The ratio of cement by volume for standard concrete is one part Portland cement, three parts aggregate and three parts sand. General purpose mortar requires one part Portland cement, one part hydrated lime and three parts sand.
"Cement" typically refers to Portland cement, which is the binding component of concrete and mortar. Concrete and mortar are mixed according to intended use. The concrete strength needed for a highway is different from that needed for a patio. Mortar usually is available in three premixed varieties. General-purpose mortar is referred to as type N. It has a compression strength of 750 psi. Type S mortar has a strength of 1800 psi. Type M is rated at 2500 psi. Each has a different mixing ratio of Portland cement, lime and sand, and each type is used for different masonry jobs.
General-purpose masonry is only used for above-ground projects. Any masonry bonding of brick or blocks below ground level requires the S or M types of mortar mixes. There are other ratio mixes for special use mortars. An O type mix is used for pointing and has a 350 psi strength. K type mortar is used in the restoration of very old masonry, such as historical buildings. It only has a 75 psi strength. O and K mixes are not suitable for any load-bearing purposes. Waterproof Portland cement is needed for mixing mortar intended to bind glass blocks.