Use a ratio of 3 parts sand to parts cement. Adding hydrated lime to the mixture will reduce shrinkage cracks. It will also make the mortar more waterproof. If you add lime, the mixture will be 15 gallons of sand, gallons of cement and gallons of lime. Mix the ingredients together before adding water.
Sand is heterogeneous if you examine it in the palm of your hand, yet seems homogeneous if you view an entire beach. Nearly any mixture, viewed on a molecular scale, is heterogeneous. Math is applied to determine whether a mixture is homogeneous or heterogeneous.