Despite a current trend back towards the use of lime many people are still wary of it and opt for what is perceived as the safe option of using a cementitious mortar. This is invariably a poor choice for any historic building for a number of reasons, which would take pages to explain in full but we will spare you and give a brief case against the practise.
The golden rule is that the mortar should never be harder than the masonry it encases. The mortar should be designed to be sacrificial. Once applied the cement will harden to surpass that of most historic bricks, and with time a rounding of the brick arises will occur. Cementitious mortars do not allow moisture to pass through them. The fact that the mortar will not allow moisture in is part of their attraction, however historic buildings were designed to breathe and to cope with water ingress.
Sealing the wall with cement traps the moisture in and leads to internal failure of the mortar and ultimately to the detriment of the masonry. As the moisture within the wall cannot be released through the cement it tries to make its way through the next most permeable substance which will be the brick, this can lead to spalled brickwork where the face completely deteriorates to leave a crumbly hollow. This can sometimes be rectified by carefully extracting the brick, turning it round and setting it in a fresh mortar.
Mortar, cement, lime