10 Common Mistakes People Make While Working on their Historic Building.
        1. Repointing Bricks with Modern Mortar
                Masonry is one of the most durable historic building materials, however it is also very susceptible to damage as a result of improper maintenance or repair. Mortar is used to bond together masonry units. The interaction between mortar and brick or stone is complex and often misunderstood.  Historic mortar (lime based) was generally soft and readily allowed water or vapor to pass. Modern mortar (Portland cement) is very hard and slow to transmit water or vapor.
             Portland cement works well with modern brick or stone, but causes problems if used with older, hand-packed brick (which were fired at low temperatures and are fairly soft).
              Repointing historic masonry with Portland cement mortar can create a bond that is stronger than the historic material, resulting in a differing coefficient in expansion and porosity, causing damage. This pairing can cause rapid deterioration and failure in hundreds of years old masonry.
             When in doubt, use soft mortar.