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Roman Concrete: Why 2,000-Year-Old Harbors Still Stand | History for Sleep with Rain Sounds · Sleepless History: Sleep Documentaries

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[SPEAKER_00]: Welcome to Sleepless History.
0:02
[SPEAKER_00]: Tonight we are going to spend some time with one of the most patient materials in human history.
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[SPEAKER_00]: It does not glitter like gold and it does not ring like bronze.
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[SPEAKER_00]: It is gray and rough and mostly hidden underwater.
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[SPEAKER_00]: It is Roman concrete.
0:22
[SPEAKER_00]: More specifically, it is the concrete the Romans poured into the sea to build their harbours, their break waters, and their peers.
0:31
[SPEAKER_00]: Some 2000 years ago.
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[SPEAKER_00]: A remarkable amount of it is still there.
0:35
[SPEAKER_00]: Waves have washed over it every day for 20 centuries, and it has not given way.
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[SPEAKER_00]: In some places, it seems to have grown stronger with time.
0:46
[SPEAKER_00]: Meanwhile, many of the concrete pears and seawalls built in the last 100 years are cracking, flaking and rusting from the inside out.
0:55
[SPEAKER_00]: So tonight, we will ask a simple question, slowly, and we will take our time with the answer.
1:03
[SPEAKER_00]: how did the Romans do it?
1:05
[SPEAKER_00]: What was in their recipe?
1:07
[SPEAKER_00]: Why did the knowledge seem to fade?
1:10
[SPEAKER_00]: And what have scientists learned in recent years, looking at tiny white flecks in ancient stone, about a material that can repair its own cracks?
1:22
[SPEAKER_00]: We will move through it gently in five chapters.
1:25
[SPEAKER_00]: We will visit a harbour on the coast of Judea,
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[SPEAKER_00]: Walk the volcanic shores near the Bay of Naples, look through the microscope at crystals that formed in the dark over many centuries.
1:39
[SPEAKER_00]: Consider why our own concrete struggles in salt water and finally sit with the small white stones that seem to hold the last piece of the puzzle.
1:50
[SPEAKER_00]: Before we begin, if you enjoy these long calm journeys through the past, you are welcome to subscribe.
1:57
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[SPEAKER_00]: It genuinely helps the show and I read everyone.
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[SPEAKER_00]: Lastly, before we begin tonight's story, I wanted to say that sleepless history is independent with no network behind it, just careful research and quiet evenings.
2:23
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2:46
[SPEAKER_00]: Thank you all so much.
2:56
[SPEAKER_00]: on the eastern shore of the Mediterranean Sea, along the coast of what is now considered Israel.
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[SPEAKER_00]: There is a stretch of beach that has almost no natural shelter.
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[SPEAKER_00]: The coastline runs long and straight.
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[SPEAKER_00]: There are no deep bays, no rocky headlands curling out to protect a fleet of ships.
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[SPEAKER_00]: San drifts north along the shore with the current and the wind comes in from the open sea.
3:24
[SPEAKER_00]: For a sailor, it was a difficult place to find safety.
3:28
[SPEAKER_00]: For a king who wanted a great port,
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[SPEAKER_00]: It was an even more difficult place to build one.
3:34
[SPEAKER_00]: In the years before the birth of Christ, that King was Herod, often called Herod the Great.
3:41
[SPEAKER_00]: He ruled Judea as a client of Rome, which meant he held his throne with the approval of the Roman Emperor Augustus.
3:48
[SPEAKER_00]: Herod was an ambitious builder.
3:51
[SPEAKER_00]: He built fortresses, palaces, and a vast expansion of the temple in Jerusalem.
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[SPEAKER_00]: And on this long, unsheltered coast, beginning around 22 years before the birth of Christ, he decided to build a city, and beside it, a harbour that could hold its own against the open sea.
4:11
[SPEAKER_00]: He named the city Caesaria, in honour of Augusta Caesar.
4:16
[SPEAKER_00]: He named the harbour Sebastian, which is the Greek form of the name Augustus.
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[SPEAKER_00]: There was no natural harbour to improve.
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[SPEAKER_00]: So Herod's engineers made one from nothing.
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[SPEAKER_00]: They built two enormous breakwaters that reached out into the sea like a pair of arms, enclosing a sheltered basin of calm water.
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[SPEAKER_00]: the northern one was shorter.
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[SPEAKER_00]: Between them near the tips there was an entrance facing north.
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[SPEAKER_00]: away from the worst of the prevailing winds and waves.
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[SPEAKER_00]: Along the tops of the break waters, there were keys, storehouses, towers, and walkways.
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[SPEAKER_00]: Ships from across the Mediterranean could tie up there, unload their cargo and rest.
5:10
[SPEAKER_00]: The ancient historian Joe Seifus, who wrote about Herod's reign, described the harbour with admiration.
5:18
[SPEAKER_00]: He wrote that it was larger than the famous harbour at Pyreus near Athens, and that the king had overcome nature itself.
5:28
[SPEAKER_00]: He described great blocks being lowered into water that was roughly 20 fathoms deep, which is to say a depth of many meters.
5:39
[SPEAKER_00]: And he described the break waters rising up from the seabed to stand above the waves.
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[SPEAKER_00]: For a long time, readers wondered how much of that was exaggeration.
5:51
[SPEAKER_00]: It sounded almost too grand.
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[SPEAKER_00]: Then, in the 20th century, divers began to explore the waters of Caesarion.
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[SPEAKER_00]: Beneath the surface, lying on the sea bed, they found the remains of the breakwaters.
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[SPEAKER_00]: Much of the harbour had slumped and settled over the centuries.
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[SPEAKER_00]: The sea floor there is sandy, and there may also have been earthquakes or a slow subsidence of the ground beneath.
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[SPEAKER_00]: The great arms of the harbour had sunk and broken apart in places.
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[SPEAKER_00]: But the pieces were still there.
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[SPEAKER_00]: And many of those pieces were not cut stone at all.
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[SPEAKER_00]: They were enormous blocks of concrete.
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[SPEAKER_00]: Some of these blocks were the size of a small room.
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[SPEAKER_00]: The divers found that they had been cast inside wooden forms, a kind of large timber box, which had been floated out into position, sunk to the bottom, and then filled with concrete right there in the sea.
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[SPEAKER_00]: In some places, the wood of those forms was still preserved, pressed against the concrete.
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[SPEAKER_00]: It's grain still visible.
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[SPEAKER_00]: The concrete inside had set underwater around 2,000 years ago.
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[SPEAKER_00]: and it had been sitting in saltwater ever since.
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[SPEAKER_00]: When archaeologists and engineers later studied samples of this concrete, they found something else that was quite surprising.
7:19
[SPEAKER_00]: The volcanic ash that had been used in the mixture did not come from anywhere near Judea.
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[SPEAKER_00]: It had come from Italy, from the region around the Bay of Naples, more than 2,000 kilometers away by sea.
7:34
[SPEAKER_00]: Herod's builders, or the Roman engineers who assisted them, had shipped enormous quantities of Italian volcanic ash across the Mediterranean in the holds of cargo ships, specifically to make this concrete.
7:49
[SPEAKER_00]: They did not use local sand.
7:52
[SPEAKER_00]: They went to the trouble of importing a very particular kind of earth.
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[SPEAKER_00]: That tells us something important.
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[SPEAKER_00]: It tells us that by this time the builders of the Roman world knew very well that this ash was special.
8:08
[SPEAKER_00]: They knew that ordinary mortar, the kind used to hold bricks and stones together on dry land, would not behave the same way underwater.
