In a recent repointing project, we ground all joints out to a depth of approximately 3/4 inch. When the grinding was complete, it appeared that we had removed virtually all of the mortar from the joints. However, after the dust was blown out of the joints using compressed air and the walls were wetted prior to placing the new mortar, several slivers of mortar appeared on the top and bottom surfaces of some brick within the joints to be repointed.
The architect involved in the project had us grind off these slivers of mortar. If the slivers are still on the brick, it must mean that they were bonded well.
Won’t the new mortar bond to the existing mortar? Why is it necessary to remove the slivers of mortar on the brick? Won’t this action increase the chances of causing permanent damage to the brick units?
Just because there is mortar remaining on the brick masonry doesn’t mean that it is well bonded. I have been involved with several repointing projects where many mortar slivers were initially left on faces of the units. These mortar slivers were generally not well bonded, but could be easily chipped off using a knife.
Adding new mortar to old mortar that is poorly bonded to the original substrate does not provide a good watertight joint in the long run. Although there may be increased risk of damaging the brick units by additional grinding, leaving this mortar in place permits more water penetration, which shortens the life of the repointing, and may result in the need to repoint the masonry sooner.
I typically recommend removing the dust with compressed air and then rinsing the joints with water so that these mortar slivers can be seen and removed.