Masonry Magazine April 1980 Page. 22
6. Bond mortar to wire cut or roughened surfaces rather than to a die skin.
Durability
To increase the durability of mortar cubes:
1. Increase portland cement content.
2. Add water repellents (stearates).
Extensibility
For greatest resiliency and flexibility, although not necessarily maximum ultimate strength, increase lime content of mortars.
Volume Changes
Volume changes are caused by hardening, cyclic wetting and drying, temperature change, and unsound ingredients. Only the latter two appear significant. Provide for expansion and contraction in design; avoid unsound ingredients.
Strength
Compressive strength is measured by forming and testing cubes and standard briquets of mortar (ASTM C 109 and C 348). Compressive strength is less important than bond and is not directly related to most other mortar properties. However, because tests are simple and reliable, strength is one basis for comparing mortars. To increase compressive strength:
1. Increase cement content; decrease water; use a low water-cement ratio.
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 MORTAR USES
General. Selection of a particular mortar type is usually a function of the needs of the finished structural element. Where high winds are expected, high lateral strength is required and, hence, mortar with high tensile bond strength is chosen. For loadbearing walls, high compressive strength may be the governing factor, or considerations of durability, color, flexibility, etc. may be of utmost concern. Factors which improve one property of mortar often do so at the expense of others. For this reason, when selecting a mortar, evaluate properties of each type and choose that mortar which will best meet particular end-use requirements. No single type of mortar is best for all purposes. Recommended general uses for mortars are discussed in the following paragraphs.
Type N Mortar
Type N mortar is a medium strength mortar suitable for general use in exposed masonry above grade. It is specifically recommended for parapet walls, chimneys and exterior walls wherever these are subject to severe exposure.
Type S Mortar
Tests indicate that the tensile bond strength between brick and Type S mortar approaches the maximum obtainable with cement-lime mortars. Type S mortar also has reasonably high compressive strength. It is recommended for use in reinforced masonry, for unreinforced masonry where maximum flexural strength is required, and for use where mortar adhesion is the sole bonding agent between facing and backing.
Type M Mortar
Type M mortar has high compressive strength and somewhat greater durability than other mortar types. It is specifically recommended for unreinforced masonry below grade and in contact with earth such as foundations, retaining walls, walks, sewers and manholes.
Type O Mortar
Type O mortar is a low strength mortar suitable for general interior use in nonload-bearing masonry. It may be used for loadbearing walls of solid masonry where compressive stresses do not exceed 100 psi, provided that exposures are not severe. In general, do not use Type O mortar where it will be subject to freezing action.
Cavity Walls
Where wind velocities will exceed 80 mph, use Type S mortars. For locations where lesser winds are expected, use Type S or Type N.
Tuck-Pointing Mortar
Use only prehydrated mortars. To prehydrate mortars, thoroughly mix all ingredients except water; then, mix again, adding only enough water to produce a damp unworkable mix which will retain its form when pressed into a ball. After 1 to 2 hr, add sufficient water to bring it to the proper consistency; that is, somewhat drier than conventional masonry mortars.
For best results, duplicate the original mortar proportions. When in doubt, use prehydrated Type N mortar. For a more complete discussion of tuck pointing, see Technical Notes 7. "Dampproofing and Waterproofing Masonry Walls".
Dirt-Resistant Mortar
Where resistance to staining is desired, add aluminum tristearate, calcium stearate or ammonium stearate to the construction mortar in the amount of 3 percent of the weight of the portland cement.
Where maximum dirt resistance is desired, use mortar or grout consisting of 1 part portland cement, 1/8 part lime, and 2 parts graded fine (80 mesh) sand, proportioned by volume. To this add aluminum tristearate, calcium stearate or ammonium stearate equal to 2 percent of the portland cement by weight.