Benefits of using charcoal in a cob mix ?

Wondering if there are any advantages to using charcoal in a cob mix ? A charcoal slurry painted over a typical cob wall ? Or perhaps charcoal mixed directly with the clay, sand, straw ? Anyone have experience adding charcoal in some form or fashion to their cob structure ?

Since I make lots of biochar, I am inclined to use it in all sorts of building materials. So far, I've tried it in lime mortar, lime plaster, as aggregate in concrete, and slipstraw infill panels. The next thing I will try it in will be either cob or earth plaster, depending on which project gets out of the concept phase and into implementation first.

Some of the good things it will do include replacing dense aggregates like rock or sand, for a lighter weight with high tensile performance, and a more insulating mix. It also enhances breathability and moisture control. And if you're looking to limit EMF, a plaster with lots of char in it will help in a similar manner to a Faraday cage.

Biochar added to lime mortar or plaster is a pozzolan, which modifies the lime to give a harder and more durable finish.

In the book, "Burn", Albert Bates and his co-author Kathleen Draper? explain how adding biochar to cement makes it stronger.  It is also a useful way of sequestering carbon.  So I guess I am mostly confirming Phil's suggestion about using it in building materials.
John S
PDX OR

Ray Sackett wrote:Wondering if there are any advantages to using charcoal in a cob mix ? A charcoal slurry painted over a typical cob wall ? Or perhaps charcoal mixed directly with the clay, sand, straw ? Anyone have experience adding charcoal in some form or fashion to their cob structure ?

Here is an interesting article about using biochar/clay paints and plasters. https://www.biochar-journal.org/en/ct/3

I found that on the natural building blog -- my favorite resource for natural building.

This thread quickly stopped making a difference between charcoal and biochar. Is it possible to confirm that people are in fact talking about adding charcoal to various raw material mixes?

One of the key differences between charcoal and biochar is that charcoal is meant to be reignited.  It's supposed to have some wood left in it, so that it can make a fire.  Biochar might be able to ignite, but that is not it's purpose. When the OP talked about making a slurry, it seems quite difficult with traditional charcoal, but quite easy with biochar. I can't confirm that this process would work with charcoal, but I know it could work with biochar.  Many people call it "char" until you crush and inoculate it. I'm ok with that.  So then I could say that I know it could work with "char" designed for biochar, but I don't know if it would work with traditional charcoal.
John S
PDX OR

Since the OP is about *cob* I would recommend against it. Charcoal does not bond very well and would weaken the cob.

It also holds lots of water. I would think this would cause the wall to absorb rain rather than shed it. Not a good thing in my thinking.

H Seymore wrote:This thread quickly stopped making a difference between charcoal and biochar. Is it possible to confirm that people are in fact talking about adding charcoal to various raw material mixes?

Re: comments about charcoal vs biochar and cob / slurry / plaster / paint / cement / brick, the article I linked is specifically talking about biochar, and discusses its uses in clay paint, plasters, brickmaking, cement or lime on exterior, earthen clays on the interior. It is based on work at the Ithaka Institute, and discusses water absorption, insulation, ratios, etc. Worth reading the article.

In my understanding, a small amount of char in the cob mix would be OK for carbon sequestration purposes, but- as stated above- it has water retention and bonding issues that would be deleterious to the overall mix if used in too high a quantity.

A better use may be in the weather-proof lime plaster layer, coating the cob. The charcoal acts a a polozon (sp?) in the mix and as a darkening agent for areas where solar gain is desired... such as a garden bed wall or a South facing wall in a cold climate.

"A better use may be in the weather-proof lime plaster layer, coating the cob. The charcoal acts a a polozon (sp?) in the mix and as a darkening agent for areas where solar gain is desired... such as a garden bed wall or a South facing wall in a cold climate."

Do you have any articles to refer us to which would go into more detail as to making the mixture of weather-proof lime plaster with biochar? Sounds like an excellent use in passive solar.
thanks

I see some terms being brought into this thread that may lead to confusion.  There is a very small level of study regarding bio char in concrete.  It was also claimed that bio char acted as a pozzolan in Portland cement.  I am not totally confident that is the case.  There was mixing of the terms bio char and charcoal, another concern.  While both are very high carbon, charcoal needs to be ground quite small to properly mix in a cement otherwise it will lighten the mix and reduce overall strength, not act as a pozzolan, which increases strength.  I believe this misconception may arise from confusion between bio char and fly ash.  Fly ash and microsilica both act in Portland cement to develop a much stronger and more water resistant final product.  This action does not arise from the carbon in the fly ash, but from the silica therein.  That is a pozzolan reaction; silica provides the actual chemical bonding that provides enhanced properties.  I did note a couple of studies where the authors claimed use of bio char produced from rice hulls to enhance cement.  That was likely sloppy science as rice hulls are generally quite high in silica content and the resulting product was less bio char and more fly ash.

