Tikalon Blog by Dev Gualtieri

Roman Concrete

August 7, 2017

Astronomy was well-developed in antiquity, since the dome of heaven was quite mysterious, and it demanded serious study. However, there's not much detail that can be discerned with the unaided eye, so knowledge of the stars started with a few facts, and then it developed into speculative areas such as astrology. In contrast, materials science allowed experiment with the primitive processing tools available the time, and the product of such science was often useful.

Because of the ubiquity and general utility of stone, stone tools persisted for quite a while. The Stone Age of human culture persisted for a little more than 3 million years. Metalworking began in earnest about 2,000 BC, and the ancients were quite good at metallurgy, since metals were easy to form into vessels, works of art; and, of course, weapons. The first metals to be worked were those like gold, silver, and copper that existed as native metal in ores.

Eventually, other metals, such as iron, were released from ores by smelting, so we have a progression of useful metals as the ages of Man, expressed by the Greek poet, Hesiod, in his Works and Days; namely, Gold, Silver, Bronze, and Iron. Hesiod viewed the diminished value of the metals in this progression as a reflection of the degeneration of the state of man from a golden age, to a crass age of iron.

Hesiod, Works and Days, lines 175-176. "For now truly is a race of iron, and men never rest from labor and sorrow by day, and from perishing by night; and the gods shall lay sore trouble upon them." (Greek and English translation from Project Perseus, Tufts University.)[1

Still, Man was quickly able to refine his metallurgy even in this bleak age of iron to produce some incredible artifacts. The 1600 year old Iron pillar of Delhi, India, is a 6.5 ton iron stele about seven meters (23 feet) high that tapers from a diameter of about a meter at its base to about 0.7 meters at the top. This object has high corrosion resistance, a consequence of the iron's high phosphorous content, induced by the phosphorus content of the firing wood. Phosphorous is one of the ingredients of CORTEN steel, a modern alloy that's passivated by a phosphorous phase. The composition of CORTEN-A is

Fe_xC_0.12Si_0.25-0.75Mn_0.20-0.50P_0.07-0.15S_0.03Cr_0.50-1.25
Cu_0.25-0.55Ni_0.65.

If you look around the world, most of what you see isn't metal, it's concrete. In 2007, members of The Materials Society (TMS), declared that concrete was the sixth most important material in history.[2] John Smeaton is credited with inventing modern concrete in 1775, but the ancients were using concrete mixtures long before that. Some of this ancient concrete was superior to modern concrete in durability, surviving 2,000 years of seawater attack and wave action, and its manufacture was environmentally friendly. I wrote about ancient concrete in an earlier article ("In Search of... Ancient Concrete," July 1, 2013).

An early mention of concrete is found in de Architectura by the Roman civil engineer and military engineer, Vitruvius (c. 75 BC - c. 15 BC), as follows (Latin text from ref. 3. The translation is my own)[3]

Est etiam genus pulveris quod efficit naturaliter res admirandas. nascitur in regionibus Baianis et in agris municipiorum quae sunt circa Vesuvium montem. quod commixtum cum calce et caemento non modo ceteris aedificiis praestat firmitatem, sed etiam moles cum struuntur in mari, sub aqua solidescunt. (de Architectura, 2.6.1)

There is a type of sandy earth that, because of its nature, produces wonderful results. It is native to the region of Baiae, and the fields and cities around Mount Vesuvius. This material, when mixed with lime and rubble, not only gives strength to buildings, but also to breakwaters set in the sea, and it solidifies underwater.

The mention of the volcano, Mount Vesuvius, famous for its 79 AD eruption that buried Pompeii, gives a quick clue as to how the ancients created such concrete without today's technology. A large international team of scientists from the University of Utah (Salt Lake City, Utah), Western Washington University (Bellingham, Washington), Southeast University (Nanjing, People's Republic of China), Xi'an Jiaotong University (Xi'an, People's Republic of China), Harbin Institute of Technology (Harbin, People's Republic of China), the Università degli Studi di Napoli Federico II (Naples, Italy), and the University of California (Berkeley, California) has just published a thorough study of this ancient concrete.[4-8]

Volcanic sites in Italy.

(Base map Via Wikimedia Commons. Data from Ref. 4)[4

Modern concrete is based on Portland cement, a material that's processed at high temperatures (>1,300 °C). Such processing involves a severe environmental burden, since the making of cement is responsible for about 5% of global carbon dioxide (CO₂) emissions.[7] The Roman concrete was made from volcanic ash, lime (calcium oxide, CaO), and seawater, and the only energy needed is for heating of limestone to make lime.[5] These three ingredients react together in what's called a pozzolanic reaction, named after the city of Pozzuoli in the Gulf of Naples.[6] Roman historian, Pliny the Elder, wrote in his Naturalis Historia that this concrete becomes "a single stone mass, impregnable to the waves and every day stronger."[6]

Roman concrete pier at Portus Cosanus, Orbetello, Italy.

Imagine Pliny in short pants. This is a pier at Portus Cosanus in Orbetello, Italy. The Roman concrete of this structure has been carefully studied at Lawrence Berkeley National Laboratory. (DOE/Lawrence Berkeley National Laboratory image by J.P. Oleson.

While modern concrete corrodes, especially in the presence of sea water, this ancient concrete appears to strengthen from exposure.[7] The key to this concrete's millennial stability is its growth of crystals of aluminum tobermorite and phillipsite which are arrayed as fine, interlocking fibers and plates that resist fracture.[5] Underwater volcanoes, such as the Surtsey Volcano in Iceland, produce these same minerals.[5]

The research team suggests that a concrete recipe based on Roman concrete could be useful for construction of modern seawalls and other ocean-facing structures.[5] It's presumed that the lime and volcanic ash were completely reacted at the start, but a period of aluminum tobermorite and phillipsite formation continued.[5] Such a formulation might also be useful for safeguarding hazardous waste. Radioactive waste needs to be kept stable under extreme environments for long times.[5]

Plates of Al-tobermorite in a calcium-aluminum-silicate-hydrate (C-A-S-H) matrix.

Plates of aluminum tobermorite in a calcium-aluminum-silicate-hydrate (C-A-S-H) matrix.

(University of Utah image by Marie Jackson.

Concrete is big business, with sales of about $50 billion in the US alone in 2015.[5] Portland cement production was about 80.4 million tons in 2015, the equivalent of twelve Hoover Dams.[5] Since the energy requirement for production of Roman concrete is smaller, there would be a considerable environmental saving.[7] However, there are impediments to modern production of Roman concrete, not the least of which is the availability of suitable volcanic rocks.[7] Also, the precise recipe is not known. Marie D. Jackson, lead author of the paper and a professor of Geology and Geophysics at the University of Utah, has searched through ancient Roman texts to find the recipe, to no avail.[6]

Nevertheless, Jackson is mixing seawater from the San Francisco Bay and volcanic rock from the Western United States to find the right formula.[5-6] Even if a proper mixture can be found, there's the problem that Roman concrete develops its strength over time, and it has an initial compressive strength that's smaller than that of modern concrete.[6] This work was funded by the United States Department of Energy's Office of Science, and the National Science Foundation.[5]

Roman Concrete

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