Is my soil good enough for cob / alternative clay-free mix for refractory mortar?

I posted a few days ago that I'm building a rocket oven. I'm building the J-tube out of brick. The book I'm following says that I need to have a mortar with refractory properties for the bricks in the J-tube. Additionally, I'll need some sort of insulation to cover the outside of the brick. I have two questions here:

1. I dug up a little soil from my yard. The soil is about 10% clay based on measurements from a jar test. I was able to make a little snake and hold it from the top. It basically completely failed the ribbon test, though. How much clay do I need in my soil to be able to use the soil as a mortar for the J-tube?
2. If my soil doesn't sound like the right kind of stuff here, are there any other mortar mixes that could work? I've looked at mixes involving wood ash and grog, but the grog is pretty much just as hard to source as fireclay. I've also considered kitty litter (bentonite). Thoughts there?

I'm on a super tight budget, which is making this harder. But I'm also embracing the low budget. Maybe I can learn something that someone with a big budget would not have considered because they didn't need to worry about the money.

You may have to look around a lot to find clay in the ground. One thing I noticed when I got my property was the sand road going to it had a number of ant hills around, and a couple of them had red clay mixed in, while others didn't. So I could benefit from their effort to find places where clay is much closer to the surface. It's possible right where you live doesn't have any easy access to good clay, but perhaps not too far away is some place like a construction site that is digging stuff up where you can see they have clay. From there you could ask the workers if you can grab a sample, or perhaps ask other people nearby.

If you can find anyone working with pottery, you may be able to get your hands on some free broken pieces you can turn in to grog. I tried several methods and they all had problems. I didn't like a lot of them, so I resorted to using the 'anvil' face of a vise and a blacksmith hammer. It took entirely too long, but I wanted it done and eventually I got there. In my case I was making an aluminum melting furnace, so the particulars of how I went about many things wouldn't be the same as a J-tube fire box.

I ended up finding a local pottery supply store that was a small business running out of a space on the back of a strip mall. I never would have found it without a bunch of searching online. If all else fails then that may be a good option. Another idea is to find a local potter and see if you can get a sample of clay or work out some sort of trade. In places where they use oil furnaces, you may be able to get leftover refractory cement from an installer. There may even be someone else interested in the same materials like a knife maker, metal caster, or another mass heater enthusiast where you could possibly buy/trade for  their leftover refractory cement.

Andrew Lubrino wrote:The soil is about 10% clay based on measurements from a jar test.

20-30% is usually the right amount. Please do not give up with your search though. Originally I thought that all I have is sandy loam with 12% clay, but it turned out that I have found small deposits of rich clay soil that can be fired into bricks. Look at lowest spots on your property and dig at least 50-100 cm.

Andrew Lubrino wrote:I've also considered kitty litter (bentonite).

Having the smallest particles of all clays, bentonite is extremely plastic so the drying mortar would shrink order of magnitude too much than desired. It can be added in small amounts to plasticize some coarser clays.
Bentonite on DigitalFire

If you shared your approximate location we could help with sourcing the powdered clay.

My luck has turned in the past few days. I found a guy on Facebook giving away perfect condition, unused firebrick (or at least I think they're firebrick from the little research I could find. The bricks say "Valentine" on them). I have literally hundreds of these fire bricks. On the clay front, I just needed to learn how to look. I found some very clay rich soil and collected about a bucket worth. I think I have a pretty good supply of clay now.

So now I'm wondering if there are better designs that I can work with here. I have some really high quality materials on my hands. I don't want to waste them. Any suggestions? I'm currently using Tim Barker's plans, and those look good. Just wondering what else is out there.