NHL3.5 or NHL5 to grout joints in natural cleft sandstone (bluestone)

[mrblint]

I'm in the US, in the mid-Atlantic area, and am in the process of bedding some reclaimed natural cleft bluestone patio slabs using St Astier NHL5 in a ratio of 2 parts NHL to 5 parts sand (by volume). We had several weeks of rain, a while back, which delayed things, so I'm only about two thirds of the way done, and the cold weather is coming. It was 32° F (0° C) last night but around 60° F (16° C) today. I'm not sure whether to continue with the NHL5 for the grout or use NHL3.5 instead. The bluestone is fairly soft stone.

The patio will receive foot traffic only. We do have cold winters with freeze/thaw. One of my goals is to have any moisture below the slabs be able to escape really easily through the NHL. Below the NHL bed is a cured portland cement sand topping base. Another goal is to mix in some pigment that will work with the varied colors of the bluestone slabs. Some are "lilac" which is like a brownish red, some are greenish, some are dark blue, some a dark rusty brown-orange like a whisky. The "concrete sand" I'm using is a pale orangy color, and I've mixed it with some coarser sand (max 3mm grains) that is beige, and when the NHL5 bedding mortar dries, it's a light ochre. For the grout, I'd like to darken it to a dry cocoa powder color, or even darker, a burnt umber, if that's even possible. I've been told to expect pastel shades.

Open to suggestions and advice, since I've never done this before.

 

[88v8]

Sounds as if this is going to be a very classy job.

In general, the mortar should be softer than the stone. So I would not use NHL5 for pointing, although it will do no harm for bedding.

The sand, needs to be sharp. Soft sand will give very little strength. That said, an over-rich mix will lack flexibility and for pointing I would be looking at 4:1.

Weather... when the temp goes below 40F, it's time to stop and leave it until next spring. The lime will freeze and in a season or two it will start to crumble. Lime work in freezing temperatures is self-defeating, and while it's irritating to leave a job half done, sometimes that's just the way it has to be. Using NHL5 as a workaround is really not the way to go. Even if it cures before freezing - unlikely as hydration takes days or weeks - it will be too hard.

Tinting... my only experience of tinting, was where I had to match the (untinted) mortar in a section of floor. There, I made up six test strips on a piece of plywood with 1" battens nailed on, using different sand/lime until I got a match. There was no tint, but the different materials certainly made a big difference to the outcome.
If you are adding a tint, that is another variable, so I would make up several test strips keepng a note of what went into each one, and then be careful with your gauging of sand, lime, water, tint, to reproduce the result.