Mix concrete mix by hand or machine. Add approximately 3⁄4 of a gallon (2.8 liters) of clean water for each 80-pound (36.6 kilograms) bag of concrete mix and mix thoroughly. If more water is needed to obtain a firm, workable mix, add small amounts at a time until desired consistency is achieved.
Flagstone mortar mix is made by mixing sand, hydrated lime, and Portland cement in precise quantities. The hydrated lime is often left out but it adds long-term strength and durability. DEAR TIM: I'm installing flagstone and some paving brick on top of a concrete patio.
A concrete mixture ratio of 1 part cement, 3 parts sand, and 3 parts aggregate will produce a concrete mix of approximately 3000 psi. Mixing water with the cement, sand, and stone will form a paste that will bind the materials together until the mix hardens.
Therefore, for light use patios, a screed bed of a sharp sand and cement mix (a ratio of 6:1 is more than adequate) can be used as oppose to creating a mortar bed. If preferred, manufactured paving can also be laid on a mortar bed, but it is not always necessary.