[ Page 3 of 10 ]

Advertising supplement provided by Portland Cement Association

by Jennifer Grover Prokopy

Desirable Mortar Properties

To ensure good workmanship and proper structural performance of a building, masonry mortars must embody a variety of properties:

Workability. Experienced masons agree that workability is perhaps the most important property of masonry mortar. Mortar should spread easily, cling to vertical surfaces, extrude readily from joints, remain workable for sufficient time, and permit easy positioning of masonry units without subsequent shifting.

 

Parts By Volume Mortar Type Portland Cement or Blended Cement Masonry Cement or Mortar Cement Type Hydrated Lime or Lime Putty Aggregate M S N M 1 --- 1 --- --- 1 1 --- --- --- --- --- --- --- 1/4 4 1/2 - 6 2 1/4 - 3 2 13/16 - 3 3/4 S 1/2 --- 1 --- --- 1 --- 1 --- --- --- --- --- --- Over 1/4 - 1/2 3 3/8 - 4 1/2 2 1/4 - 3 * N --- 1 --- --- 1 --- --- --- Over 1/2 - 1 1/4 2 1/4 - 3 * O --- 1 --- --- 1 --- --- --- Over 1 1/4 - 2 1/2 2 1/4 - 3 * Table 1. Proportion Specification for Mortar** * The total aggregate shall be equal to not less than 2 1/4 and not more than 3 times the sum of the volumes of the cement and lime used. ** Adapted from ASTM C270. Notes: 1. Under ASTM C270, Standard Specification for Mortar for Unit Masonry, aggregate is measured in a damp, loose condition and 1 cu ft of masonry sand by damp, loose volume is considered equal to 80 lb of dry sand (in SI units 1 cu m of damp, loose sand is considered equal to 1280 kg of dry sand). 2. Mortar should not contain more than one air-entraining material.

 

Location Building Segment Mortar Type Recommended Alternative Exterior, above grade Load-bearing walls Non-load-bearing walls Parapet Walls N O** N S or M N or S S Exterior, at or below grade Foundation walls, retaining walls, manholes, sewers, pavements, walks, and patios S M or N Interior Load-bearing walls Non-load bearing partitions N O S or M N Table 2. Guide for the Selection of Masonry Mortars (United States)* * Adapted from ASTM C270. This table does not provide for specialized mortar uses, such as chimney, reinforced masonry, and acid-resistant mortars. ** Type O mortar is recommended for use where the masonry is unlikely to be frozen when saturated or unlikely to be subjected to high winds or other significant lateral loads. Type N or S mortar should be used in other cases. † Masonry exposed to weather in a nominally horizontal surface is extremely vulnerable to weathering. Mortar for such masonry should be selected with due caution.

Board life. This is the amount of time that freshly mixed mortar retains its desired workability. Board life should be acceptable in both hot and cold weather.

Water retention. Mortar with good water retention not only resists rapid loss of mixing water, but also is more workable and remains so for a longer period of time. Poor retention often results in rapid stiffening, making weather-tight joints nearly impossible to achieve.

Absorption. The less absorbent the hardened mortar is, the more able it is to resist chemical attack, freeze-thaw damage, and staining.

Consistent rate of hardening. Rapid hardening makes masonry placement more difficult; slow hardening can impede the overall work progress. A consistent rate of hardening not only allows the mason to work at an optimal pace, but also contributes to greater color consistency.

Durability. A good mortar joint will stand up well to repeated exposure to adverse conditions over the long term, including freeze-thaw cycles or aggressive chemical environments.

Compressive strength. The type and quantity of cementitious material plays a large role in determining compressive strength of mortar. Compressive strength increases with an increase in cement content, and decreases with an increase in air entrainment, lime or water. However, higher compressive strength does not necessarily improve the overall performance of the mortar.

Permeability. Related primarily to workmanship and design, permeability is kept to a minimum through care in construction, including proper tooling techniques.

Bond. Measured by extent (degree of contact between mortar and masonry units) and strength (the amount of force required to separate units), bond quality is influenced by a number of factors, most importantly workmanship.

Volume change. All masonry mortars experience very small amounts of drying shrinkage as they harden. Good mortar design minimizes shrinkage. Control joints are used to reduce cracking caused by drying shrinkage.

Appearance. The overall appearance of a masonry structure is affected by uniformity of color and shade of mortar joints. Careful measurement of materials, thorough mixing, and proper tooling of joints ensures uniform mortar color.