Primitive Technology: Wood Ash Cement

Primitive Technology: Wood Ash Cement - Creating wood ash cement from scratch

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I developed an experimental cement from made only from re-fired wood ash as its cementitious material. It was mixed with crushed terracotta as an aggregate and formed into a cube. The cement set hard after 3 days and did not dissolve in water after this period.
Process: First I burnt bark and leaves in a kiln at high temperatures to produce well burnt, mostly white wood ash. The ash was then mixed into water and stirred well. The excess water was poured off and the resulting paste was made into pellets and allowed to dry. A pellet was then re-heated in the forge until it glowed about orange hot. This was then taken out, cooled and dropped in a pot of water. The pellet dissolved and boiled due to a chemical reaction with the water. The paste was stirred and crushed terracotta (old tiles from previous projects) was added and mixed to form a mouldable mortar. This was formed into a cube and allowed to set for three days (in the video, a cube made exactly the same way 3 days previously was used due to time constraints). The resultant cube was strong and made a slight ringing sound when tapped with a finger nail. It was placed in water for 24 hours to simulate a very heavy rain event and did not dissolve or release residues into the water.
My current theory: The main component of wood ash consists of calcium in some form (e.g. calcium carbonate, calcium oxide). This can be up to 45% from my research. Calcium is in higher concentration in the bark and leaves of a tree. When the ash is mixed with water, the soluble component of wood ash (10% pot ash) dissolves into the water. But seeing that it does nothing for the cementing process, it is drained off leaving the insoluble calcium (and other components) in the paste. Doing this probably raises the relative percentage of calcium in the paste to about 50% or more. Most of the other 50 % consists of silica and alumina which are pozzolans, materials that chemically react with calcium hydroxide to increase the durability of the cement product. The paste was then made into a pellet and fired again to high temperature to convert all the calcium compounds to calcium oxide. It also reduces any charcoal in the pellet to ash if it hadn’t already been burnt the first time. This step seemed important as un-fired ash pellets only partially hardened and would fall apart in water, though retaining a weak undissolved 5mm thick crust. I can only surmise that re-firing the ash just gave a greater conversion of the calcium components to calcium oxide. The pellet is slaked in water converting the calcium oxide to calcium hydroxide. This cement was mixed with crushed terracotta which may also help in some way that I’m not aware of as I only did this one experiment and did not test other aggregates yet (e.g. sand, gravel etc.). Terracotta is porous and might hold together better than other materials. The mixture is allowed to set in air where carbon dioxide reacts with calcium hydroxide to form calcium carbonate cementing the aggregate together. After this, the cement will not dissolve in water.
Use: I think this material might have a potential use as a mortar holding rocks or bricks together in wet environments where limestone or snail shells are unavailable for making cement. Wood ash is a pretty ubiquitous material to most natural environments inhabited by people using biomass fuels. Wood ash cement turns a waste product into a valuable building material. From my research, wood ash is already being used as a partial replacement for cement in the building industry without decreases in strength of the final product. But I’ve only just started experimenting with it and don’t know its full capabilities and limitations. Calcium content of wood ash differs depending on the species of tree, the part of the tree burnt and the soil it’s grown on. Cautious experimentation is still required before committing to a hut built from this material.

[primitivetechnology]

I think I have the chemistry right. If any chemists are watching correct me if I'm wrong. The calcium compounds are converted to calcium oxide in the second firing, turn to calcium hydroxide in the water and then set to calcium carbonate in the air. There maybe a pozzolanic effect due to the alumina and silica in the ash too. The end product is a cube made only from wood ash (fired twice) and crushed pottery. It sets hard after 3 days and will not dissolve in water even when fully submerged for 24 hours. Thanks.

