Mortar Types

Reference data and engineering information about mortar types for building and construction applications.

mortartypes

Overview

Engineering reference data for Mortar Types in building construction.

$U = \frac{1}{R_{total}}$

Overall heat transfer coefficient.

$Q = U A \Delta T$

Heat transfer through building envelope.

The following table summarizes the compressive strength requirements and material proportions for each ASTM mortar type as specified in ASTM C 270.

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ASTM C 270 Mortar Type Specifications
Property	Type M	Type S	Type N	Type O
Compressive Strength (psi)	2500	1800	750	350
Volume Ratio (C:L:S)	1:1/4:3	1:1/2:4½	1:1:6	1:2:9
Cement (by volume)	0.33	0.22	0.17	0.11
Lime (by volume)	0.08	0.11	0.17	0.22
Sand (by volume)	1	1	1	1
Cement (by weight)	31.3	20.9	15.7	10.4
Lime (by weight)	3.3	4.4	6.7	8.9
Sand (by weight)	80	80	80	80

Source: ASTM C 270

ASTM C 270 defines four mortar types with decreasing strength and increasing workability:

Type M — Highest strength (2500 psi). Used for below-grade masonry, foundations, and retaining walls where high compressive strength and resistance to soil pressure are required.
Type S — Medium-high strength (1800 psi). Used for structural load-bearing walls, at or below grade, and for lateral resistance to soil or wind loads.
Type N — Medium strength (750 psi). General-purpose mortar for above-grade exterior walls and interior load-bearing walls exposed to weather.
Type O — Lowest strength (350 psi). Used for interior non-load-bearing walls, repointing, and historic restoration where high strength is not required.

As mortar type increases in strength (from O to M), the cement content increases while the lime content decreases:

Higher cement content produces stronger, less workable mortar with lower water retention
Higher lime content produces more workable, flexible mortar with better bond to masonry units
Sand content remains constant at 1 part by volume (80 parts by weight) across all types