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Build Like a Roman
Lime, Mortar, and Concrete
Ep. 004 - Roman Lime, Pozzolana and Concrete
In this episode, the Build Like A Roman Podcast episode looks at the Roman use of Lime mortars, Pozzolans, a volcanic ash which modified the nature of lime, and how they produced concrete.
The focus on this episode is lime from calcium based limestone. We'll look at limes derived from Dolomitic and Magnesium in the future. There deserve their own episode, given the difference between them and calcium limes. For one, they need vastly extended slaking time compared to calcium to avoid post construction expansion and other problems.
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The Episodes definitions:
Opus signinum = Hydraulic flooring/render with crushed pottery in lime binder. Key in baths and water features; related to opus signinum.
Opus Caementicium = Concrete (lime + aggregate)
Binder – The cementitious component that coats and glues aggregate grains, hardening by chemical reaction and/or carbonation.
Mortar – A plastic mix (binder + aggregate + water, sometimes additives) used for bedding and pointing masonry; hardens to a rigid, adhesive matrix.
Plaster – As a noun, a finishing material for interior (and sometimes exterior) surfaces; as a verb, the act of applying a plaster/render coat.
Stucco / Render – Historically imprecise. Can be lime-, gypsum- or cement‑based; composition and regional usage vary
Lime - Non-hydraulic lime (Air lime): Produced by burning relatively pure limestones. Sets slowly by carbonation. Common in Europe, Middle East, and India until the late 19th century.
Lime - Hydraulic lime (Water lime): Produced from impure limestones containing clay/silica. Hardens under water. Widely used in Roman and later European construction.
Gypsum - Used for internal plasters and decorative finishes. Rapid setting but not durable in damp conditions.
Portland Cement
- Patented in 1824 (Joseph Aspdin, Leeds).
- By late 19th century, became dominant due to strength and speed of setting.
- Conservation issue: often too rigid and dense compared to historic lime mortars, leading to damage in traditional masonry.
Sands
- Siliceous sands: durable, sharp, but chemically inert.
- Calcareous sands: softer, more chemically compatible with lime binders, but less durable.
- Volcanic sands: reactive, good pozzolanic properties.
Plasticity – Ability to undergo permanent deformation without cracking
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