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Roman Concrete

February 20, 2023

As any homeowner knows, a house will fall into ruin without continual maintenance. As they say, a stitch in time saves nine; so, that small water leak under the kitchen sink can lead to a water damaged floor if not repaired quickly enough. A well maintained house might have a lifespan of a century or two, and few structures will persist over millennia. Only one of the Seven Wonders of the Ancient World still exists. The remaining six were destroyed by such things as fire and earthquakes.

The Seven Wonders of the Ancient World
Wonder Date Demise
Great Pyramid of Giza c. 2575 BC Still in existence
Hanging Gardens of Babylon c. 600 BC After 1st century AD
Statue of Zeus at Olympia 435 BC 5th-6th centuries AD
Temple of Artemis at Ephesus c. 550 BC 262 AD
Mausoleum at Halicarnassus 351 BC 12th-15th century AD
Colossus of Rhodes 292-280 BC 226 BC
Lighthouse of Alexandria c. 280 BC 1303-1480 AD

Map of the seven wonders of the ancient world

map of the Seven Wonders of the Ancient World. Despite its description and pictorial depiction in one of my childhood Golden Books, there's reason to believe that one of these, the Hanging Gardens of Babylon, did not really exist. (Map via the OpenStreetMap Foundation, released under the Open Data Commons Open Database License, and modified using Inkscape. Click for larger image.)

The permanence of the modern world is a consequence of the use of concrete as a building material. Water is the most used substance in our modern world, but concrete is the second. Concrete is formed as a mixture of cement and various aggregates, such as sand, gravel, and crushed stone. Sand is a preferred aggregate, since its small size leads to lesser need for the more costly cement. As I wrote in a previous article (Sand, October 18, 2021), concrete can't be made from desert sand, since the grains are too smooth. The following table shows the tremendous quantities of cement manufactured in a single year.[1-2]

Portland and Masonry Cement (2021)
Country Thousand
Metric Tons
Country Thousand
Metric Tonss
United States 92,000 Brazil 65,000
China 2,500,000 Egypt 40,000
India 330,000 Indonesia 66,000
Iran 62,000 Japan 52,000
South Korea 48,000 Mexico 50,000
Russia 56,000 Saudi Arabia 55,000
Turkey 776,000 Vietnam 100,000
Other countries 810,000 World total 4,400,000

As can be seen from the table, China's need for cement is much larger than that for other countries; and, all that cement needs a like quantity of sand to make concrete. China had dredged large quantities of sand from the bed and shores of Poyang Lake in the Chinese province of Jiangxi, causing an environmental disaster. Now, Chinese dredgers have been taking sand in the territorial waters of Taiwan, Japan, and other countries.

Concrete has been used for more than three millennia. The royal palace of Tiryns, Greece, which dates to around 1300 BC, had concrete floors.[3] Roman concrete, which was in common use from about 150 BC and possibly used a century earlier, has been of special interest to materials scientists because of its durability and persistence to the present day. It's been written that "Roman-era concrete is the iconic embodiment of long-term physicochemical resilience."[4] There's even a website dedicated to Roman concrete.[5]

Roman concrete is durable possibly from the incorporation of volcanic ash, known as pozzolanic ash (pulvis puteolanus), that prevents crack propagation. As Vitruvius (80-70 BC - post 15 BC), who famously stated that architects deserve more honor than wrestlers, wrote in Book II, Chapter 6, of his treatise, De Architectura (On Architecture),[6]

Est etiam genus pulveris, quod efficit naturaliter res admirandas. nascitur in regionibus Baianis in agris municipiorum, quae sunt circa Vesuvium montem. quod commixtum cum calce et caemento non modo ceteris aedificiis praestat firmitates, sed etiam moles cum struuntur in mari, sub aqua solidescunt. hoc autem fieri hac ratione videtur, quod sub his montibus et terrae ferventes sunt et fontes crebri, qui non essent, si non in imo haberent aut e sulphure aut alumine aut bitumine ardentes maximos ignes. igitur penitus ignis et flammae vapor per intervenia permanans et ardens efficit levem eam terram, et ibi quod nascitur tofus exsurgens, est sine liquore. ergo cum tres res consimili ratione ignis vehementia foratae in unam pervenerint mixtionem, repente recepto liquore una cohaerescunt et celeriter umore duratae solidantur, neque eas fluctus neque vis aquae potest dissolvere.

There is a species of sand which, naturally, possesses extraordinary qualities. It is found about Baiae and the territory in the neighbourhood of Mount Vesuvius; if mixed with lime and rubble, it hardens as well under water as in ordinary buildings. This seems to arise from the hotness of the earth under these mountains, and the abundance of springs under their bases, which are heated either with sulphur, bitumen, or alum, and indicate very intense fire. The inward fire and heat of the flame which escapes and burns through the chinks, makes this earth light; the sand-stone (tophus), therefore, which is gathered in the neighbourhood, is dry and free from moisture. Since, then, three circumstances of a similar nature, arising from the intensity of the fire, combine in one mixture, as soon as moisture supervenes, they cohere and quickly harden through dampness; so that neither the waves nor the force of the water can disunite them.