[davidhoffmann]

Hello, I’m chemist specialized in chemistry of construction. What you did is a pozzolanic cement. Your mix with lime react with the calcined clay (your clay certainly contained some kaolinite which was partly turned to metakaolin because your furnace not enough hot to fully burn the clay). It’s some kind of Roman cement. If you would add some clay and limestone into the wood fire, you could make some kind of hydraulic powder, just the setting and performance wouldn’t be regular as you can’t finely adjust the composition but it would be enough to make some mortar. Also congratulation for your work :) that’s very nice

[thomashongshagen]

as an additional project for anyone that wants to do this, the water that you pour of the ash can be used for other projects, it contains lye which can be mixed with fat to create soap, but is also used to preserve and even prepare food in some parts of the world.

[thewombat]

Props for putting the crafting recipe at the end

[EliminatorPerformance]

I recently learned that enabling captions on your videos completely changes everything! Amazing!

[bellalunaoxox]

I suffer from insomnia but watching your videos at night helps sometimes. There’s a calmness and humbleness to them. The fire cracking, water rushing, it’s the primitive things that bring me down to earth and relax my mind😉

[thehouseofcrumblingidols]

As someone who builds concrete walls and foundations, this really inspires me. Ive wanted to figure out an alternative for doing footings using natural materials, as well as building earthship type structures and something akin to this is what I'm leaning towards. It would take tons of prep, but I live in the wealthy side of the Appalachian mountains and the bougie hippies might be willling to foot the bill. Thank you Primitive Technology!

[mr.mentat.x]

I just discovered your channel... and out of years skimming youtube... this is the first channel I can focus on each episode and practically write notes. I bought your book and I am hooked... this is AMAZING!

[mahirorigami]

I don't think I will ever get tired of watching your videos

[jeremyporterfield]

Primitive Technology, I know you've got loads of comments to read, but concrete technologist here - from a mix design perspective, as I believe I've read in another comment that you posted, you want to use very little water when hydrating the pellets. This is your water:cement (w/c) ratio, and determines your strength. You basically want to use just enough water to get cementitious material mixable - a .45 w/c is roughly equivalent to 4000 psi. However, it's based on volume, not weight, so if you have a pellet of a certain size, you'll want to use a comparable "pellet" of water that's about half of that for high strength. It is much harder to work and shape at that ratio though. If you do this, if possible, after the piece has hardened to the touch, you can get more strength by submerging the item in water for a few days to further hydrate any remaining cementitious particles.

[AmblingAloof]

Excellent. I have been learning about making lime from seashells etc but was unaware that the calcium carbonate is also available from wood ash. Thank you for sharing this valuable information.

[andrewprahst]

Hey John I've been watching you for awhile now and am re-watching. Have you figured out how to make glass with primitive tools? I'd love to see that.
Good luck with the Iron Age!

[dylanbassnett]

Primitive Technology: 8.5 Mil Subs without uttering a single word

[wolfhound]

I think primitive technology is one of the most advanced. The reason is you can access it everywhere and always utilize it. It is easy to maintain and repair when needed. It provides the means to live somewhat comfortably in times of dire need. Knowledge everyone should know for the time you may find yourself unexpectedly stranded. You might be able to survive depending on the climate.

[shimonnygaard]

Your time and effort is really exceptional. You may or may not be aware of some great research material. One, the “ Millers plastering plain and decorative. And English heritage practical building conservation.
Mortars, renders and plasters.
Books worth their weight in gold!!
Keep up the amazing work.

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Up Next:
Primitive Technology: Wood Ash Cement House

[nocturnal]

Before you start building some tower, I just want to tell you that the height limit is only 256 blocks.

[josephsimeon]

You never cease to amaze me with all of the knowledge that you share. Glad I found your channel

[robsierra]

This is the knowledge that we need now in 2020, thanks for sharing

[realhero]

3 Year ago: Simple shelter
2 Year ago: Tools
1 Year ago: Pottery and some stuff
Now: Cement
1 Year later: Generator
2 Year later: New city
5 Year later: Nuclear bomb
10 Year later: New World Order