[Cob] wood ashes and concrete

[lucynda riley]

  i found a recipie through a self sufficient living website I am a member 
of to make a type of concrete using salt, water, and wood ashes.  It 
actually worked pretty well.  I made a small dap of it on a cookie sheet and 
had to smash it with a hammer before I could break it loose.  Ofcourse it 
crumbled into pieces after that.  Through some experimenting i learned that 
its hard like concrete until you break the shiny surface on top, then it 
crumbles.  Its also becomes soft again once it gets wet.
  Is there anything I could add to it to cause it to not soften when it gets 
wet or to crumble into pieces once the surface is broken?