COMMUNITY - FORUMS - GENERAL DISCUSSION
Crafting Complexity Journal #2: Concrete
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But other creations belong to the Earth itself. For who could marvel enough that on the hills of Puteoli there exists a dust (pulvis)—so named because it is the most insignificant part of the Earth—that, as soon as it comes into contact with the waves of the sea and is submerged, becomes a single stone mass, impregnable to the waves and every day stronger

~ Pliny the Elder, Naturalis Historia

The dome of the Roman Pantheon - the largest unreinforced concrete structure on earth

The second entry in the Crafting Complexity Journal is on one of my favorite ancient marvels: concrete. The Wikipedia article is very good, but I’ll try to go a little beyond that to show how we have only very recently begun to understand exactly how ancient concrete actually worked. I'll also draw a few connections between the short history presented here and how we might see it expressed by architects and others in Chronicle of Elyria.

After all the millennia of monumental construction, it was the discovery and systemic development of concrete in Rome around 300 BCE that finally freed men (for a time) from the laborious tasks of stone-cutting and movement. Here was stone- in all appearance and durability like some sedimentary rock cut from a quarry, but able to be produced at the point of demand and in a formable state. The technology was almost unique to Rome; indeed, it was almost dependent on Rome and the volcanic ash common to Italy’s geography. The ash was exported all over the Roman world for architecture, bridge building, and monuments. When Rome fell, the craft of concrete-making fell with it, and it was not to be discovered again for over a thousand years.

Roman concrete

By the time of Rome, many societies had observed that limestone could be burnt by fire into a different substance called “quicklime”. (The chemical transformation here, for those interested, is from the carbonate to the oxide: CaCO3 -> CaO + CO2). Quicklime is useful for many things: lowering the acidity of the soil, refining metals, and when mixed with water it becomes cement, which will harden over time and stick to what it coats. The Roman innovation was to mix quicklime cement with the volcanic ash that had been deposited over eons by local volcanos. There were two primary ways in which this was done, as described by the architect Vitruvius and the natural historian Pliny the Elder.

In the first century AD, the Caesar Augustus began a campaign of restoration and renewal of the monuments of Rome, and in this era concrete usage was universal. The recipe was simple: 4 parts of either brick rubble or spongy volcanic rock called pumice as aggregate fill + 3 parts volcanic ash from the Albin Hills south of Rome + 1 part quicklime + water. This mixture is now known as Imperial monument concrete, and much of it has survived two thousand years of daily use into the present day.

Natural earthquake resistance

Even more impressive, any structure of Roman concrete now standing has survived at least half a dozen earthquakes. Modern day concrete would be challenged by these conditions, to say the least, and we have the benefit of iron rebar rod that the Romans did not have. Our recipe is different, though, and the Roman version produced upon curing a type of mineral called strätlingite that is not seen in modern concrete. The plate-like, fibrous nature of this mineral gives the containing concrete a natural resistance to cracking[1]. The budding architect should take note: cut stone upon cut stone is a brutally simple way to build, but true durabilty comes with concrete properly applied.

Concrete chemistry is complex. Almost all variations of the class of materials we call concrete contain the same three basic building blocks: silica, or sand, with the chemical formula SiO2, lime, or CaO, and water. These three substances can react to form dozens of different minerals depending on the proportions and conditions. In the Roman concrete, two more variables are added: aluminum and sodium are present in the volcanic ash. It is these that allows strätlingite to form, and it is strätlingite that gives the concrete of Rome its toughness in the face of both earthquakes and Father Time.

Seawater concrete

The other concrete was defined by the addition of seawater, and it defined Rome’s ports as centers of trade in the Mediterranean. Many of these ports still stand today; the original concrete still undiminished by seas that would reduce modern concrete to sand in a matter of decades. The recipe again was simple enough, using the traditional ingredients: 3 parts pumice + 2 parts Puteoli ash + 1 part quicklime + seawater. The ash for this concrete was taken from the volcano at Puteoli, near the one that would later bury Pompeii.

This concrete was defined by a slightly different mineral in the cured mixture: the additional sodium present in the seawater favored the creation of aluminum tobermorite, which requires alkaline elements like potassium or sodium. The resulting concrete can be submerged in the ocean without risk of it soaking up the sodium or chlorine that both give the sea its salty familiarity and spell eventual death to the concrete that we use today. Here the natural scientist or classically trained explorer might note cementitious conglomerates formed in the water of a quiet bay, as the Romans may have done at the birth of their empire.

By this point you might be wondering if the impressive resilience of Roman concrete means that volcanic ash is required for the creation of man-made stone in antiquity. The answer is no- you can make do with simple quicklime and pebbles in a pinch. Conversely, founding a city by the sea on the hillside of an ancient volcano will risk disaster, but you might just launch an empire!

  1. M.D. Jackson, et al, “Mechanical resilience and cementitious processes in Imperial Roman architectural mortar,” Proceedings of the National Academy of Sciences 111 [52] (2014), pp. 18484-18489 Link
  2. M.D. Jackson, et al, “Unlocking the Secrets of Al-tobermorite in Roman Seawater Concrete,” American Mineralogist 98 (2013) pp. 1669-1687 Link

If you found this post interesting and informative, please check out the series!

5/21/2016 12:33:55 AM #1
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As always, I hope you enjoyed the read about an ancient craft, and I hope you like the parallels that I have drawn to how this craft might find integration in Chronicles of Elyria!

