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.