Our company has been performing repairs on a sandstone colonnade. The repairs were to include repointing the sandstone cladding with a Type N portland cement lime mortar. Another part of the repair was to core holes in the colonnade to insert grouted pins.
When coring was being performed on the colonnade, water from the process began to leak out of the pointed mortar joints between the stone blocks on the columns. The contractor ground out the Type N mortar joints and "temporarily" repointed them with Type S mortar. This action halted the leakage. The contractor and the office of historic preservation wanted to leave the Type S mortar in place to avoid regrinding the joints a third time.
I want to make sure that we are not creating a condition for spalling of the stone blocks. The stone was tested in our lab and found to have compressive strength of 9700 psi. The Brick Industry Association Technical Note 8 states that Type S mortar has a compressive strength of 1800 psi. It doesn't appear that there will be a problem leaving the Type S in place.
Am I missing something?
My concern is not with the relationship between the compressive strength of the mortar and that of the stone, but rather the compressive strength of the new mortar compared to the compressive strength of the original behind the pointing mortar. Repointing mortar should never be stronger or "harder" than the original behind it to prevent stress concentrations from developing on the outer edge of the masonry.
These stress concentrations can cause spalling of the stone, even if the compressive strength of the mortar is less than that of the stone. Whether or not the stone spalls depends on the magnitude of stresses in the column and the difference between the compressive strength of the repointed mortar and the original mortar.
I would recommend repointing with a mortar that is weaker than the original mortar in the stone. Leaving in stress concentrations, which may result in spalling of the stone, may do far more damage than minor chips that may occur during removal of repointing mortar.