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[SPEAKER_00]: They knew that if you wanted to pour stone into the sea and have it hard on there, you needed the dust from the volcanic hills near Naples.
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[SPEAKER_00]: and they trusted that knowledge enough to ship it across the sea by the ship load.
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[SPEAKER_00]: Caesaria was not unique.
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[SPEAKER_00]: It was simply one of the most ambitious examples.
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[SPEAKER_00]: All around the Mediterranean, the Romans built harbors, breakwaters, fish plums, and pears using this same kind of concrete.
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[SPEAKER_00]: At Cosa on the coast of Tuscany, there are remains of a harbour with concrete pears.
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[SPEAKER_00]: At Bae, the old seaside resort on the Bay of Naples, where wealthy Romans kept their villas.
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[SPEAKER_00]: There are concrete foundations and walls now lying underwater, because the land there has slowly sunk.
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[SPEAKER_00]: At Portis, near Rome, the Emperor Claudius began a vast new harbor at the mouth of the Tiber River.
9:17
[SPEAKER_00]: and his engineers, according to ancient writers, sank a great ship filled with concrete to serve as the foundation for a lighthouse.
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[SPEAKER_00]: At Antium, which is now called Anzio,
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[SPEAKER_00]: There are harbour works associated with the Emperor Nero.
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[SPEAKER_00]: At POSWOLY itself on the Bay of Naples, there were rows of concrete pears stretching out into the water, supporting archers above them.
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[SPEAKER_00]: Imagine standing on one of those pears on a quiet evening.
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[SPEAKER_00]: The sea moves gently beneath you.
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[SPEAKER_00]: Small waves lap against the gray stone, and draw back, and lap again.
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[SPEAKER_00]: Seaweeds swayes at the waterline.
10:00
[SPEAKER_00]: little crabs move in and out of the crevices.
10:04
[SPEAKER_00]: It seems like nothing is happening at all.
10:07
[SPEAKER_00]: And yet, in the dark interior of that stone, something is happening very slowly.
10:13
[SPEAKER_00]: Sea water is seeping in through tiny pores.
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[SPEAKER_00]: Minerales are dissolving and reforming.
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[SPEAKER_00]: New crystals are beginning to grow.
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[SPEAKER_00]: Too small to see.
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[SPEAKER_00]: in the spaces between grains.
10:29
[SPEAKER_00]: That slow invisible activity is at the heart of our story.
10:33
[SPEAKER_00]: But the Romans did not know about crystals.
10:36
[SPEAKER_00]: They did not have microscopes or chemistry as we understand it.
10:41
[SPEAKER_00]: What they had was experience, careful observation and a handful of written descriptions that have come down to us.
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[SPEAKER_00]: So in our next chapter, we will turn to those descriptions and to the volcanic hills where the story really begins.
11:00
[SPEAKER_00]: Chapter two, the dust of puteoli.
11:05
[SPEAKER_00]: On the northern side of the Bay of Naples, there is a region of low hills, craters, hot springs, and steaming vents, the ground there breathes.
11:15
[SPEAKER_00]: In some places, the earth is warm to the touch.
11:18
[SPEAKER_00]: sulfurous gas rises from cracks in the rock.
11:21
[SPEAKER_00]: The ancient Greeks who settled in this area long ago called it by a name that later became the Flegreian fields, which means the burning fields.
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[SPEAKER_00]: It is a volcanic landscape, and it has been for a very long time.
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[SPEAKER_00]: Over tens of thousands
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[SPEAKER_00]: eruptions here have covered the land with thick layers of ash or pumice, and a kind of soft volcanic rock called Tuff.
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[SPEAKER_00]: In the middle of this landscape was a busy harbour town.
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[SPEAKER_00]: The Greeks had called it Dikertia.
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[SPEAKER_00]: The Romans called it Puteoli.
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[SPEAKER_00]: Today it is called Potsooli.
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[SPEAKER_00]: For a long stretch of Roman history,
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[SPEAKER_00]: Before the Great Harbour at Portus was finished, Puteoli was one of the most important ports in all of Italy.
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[SPEAKER_00]: Grainships arrived there from Egypt.
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[SPEAKER_00]: Merchants from across the eastern Mediterranean kept warehouses and shrines in the town and all around it on the hillside and in the quarries, there was ash.
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[SPEAKER_00]: The Romans called it Poulvis Puteolainus, which simply means the dust of Puteoli.
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[SPEAKER_00]: We now call it Potsalana, after the modern name of the town.
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[SPEAKER_00]: And in English, we have borrowed the word to describe a whole family of materials.
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[SPEAKER_00]: A Potsalan today is any substance that behaves the way this ash does.
12:49
[SPEAKER_00]: But the original, the one that gave its name to all the rest, came from here, from the burning fields near Naples.
12:56
[SPEAKER_00]: What made it special to understand that we need to take a step back and think about line.
13:03
[SPEAKER_00]: For thousands of years before the Romans, people in many parts of the world had known how to make line.
13:10
[SPEAKER_00]: You take limestone, which is mostly calcium carbonate, the same material as sea shells and chalk.
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[SPEAKER_00]: You break it into pieces and you stack it in a kiln and you heat it for a long time with a hot fire.
13:24
[SPEAKER_00]: Roman lime kills were often tall structures of stone loaded from the top with a fire burning below.
13:31
[SPEAKER_00]: The lime stone was heated to something like 900 degrees Celsius, sometimes more, and kept there for days.
13:39
[SPEAKER_00]: As it heated, it gave off carbon dioxide gas, and what was left behind was
13:54
[SPEAKER_00]: Quick climb is not a gentle material.
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[SPEAKER_00]: When it meets water, it reacts vigorously.
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[SPEAKER_00]: It hisses and steams and grows hot.
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[SPEAKER_00]: Sometimes hot enough to boil the water around it.
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[SPEAKER_00]: This process is called slaking.
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[SPEAKER_00]: When Quick climb is slaked with enough water, it becomes slaked line, which is calcium hydroxide.
14:18
[SPEAKER_00]: Builders would often slake their lime in pits, letting it sit as a thick, creamy party, sometimes for weeks or months before using it.
14:28
[SPEAKER_00]: If you mix slaked lime with sand and water, you get a lime mortar.
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[SPEAKER_00]: You can spread it between bricks or stones and over time it will harden, but it hardens in a particular way.
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[SPEAKER_00]: It slowly draws carbon dioxide back out of the air and turns back into calcium carbonate, which is to say, back into a kind of limestone.
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[SPEAKER_00]: The circle closes, stone becomes lime and lime becomes stone again.
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[SPEAKER_00]: This is a lovely patient process, but it has a limitation, it needs air.
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[SPEAKER_00]: Deep inside a thick wall or underwater, lime mortar hardens very slowly, if at all.
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[SPEAKER_00]: Put ordinary lime mortar into the sea, and it will simply wash away or remain soft for a very long time.
15:20
[SPEAKER_00]: Now, add the dust of puteoli.
15:24
[SPEAKER_00]: Volcanic ash is made when molten rock is blasted into the air and cools very quickly.
15:31
[SPEAKER_00]: Because it cools so fast,
15:33
[SPEAKER_00]: Much of it does not have time to form orderly crystals.
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[SPEAKER_00]: Instead, it becomes a kind of glass, a disordered, frozen arrangement of silicon, aluminum, and oxygen, along with other elements, like potassium, sodium, and calcium.
15:51
[SPEAKER_00]: This volcanic glass is full of chemical potential.
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[SPEAKER_00]: It is, in a sense, unfinished.
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[SPEAKER_00]: When it is mixed with lime and water,
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[SPEAKER_00]: The lime makes the water strongly alkaline, and in that alkaline water, the glassy ash begins to dissolve and react.
