There are three types of mortar: cement mortar, lime mortar and cement-lime mortar. Cement mortar is the strongest and sets the fastest. Lime mortar is highly workable and sets slower than cement mortar, but it is the weakest of the three types and is rarely used today. Cement-lime mortar combines the best of both worlds -- the strength of cement mortar and the workability of lime mortar.
It is important to properly store the raw mortar materials prior to mixing. Materials should be kept in a shed or garage with a dry floor. If cement gets wet, it will harden, and if sand gets wet it may interfere with the amount of water needed to mix the mortar. Lay the sand and cement bags on pallets if possible.
Mortar mix is one part cement to three parts sand. If you are mixing a cement mortar, use Portland cement. If you are mixing a cement-lime mortar, use masonry cement, which is a dry mix of Portland cement and hydrated lime. The sand should be a clean, fine sand aggregate. All of the materials needed to make mortar are available in bags at home improvement stores.
The water used to mix mortar must be clean and free of debris. A good rule of thumb is that if you wouldn't drink it, don't use it to mix your mortar. The water added to mortar makes it workable and begins the chemical process that makes it set. Using too much water will weaken the mortar. For every sack of cement you use in your own mortar mix, add 7.5 gallons of water. For pre-mixed mortar, add water according to the manufacturer's directions.