Hydrated Lime Mortar
When people refer to lime mortar they're generally referring to mortar made with hydraulic lime or fat lime putty, sand and no cement. However, many modern bricklayers and builders have experienced hydrated lime which is a little different.
Hydrated Lime is readily available at the local builder's merchant and can be used to increase the workability of cement mortar. It is used more widely in schools, colleges and training centres where it is mixed with sand (no cement) to make a mortar that does not set and is reused again and again - ideal for teaching brickwork.
Naturally, many people new to lime mortar assume that hydrated lime is the key ingredient, they know by experience that they'll need cement to introduce strength and setting power. Unfortunately this couldn't be further from the truth and is pretty much contrary to the use of lime mortars altogether.
Lime mortars contain hydraulic lime or fat lime putty (non-hydraulic lime). These two types of lime do indeed set with no need of cement, albeit more slowly than cement. The objective when using a lime mortar is eliminate the use of cement which is unnecessarily strong, rigid and is not breathable.
Natural lime mortars are flexible, allowing for movement in the building and thus preventing cracks in the masonry. Furthermore, they are breathable (vapour permeable), drawing the vapour present in the masonry back out into the air. This breathability naturally draws out damp and also helps to prevent freeze thaw action.