Coffee Grounds in Cob

I saw this article about using coffee ground biochar in concrete: https://www.cnn.com/world/spent-coffee-grounds-could-make-concrete-stronger-scn-climate-spc/index.html

I was wondering if it would work in cob. After all, there are places where sand is a valuable resource (for example, in Senegal there are sand "poachers" who deal in illegally harvested sand).

That is an interesting study... I do not have any first hand experience but I have found a similar thread that might help you develop some answers!

https://permies.com/t/49545/added-biochar-cob-adobe-walls

Hi Randy;
Interesting idea.
I see a few logistic problems though.
On average it takes three times the sand as it does clay to create cob.
Proper sand for making cob is not finely ground but rather it is a larger grain with sharp points.
Coffee is generally very fine ground.
Where would someone acquire course ground coffee in sufficient quantities?
Even using fine ground coffee you are still looking at a large amount needed.
In some parts of the world, sand may be a valuable commodity ... in most places, it is easily available.

thomas rubino wrote:Hi Randy;
Interesting idea.
I see a few logistic problems though.
On average it takes three times the sand as it does clay to create cob.
Proper sand for making cob is not finely ground but rather it is a larger grain with sharp points.
Coffee is generally very fine ground.
Where would someone acquire course ground coffee in sufficient quantities?
Even using fine ground coffee you are still looking at a large amount needed.
In some parts of the world, sand may be a valuable commodity ... in most places, it is easily available.

Thanks, Thomas. All good points. Where I build in the American Southwest I have an unlimited supply of sand, but it's lousy for cob because it's round (sand dunes). My earth is clay-rich, but I have to add aggregate to make it hold together. I'd hoped that coffee would have sharp points, but you're probably right.

The article suggests that the main benefit is that the biochar coffee grounds rehydrate the concrete as it sets, preventing micro-cracking and strengthening it. It suggests that using 15% biochar (replacing sand) results in a 30% increase in compressive strength when compared to 'regular' concrete. This sounds a bit too good to be true, to me, but it does make sense on paper.

Given cob's tendency to crack as it dries, I think a similar addition (perhaps ~15%) would be beneficial. It would slow down the drying time at any rate and this alone could be helpful in hot conditions.

Too much coffee (at the expense of sand) is going to be a detriment to the compressive strength of the mixture as charcoal is porous and will crush down. If there is a benefit to using it, I would guess that the perfect proportion will vary based on the specific clay, sand and climate it is being used in.

As for obtaining coffee grounds, that is actually very easy in my part of the world. Most cafes will store it in coffee bean bags and these are often stacked for people to take. I've also asked cafes to save their grounds for me and they have always been happy to do so. I think I could collect upwards of 10kg/day if I tried without too much effort and maybe even without driving.

The coarseness of the grounds is an interesting question. Espresso does tend to be finely ground but filter, 'pour over' and french press coffee should be coarser. Depending on the cafes near you they might use those methods.

I have had extensive experience with mud bricks in Australia.
You may know it as adobe bricks.
Soil tests of your mud are important, if the clay content is too high or its an expansive clay it will be unsuitable as a building material, since it will crack.
I can say that if your cob is cracking a soil test will help.
I can add that in Victoria, Australia we have a shortage of suitable sand for concrete production.