This is a section of wall my wife has been working on. It's about 200 years old. It was originally built from local low grade stones of any old shape and size, with lime mortar between the stones. Two leaves of stones, with river mud and gravel in between the leaves. Over the decades the lime washes out and the stones sink on to each other and various people repoint the wall with a variety of appropriate and inappropriate mortars.
Repointing the wall as long as it hasn't slumped too badly is very simple. You gently tap out any old mortar, especially any made with modern cement, (hoping the stone doesn't fall out), wash away anything lose and get the stone good a wet. Mix your lime with sand and water (ratio varies), and make a paste. Squish the mortar into gaps and allow it to start to set. Tap it in with a wire brush and remove any excess. Takes about 2 days to achieve full strength. It's very easy, but the slow curing times makes it a bit of a wait, which is why people prefer to add Portland cement to the mix.
Traditional lime has a number of advantages:
Lower CO2 production to make
It's porous so the wall breaths properly
Lime is softer than the stones so the mortar breaks not the stone - you don't need expansion joints for example
Lime naturally repairs micro fractures - cement doesn't
It has a nicer natural colour (it's not grey)
It has some disadvantages though:
It's nasty to work work with because it's so caustic
It's slow to set - so it takes more time to work with
It's not as strong as Portland cement - so can't be used for very large structures
Anyhow for mono Monday I present a section of repointed and rustic wall. My wife did this while I worked indoors doing IT things for a client, my work was far more dull, but it does pay for the lime and the sand.