It is not possible for one mortar mix to provide all the required characteristics to the maximum extent. For example, great strength can be achieved using a 3:1 sand and cement mortar but this is at the expense of poor workability, for one thing. For a given set of circumstances, then, the ideal mortar mix has the following characteristics:
• it provides an appropriate balance between good workability and plasticity retention for the bricklayer;
• a good bond with the bricks or blocks being laid;
• it allows joints to be compressed and sealed against the driving rain and wind;
• its appearance is complementary to that of the bricks;
• it is sufficiently durable for its location or use and of adequate strength, but slightly less strong (and certainly no stronger) than the bricks or blocks being bedded. (This makes the choice of bricks a key determining factor in terms of mortar mix design.)
The final strength of the mortar is determined by the strength of the brick or block to be bedded in it. The mortar strength should roughly match that of the brick or block but it should not be greater, so that any cracking from movement occurs in the joints and not in the bricks, thus making remedial repairs easier.
Typically, these criteria can be most easily met by using a mix containing cement, hydraulic lime and sand, in proportions appropriate for its use.