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Mortar

A mixture of cement, sand, and water used to bond masonry units together and fill joints

Mortar is a mixture of cement, sand, and water used to bond masonry units (bricks, blocks, stone) together and fill joints between them. Unlike concrete, mortar contains no coarse aggregate and is designed for adhesion and workability rather than compressive strength. Unlike grout, mortar is stiff enough to stay in place on vertical surfaces without flowing.

Why It Matters

Mortar is the glue that holds masonry walls together. The right mortar type ensures structural integrity, weather resistance, and durability. Using the wrong type—too strong for soft brick, too weak for load-bearing walls—causes premature failure. Mortar joints also serve as the primary moisture barrier in masonry construction.

For concrete repair, mortar is sometimes confused with patching compounds or grout. While mortar can fill small voids, purpose-made repair products bond better to existing concrete and match its properties more closely.

Technical Details

Mortar types (ASTM C270):

  • Type M (2,500 PSI): Highest strength. Below-grade, foundations, retaining walls.
  • Type S (1,800 PSI): High strength with good bond. Structural walls, exterior at grade.
  • Type N (750 PSI): Medium strength. General-purpose above-grade, most residential work.
  • Type O (350 PSI): Low strength. Interior, non-load-bearing, repointing historic masonry.

Composition (proportion spec):

  • Cement, hydrated lime, and sand in varying ratios
  • Type N: 1 part cement, 1 part lime, 6 parts sand
  • Type S: 1 part cement, 0.5 part lime, 4.5 parts sand
  • Pre-mixed bags available for DIY convenience

Key differences from concrete and grout:

  • No coarse aggregate (sand only)
  • Higher air content for workability
  • Lower strength than concrete (by design)
  • Stiffer than grout—won't flow into voids
  • Designed for bonding, not structural mass
  • Grout - More fluid cement mixture for filling voids
  • Cement - Binding ingredient in mortar
  • Aggregate - Mortar uses fine aggregate (sand) only

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