16:11
[SPEAKER_00]: The silicon and aluminum from the ash combine with the calcium from the lime, and together they form new minerals, binding compounds that knit the whole mixture together into a solid mass.
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[SPEAKER_00]: And here is the important part.
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[SPEAKER_00]: This reaction does not need air.
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[SPEAKER_00]: It happens in wet, dark places.
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[SPEAKER_00]: It happens at the bottom of the sea.
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[SPEAKER_00]: A mixture of lime and volcanic ash will set underwater and keep hardening underwater.
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[SPEAKER_00]: In modern language, we would say that it is hydraulic.
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[SPEAKER_00]: The Romans did not use that word, but they understood the effect perfectly well.
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[SPEAKER_00]: We know they understood it because they wrote it down.
17:00
[SPEAKER_00]: an architect and engineer named Vitruvius wrote a long work in ten books called Day Architecture, which means on architecture.
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[SPEAKER_00]: He dedicated it to Augustus.
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[SPEAKER_00]: It covers a great many subjects, from the proper design of temples to the building of siege engines, from the planning of cities, to the making of sundials.
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[SPEAKER_00]: And in the second book, he pauses to discuss this remarkable dust.
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[SPEAKER_00]: Vituvius explains that there is a kind of powder found in the region around Bae, and in the lands around Mount Visuvius.
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[SPEAKER_00]: that produces wonderful results.
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[SPEAKER_00]: When it is mixed with lime and broken stone, he says.
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[SPEAKER_00]: It not only makes building strong, but also, when peers are built in the sea, they set hard under the water.
17:51
[SPEAKER_00]: He then offers an explanation.
17:54
[SPEAKER_00]: using the science of his day.
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[SPEAKER_00]: He believed that beneath those hills there were great fires and that the heat of those fires had burned the earth and driven out its moisture, leaving it thirsty and porous.
18:10
[SPEAKER_00]: When this thirsty dust met line which had also been through fire and then met water.
18:16
[SPEAKER_00]: The two parts eagerly drew the water in and bound together into a single solid.
18:22
[SPEAKER_00]: His explanation was not correct in the way a modern chemist would describe it, but it was not foolish either.
18:30
[SPEAKER_00]: He had correctly noticed that the dust came from a volcanic region, that fire and heat were involved in its making, and that both the lime and the ash had been transformed by heat before they met.
18:44
[SPEAKER_00]: He was trying with the tools he had to explain something real, a little later in his fifth book, Vitruvius turns specifically to Harbus.
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[SPEAKER_00]: He describes how to build a pier or a breakwater out in the sea.
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[SPEAKER_00]: First, he says, you should bring the powder from the region between Qme and the
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[SPEAKER_00]: to one part of lime.
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[SPEAKER_00]: Then, in the place where the pier is to be built, you should drive down into the water, a set of enclosures made of oak steaks tied together with chains and firmly braced.
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[SPEAKER_00]: Within these enclosures you level the bottom and then you throw in the mixture of mortar and rubble, until the space inside is filled with solid masonry.
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[SPEAKER_00]: two parts ash to one part line.
19:42
[SPEAKER_00]: That ratio written down more than 2,000 years ago is one of the most famous lines in the history of building materials.
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[SPEAKER_00]: Modern researchers have found that the actual concrete in many Roman harbors is broadly consistent with this kind of mixture, though of course every site varied and every crew of workers had its own habits.
20:08
[SPEAKER_00]: The Roman writer Pliny the Elder was compiling his enormous natural history, a vast collection of everything he could learn about the world.
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[SPEAKER_00]: He too.
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[SPEAKER_00]: mentions the dust of Puteoli.
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[SPEAKER_00]: He observes with evident wonder that this dust, once it meets the waves of the sea and is submerged, becomes a single stone mass, impregnable to the waves and grows stronger every day.
20:37
[SPEAKER_00]: Pliny was a naval commander as well as a writer.
20:41
[SPEAKER_00]: He was stationed at Missineum on the Bay of Naples in command of the fleet there.
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[SPEAKER_00]: He would have seen these
20:50
[SPEAKER_00]: and it was there in the year 79 that he sailed out across the bay toward Mount Vesuvius during its great eruption and did not return.
21:00
[SPEAKER_00]: The same volcanic forces that had given the Romans their wonderful dust were in the end what took him.
21:07
[SPEAKER_00]: There is one more ingredient we should gently mention before we move on.
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[SPEAKER_00]: Roman concrete was not just lime and ash.
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[SPEAKER_00]: It also contains chunks of stone, which the Romans called K-A-Menta.
21:20
[SPEAKER_00]: That word is the origin of our words cement.
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[SPEAKER_00]: Though for the Romans, it meant the rough pieces of rubble, not the binding paste.
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[SPEAKER_00]: In marine structures, these chunks were often pieces of volcanic tough, sometimes as large as a fist or larger.
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[SPEAKER_00]: the Roman method was not quite the same as ours.
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[SPEAKER_00]: Today we usually mix everything together into a fluid slurry and pour it.
21:48
[SPEAKER_00]: The Romans often laid their concrete in layers.
21:51
[SPEAKER_00]: They would place a layer of mortar than a layer of rubble, then tamp it down firmly, then add more mortar.
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[SPEAKER_00]: and so on building up the mass.
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[SPEAKER_00]: In underwater forms, the mixture was lowered into place and the heavy chunks settled into the thick sticky mortar.
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[SPEAKER_00]: The result was a material quite different from what we brought today.
22:16
[SPEAKER_00]: It was not smooth or uniform.
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[SPEAKER_00]: If you look at a cut piece of Roman marine concrete, it looks like a kind of dark, speckled pudding stone.
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[SPEAKER_00]: There are gray chunks of tough, set in a matrix of pale immortal, and scattered throughout that mortar.
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[SPEAKER_00]: If you look closely, there are small white lumps, a few millimeters across.
22:40
[SPEAKER_00]: For a long time, people who studied Roman concrete assumed that these white lumps were a sign of careless work, little bits of line that had not been properly mixed in.
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[SPEAKER_00]: We will come back to those small white lumps at the end of our story.
22:57
[SPEAKER_00]: So the Romans had a recipe, they had a source
23:03
[SPEAKER_00]: they had lime kills, timber forms, ships to carry materials and skilled crews who knew how to do the work, and they built harbors that have lasted for 2,000 years, but knowing that they lasted is not the same as knowing why.
23:19
[SPEAKER_00]: For that, we would have to wait a very long time until people began drilling into the old harbors themselves and looking at what they found
23:31
[SPEAKER_00]: Chapter 3, Stone That Grows.
23:37
[SPEAKER_00]: In the early 2000s, a group of researchers set out to study Roman marine concrete systematically, in a way that had not really been done before.
23:48
[SPEAKER_00]: The project was called Roma cons, which is short for the Roman maritime concrete study.
23:56
[SPEAKER_00]: It was led by a small team that included the archaeologist and engineer Christopher Brandon, the classical archaeologist John Peter Olson, the historian
24:07
[SPEAKER_00]: and, as the work developed, the geologist Marie Jackson, who would go on to become one of the most important voices in the study of Roman concrete.
24:18
[SPEAKER_00]: The project received support from an Italian cement company, which had its own practical interest in understanding how an ancient material could last so long in the sea.
24:29
[SPEAKER_00]: Their approach was simple, but it took a great deal of
24:37
[SPEAKER_00]: and they drilled.
24:38
[SPEAKER_00]: Divers took specialised drilling equipment down to the old brake waters and pears, and they extracted long cylinders of concrete called cause from deep inside the structures.
