Lime
Portland cement has been the most common type of cement for about a hundred and fifty years. Cement is the key ingredient in concrete and mortar. Lime is a more traditional 'binder' then OPC (ordinary portland cement) and has the following key advantages -
Not as brittle - this allows for slight movement
Breathable - porous structure that wicks water away
Environmental - production uses less energy
Environmental - absorbs CO2 as it sets
Sustainable - allows masonry to be recycled when the building comes to the end of its life
Masonary Friendly - Mortar should be softer then the surrounding stone/bricks as frost damage or thermal expansion should damage the mortar rather then the masonary
I'll add more to this section later. For now, here are some useful and informative links-
ty-mawr Lime
Wikipedia: Lime Mortar
Lime Technology
A Conservation Engineer's View Of Lime Mortar
Lime colour samples - different types of sand, result in a different coloured mortars
Also see info about my stonework