting, but much slower then predicted. This may be due to the large amount of water, high clay fraction, or cool temperatures, or all of the above. (Or maybe a tiny bit of residual MgO cement on the mixing paddle, or a bit of frosting residue in the bucket! Maybe I am on to something!)

Is it still in forms?
If it is slow setting but still stiff enough to remove the forms,that's a pretty good outcome.

It is still in the molds (yogurt containers.) I can't get the 'bricks' to come out, they are stuck. It is fairly stiff, rather leathery. I can dent it with my thumb, but not deeply. If this was formwork I could have removed the forms within an hour of pouring.

However, I'm concerned that this leathery state is as hard as it will get when moist. I'm sure it will be rock hard when it dries out, but if it gets wet again and reverts to this consistency, I'm not sure if it will work for what I hope to use it for. As far as I know, there all the chemical reactions should be done by now, and hardness produced by drying would be lost when the brick is rehydrated. But maybe I'm wrong about that; as I said above, I don't really understand the chemistry of this stuff.

I'm planning some more experiments: I'd like to try acid activating the clay ahead of time; adding some non absorbent materials such as sand to reduce the amount of water needed; adding perlite to make an insulating brick; modifying the clay with magnesium oxide or potassium chloride before adding the plaster of Paris; and using wood ash as an ingredient in the mix.

Of course, at some point the amount of expense and complexity would increase till I would be better off just pouring portland based concrete. It is unfortunate that portland cement does not work as well with clay soils as with sandy ones.

They seem to be slowly getting harder. Pretty soon I'm going to knock them out of their molds, and test them compared to the lime hardened bricks I made a week or so ago.

Next step will be to mix in a bunch of paper or perlite to see if I can get an insulating brick. Does anyone know how to test R value?

The bricks are pretty hard. I put one in the freezer overnight while it was still damp. It got a little crumbly on the surface, especially where it had not been rammed hard into the mold leaving voids.

However, one of my lime modified soil bricks simply fell apart in the freezer, and the alker one was still solid. So that was a success.

I've been doing some research, and I find that plaster of paris will definitely disintegrate over time if exposed to water.  Is Alker any different? If I build these raised beds out of Alker, will they just dissolve? I'm going to put an Alker brick in a bucket of water tonight to find out.

Well, after an hour in a bucket of water the alker brick had totally fallen apart. So it is not useful for what I want to do (raised beds) but might be useful for other building projects; it was impressively hard almost right away.

Hello Gilbert
Did you ever follow up on the experiment on Alker? Any luck with the ones you did make and if they lasted long. I shall be doing some experiments myself here so very curious.

No, I didn't carry the experiments any further. After I realized that the material was not suitable for outdoor raised beds I stopped working with it.