24:51
[SPEAKER_00]: Over the course of the project, they collected cause from around a dozen sites, including Cessaria, Cosa, Bia, Portus, Anzeo and several others in Italy, Greece, Turkey and Egypt.
25:06
[SPEAKER_00]: Each core was a kind of time capsule.
25:10
[SPEAKER_00]: It held a sample of the concrete exactly as it had existed.
25:15
[SPEAKER_00]: Deep inside the structure, after 2,000 years in the sea, the cores were carefully locked, measured, and sent to laboratories for analysis.
25:25
[SPEAKER_00]: Researchers examined the composition of the ash, looking at its chemistry to figure out where it had come from, and again and again.
25:34
[SPEAKER_00]: The answer pointed back to the Bay of Naples, harbours in Judeo, in Egypt, in Greece, all over the Roman world had been built using ash from the region around Puzzweoli and Vassuvius.
25:48
[SPEAKER_00]: The trade in this volcanic dust had been truly Mediterranean in scale.
25:54
[SPEAKER_00]: The team also did something practical.
25:58
[SPEAKER_00]: In 2004, they built a small replica of a Roman marine concrete structure in the Harbour at Brindisi in southern Italy using methods and materials as close as possible to the ancient ones.
26:13
[SPEAKER_00]: They built a timber form mixed lime and volcanic ash and placed it under water following the instructions of Vitruvius.
26:23
[SPEAKER_00]: It was a way of testing whether the ancient descriptions actually worked
26:28
[SPEAKER_00]: And of watching the material set and mature in a real marine environment, it did work.
26:35
[SPEAKER_00]: The concrete set underwater, as vitruvius had said it would.
26:39
[SPEAKER_00]: But the most fascinating discoveries came later.
26:42
[SPEAKER_00]: When researchers looked very closely at the ancient cause, at a scale far smaller than the eye can see.
26:50
[SPEAKER_00]: Marie Jackson and her colleagues took small slices and fragments of the ancient concrete.
26:57
[SPEAKER_00]: and examine them using powerful tools.
27:01
[SPEAKER_00]: Some of that work was done at the Advanced Light Source, a facility at the Lawrence Berkeley National Laboratory in California, where intense beams of x-rays can be focused on tiny spots of a sample, revealing the arrangement of atoms in its minerals.
27:17
[SPEAKER_00]: They used electron microscopes to see the shapes of crystals only a few millions of a meter across.
27:24
[SPEAKER_00]: They used chemical mapping to see which elements were present in which tiny regions of the material.
27:32
[SPEAKER_00]: What they found was a kind of hidden landscape inside the Roman concrete.
27:37
[SPEAKER_00]: in the pores and cavities of the mortar, in the spaces around the fragments of volcanic ash, and inside the old lime lumps, there were delicate crystals, some were flat, thin plates, some were clusters of tiny prisms.
27:54
[SPEAKER_00]: These crystals were minerals with names that sound, perhaps, a little unfamiliar.
28:07
[SPEAKER_00]: Let us pause for a moment with Tobra Morite, because it is a rather special mineral.
28:13
[SPEAKER_00]: It takes its name from Tobra Mori, a small harbour town on the Isle of Mal, off the west coast of Scotland, where it was first described in the 1800s.
28:24
[SPEAKER_00]: In nature, it is fairly rare.
28:27
[SPEAKER_00]: It tends to form in unusual conditions, sometimes in and around volcanic rock that has been altered by warm water.
28:37
[SPEAKER_00]: It is a calcium-silicate hydrate, which places it in the same broad family as the binding material in modern cement.
28:46
[SPEAKER_00]: But unlike much of the binding material in modern cement, which is poorly ordered, almost like a gel, tobermoreite forms proper crystals.
28:55
[SPEAKER_00]: Its layers are neatly arranged, and the kind found in Roman concrete,
29:00
[SPEAKER_00]: contains a luminum as well which seems to make it especially stable.
29:05
[SPEAKER_00]: Modern industry can make to a Burmerite but usually only by heating materials under pressure at temperatures well above the boiling point of water in a kind of industrial pressure cooker.
29:17
[SPEAKER_00]: It does not normally form easily at the cool temperatures of the sea floor.
29:23
[SPEAKER_00]: and yet, here it was, growing inside Roman harbour concrete that had been sitting in cool Mediterranean sea water for 2,000 years.
29:36
[SPEAKER_00]: Philipsite is a different kind of mineral.
29:39
[SPEAKER_00]: It belongs to a family called Zealights, minerals with an open, cage-like structure full of tiny channels.
29:47
[SPEAKER_00]: Philip site is common in volcanic rocks that have been altered by water, and it is also found on the deep sea floor.
29:55
[SPEAKER_00]: In the Roman concrete, it was growing in pores and cracks, and in the altered remains of volcanic glass.
30:03
[SPEAKER_00]: In a study published in 2017,
30:06
[SPEAKER_00]: Marie Jackson and her colleagues offered an explanation for how these minerals came to be there, and it is a beautiful one.
30:15
[SPEAKER_00]: When the Romans first mixed and placed their concrete, the reaction between lime and volcanic ash began, producing the primary binding material and hardening the mass.
30:28
[SPEAKER_00]: that was the first stage, and it happened relatively quickly, over days, weeks and months.
30:35
[SPEAKER_00]: But the story did not end there, over the following years and then decades and then centuries, seawater slowly percolated into the concrete, it seaped through tiny pores and fine cracks.
30:49
[SPEAKER_00]: And as it moved through the material, it did something that, in most modern concrete, would
30:56
[SPEAKER_00]: it dissolved things.
30:58
[SPEAKER_00]: It gradually dissolved some of the remaining volcanic glass in the ash and some of the leftover lime, but the dissolved material did not simply wash away.
31:10
[SPEAKER_00]: The fluid inside the concrete became a kind of mineral-rich solution, and in the sheltered spaces of the pores, new crystals began to grow out of it.
31:21
[SPEAKER_00]: Illuminous tober morite and fill-up sites slowly formed, filling cavities, lining cracks.
31:28
[SPEAKER_00]: and interlocking with the surrounding material.
31:31
[SPEAKER_00]: In other words, the concrete was not simply sitting still in the sea, slowly wearing away.
31:38
[SPEAKER_00]: It was reacting with the sea.
31:40
[SPEAKER_00]: Sea water, which we usually think of as an enemy of building materials, had become a kind of partner.
31:47
[SPEAKER_00]: The concrete and the sea were engaged in a very slow chemical conversation and the result of that conversation was new mineral growth that reinforced the material from within.
32:00
[SPEAKER_00]: Jackson and her colleagues compared this in some ways to the natural processes that turn volcanic ash deposits into solid tough rock over geological time.
32:12
[SPEAKER_00]: The Roman builders, without knowing it, had set in motion something close to a geological process, and then left it to continue on its own.
32:23
[SPEAKER_00]: This gives a gentle new meaning to plinney's old observation.
32:28
[SPEAKER_00]: He had written that the dust of Puteoli once submerged grows stronger every day.
32:35
[SPEAKER_00]: For centuries, that might have sounded like a poetic exaggeration.
32:40
[SPEAKER_00]: But at the scale of crystals in the dark interiors of the old break waters, it appears that something like it was actually true.
32:49
[SPEAKER_00]: It is worth being careful and honest here.
32:52
[SPEAKER_00]: Roman concrete is not a magical material.
32:55
[SPEAKER_00]: It is, in some ways, rather weak.
32:58
[SPEAKER_00]: If you tested a sample of Roman marine concrete for how much weight it can bear under compression, it would generally not match the strength of a good modern structural concrete.
