In my opinion the best mortar to use for this is NHL 5, 2 soft sand and 1 sharp. NHL lime is different to hydrated lime in that it sets to a strength over a period of around a month. Putting hydrated in any cement mortar offers no additional movement or breathability because it is countered by the cement. Cement soaks up water and holds it and will over time cause the stone to erode. 90% of our work is of a traditional nature so can vouch for this mortar, the only thing regarding this mortar in order for it to remain solid when finished it needs to be compacted i recommend you build the wall leaving all joints flush and then beat the mortar with a churn brush when nearly dry.
Regards
Darren Allanson

Hi there i was always told the mortar should never be harder than the product being layed to alow it to move the same as the product so if it was sandstone i would say 5 parts sand and 1 part cement still will be strong enough. many thanks sean

I disagree with Nick. External walls are prone to drastic temperature changes and extreme amounts of moisture. So the cement binding the stone should be more flexible. I recommend a 1:1:3 mix - cement, lime, sand
Lime has benefits because of its flexibility and its porosity; it helps moisture that has penetrated a wall to evaporate. Lime is also less prone to crack than cement. Lime helps to repair fine cracks by allowing carbon dioxide to penetrate which then reacts with free lime to harden and close the cracks.

4 to 1 mix is far too strong. a 6 or 8 to 1 with an added 1 part lime is perfect for stone. in your case i would go for 6 sand, 1 cement, 1 lime. make your mix stiffer than brickwork mortar and keep your joints as small as possible, fill all joints as you go. at the end of the day scratch off the joints flush and brush with a soft brush.

I personally would use hydrated lime with sharp sand for a grey colour stone so that you don't need to use cement as the lime will harden over time but will still be able to breathe. If the stone is over a lighter or yellow colour then try using a washed sand to match the stone colour. Try 4 and 1 as the gauge you will still be able to point this up the next day or brush it off to get an old authentic look to the stone.

In my opinion do not use a cement based mix, as previously mentioned this is due to the fact that cement does not have any type of breathability. This usually causes any moisture trapped in the wall to be vented through the stone, this then causes the stone to erode through moisture transportation and the process known as freeze thaw. Lime also survives and weathers a lot longer than cement, I have worked on a 1500 year old clapper bridge where the mortar was still bedding the stones satisfactorily.

If the stone is of a very hard and dense nature you can use a NHL 5 lime. However usually I find a NHL 3.5 is preferable mixed with a 2.5 sand to 1 lime ratio. Usually the sand is 2 parts coarse (ish) building sand and half part fine sand. Different colours can be achieved by using different sand colours

I would use a 4/5 soft yellow building sand to 1 part cement.
Reason is you state it is a wall/garden feature, if you have soil behind it or below it wont last long with lime in the mix.
Soft yellow sand wont be much stronger than the stone.
We use lime in stone work above dpc
Walcrete was phased out because it contained lime and people were using it below ground,and not cement.
These are my preferences, but different parts of the country have different types of sand, stone, as mentioned there is nothing wrong with adding coarse sand or plastering sand, but I wouldnt add lime to be used below or against ground.

If you are not putting in a footing go for a lime based mortar ,it will take longer to set but will allow movement ,I would use a 4 sand (as sharpe as you like for texture) 1 lime if you like add 1/2 of cement to aid setting, dont make it any stronger or you are defeating the point of lime but you will need to protect from frost till it sets. if using a footing (on concrete) 5 sand 1 cement if you dont mind cracks over time but dont make a wet mix it only needs to be damp enough to hold together when squeezed in your hand

has with all stones they are different mixtures.everyone uses lime in mixes ,but depends where the wall is being built.if in a garden then use lime in your mix.i would,t use a lime mix if it was on a main drag roadway,because it would,t take long to start corroding pretty quick,so look where it is going to be built then you can judge what mix is ok,thanks nick

sharp sand and cement 4:1 and add waterproofer with built in hardener. it will last forever and you will be able to jetwash it as well.

Nice well graded aggregate with an nhl 3.5 will be fine. Mix at 2.5 sand to 1 lime.

Allanson knows his stuff, I agree. Good luck.

Darren Allanson has the right answer.

I mean I'm quite shocked at some of the answers on this thread. And especially the top rated answer for stone generally speaking cement is too hard for most.
3.5 nhl is fine for generally all building purposes when it comes to stone.
It's the general standard. At a 4 to 1 ratio (3.5nhl over time due to hardeners kn the mortar goes off as hard as cement) so to the person above mentioning about a cement lime and hardener mix do not do that! Like seriously don't.
Unless your in harsher climates nhl 5 is for limecrete not building.
Nhl 2 is absolutely fine for building it has the same compressive strength just has barely any hardener but it does go off over a longer time. And nhl 2 is the general standard for conservation purposes.

3.5 if your doing garden walling you will be more than fine just make sure to cover when it's below 3 degrees during curing time.
I haven't gone into immense depth because you can easily find this information online. Where some of this advice above is so very wrong