Mortar Mix Designs

How Is Lime Specified Into Mortars?

Portland cement-lime mortars are specified in two steps. First, the mortar type must be determined based on the strength required for the application. Secondly, a choice must be made between specifying proportions or properties listed in ASTM C270. 

The ASTM Standard C270 (Mortar for Unit Masonry) provides the basis for specifying cement-lime mortars. This specification provides the basis for five different mortar types (Type K is listed in section X3 of the appendix) depending on the strength of mortar needed for an application. The names for these mortar types, developed in 1954, were based on alternating letters of the phrase "MASON WORK". Type M mortar has the highest strength. Type K mortar has the lowest compressive strength. 

The Appendix of ASTM C270 provides a reference to which mortar type should be used in some general applications. A version of this list is shown in Table 1.

    Mortar Type
Location Building Segment Recommended Alternative
Exterior, above grade Load-bearing wall
Non-load bearing wall
Parapet wall
S or M
N or S
Exterior, at or below grade Foundation wall, retaining wall,
manholes, sewers, pavements,
walks, and patios
Sc M or Nc
Interior Load-bearing wall N S or M
Non-bearing partitions O N
  1. This table does not provide for many specialized mortar uses, such as chimney, reinforced masonry, and acid resistant mortars.
  2. 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.
  3. Masonry exposed to weather in a nominally horizontal surface is extremely vulnerable to weathering. Mortar for such masonry should be selected with due caution.

The materials which could be used in a cement-lime mortar are defined in ASTM C270 (Mortar for Unit Masonry).

  • Portland Cement - Types I, IA, II, IIA, III or IIIA of ASTM Specification C150 or
  • Blended Hydraulic Cement - Types IS, IS-A, IP, IP-A, I(PM), I(PM)-A of ASTM Specification C595/595M
  • Quicklime - ASTM Specification C5 
  • Lime Putty - ASTM Specification C1489
  • Hydrated Lime - Type S or SA of ASTM Specification C207. Note: If a Type SA Hydrated Lime product is used, air entrained portland cement products should not be used.

Aggregates - ASTM specification C144

ASTM Standard C270 provides both a proportion specification and a property specification for each mortar type. 

The proportion specification provides a recipe based on volume. For cement-lime mortars, the proportion specification will indicate the volume of portland cement followed by the volume of hydrated lime and finally the volume of sand. For example, a 1:½:4½ mix contains 1 cubic foot of portland cement plus ½ of a cubic foot of hydrated lime and 4½ cubic feet of sand. For the purposes of determining volumes, ASTM C270 provides typical bulk densities for portland cement, hydrated lime and sand. These densities are shown in Table 2. Table 3 details the recipes required for each mortar type specified by proportion. Type N hydrated lime products must be laboratory tested to insure that they meet the property specification for the applicable mortar type in order to be acceptable for the proportion specifications. 

Mortar Component Bulk Density (lbs/ft3)
Portland Cement 94
Hydrated Lime 40
Mason's Sand, damp & loose 80 (dry)
Mortar Type Proportions by volume
(cementitious materials)
Aggregate Ratio - Measured in damp, loose conditions
Cement Lime
M 1 ¼ Not less than 2¼ and not more than 3 times the sum of the separate volumes of cementitious materials
S 1 Over ¼ to ½
N 1 Over ½ to 1¼
O 1 Over 1¼ to 2½

The property specification requires that the mortar exhibit certain characteristics when tested under laboratory conditions. As seen in Table 4, compressive strength, water retention and air content tests are required to be performed on the mortar mixed in the laboratory. Since jobsite water additions may not be the same as those in the laboratory, the properties of field mixed mortar cannot be compared to the property requirements of ASTM C270. 

Portland cement-lime mortars should be specified by either the property or the proportion specification but not both. When neither the proportion or property specifications are specified, the proportion specifications govern. 

ASTM C270 Property Specificationa

Mortar Type Average Compressive Strength at 28 Days (min. psi) Water Retention
(min. %)
Air Content
max. %
Aggregate Ratio
M 2,500 75 12 Not less than 2¼ and not more than 3 times the sum of the separate volumes of cementitious materials
S 1,800 75 12
N 750 75 14b
O 350 75 14b
  1. Laboratory Prepared Mortar Only
  2. When structural reinforcement is incorporated into cement-lime mortar, the maximum air content shall be 12%

For the complete ASTM C270 document, visit their website.

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