33:10
[SPEAKER_00]: It sets slowly, it gains strength slowly.
33:14
[SPEAKER_00]: It would be a poor choice for a modern skyscraper, or for a highway bridge carrying heavy trucks.
33:22
[SPEAKER_00]: The Romans compensated by building massively with enormous thick blocks and walls.
33:29
[SPEAKER_00]: Their harbors were not elegant slender structures.
33:33
[SPEAKER_00]: They were great heavy masses, designed to sit in the sea and simply endure, and not all Roman harbour concrete survived.
33:43
[SPEAKER_00]: Much of it has slumped, broken, or been buried by sand.
33:48
[SPEAKER_00]: Some harbors silted up and were abandoned.
33:51
[SPEAKER_00]: Some structures were deliberately dismantled for building materials in later centuries.
33:58
[SPEAKER_00]: What we see today is to some extent what happened to last.
34:04
[SPEAKER_00]: But even with those caveats in mind, the durability of the surviving material is genuinely remarkable.
34:11
[SPEAKER_00]: Its ability to sit in sea water for two millennia and to use the sea itself as an ingredient in its own ongoing chemistry is something very few modern building materials can claim.
34:24
[SPEAKER_00]: So we now have a sense of what was happening inside the ancient stone.
34:30
[SPEAKER_00]: But this naturally raises another question.
34:34
[SPEAKER_00]: One that sits a little uncomfortably beside all this admiration.
34:39
[SPEAKER_00]: If the Romans had such a durable recipe, what happened to it?
34:44
[SPEAKER_00]: And why is it that our own concrete?
34:46
[SPEAKER_00]: so much stronger on the day it is poured, so often seems to struggle in the sea.
34:52
[SPEAKER_00]: That is where we will drift next.
34:57
[SPEAKER_00]: Chapter 4.
34:59
[SPEAKER_00]: The long forgetting and the rusting heart.
35:04
[SPEAKER_00]: It is often said that the recipe for Roman concrete was lost.
35:09
[SPEAKER_00]: That is a lovely, mysterious phrase, and it is not entirely wrong.
35:15
[SPEAKER_00]: But it is not entirely right, either.
35:18
[SPEAKER_00]: The truth is a little more gradual.
35:21
[SPEAKER_00]: The text of Vitruvious was never completely lost.
35:25
[SPEAKER_00]: Copies of on architecture were made by hand in monasteries through the Middle Ages and the work was read and studied by scholars.
35:34
[SPEAKER_00]: In the early 1400s, a manuscript of Vitruvious
35:39
[SPEAKER_00]: and widely circulated among the architects of the Italian Renaissance, who treated it almost as a sacred text.
35:47
[SPEAKER_00]: Its first printed edition appeared in the late 1400s, so the famous ratio, two parts ash to one part line, was available to anyone who could read Latin and find a copy.
36:00
[SPEAKER_00]: Line mortar was never forgotten, either.
36:04
[SPEAKER_00]: Builders across Europe continue to burn limestone in kilns and make line mortars for centuries after the fall of the western Roman Empire.
36:13
[SPEAKER_00]: Carcels, churches, cathedrals and town walls were all built with line mortar.
36:20
[SPEAKER_00]: What faded was something more specific.
36:23
[SPEAKER_00]: It was the large-scale organized use of potholanic concrete, especially for massive marine structures and great concrete folds.
36:33
[SPEAKER_00]: and the whole network of knowledge, trade, labour, and habit that had supported it.
36:39
[SPEAKER_00]: Building a Roman harbour required a great deal more than a recipe.
36:44
[SPEAKER_00]: It required reliable supplies of the right ash, often shipped across the sea.
36:50
[SPEAKER_00]: It required large, well-organized workforces, experienced crews who knew how to prepare lime,
37:02
[SPEAKER_00]: and how to place the concrete under water.
37:05
[SPEAKER_00]: It required wealth and a state or a powerful patron willing to spend it on infrastructure.
37:12
[SPEAKER_00]: As the Western Roman Empire fragmented in the fourth and fifth centuries, many of those conditions slowly fell away.
37:20
[SPEAKER_00]: Long distance trade became more difficult.
37:24
[SPEAKER_00]: Large public building projects became rare.
37:27
[SPEAKER_00]: Skills that had been passed from Master to Apprentice in the building trades were not always passed on.
37:34
[SPEAKER_00]: Much of what was lost was not written knowledge at all, but practical hands-on knowledge.
37:40
[SPEAKER_00]: The sort of thing a worker knows in their hands rather than on the page.
37:45
[SPEAKER_00]: Then, in the 1700s, interest began to return,
37:53
[SPEAKER_00]: with the sea.
37:54
[SPEAKER_00]: Off the coast of Plymouth, in southwestern England, there is a treacherous cluster of rocks called the Eddie Stone Rocks.
38:02
[SPEAKER_00]: Ships have been wrecked there for a very long time.
38:05
[SPEAKER_00]: Several lighthouseers had been built on the rocks, and each had been destroyed, one by a great storm, another by fire.
38:14
[SPEAKER_00]: In the 1750s, a young engineer named John Smeter
38:22
[SPEAKER_00]: He knew that the lighthouse would stand in a place where it would be constantly battered by waves and soaked by spray and he wanted a mortar that would set and hold in the wet.
38:35
[SPEAKER_00]: Smitten was a careful experimenter.
38:39
[SPEAKER_00]: He tested many different kinds of line from different sources to see which would harden underwater.
38:47
[SPEAKER_00]: He found that Lyme's made from Lyme's stone, containing a certain amount of clay, behaved differently from pure Lyme's.
38:57
[SPEAKER_00]: They had a natural hydraulic quality, and for his lighthouse, he used a mortar that combined a hydraulic line with Potsalana imported from Italy, from a place called Sivitavakia, north of Rome.
39:12
[SPEAKER_00]: His lighthouse was finished in 1759.
39:16
[SPEAKER_00]: It stood on the eddy stone rocks for more than a century until the rocks beneath it rather than the tower itself began to erode.
39:26
[SPEAKER_00]: The upper part of the tower was then taken down and rebuilt on land in Plymouth, where it still stands today, looking out over the sound.
39:36
[SPEAKER_00]: Smeeton's experiments helped begin a new era of research into hydraulic building materials.
39:42
[SPEAKER_00]: Over the following decades, engineers and inventors in Britain and France
39:47
[SPEAKER_00]: experimented with various mixtures and burning methods.
39:52
[SPEAKER_00]: And in 1824, a bricklayer from Leeds, named Joseph Astin, took out a patent for something he called Portland cement.
40:02
[SPEAKER_00]: He chose the name because he thought the hardened material resembled Portland stone, a fine pale limestone, quarried on the Isle of Portland in the south of England.
40:16
[SPEAKER_00]: which was a very prestigious building stone at the time.
40:20
[SPEAKER_00]: Asptons early product was not quite the same as modern Portland cement.
40:26
[SPEAKER_00]: Over the following decades, other manufacturers improved it, particularly by burning the raw materials at much higher temperatures.
40:36
[SPEAKER_00]: Modern Portland cement is made by heating a carefully proportioned mixture of limestone and clay in a long rotating kiln to around 1450 degrees Celsius.
40:52
[SPEAKER_00]: At that intense heat, the materials partially melt and fuse into hard little nodules called clinker.
40:59
[SPEAKER_00]: The clinker
41:00
[SPEAKER_00]: is then ground into a fine grey powder mixed with a small amount of gypsum and bagged.
41:07
[SPEAKER_00]: That powder is cement.
41:09
[SPEAKER_00]: When you mix it with water, sand and gravel, you get concrete.
