they make a chemical liquid "Hardener" for mprtar now, just like the hardener for rotten wood. It gets pulled through the spalling concrete via the increased permeability. Then I guess it sets up with a chemically stable gel like those silica gels that harden ito ROCKS over night.
Pretty nifty if you hve an old stone barn or outbuilding that is beginning to deteriorate. Then, on top, (I assume that the mortar is faced a bit), its filled in with new concrete with some of the same silca gel