Hello I did the subsoil & water in a jar test and can see no clay in my soil. It crumbles if I try to make sausages.

I need a cob bench for a rocket mass heater. I have a free supply of local chalky rock.

Any advice about best way to fill my bench and make cob mortar v. Much appreciated.

I have some cement left from a previous project so I was thinking of replacing the clay with the cement. To clarify. I am not discussing  the stove parts. Just the bench which will store the heat from the hot air vented.

You can't very well do without clay if you want a proper cob, from what I read on the subject. I think some methods use animal manure, specifically cow, I think, but some in your situation who need more clay in their soil makeup sometimes bring it in from outside, bought or dug up from a clay-rich area, and mix it in.

-CK

When we starrted building our school out rammed earth, we used a seam of what we thought was clay very nearby, to mix with the sandy onsite soil. We made a bunch of adobe bricks with different ratios, let them dry, then tested them in various ways: rubbing, kicking corners, dropping, etc. Then we chose the ratio of the bricks that did best, and built our whole school, two story buildings. Years later we learned that technically that's not clay, it's fine slt. But it works fine in earth building: the walls are strong and durable.

The proof of the pudding is n the eating -- so try making some bricks with materials you have on hand in different ratios, and see if any are good enough to use.