Question
I want to lay 1/2" red flagstone with 1/2" joints on my existing concrete walkway.   My climate is    -20 to 90F. What cement mix or material and method do you recommend for this task?


Answer
I would use one of two products to directly attach these flagstones to the existing concrete - if the area is small, I抎 use a Quick-set mortar, available at any larger lumber retailer.  For a larger area, I抎 use site-mixed  mortar, made up of  Portland cement/sand/water, with a dash of hydrated lime (for workability & better water-proof characteristics, and it also cuts down on shrinkage).  Rental yards have mixers for about $30-$40/day.  The stone supplier where you bought the stone may also have a handy flyer on suggested methods of installation.

The exact receipt for mortar is a bit up to you - the general one is 3:1 & 4:1 sand/cement.  Many masonry contractors have 搕weaked?this for local conditions.  I抳e also seen the mix using hydrated lime as 15:6:1 of sand/cement/lime.

Either route, the weather is your friend or enemy.  You want a cool day, so the mortar doesn抰 dry too quickly, and not cold enough to freeze it.  Too hot, and it will set too rapidly.   

Once the stone is down and set, you抣l want to grout those ? gaps, which is a different process. I have no other specific tricks or tips, so if you need exact help, I抎 consult a mason, or the fine folks on the other side of the counter at the masonry supply store.  If you live in a remote area, where these stores are non-existent, you can check out this link that may help you with extra details.

http://www.dulley.com/diy/dw106.htm

Good luck! -Marc