The Brew Cabin

Tony, I am going to lay some Indian sandstone Flags (or slabs as the civilised southerners say) of about 40 sq M.
As your site says go for the 'darker' mortar, what mix/cement/sand do I use for this to make the mortar 'dark' (beginner)???

'Slabs' are large bays of concrete. It may be just about acceptable for you soft southern jessies to use the term 'slabs' for the little patio flags, but stone flags are definitely flags - as in flagstone. You never hear the term "slabstone", do you? ;)

To achieve a dark mortar, you have two options - you use either a dark sand (or PFA) or you use a dye. Dark sands are a bit of a rarity in Britain and Ireland, but PFA is readily available, if you have somewhere to put the minimum order of 20 tonnes. So, for most jobs, a dye is used, and the easiest colour to use is the black dye sold by most BMs, even in the wastelands and scrub of the southern counties, for around a fiver per kilo.

If you add the black dye to a yellow/buff building sand, you get a earthy brown tone. If added to a red building sand, you get a reddy-brown toned mortar, but, if you add it to a silver or white sand, then you get a grey or even a dark grey mortar. You can buy pre-packs of a definitely black lime mortar from the better BMs, and this is a good option for slightly larger jobs, as it guarantees shade consistency, which can be a bit hit and miss when you're relying on the very unscientific units of 'spoonfuls, spadefuls and bucketfuls' to mix a mortar on site.

Be warned: it takes a lot of black dye to make a very dark mortar and, if this is the route you choose, you need to be pretty accurate with your measuring. If you get too much or too little cement, or dye, then the shade of different batches of the cured mortar can be significantly different. And remember - a dyed mortar will typically get lighter as it cures - so, what looks a rich, deep colour in the wet, can look pale and pastel-like once set.