This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
152. Pure, rich, or "fat" lime is the product of the calcination, or burning, of limestone or marble. By this burning, the carbonic acid and water are driven from the stone in the form of vapor, and the residue is ordinary lime. Pure water-slaked lime has no cohesive power, but on the addition of sand, the carbonic acid in the atmosphere is slowly absorbed by the hydrates in the lime, and finally they resume the crystalline form of the original carbonates, and solidify around the particles of sand with which they are in contact. Hence the mortar on the outside of a wall hardens first, and this hardening process continues until the whole mass of mortar in the wall is affected.
Both cement and lime should be kept in a dry place, as exposure to the air causes lime to air-slake. The action of the atmosphere reduces the lime to a powder, as when slaked by water, but without heating and but little swelling.
153. Hydraulic limes, containing from 10 to 20 per cent. of silicates, when mixed into mortar, will harden in either air or water, but somewhat slowly under water. They slake to some small extent, but not rapidly, and harden through chemical action through the whole mass at the same time. These mortars should not be allowed to stand any great length of time, as they take an initial set, which when disturbed by remixing, materially diminishes their ultimate strength. Hydraulic lime mortar is usually made of 1 part hydraulic lime to 3 parts sand.