AGGREGATES FOR NEW MORTARS
Designing a new lime mortar gives far greater freedom to the practitioner, who may first select an appropriate aggregate before deciding the type and ratio of the binder.

The performance of a mortar is heavily dependent upon the quality of the aggregate which should, under ideal conditions, be sharp in particle profile and well graded through the selected range of sizes. Sharp aggregates are preferred because rounded particles do not interlock with one another as well as sharp particles, and rounded particles also tend to produce larger voids.

The purpose of the binder is to coat each grain of aggregate and to fill any voids that may be present. The finer the particle size, the greater the surface area of the particles and the greater the quantity of binder required. As the range of particle sizes increases, the surface area of the whole becomes smaller, thus requiring less binder. The strength of a mortar is derived mainly from the selection of well graded sharp aggregate, and performance tends to become poorer as the binder content increases.

A 1:3 MORTAR MIX IS NOT NECESSARILY THE BEST
The ratio of aggregate to binder may be determined by a 'surface area and void' test. This test may be carried out using clean cold water in the simple procedure illustrated on the left (figure 3).

Article reprinted from "The Building Conservation Directory 1999"