mixing hydrated lime into an adobe mix, or cob

I want to build some outdoor furniture out of earth; if I add enough hydrated lime to the mix will cob or adobe become weather proof? My soil is quite clayey and tends to crack. Can I get around this without importing sand? Would it be easier to avoid the cracking problems with pre dried adobe brick, or with monolithic cob?

What you are talking about is turning it into caliche (https://en.wikipedia.org/wiki/Caliche).  And yes, caliche is quite a durable building material, but not so great to try to grow things in.  You are a bit north of the zones of caliche in the southwest, but if you ever get down to eastern New Mexico or West Texas, you can run into formations of it.

How much lime do you have to add to your soil to get it to firm up?  I would start at 2% by volume and see what that does, and then scale up from there, if needed.  When lime is used to stabilize road beds, they generally work in the 3-6% range.  Here's a reference on Lime Treated Soil Construction which can get you started.

I wonder if tadelakt over cob would work?

I've researched Tadelakt a bit. However, for this use, I'm wary of using any kind of water proofing coating over plain cob due to the potential for failure if water did seep in somehow.

Also, I need to counteract the tendency of my clay soil to shrink, swell, and crack. I'd rather not haul in tons of sand to mix with it. It would be a lot easier to haul in a smaller amount of lime.

But I am looking into using water glass as an exterior coating on already water resistant stabilized brick.

John, in the lime treated soil construction link, it recommends mixing the lime into the soil, leaving it to sit for, if I remember, two days, and then compressing it. This allows the lime time to modify the clay.

Would it be advisable to do the same with adobe bricks? Or would all the ability of the lime to "set up" have been lost by then?

Today I used Home Depot hydrated bagged lime to mix with some clay subsoil and make some test batches. I made two mixes, on of about 5% and another of about 2.5% lime content. I rammed a couple of yogurt containers full of each to act as test bricks, and left some of each batch un compacted.

The first thing I found was that it was difficult to mix damp clay and lime. The clay turned into little balls. (I was mixing by hand.) I will have to figure out a way to stop this from happening in a cement mixer.

I plan to turn the "bricks" out of the molds and let them harden for a week, exposing them to air but keeping them damp, to allow the lime to absorb CO2. Then I will put one damp block in the freezer, and one in some water, from each batch of mix, and see how they behave. I will let the un compacted mix react in a closed container for a few days, then remix and ram some bricks out of that.

A final though is that I've heard hydrated lime from the big box stores often has absorbed CO2 from the air and is no longer reactive. So if this does not work, I will have to try some fresher lime.

I need to actually test my rocks or my land but if i crush it up and wet down this caliche will it form back to cemented material again or would I have to ammend it.  example of what am talking about of my land.  I know I have plenty of this stuff on my land.