The classification was widely accepted and is that used in this table Table 2. The mortar mixes shown in Table 2 are recommendations based on practice but, whilst conforming in general with suppliers' recommendations, they should not be taken as literal for every circumstance. The point has already been made that the selection of aggregate is of vital importance. Clean, well-graded, sharp sand is the backbone ingredient, while other aggregates have important, but supportive roles. Site and laboratory tests have demonstrated that it enhances frost resistance, although too high a percentage will increase porosity and may tend to decrease flexibility.
Soft sand is an aid to workability and can contribute significantly to the final colour.
Porous limestone or brick are added to aid carbonation and, by pre-soaking, to feed the mix with water and retard drying. In addition, they aid the development of a macroporous structure which enhances resistance to salt crystallisation and frost damage. The grading should be similar to that for the sand. Suitable porous limestones are Portland, Bath, Cotswold or Lincolnshire. Tough impermeable, compact limestones are unsuitable.