What Kind of Mortar to Lay Flagstone Over Concrete? | Hunker
It is a combination of sand, cement and polymers. You can trowel it on using a notched trowel with a size that is dependent on the size of the flagstone with which you are working. Large-bodied stones such as flagstone generally require a minimum trowel notch of 1/2-inch, which provides sufficient mortar for the flagstone to use as a bed.