41:14
[SPEAKER_00]: Portland cement was and still is a tremendous achievement.
41:19
[SPEAKER_00]: It sets quickly and reliably.
41:21
[SPEAKER_00]: It becomes very strong.
41:23
[SPEAKER_00]: much stronger in compression than Roman concrete within days or weeks.
41:29
[SPEAKER_00]: It is consistent so that engineers can predict exactly how it will behave.
41:35
[SPEAKER_00]: It can be manufactured in enormous quantities and shipped anywhere.
41:40
[SPEAKER_00]: It became the foundation of modern construction.
41:43
[SPEAKER_00]: Nearly everything around us,
41:51
[SPEAKER_00]: Today, it is one of the most widely used materials on earth, second only to water, and then in the late 1800s came another innovation that transformed concrete entirely.
42:07
[SPEAKER_00]: People began embedding iron and later steel inside it.
42:12
[SPEAKER_00]: Concrete is very strong when you squeeze it, but it is weak when you pull on it or bend it,
42:20
[SPEAKER_00]: is excellent at resisting pulling forces.
42:24
[SPEAKER_00]: By placing steel bars called reinforcing bars or rebar inside the concrete, engineers created a composite material that could resist both squeezing and bending.
42:38
[SPEAKER_00]: This is reinforced concrete.
42:41
[SPEAKER_00]: It allowed for long bridges, thin floor slabs, tall towers, and elegant cantilevers that seemed to float in the air.
42:51
[SPEAKER_00]: It was something the Romans never had and could never have built, but reinforced concrete carries within it a vulnerability.
43:00
[SPEAKER_00]: and it is this vulnerability, more than anything else, that explains why so many modern peers and sea walls seem to crumble in the sea.
43:09
[SPEAKER_00]: When concrete is fresh and healthy, it is very alkaline inside.
43:14
[SPEAKER_00]: That alkaline environment actually protects the steel.
43:18
[SPEAKER_00]: It causes a very thin, stable film to form on the surface of the steel, a passive layer that prevents rust.
43:26
[SPEAKER_00]: As long as that layer stays intact, the steel is safe, even though it is surrounded by damp concrete.
43:33
[SPEAKER_00]: Sea water, however, is full of salt, and salt contains chloride.
43:40
[SPEAKER_00]: Over time, chloride ions from the sea slowly work their way into the concrete.
43:47
[SPEAKER_00]: They travel through the tiny pools carried by moisture.
43:51
[SPEAKER_00]: driven by the constant wetting and drying of tides, spray and waves.
43:57
[SPEAKER_00]: The process is slow, it might take years or decades.
44:02
[SPEAKER_00]: But eventually, the chloride reaches the steel.
44:06
[SPEAKER_00]: And when enough chloride gathers at the surface of the steel, it breaks down that protective film.
44:12
[SPEAKER_00]: The steel begins to rust.
44:14
[SPEAKER_00]: Rust is not just a discoloration.
44:18
[SPEAKER_00]: When steel carones, the rust that forms takes up considerably more space than the original metal, several times more.
44:26
[SPEAKER_00]: So as the steel bar rusts, it swells.
44:33
[SPEAKER_00]: The concrete, as we said, is weak when it is pulled apart.
44:38
[SPEAKER_00]: The swelling rust cracks the concrete from the inside.
44:42
[SPEAKER_00]: Those cracks let in more water, more salt and more oxygen, which speeds up the rustling, which causes more swelling, which causes more cracking.
44:52
[SPEAKER_00]: Eventually, pieces of the concrete surface break away entirely.
44:57
[SPEAKER_00]: This is called spalling.
45:00
[SPEAKER_00]: You may have seen it on an old pier, a parking garage, or a bridge support near the sea.
45:07
[SPEAKER_00]: There are patches where the gray surface has fallen away, revealing orange brown rusted steel bars beneath.
45:15
[SPEAKER_00]: It is a slow cycle.
45:17
[SPEAKER_00]: Nothing dramatic happens on any particular day.
45:21
[SPEAKER_00]: But year after year, the heart of the structure rusts.
45:25
[SPEAKER_00]: and the stone around it gives way.
45:27
[SPEAKER_00]: Many marine concrete structures of the 20th century were designed with a service life of perhaps 50 years, or in some cases, a hundred, maintaining them, patching them, and eventually replacing them costs a great deal of money and effort around the world.
45:46
[SPEAKER_00]: So we can see that the comparison between Roman harbours and
45:50
[SPEAKER_00]: and modern peers is not quite a fair fight in either direction.
45:56
[SPEAKER_00]: The Romans did not use steel, their concrete had no rusting heart.
46:00
[SPEAKER_00]: It was a thick, heavy, un-reinforced mass, and so the particular cycle that troubles modern marine concrete simply could not happen to it.
46:11
[SPEAKER_00]: On the other hand, modern concrete does things that Roman concrete could never do.
46:16
[SPEAKER_00]: It carries enormous loads, spans great distances, and can be built quickly.
46:22
[SPEAKER_00]: It was never meant to sit in the sea as a massive block for 2,000 years.
46:28
[SPEAKER_00]: It was meant to do far more demanding work, faster, with less material.
46:33
[SPEAKER_00]: But there is still a real and interesting difference in chemistry, and it is worth resting
46:41
[SPEAKER_00]: In modern Portland cement concrete, seawater is almost entirely an enemy.
46:47
[SPEAKER_00]: The reactions it triggers tend to weaken the material over time.
46:52
[SPEAKER_00]: In Roman marine concrete, seawater became, in part, an ally.
46:58
[SPEAKER_00]: The reactions it triggered with the volcanic ash and the lime produced new minerals that strengthened and consolidated the material.
47:07
[SPEAKER_00]: The Romans, working by experience, rather than theory, had chosen ingredients that got along with the sea.
47:16
[SPEAKER_00]: There is also a difference in how the two materials are made and it has become more important in our own time.
47:24
[SPEAKER_00]: Making Portland cement requires those very high kiln temperatures around 1,450 degrees, and it also releases large amounts of carbon dioxide, both from the fuel burn to heat the kilns, and from the limestone itself as it breaks down.
47:44
[SPEAKER_00]: Sement production is responsible for a significant share of the carbon dioxide, released by human activity every year.
47:53
[SPEAKER_00]: often estimated at around 7 or 8% of the global total.
47:58
[SPEAKER_00]: Roman lime was burned at lower temperatures and in their marine concrete, a large part of the mixture was volcanic ash.
48:07
[SPEAKER_00]: which needed no burning at all.
48:09
[SPEAKER_00]: It had already been through the fire of the volcano.
48:13
[SPEAKER_00]: For all of these reasons, in the late 20th and early 21st centuries, scientists and engineers began to look back at Roman concrete with fresh curiosity, not with the idea of abandoning modern concrete, but with the idea of learning from the old one.
48:33
[SPEAKER_00]: And as they looked more closely, one detail kept catching their eye.
48:39
[SPEAKER_00]: Those small, white lumps scattered through the mortar.
48:44
[SPEAKER_00]: The flecks that had so long been dismissed as mistakes.
48:48
[SPEAKER_00]: In our final chapter, we will turn to them at last.
48:55
[SPEAKER_00]: Chapter 5.
48:56
[SPEAKER_00]: The white flecks.
49:00
[SPEAKER_00]: If you take a piece of Roman concrete from a harbour or a wall or a tomb and you cut it open and polish the surface, you will see the grey and brown chunks of volcanic rock, the paler mortar around them, and those little white inclusions.
49:20
[SPEAKER_00]: They are usually somewhere between a millimeter and a couple of centimeters across.
49:26
[SPEAKER_00]: They are soft looking.
