Two years ago, I laid a flagstone patio on stone dust and filled the joints with mortar. That is, I dumped the mortar on dry, swept it into the joints with a broom and sprayed it with a hose.
Since then, the mortar seems to be flaking away. Both this spring and last, I started the year by sweeping away lots of little fragments of the stuff. The flagstones don't seem to have moved. Is there an obvious reason this is happening?
Thanks
Mark