Mortar

Summary

Mortar is a workable paste of cement or lime, sand, and water that bonds masonry units and fills the joints between them. It is the weaker, sacrificial part of a masonry wall, so the mortar type chosen controls strength, flexibility, and repairability.

Cols

Image

Type M, S, N, O, K: the five standard mortar types, M strongest and K weakest
Lime mortar: lime and sand binder, hardens by carbonation
Portland cement mortar: cement binder, overtook lime mortar by 1930
Sacrificial joint: mortar is weaker and cheaper to repair than the units

History and demand

First uses in construction

10th millennium BC (Jericho) and 8th millennium BC (Ganj Dareh): earliest mud and clay mortars
Circa 6500 BC: Mehrgarh sun-dried brick mortar
Before 2600 BC: gypsum mortar at Mohenjo-daro
1824: Joseph Aspdin patented Portland cement, British Patent No. 5022 (21 October)
By 1930: Portland cement mortar overtook lime mortar in popularity

Composition and production

Raw materials

Cement and lime binders, sand, and water

Grades and types

Type	Cement : lime : sand by volume	Approximate compressive strength
M	1 : 1/4 : 3-1/2	2,500 psi
S	1 : 1/2 : 4-1/2	1,800 psi
N	1 : 1 : 6	750 psi
O	1 : 2 : 9	350 psi
K	1 : 3 : 12	75 psi

Standards

Material specifications

ASTM C270: mortar for unit masonry
ASTM C887: surface-bonding mortar

Uses in buildings

Building elements

Brick Veneer Walls: bed and head joints
CMU Walls: bed joints and grouted cells

Pros and cons

Pros

Weaker mortar is cheaper to repair than the units

Cons

Type K reaches only about 75 psi; type M about 2,500 psi

Manufacturing and cost

Main manufacturers and suppliers

Trade association: The Masonry Society.

Sources

Wikipedia: Mortar (masonry), https://en.wikipedia.org/wiki/Mortar_(masonry)
Wikimedia Commons: Throwel in a bucket.JPG, photo by Stanisław Skowron (Public domain), https://commons.wikimedia.org/wiki/File:Throwel_in_a_bucket.JPG