49:28
[SPEAKER_00]: a bit chalky.
49:29
[SPEAKER_00]: They appear in Roman concrete from many different sites and many different periods.
49:35
[SPEAKER_00]: These are called lime clasts.
49:38
[SPEAKER_00]: For a long time, the most common explanation was that they were simply lumps of lime that had not been properly mixed into the water.
49:47
[SPEAKER_00]: Perhaps the workers had been hasty, perhaps the lime had not been fully slaked, or had clumped together, it was a sensible assumption.
49:57
[SPEAKER_00]: In modern construction, a lump of unmixed material would be considered a defect, and a sign of poor quality control, but some researchers began to feel that this explanation did not quite fit.
50:11
[SPEAKER_00]: The Romans were extremely careful about their building materials.
50:15
[SPEAKER_00]: The truvius himself wrote at length about how lime should be selected and prepared.
50:21
[SPEAKER_00]: Roman builders produced carefully graded mixtures and followed well-established practices over many generations.
50:29
[SPEAKER_00]: Would they really have made the same sloppy mistake, again and again, across so many sites and so many centuries?
50:39
[SPEAKER_00]: And if these lumps were a flaw, why did the concrete that contained them last so remarkably well?
50:46
[SPEAKER_00]: At the Massachusetts Institute of Technology, a material scientist named Admir Massick had become deeply interested in these questions.
50:56
[SPEAKER_00]: Working with colleagues from Harvard University and from laboratories in Italy and Switzerland, including a researcher named Linda Seymour, who led much of the detailed work.
51:08
[SPEAKER_00]: He and his team decided to take a very close look at the lime clasts themselves.
51:15
[SPEAKER_00]: For some of their studies, they used samples of concrete from the ancient city walls of Prevernam.
51:22
[SPEAKER_00]: an archaeological site in central Italy, south of Rome.
51:26
[SPEAKER_00]: The walls there had stood for around 2,000 years.
51:30
[SPEAKER_00]: The team used a whole range of techniques.
51:33
[SPEAKER_00]: They used large area scanning electron microscopy to map the samples at high resolution.
51:40
[SPEAKER_00]: They used a technique called energy-dispersive x-ray spectroscopy to see which chemical elements were present in each tiny region.
51:51
[SPEAKER_00]: They used x-ray diffraction to identify the minerals and they used a method called confocal raiman imaging which shines a laser onto a sample and reads the faint signals that come back to identify the chemical compounds present, point by point what they found
52:12
[SPEAKER_00]: When they looked inside the lime clasts was that these little white lumps were not simply unmixed slate-lime.
52:20
[SPEAKER_00]: They were made of various forms of calcium carbonate, and they had an unusual internal structure.
52:27
[SPEAKER_00]: They were brittle and full of tiny cracks and pores, a kind of fractured porous texture, and their chemistry and structure strongly suggested that they had formed at high
52:42
[SPEAKER_00]: that these lumps were the signature of a particular way of making the concrete.
52:47
[SPEAKER_00]: One that differed from how many people had long assumed the Romans worked.
52:52
[SPEAKER_00]: The standard assumption had been that the Romans slaked their line first, turning quick climb into a smooth party with water, and then mixed that party with the volcanic ash.
53:04
[SPEAKER_00]: But the evidence from the line clasps pointed toward a different method.
53:09
[SPEAKER_00]: It suggested that the Romans, at least in many cases, mixed quick line, the fiercely
53:22
[SPEAKER_00]: and then added water.
53:24
[SPEAKER_00]: The team called this method hot mixing.
53:27
[SPEAKER_00]: Remember what quick climb does when it meets water.
53:31
[SPEAKER_00]: It reacts vigorously and it releases a great deal of heat.
53:36
[SPEAKER_00]: In a batch of concrete made by hot mixing, that heat would have raised the temperature of the whole mixture considerably and the researchers proposed that this heat did several useful things at once.
53:49
[SPEAKER_00]: It's sped up the chemical reactions, allowing the concrete to set more quickly.
53:56
[SPEAKER_00]: It encouraged the formation of certain high temperature compounds that would not form otherwise.
54:04
[SPEAKER_00]: And, as the lime reacted unevenly in the hot, rapidly changing mixture, it left behind these characteristic lumps.
54:13
[SPEAKER_00]: the lime clasts, which were not fully incorporated into the rest of the mortar.
54:19
[SPEAKER_00]: In other words, the white flecks were not a mistake.
54:22
[SPEAKER_00]: They were a natural result of a hot, fast, energetic way of making concrete.
54:29
[SPEAKER_00]: And more than that, the researchers came to believe that the lime clasts served a remarkable purpose.
54:36
[SPEAKER_00]: Think about what happens when a crack begins to form in a piece of concrete.
54:41
[SPEAKER_00]: Tiny cracks are almost inevitable in any concrete structure, over time.
54:48
[SPEAKER_00]: They conform as the material dries or as it settles, or as it is stressed by the ground moving or the waves pushing.
54:57
[SPEAKER_00]: In most modern concrete, a small crack is the beginning of trouble.
55:02
[SPEAKER_00]: It lets in water, and with water comes salts and other substances, and gradually the crack widens and the damage spreads.
55:12
[SPEAKER_00]: But in Roman concrete, a crack moving through the mortar would often run into one of these lime clasts.
55:20
[SPEAKER_00]: And because the clasts are brittle, with a larger surface area than the surrounding material
55:26
[SPEAKER_00]: and full of tiny weaknesses, the crack would tend to travel into them.
55:30
[SPEAKER_00]: When water then entered the crack, it would reach the calcium-rich material of the classed.
55:36
[SPEAKER_00]: Some of that calcium would dissolve into the water, forming a calcium-rich solution, and that solution could then react.
55:44
[SPEAKER_00]: Either by drawing in carbon dioxide and recrystallizing as calcium carbonate, or by reacting further with the volcanic ash around it.
55:54
[SPEAKER_00]: Either way, new mineral material would form inside the crack, gradually filling it in, and sealing it closed.
56:01
[SPEAKER_00]: The lime clasts, in this view, were a built-in reservoir of calcium scattered all through the concrete, waiting.
56:10
[SPEAKER_00]: Where no crack ever came, they simply sat there, but wherever a crack opened and water came in, a nearby clast could release its stored calcium and help heal the damage.
56:23
[SPEAKER_00]: The concrete could in a small but real way repair itself.
56:28
[SPEAKER_00]: This was an elegant idea, but the team wanted to test it, so they made their own concrete.
56:35
[SPEAKER_00]: They prepared samples using a hot mixing method with quicklime, following what they believed was the ancient approach, and they prepared other samples without quicklime as a comparison.
56:48
[SPEAKER_00]: Then they deliberately cracked the samples.
56:52
[SPEAKER_00]: and they ran water through the cracks.
56:55
[SPEAKER_00]: Over the following two weeks or so, the results were clear.
56:59
[SPEAKER_00]: In the hot mixed concrete, the cracks gradually healed.
57:03
[SPEAKER_00]: Mineral deposits formed inside them, and eventually the water could no longer flow through at all.
57:10
[SPEAKER_00]: In the samples made without quick climb, the cracks did not heal, the water kept flowing.
57:16
[SPEAKER_00]: This work was published in January of 2023, in the journal Science Advances, and it drew a great deal of attention around the world.
57:27
[SPEAKER_00]: For many people,
57:28
[SPEAKER_00]: It was the first time they had heard that a 2,000-year-old material might have been in a sense, self-healing.
57:36
[SPEAKER_00]: It is worth pausing to notice how this discovery fits together with the earlier work on harbour concrete.
