Mortar (masonry)
Workable paste that hardens to bind masonry units.
Mortar is a workable paste that hardens to bind building blocks such as stones, bricks, and concrete masonry units, filling and sealing irregular gaps, spreading weight evenly, and sometimes adding decorative colors or patterns. In its broadest sense, the material includes pitch, asphalt, soft clay, and cement mortar. The term originates from the Old French word *mortier*, meaning "builder's mortar, plaster; bowl for mixing." Cement mortar cures into a rigid aggregate structure, but it is intentionally weaker than the building blocks it joins, serving as a sacrificial element that is easier and less expensive to repair. Bricklayers typically mix sand, a binder, and water to create mortar. Since the early 20th century, Portland cement has been the most common binder, though ancient lime mortar is still used in specialty new construction. Lime, lime mortar, and gypsum (plaster of Paris) are particularly employed in the repair and repointing of historic buildings to match the original materials in performance and appearance.
The earliest mortars were mud and clay, found in 10th millennium BC structures at Jericho and 8th millennium BC buildings at Ganj Dareh. Evidence from Mehrgarh in Baluchistan (modern-day Pakistan) shows sun-dried bricks bound with mortar around 6500 BC. Gypsum mortar, made from gypsum at a lower firing temperature, sets faster than lime mortar and was common in ancient brick arch and vault construction, though it is less durable in damp conditions. In the Indus Valley civilization, sites like Mohenjo-daro (before 2600 BC) used a light grey gypsum cement containing sand, clay, traces of calcium carbonate, and a high percentage of lime for wells, drains, and important buildings, while bitumen mortar appeared less frequently, such as in the Great Bath. Early Egyptian pyramids of the Old Kingdom (around 2600–2500 BC) used mud and clay or clay and sand mortar for limestone blocks; later pyramids used gypsum or lime mortar, a soft mixture of plaster and sand. Second-millennium Babylonian constructions employed lime or pitch. In Greece, a pozzolanic mortar—lime-based with volcanic ash that hardens underwater—was used around 500 BC in the underground aqueduct of Megara. The Romans later improved pozzolanic mortar and also developed a denser mortar using crushed terra cotta instead of volcanic ash. Around 500 CE, Chinese builders combined sticky ri
- first_known_use
- 10th millennium BC (Jericho)
- common_binder_since_early_20th_century
- Portland cement
- ancient_binder_still_in_use
- Lime
- types_in_united_states
- M, S, N, O, K
Lore & Background
Mortar is a workable paste that hardens to bind building blocks like stones, bricks, and concrete masonry units, filling irregular gaps, spreading weight evenly, and sometimes adding decorative colors or patterns. The word derives from the Old French *mortier*, meaning "builder's mortar, plaster; bowl for mixing." The earliest mortars, from the 10th millennium BC at Jericho and the 8th millennium BC at Ganj Dareh, were made of mud and clay. Evidence from Mehrgarh in Baluchistan shows sun-dried bricks bound with mortar around 6500 BC. Gypsum mortar, or plaster of Paris, was common in ancient structures; it requires a lower firing temperature than lime mortar and sets much faster, making it useful for brick arches and vaults, though it is less durable in damp conditions. In the Indus Valley civilization, gypsum cement containing sand, clay, calcium carbonate, and lime was used for wells, drains, and important buildings, while bitumen mortar appeared in places like the Great Bath at Mohenjo-daro. Early Egyptian pyramids used mud and clay mortar; later ones used gypsum or lime. Babylonian constructions employed lime or pitch. The Greeks developed pozzolanic mortar, a lime-based mix with volcanic ash that hardens underwater, used in a reservoir at Megara around 500 BC. Romans later improved this hydraulic mortar and also used crushed terra cotta to create a denser, water-resistant mortar. In ancient China, sticky rice soup was mixed with slaked lime around 500 CE to produce a stronger, more water-resistant mortar. The art of hydraulic mortar was lost for nearly two millennia after the Roman period; during the Middle Ages, only lime was used, leaving structures vulnerable to rain. Ordinary Portland cement mortar, patented in 1824, became dominant by 1930, setting hard and quickly for faster construction with less skilled labor, but it is unsuitable for repointing historic lime-mortar buildings, which require the flexibility and breathability of lime.
Reader's Guide
Mortar's significance lies in its role as the sacrificial element in masonry, being weaker and easier to repair than building blocks. The development of Portland cement mortar in the 19th century revolutionized construction, allowing faster building with less skilled labor, but its hardness and impermeability make it unsuitable for repairing historic lime mortar structures, which require flexibility and breathability. The loss of hydraulic mortar knowledge for nearly two millennia highlights the fragility of technical knowledge. Modern mortar types, standardized in the US as M, S, N, O, and K, offer varying strengths, and polymer cement mortars are used for concrete repair, though they may trap moisture in traditional walls.
Did You Know?
- The word 'mortar' comes from Old French 'mortier', meaning 'builder's mortar, plaster; bowl for mixing' (13th century).
- Gypsum mortar was used in ancient brick arch and vault construction because it sets much faster than lime mortar.
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