Apply small amounts of water over a short period until it has absorbed water deep into the wall.
Moist basement concrete wall repair crumbling.
Wash the walls with an acidic solution check on amazon to neutralize the salt completely and allow to dry for a few hours.
The walls in my 63 yr old full walk out basement are crumbling in spots.
First off brush off any loose material from the basement walls.
If dry the moisture is originating elsewhere in the basement most likely from a basement shower easily remedied by installing a vent fan in the bathroom to direct steam outdoors.
Now you should apply water to the wall where you will be applying the mortar.
The walls have been painted and the wall is crumbing in several places behind the paint and salt like particles have formed.