Lime mortar as opposed to cement mortar and gypsum plaster helps brick and stone lose moisture. This is more commonly known as allowing the building to breath. Lime mortar works via a process called the lime cycle. Limestone (calcium carbonate) is burned and turns to calcium oxide. When water is added to calcium oxide it turns to calcium hydroxide. Calcium hydroxide can be added to an aggregate and used as a lime mortar. At this point carbon dioxide forces out the water to turn the lime mortar back to calcium carbonate. This allows buildings to breath and helps keep the building warm and dry. Lime is burned at temperatures much lower than the high temperatures used for burning portland cement and draws in carbon from the air. So as well as allowing the building to breath lime is also kinder to the environment.