What Mortar Works Best With Flagstone Outside? | Hunker
Mix parts Portland cement, 3 parts sand, part hydrated lime. Mix only as much mortar as you will use in 1 or 2 hours and do not add more water if mixture starts to get thick. A good outdoor hydrated lime mortar recipe is: 3 buckets of 5 gallon size of fine sand, gallons of Portland cement and gallons of hydrated lime.