Adding sawdust,paper or charcoal to clay

So I'm digging holes at my yarden  and finding lots of clay and rock, which I would like to build something with.
I already plan to wet process the clay, which will leave me with something akin to slip.
I think sawdust plus slip will make a great insulating material, but I'm curious about adding charcoal to the mix.
Could charcoal replace sand in the mix?

Does anyone here have experience with these materials?
How about other free materials?

I discovered long ago that adding sawdust to clay and lime plasters added strength and sped up the drying time. Ten years ago I also started adding biochar (not charcoal, which is a fuel) as a fine powder (chunky is hard to trowel evenly). That was even better and had the added benefits of freshening the air, blocking EMF smog, and arresting mold. The reason it is stronger is because lime (CaO) or hydrated lime (CaOH) that you would want in a paint or plaster were made by crushing and baking limestone (CaCOH) to drive off the carbon as CO and CO2. But the lime misses its carbon. It had been together for millions of years. So it grabs C from the air and moisture and as the lime cures in a wall it gets harder and stronger, returning to limestone. If you put fresh sawdust or biochar into the wet mix, that happens much faster, by about 50 years. It is a great addition!

I'm getting ready to do an earth building project that will incorporate cob and wood shavings as infill in a stick frame interior partition, cob for wattle and daub mass over insulated exterior walls, and earthen plaster on the entire inside. I started by making a couple of test pucks with the silty clay subsoil that I will be using, to get a feel for shrinkage and strength. Then I added some biochar to the cob for comparison.

One word: Wow.

The unaltered cob shrank by about 10%. Not too bad, and about what I expected. But one of the pucks broke in half as it was drying, and both were weak when fully dry (easy to break by hand). The 1/3 (by volume) biochar mix dried into a rock-hard state and required a hammer to smash it. 2/3 biochar yielded a much lighter product that still could not be broken by hand. The particle size range of the biochar was about 2-15 mm, and the fines were screened out.

The density and tensile strength improvements were impressive, to say the least. I've now made about 1.25 cubic meters of biochar without quenching with water (put a stainless steel lid on top of the kontiki to exclude air) and graded it into my different size ranges.

What I plan to do is use 1-5 cm chunk biochar in the cob-wood infill mixture, 2-10 mm in the wattle and daub mix, and <2 mm for the plaster. I'll seal it with wheat paste or casein wash to prevent dusting. I might also put large chunks in a 5 cm layer over the concrete slab to provide insulation for a wood floor.

I'll post photos and findings along the way.

These reports are very promising!
I've been quenching my char with rainwater.
Some say quenching somehow opening the pores in the charcoal better.
Phil, do you think smothering makes for a harder charcoal?
Albert, do you inoculate your charcoal  with something to make it into biochar?

Since this is for construction material, I wasn't concerned with the opening of the pore structure that steam from water quenching facilitates. It doesn't seem to be any harder or denser overall compared to my standard batches with similar feedstocks (lots of fruit tree prunings, willow and various hardwoods, and a little pine). I can crumble most of the pieces pretty easily in my hand. By the way, I call everything I make biochar, as I use the IBI definition (high temperature process and I don't intend to burn any of it). Whether it's inoculated and how is an application detail...some gets co-composted, most gets soaked with manure/seaweed effluent, and some goes straight into the chicken coop.

Hi fellow permies,
great to read, that someone already tried sawdust/wood shop residue and charcoal in cob.
I have some questions.
If the sawdust isn't really dust, but rather shavings, do you think it can play the roll of fiber in the cob?
Did you soak it before adding to the cob?

Did anyone try cardboard as fiber? There's always so much of that stuff readily available at the dumpster. I was thinking about soaking it in water, roughly mixing it (not as small as for paper mache) and adding it to the cob mix.

If I get around to it, I'll report. If anyone has done it already I'd be thrilled to hear about it.

Benjamin, a mate of mine is an experienced earth builder and his "secret sauce" for lime plaster is paper pulp. He left me the remainder of a bucket once and that was a real joy to use. Turning cardboard back into pulp would give similar results in a cob mix, I think. I don't think sawdust has enough length to help, but will add some insulating value. Chainsaw or planer shavings would be a far better additive if you're after tensile strength, especially if they go with the grain.

By the way, on my current project the framing is up and I'm almost done with the pallet box sections. A bit of wiring to do, and maybe the ceiling insulation, and then the actual cob work commences. I'll start a thread to show progress soon.