Lime mortar is composed of lime and an aggrigate such as sand then mixed up with water. Dating back to where lime mortars seem to have originated you would be going back 6,000 year to Egyptian times where evidence shows lime plasters on the inside of the pyramids at Giza. Also used by Ancient Rome and Greece where it replaced clay and gypsum mortars being used at the time.
During the 19th century with the introducton of portland cement, Lime mortars in new construction and the building trade in genral slowly decreased with cements high strength, ease of work and its low costs. Little did they know this would be the downfall in softer buildings such as natural stone where lime mortars should never have stopped being used.
Depite lime mortars enduring utility over time centuries long, Its effectiveness as a building material only recent times have provided studies enough to prove there is no other way to build with your natural stones.
When there is a foreign material used in sandstone like cement, their own qualities lead directly to the premature deterioration of softer historic stoneworks.