Bio char would be very useful on the surface of a cob wall to provide coloration, although to prevent it transferring to other items placed against it, it might be necessary to coat it with an overlay since carbon has very weak Van Der Waals forces.  Carbon has high inter-molecular strength in two dimensions when in the form of graphite and in three dimensions as diamond.  Another high tech strong inter-molecular form of carbon is as a buckyball.   In most cases, even the two dimensional form (graphite) tends to be weak between the layers and appears soft as a result.  From a chemical thermodynamics viewpoint, carbon is highly reactive if not in the presence of an oxidizer.  The reactivity of carbon on being buried deep in the earth can be sufficient to crack water into hydrogen and oxygen, break down pyrite into hydrogen sulfide, hydrogen and sulfate and sulfuric acid.  This process can lead to formation of caves from the bottom up and when continued deep within the earth serves to create hydrocarbons.  That is a much more involved chemical thermodynamics discussion.

Sorry for the academic-speak, but in summary.  It would seem more likely that bio char is more useful as a resource in agricultural use until more study provides the proper mix proportions in materials such as cob.  Larger charcoal seems better suited to assist in purification of air and water servicing a residence and in starting fires such as rocket mass heaters.  Just my two cents from a chemical background.  To create lighter and more insulating Portland cement, add chopped or ground recycled polystyrene foam or just add an emulsifier and water to create foam concrete.  Easy and very useful in residential builds.

I just read the article that Kimi Iszikala included from the Ithaka institute.  Apparently, like a lot of people on this thread, I wasn't previously paying attention to the details in it. It is fantastic, and includes a lot of specific information about percentages of biochar in different building mixes.  It also includes info about protecting from EMFs, and regulating temperature and humidity,  which I had really never thought about before.  Nice to see some people making this work and finding out which mixes are optimal for different uses.

JohN S
PDX OR

I read this linked article as well.  While it reads well, I have some concerns regarding how it seems to mix various properties of bio char without identifying that many of the uses require specific additives to work appropriately.  There was good referencing with respect to humidity and uses requiring a natural humidity control, but no referencing with respect to actual utility for EMF resistance, strength vs insulation values, etc.  I pulled up scientific articles on several of the claims and there found clear data regarding the differing additives and composition needed to achieve those claims.  For example, it appeared on quick review that bio char in an iron composite produced the best EMF protection, well above bio char alone.  Another minor blip in the article was the failure to identify in the first paragraph that the protection provided was for "electromagnetic" radiation and not radiation as a global term.  I totally understand that reading the entire article provided the type of radiation, but just one of my personal quirks is to attempt to ensure terms are used exactly the same way throughout an article to reduce potential for "quick reader syndrome".

Good article to use as a jump off base for study, but please, do not consider bio char a miracle component that can do anything.  Do your due diligence and determine which aspects of your construction can work best with a specific bio char recipe.  

Yes, I agree Richard Henry.

There are many uses to biochar, as there are to garlic, mint, and WD-40.  Specificity is important.  

I think that part of the reason is that for any use of biochar, carbon is sequestered.

John S
PDX OR

Yes, carbon sequestration is very important.  That is one reason I like to follow the uses of hemp in building construction.  It not only sequesters carbon when placed in a wall for insulation or on a floor or sprayed as an insulating concrete substitute (basic recipe lime + water + hemp hurd) or multiple other uses, it continues to pull and sequester carbon over time.  It also provides excellent humidity control, is resistant to insects, rodents and mold and provides consistent insulation value greater than fiberglass insulation.  It remains a bit of the unknown in the US due to outdated and inaccurate laws.  That, thankfully is changing with it now allowed in much of the country.

Richard, my claim about biochar acting as a pozzolan is derived from direct experience and I presume it is a result of the ash fraction in the char, and may also result from the high number of functional attachment points on the surfaces of the particles, which would (in theory at least) enhance the carbonation by providing nucleation sites. There is also the increased aeration from the porous structure.

Using higher ash content hardwood chars gave me a harder final product than a test batch of pine biochar.

Phil - A pozzolan is a siliceous or siliceous and aluminous material that in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds having cementitious properties. - American Concrete Institute.  Therefore, if the statement is to be true, you would have higher levels of silica and or aluminous material in oak bio char vs pine bio char.

If that is the case, and it does sound like that, it is the percentage of available silica or aluminous material in the bio char that produces that effect.  Not trying to argue, just stating facts.

And indeed, hardwood biochars have higher mineral content than pine, including the species you mention. Now, if you want a more pozzolanic char, use straw, chaff, rice husks, or bamboo, as these will all have much higher silica content than wood.

Again, I cite from direct experience but there is a large body of research out there if you want to go looking for it.

Biochar is a great addition to cob, yes it slows down the drying process but the increased humidity regulation and higher insulation value make it a worthwhile addition.