During the construction of most Capitol Hill row homes at the turn of the 20th Century, modern strong mortar was not available to brick and stone masons. Our modern Portland cement has only been recently used for a short period of time for masonry construction (in America). Prior to portland cement, lime and sand were used as mortar. They set up to harden and were able to hold stone in place. While it is not required by the Building Code to use lime mortar when pointing historic brick, it is the best practice because the brick has a density of 800 to 1800 psi and has a higher level of permeability than modern brick.
If you use Portland or modern cement with historic brick, it will lead to hard points between the brick and cause cracking when the brick is exposed to naturally occurring thermal energy forces.
Additionally, there is moisture in the existing mortar joints. Modern commercially available or retail cement has less permeability than lime mortar, which will cause moisture to build up inside the wall and cause cracking, known as “spalling.”