Masonry Magazine August 1976 Page. 18
2. Cure in absorbent molds or masonry.
3. Keep air content to a minimum.
Color. Use colored aggregates or suitable inorganic pigments. Use white cement, white aggregate and lime in white mortars. For best results with pigments, premix pigment and cement in large quantities; exercise careful control over proportioning of all ingredients. Small batch mixing may produce unacceptable color variation.
Recommended Uses. Select the mortar with the best properties suitable for a particular use. In general, Type S provides maximum bond, Type N is a good general purpose mortar, Type M provides maximum compressive strength, Type O is suitable for interior non-loadbearing construction.
APPENDIX
Proprietary mortar mixes (masonry cements) are widely used because of their convenience and generally good workability. However, there are some drawbacks to such proprietary mixes. As proprietary materials, their formulae are seldom disclosed by their manufacturers.
Basically, they usually contain portland cement, ground inert limestone as a filler, plus additives to provide workability, water retentivity and air entrainment. Because the standard governing masonry cement, Standard Specification for Masonry Cement, ASTM Designation C 91, provides no limitations on the additives (as do the ASTM specifications for portland cement and for hydrated lime), the constituents of proprietary masonry cements can vary widely among different brands.
Most masonry cements weigh about 70 lb per bag and consist of approximately equal parts of portland cement and inert limestone by weight, plus additives. When one such bag is mixed with 3 cu ft of sand as recommended, the resultant proportions, by volume, are 1 part portland cement, 2 parts ground inert limestone (essentially only a fine aggregate) and 9 parts sand. Actually, such a mix has only 1 part cementitious material to 11 parts aggregate! Workability must, therefore, be achieved by the addition of other ingredients, including air entrainment additives.
The current (1972) edition of ASTM C 91-71 requires a minimum air content of 12 percent (by volume) and limits the maximum to 22 percent. Air contents in excess of 12 percent generally result in reduced bond.
Because of the lack of limitations on the type and amount of ingredients, and the wide variation permitted in air content, the properties of brick masonry constructed with masonry cement mortars cannot be predicted with any degree of assurance. For this reason, masonry cements per se cannot be recommended. The use of any particular brand should be based on its performance record and laboratory tests of masonry assemblages.