The wood ash must come from a high temperature, well oxygenated fire. Burning wood bark is recommended.
Decant ash until left with paste, then form into balls.
Heat balls in forge or kiln until glowing red-orange. The hotter the better. (calcining)
Slake pellet in water, stirring all the while.
3 pots of sand to 1 pot of calcinated wood ash.
Make a well in sand and mix with stick, not hands, because burns.

• Primitive Technology: Wood Ash Cement - YouTube
• Primitive Technology: Wood Ash Cement & Fired Brick Hut - YouTube