Buildings built before the 1930’s generally have softer brick, which makes them a candidate for old-style lime mortar which is really the only way to go. Most older masonry structures were built using lime and sand including the Egyptian pyramids and 600 year old castles in Scotland. The composition structure and level of hardness/softness are completely different from cement or concrete which are modern building materials best suited for new construction, not renovation of historic masonry structures.
If a building or chimney is re-pointed with cement the extreme density of the materials will cause the softer coal-fired bricks to crack and will eventually show signs of structural damage and interior water penetration. It is not true that “the harder the mortar the sturdier the wall” as some contractors would have you believe! Lime mortar containing NO cement is by far the best option and the only type that we use in our restoration work.
Many contractors use Type N or Type O mortar. While these mixes consist mostly of lime, they do contain some cement which, as said, may harm your structure. So, if you have had a previous patch job, which used masonry cement, and not lime-based mortar, the patch job will probably do more harm than good over the long run because the cement can dam the joints trapping moisture inside the brick. In winter water-logged walls can freeze and crack allowing still more water to penetrate the bricks. In summer, as the brick tries to expand, its protective skin can pop off. The bottom-line is that cement will hurt, not help your walls.
You need a contractor who will know how to chisel out the old cement mortar to a proper depth and use lime-based mortar and traditional re-pointing techniques. You want your walls and chimneys to withstand the test of time like the ancient Pyramids and old Scottish castles!