57:44
[SPEAKER_00]: The two lines of research are different, but they are in harmony with one another.
57:48
[SPEAKER_00]: Marie Jackson and her colleagues had shown that Roman marine concrete over the centuries reacted with sea water to grow new, strengthening minerals like toba morite and philipsite, often inside pores and inside the old lime clasts.
58:05
[SPEAKER_00]: Admir Macyk and his colleagues showed that the lime class themselves could act as a reservoir of calcium, ready to heal cracks when water came in.
58:15
[SPEAKER_00]: Both pieces of work point toward the same broad picture.
58:19
[SPEAKER_00]: Roman concrete was not a static finished material.
58:23
[SPEAKER_00]: It was a slowly active one.
58:26
[SPEAKER_00]: It continued to change after it was poured.
58:29
[SPEAKER_00]: And in the right conditions, much of that change
58:35
[SPEAKER_00]: rather than less.
58:37
[SPEAKER_00]: There is one more gentle chapter to this story, and it came from a place we have already visited briefly in our earlier chapters, Pompeii.
58:46
[SPEAKER_00]: When Mount Vassuvius erupted in the year 79, the town of Pompeii was buried under a thick layer of ash and pumice.
58:54
[SPEAKER_00]: Everything was frozen in place, exactly as it had been in that moment.
58:59
[SPEAKER_00]: And it so happens that at
59:05
[SPEAKER_00]: Parts of Pompeii had been damaged by an earthquake some years earlier, and repairs and new building projects were underway.
59:13
[SPEAKER_00]: Some of those building sites were buried in the middle of their work, with materials still sitting where the workers had left them.
59:21
[SPEAKER_00]: In recent years, excavations uncovered one such building site.
59:26
[SPEAKER_00]: in a house that was being renovated when the eruption came.
59:31
[SPEAKER_00]: Massic and his colleagues studied the materials found there, and according to their findings, published in late 2025.
59:41
[SPEAKER_00]: The construction materials on that site included piles of dry, pre-mixed material that appeared to be quick-lime already combined with volcanic ash, ready to be wetted and used.
59:55
[SPEAKER_00]: This was the first time anyone had seen at an actual Roman building site, direct evidence of the materials prepared in the way the hot mixing theory suggested.
1:00:06
[SPEAKER_00]: It was a little like finding the ingredients of a recipe laid out on a kitchen counter, 2,000 years after the cook had stepped away.
1:00:16
[SPEAKER_00]: There is something very peaceful about that image, a building site, with its piles of grey powder, its tools, and its half-finished walls, waiting for workers who would never return.
1:00:30
[SPEAKER_00]: Beneath the ash, for nearly 2,000 years, it simply kept its secret, and then carefully uncovered and studied, it confirmed what the little white flecks in old walls had been suggesting all along.
1:00:45
[SPEAKER_00]: So where does all of this leave us?
1:00:48
[SPEAKER_00]: It leaves us first, with an answer to our opening question, and it is a satisfying one.
1:00:56
[SPEAKER_00]: Roman harbour concrete has survived for two thousand years in sea water for several reasons working together.
1:01:04
[SPEAKER_00]: The Romans used volcanic ash that reacts with lime to form a hydraulic binder, one that sets underwater and does not need air.
1:01:14
[SPEAKER_00]: They built with thick, heavy, un-reinforced masses, with no steel inside to rust and swell and crack the stone from within.
1:01:24
[SPEAKER_00]: their concrete over the centuries, reacted with seawater in ways that grew new minerals, like toba morite and philipsite, reinforcing the material from the inside.
1:01:37
[SPEAKER_00]: And their method of hot mixing with quick-lime left behind lime clasts that could release calcium and help seal small cracks when water found its way in.
1:01:47
[SPEAKER_00]: None of these alone is the whole secret.
1:01:50
[SPEAKER_00]: Together, they make up the recipe.
1:01:53
[SPEAKER_00]: It leaves us second with a clearer understanding of why our own marine concrete struggles.
1:02:01
[SPEAKER_00]: It is not that modern engineers are careless or that modern concrete is poor.
1:02:06
[SPEAKER_00]: It is that modern concrete
1:02:09
[SPEAKER_00]: was designed for different goals, speed, strength, slendoness, and it carries steel within it, which the sea slowly finds and curodes.
1:02:21
[SPEAKER_00]: The two materials were made for different worlds, and it leaves us, finally, with something hopeful.
1:02:30
[SPEAKER_00]: Because these discoveries are not only about the past,
1:02:34
[SPEAKER_00]: Engineers and scientists today are actively using what they have learned.
1:02:40
[SPEAKER_00]: Modern concrete already often includes materials that behave like the Roman Potsalana, such as fly ash, a fine powder collected from coal burning power plants, and ground slack, a byproduct of iron production, and natural volcanic ashes, and calciumed clays.
1:03:02
[SPEAKER_00]: These materials react with the lime released by cement and make concrete denser, more durable and more resistant to the intrusion of chloride and sulfate.
1:03:14
[SPEAKER_00]: Using them also means using less Portland cement, which means less carbon dioxide released into the air.
1:03:22
[SPEAKER_00]: Researchers are experimenting with concrete that include lime clasts or capsules of healing material so that modern structures might also be able to seal their own small cracks.
1:03:35
[SPEAKER_00]: Others are studying mixtures inspired by Roman marine concrete.
1:03:40
[SPEAKER_00]: for sea walls, break waters, and other structures that need to sit in the sea for a very long time, without steel and without hurry.
1:03:50
[SPEAKER_00]: So the recipe was never truly lost.
1:03:52
[SPEAKER_00]: It was set down for a while and then patiently picked up again.
1:03:56
[SPEAKER_00]: Betruvius wrote down the ratio.
1:03:59
[SPEAKER_00]: Pliny noticed that the stone grew stronger.
1:04:02
[SPEAKER_00]: Smitten tested his lines and sent for Potsalana from Italy.
1:04:07
[SPEAKER_00]: Divers drilled into the old brake waters.
1:04:10
[SPEAKER_00]: Geologists found crystals growing in the dark.
1:04:13
[SPEAKER_00]: Chemists looked closely at the white flecks and a buried building site at Pompeii offered the final confirmation.
1:04:22
[SPEAKER_00]: Each person added one small piece and one by one the pieces settled into place.
1:04:29
[SPEAKER_00]: Let us return for a last moment to where we began.
1:04:34
[SPEAKER_00]: the long, straight coast of Judea.
1:04:37
[SPEAKER_00]: The remains of Herod's harbour at Cessaria, lying beneath the water.
1:04:43
[SPEAKER_00]: The sea washes over the old concrete blocks.
1:04:46
[SPEAKER_00]: As it has every day for 2,000 years, tiny fish drift through the shadows.
1:04:53
[SPEAKER_00]: Sunlight flickers on the sand, and deep inside the grey stone, very slowly the chemistry continues.
1:05:01
[SPEAKER_00]: Sea water seeps in, minerals dissolve and reform, and crystals grow in the smallest spaces, filling them one layer at a time.
1:05:11
[SPEAKER_00]: There is no rush.
1:05:13
[SPEAKER_00]: there never was.
1:05:14
[SPEAKER_00]: The stone and the sea have come to an understanding and they will keep it for a long time yet.
1:05:22
[SPEAKER_00]: That is where we will leave our story tonight.
1:05:25
[SPEAKER_00]: The mystery has been answered.
1:05:27
[SPEAKER_00]: The pieces have found their places and the harbour rests beneath the water, quiet and whole in its own way.
1:05:35
[SPEAKER_00]: Thank you for spending this time with me here on sleepless history.
1:05:40
[SPEAKER_00]: Good night.