The pozzolanic ash is an aluminosilicate which reacts at room temperature with calcium hydroxide (Ca(OH)2) and water. This reaction creates a cement of insoluble calcium silicate hydrate and calcium aluminate hydrate compounds. The reaction of calcium silicate (Ca3SiO5) and water to produce calcium silicate hydrate (CaO.2SiO2.4H2O) and calcium hydroxide is as follows:
2Ca3SiO5 + 7H2O -> 3CaO·2SiO2·4H2O + 3Ca(OH)2

Core specimen of Roman concrete from the Roman Theater of Augustus Caesar

Core sample of Roman concrete from the Roman Theater of Augustus Caesar.

As can be seen from the core sample, the aggregates were typically far larger than those in modern concrete.

The Pantheon dome, the world's largest and oldest unreinforced concrete dome, is made from Roman concrete.

Unlike modern concrete, which achieves its highest strength within days, The strength of Roman concrete increases over several decades.

(Wikimedia Commons image by Millars.)

Binding additives to Roman concrete included quicklime (calcium oxide, CaO) and gypsum (Calcium sulfate, CaSO4). Examination of Roman concrete specimens revealed the existence of lime clasts; that is, chunks of poorly mixed quicklime, which were just thought to be an indication of of poor aggregation technique with no real consequence. However, a recent open access article in Science Advances indicates that this was a purposeful addition for added strength.[7-8]

The research team included members from the Massachusetts Institute of Technology (Cambridge, Massachusetts), Harvard University (Cambridge, Massachusetts), the Istituto Meccanica dei Materiali SA (Grancia, Switzerland), and DMAT (Udine, Italy).[7]

The special durability of Roman concrete under harsh conditions, such as earthquakes, and its use as a material for aqueducts and seawalls, has been a mystery even under modern scientific inquiry.[8] Its volcanic ash component from Pozzuoli, on the Gulf of Naples, had always seemed to be the singular determinant of its strength.[8] However, large, aggregate-scale lumps of raw lime, known as lime clasts, are also a ubiquitous and conspicuous feature of the material.[7] The presence of these lime clasts, which are not present in modern concrete, have been attributed to some possible causes, such as improper calcining of lime and insufficient mixing of the mortar.[7-8] This new study suggests that these lime clasts give Roman concrete a self-healing capability.[8]

It had been assumed that the lime incorporated into Roman concrete was first combined with water, in a process called slaking, to form a highly reactive paste-like calcium hydroxide (CaOH) from the quicklime calcium oxide.[8] However, slaked lime would not produce lime clasts; so, it appeared that Roman concrete was made using quicklime.[8] Spectroscopic examination indicated exposure to high temperature, something expected from the exothermic reaction involving quicklime.[7-8] Such hot-mixing contributed to the durability of Roman concrete.[7-8] This increased temperature significantly reduces setting times, and this would allow for faster construction.[8]

Elemental map of ancient Roman concrete

A large-area elemental map of a two centimeter fragment of ancient Roman concrete collected from the archaeological site of Privernum, Italy.

The color coding is calcium = red, silicon = blue, and aluminum = green).

A calcium-rich lime clast (red), is clearly visible in the lower region of the elemental map.

(Portion of an MIT image courtesy of the Roman concrete research team.)

The crack self-healing mechanism is based on the high surface area lime clasts that act as a source of reactive calcium.[7] The high surface area arise from the brittle nature of the lime inclusions that cause them to fracture into nanoparticules.[8] To verify this behavior, the researchers prepared their own cement mixture with the hot mixing method, and they found that cracks measuring up to 0.5 millimeter in width would self-heal.[7] The lime, reacting with water, creates a saturated solution of calcium that recrystallizes as calcium carbonate (CaCO3) to both fill cracks, or react with the volcanic ash.[8] In the experiments, cracks healed within two weeks in the simulated Roman concrete, but not in modern concrete.[8] The research team is working to commercialize their modified cement material, which might be useful in the 3D-printing of concrete.[8]


  1. Cement Statistics and Information, National Minerals Information Center, U.S. Department of the Interior.
  2. Mineral Commodity Summaries, Cement Statistics and Information 2022, United States Geological Survey.
  3. Amelia Carolina Sparavigna, "Ancient concrete works," arXiv, October 24, 2011.
  4. Jackson MacFarlane, Tiziana Vanorio, and Paulo J.M. Monteiro, "The Importance of the Ultra-alkaline Volcanic Nature of the Raw Materials to the Ductility of Roman Marine Concrete," arXiv, January 29, 2020
  5. David Moore, "The Roman Pantheon: The Triumph of Concrete," romanconcrete.com.
  6. Vitruvius Pollio, "De Architectura," F. Krohn, Lipsiae. B.G. Teubner. 1912 (via Tufts University Project Perseus). English translation, The Architecture of Marcus Vitruvius Pollio, translated by Joseph Gwilt, London: Priestley and Weale, 1826 (via the University of Chicago).
  7. Linda M. Seymour, Janille Maragh, Paolo Sabatini, Michel Di Tommaso, James C. Weaver, and Admir Masic, "Hot mixing: Mechanistic insights into the durability of ancient Roman concrete," Science Advances, vol. 9, no. 1 (January 6, 2023). This is an open access paper with a PDF file here.
  8. David L. Chandler, "Riddle solved: Why was Roman concrete so durable?" MIT Press Release, January 6, 2023.

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