These can cause potentially irreversible damage to historic buildings. Modern masonry cement based mortars were introduced in the twenties in an effort to make mixing mortar at the construction site easier. When using a prepackaged masonry cement mortar mix, the contractor simply adds water and sand. The historic alternative is to slake quicklime, age it for several weeks, and then mix with sand and water. The two prime conditions that cause joint failure are moisture and movement.
Weather is a cumulative effect, causing further deterioration.
Original mortar generally performs well in areas which have been protected from these conditions. Therefore, when specifying a repointing mortar for an old buildings, it's logical to consider the original mortar ingredients. It was then discovered that by mixing volcanic ash, river sand, or seashells in with the putty produced an excellent mortar. This mortar inherently possessed very low compressive strength, attaining between 50-300 psi in 28 days. The initial setting time, however, was slow, and the construction process was lengthy until the development of Portland cement in 1871.