Please feel free to suggest new topics for a Crafting Complexity Journal if there's anything you'd like to learn about. On tap for the future: papermaking, glass, colored paints or dyes, wiredrawing, the scythe, and porcelain.

5/21/2016 2:35:25 AM #2
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Very interesting read! Thanks for the time and thought put into these journals. Looking forward to your future ones.


5/21/2016 2:42:40 AM #3
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Excellent article again, thank you for taking the time to post these!

Another craft that I've always found interesting is that of the chandler. I grew up just up the road from what is now Yankee Candle, and there is potentially a lot of material there for a good write up.


5/21/2016 7:12:23 AM #4
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Excellent read - especially given I'm contemplating masonry as a part of my career choice - with some mining/quarrying (or similar)!

Your note about "cut stone on cut stone being brutally efficient" does strike a chord. It might do for your village hall, but your town centre, or battlements? Not a chance!


5/21/2016 11:06:42 PM #5
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Thanks all! I'll add candling to the list as well.

5/21/2016 11:12:38 PM #6
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Quality post, man. Looking forward to the candling post.


5/23/2016 6:07:47 AM #7
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Battlements are actually a nice segway into something I would have like to add to the initial post above, but it didn’t quite fit the theme.

When people now talk about concrete, they mostly mean rocks or other aggregate fill bound with a water-activated cement. That’s what’s described above, and it works well enough. Go further in masonry, and you see things like stone blocks with the cracks filled with some sort of cement-based mortar. But what if you don’t have the materials or the knowledge to make cement-based mortar? This was apparently the challenge in post-Roman Sweden. In at least one intriguing instance, manufactured glass was used to construct the inner wall of a hillfort.[1]

In the archaeology of the “Dark Ages”, vitrified fort is a term you see pop up from time to time. Scotland alone has nearly a hundred examples of stone strongholds with melted, glassy spots in the walls. Some of these are incidental- cooking fires built against the inside wall, or forge slag thrown in with the rubble fill. Some were clearly razed by a long-ago conqueror. Some may have been ceremonially destroyed by fire at the end of their useful occupation.

In a few instances, though, the glass-forming minerals poured over the fort’s rocky bones were imported to the site.So it was at Broborg, where the mineral amphibolite was melted on some sort of charcoal hearth and poured over the walls in long sections. This “deliberate and constructive act of man” [1] is still constructive in the present day: the 1500 year old walls of Broborg have something for us to learn when tackling the problem of stabilizing and disposing of nuclear waste in a way that will survive thousands of years into the future. [2]

Glass is a fun topic- I’m looking forward to tackling it in one of these journals!

  1. P. Kresten “Swedish vitrified forts - a reconnaissance study,” Fornvännen 87 (1992) pp. 1-17 Link

  2. J.L. Weaver, J.S. McCloy, J.V. Ryan, and A.A. Kruger “Ensuring longevity: Ancient glasses help predict durability of vitrified nuclear waste,” American Ceramic Society Bulletin, 95 [4] (2016) pp. 18-23 Link

2/5/2018 10:01:15 AM #8
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You certainly live up to your avatar.


2/6/2018 5:51:52 AM #9
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Posted By Poldano at 02:01 AM - Mon Feb 05 2018

You certainly live up to your avatar.

Just wait till we talk explosives! ;-)

2/7/2018 6:25:39 AM #10
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This was an interesting read and I'm definitely going to take a look at your past entries. I'm also looking forward to a piece about candle making. I'm a hobby candle maker so i've interest in it. Also researching soap making. Hot processes and cold. Not sure which one I want to try yet. Keep up the good work!


2/7/2018 7:26:38 AM #11
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I'm kinda curious as to whether concrete may actually be skipped over as the strength of wood in this world can compete with modern steel.

I am of course referring to the trees that the kypiq live in. I am not sure how this wood could be harvested, assuming the kypiqs don't stop you, as it has such great strength. If the kypiq were able to mass produce this tree and sell it, then the strength of bases will far outstrip the weapons for a long period of time.

Just something I thought of while reading this.


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2/7/2018 9:11:13 AM #12
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Posted By LukeSpyro at 11:26 PM - Tue Feb 06 2018

I'm kinda curious as to whether concrete may actually be skipped over as the strength of wood in this world can compete with modern steel.

I am of course referring to the trees that the kypiq live in. I am not sure how this wood could be harvested, assuming the kypiqs don't stop you, as it has such great strength. If the kypiq were able to mass produce this tree and sell it, then the strength of bases will far outstrip the weapons for a long period of time.

Just something I thought of while reading this.

You have not thought it through well enough. For one thing, mining ingredients for concrete generally does not risk arousing the ire of swarms of Kypiqs.


2/7/2018 3:07:32 PM #13
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Posted By LukeSpyro at 11:26 PM - Tue Feb 06 2018

I'm kinda curious as to whether concrete may actually be skipped over as the strength of wood in this world can compete with modern steel.

Well, there's a few things. As Poldano pointed out, Kypiq are rather attached to their trees and will likely object to their murder. Concrete can also fit nearly any needed shape, be it foundations, harbour pilings, mortar for masonry, etc, while wood has to be cut and shaped (and wood as strong as steel is not going to be easy to cut and shape). Finally, concrete resists fire better than wood!

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