Creating wood ash cement from scratch, an experiment

How do you make a cement-like building material that won’t dissolve in water? From scratch? In this Primitive Technology video, an experimental cement is made from crushed terracotta and refired wood ash that was burned from fallen eucalyptus bark and leaves. In addition to his process notes and research, he writes:

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.

Previously, watch making lime with Primitive Technology, barrel tiled shed & tiled roof hut, pottery from beach-harvested clay, growing cement bricks with bacteria, and from Minute Physics: Concrete does not dry out.