Roman concrete has stood the test of time. Some ancient buildings still stand after millennia. For decades, researchers have been trying to re-create the recipe that made them last — with little success. Finally, with some detective work, scientists have figured what’s behind their lasting power.
Concrete is a mix of cement, gravel, sand and water. Admir Masic is a chemist at the Massachusetts Institute of Technology in Cambridge. He was part of a team that was trying to figure out what technique the Romans used to mix those ingredients.
The researchers suspected the key was something called “hot mixing.” It uses dry bits of calcium oxide, a mineral that’s also called quicklime. To make cement, that quicklime is mixed with volcanic ash. Then water is added.
Hot mixing, they thought, would ultimately produce a cement that wasn’t completely smooth. Instead, it would contain small calcium-rich rocks. And little rocks do show up everywhere in the walls of the Romans’ concrete buildings. They might explain how those structures withstood the ravages of time.
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Masic’s team had pored over texts by Roman architect Vitruvius and the historian Pliny. Their writings offered some clues. These texts gave strict requirements for the raw materials. For instance, the limestone used to make quicklime must be very pure. And the texts said that mixing quicklime with hot ash and then adding water could make a lot of heat. No rocks were mentioned. Still, the team had a feeling they were important. Every sample of ancient Roman concrete they’d seen held these bits of white rocks, called inclusions.
Where the inclusions came from was unclear for many years, Masic says. Some people suspected the cement just wasn’t fully mixed. But the Romans were super organized. How likely is it, Masic asks, that “every operator [was] not mixing properly, and every single [building] has a flaw?”
What if, his group wondered, these inclusions were a feature of cement, not a bug? The researchers studied the bits embedded at one ancient Roman site. Chemical analysis showed that these inclusions were very rich in calcium.
And that suggested an exciting possibility: The little rocks might be helping the buildings heal themselves. They might be able to patch cracks caused by weathering or even an earthquake. They could supply the calcium needed for a repair. This calcium could dissolve, seep into the cracks and re-crystallize. Then voila! Scar healed.
Hoping nothing explodes
Hot mixing is not how modern cement is made. So the team decided to observe this process in action. Mixing quicklime with water can produce a lot of heat — and possibly an explosion. Although many people thought it was ill-advised, Masic recalls, his team did it anyway.
Step one was to re-create the rocks. They used hot mixing and watched. No big bang occurred. Instead, the reaction produced only heat, a damp sigh of water vapor — and a Roman-like cement mixture bearing small, white, calcium-rich rocks.
Step two was to test this cement. The team created concrete with and without the hot-mixing process and tested the two side-by-side. Each block of concrete was broken in half. The pieces were placed a small distance apart. Then water was trickled through the crack to see if the seepage stopped — and how long it took.
“The results were stunning,” Masic says. The blocks incorporating hot-mixed cement healed within two to three weeks. The concrete produced without hot-mixed cement never healed. The team shared its findings January 6 in Science Advances.
Ancient solution for a modern problem?
Hot mixing’s key role was an educated guess. But now that Masic’s team has cracked the recipe, it could be a boon to the planet.
The Pantheon is an ancient building in Rome, Italy. It and its soaring, detailed, concrete dome have stood for nearly 2,000 years. Modern concrete structures generally last perhaps 150 years, at best. And the Romans didn’t have steel bars (rebar) shoring up their structures.
Concrete manufacturing emits a huge amount of carbon dioxide (CO2) into the air. More frequent replacements of concrete structures means more releases of this greenhouse gas. So longer-lasting concrete could reduce this building material’s carbon footprint.
“We make 4 gigatons per year of [concrete],” Masic says. (A gigaton is one billion metric tons.) Each gigaton equals the weight of some 6.5 million houses. Manufacturing makes as much as 1 metric ton of CO2 per metric ton of concrete. That means concrete is responsible for about 8 percent of global CO2 emissions each year.
The concrete industry is resistant to change, Masic says. For one thing, there are concerns about introducing new chemistry into a tried-and-true process. But “the key bottleneck in the industry is the cost,” he says. Concrete is cheap, and companies don’t want to price themselves out of competition.
This old Roman method adds little cost to making concrete. So Masic’s team hopes that reintroducing this technique could prove a greener, climate-friendly alternative. In fact, they’re banking on it. Masic and several of his colleagues have created a company they call DMAT. It’s seeking funds to start making and selling the Roman-inspired hot-mixed concrete. “It’s very appealing,” the team says, “simply because it’s a thousands-of-years-old material.”
Power Words
More About Power Wordsannual: Adjective for something that happens every year.
atmosphere: The envelope of gases surrounding Earth, another planet or a moon.
calcium: A chemical element and alkali metal common in minerals of the Earth’s crust and in sea salt. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.
calcium oxide: A substance that gives off heat as it chemically reacts with water. Its chemical formula is CaO (which means each molecule is made up of one calcium atom and one oxygen atom).
carbon: A chemical element that is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules. (in climate studies) The term carbon sometimes will be used almost interchangeably with carbon dioxide to connote the potential impacts that some action, product, policy or process may have on long-term atmospheric warming.
carbon dioxide: (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
carbon footprint: A popular term for measuring the global warming potential of various products or processes. Their carbon footprint translates to the amount of some greenhouse gas — usually carbon dioxide — that something releases per unit of time or per quantity of product.
cement: To glue two materials together with a binder that hardens into a rigid solid, or the viscous glue used to affix the two materials.(in construction) A finely ground material used to bind sand or bits of ground rock together in concrete. Cement typically starts out as a powder. But once wet, it becomes a mudlike sludge that hardens as it dries.
chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
chemistry: The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) Chemistry also is used as a term to refer to the recipe of a compound, the way it’s produced or some of its properties. People who work in this field are known as chemists.
colleague: Someone who works with another; a co-worker or team member.
concrete: (in construction) A simple, two-part building material. One part is made of sand or ground-up bits of rock. The other is made of cement, which hardens and helps bind the grains of material together.
dissolve: To turn a solid into a liquid and disperse it into that starting liquid. (For instance, sugar or salt crystals, which are solids,will dissolve into water. Now the crystals are gone and the solution is a fully dispersed mix of the liquid form of the sugar or salt in water.)
earthquake: A sudden and sometimes violent shaking of the ground, sometimes causing great destruction, as a result of movements within Earth’s crust or of volcanic action.
greenhouse gas: A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.
inclusion: (in geology) Something trapped inside a mineral.
limestone: A natural rock formed by the accumulation of calcium carbonate over time, then compressed under great pressure. Most of the starting calcium carbonate came from the shells of sea animals after they died. However, that chemical also can settle out of water, especially after carbon dioxide is removed (by plants, for instance).
manufacturing: The making of things, usually on a large scale.
mineral: Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns).
oxide: A compound made by combining one or more elements with oxygen. Rust is an oxide; so iswater.
raw materials: The natural products and unfinished goods used to manufacture other things. These can range from minerals such as iron, coal and silica to farmed products such as cotton and oats. They can even include some processed ingredients, such as lumber, gasoline, plastics, solvents and adhesives.
technology: The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.
water vapor: Water in its gaseous state, capable of being suspended in the air.
weathering: The process of breaking down rocks, soil and chemicals (such as crude oil). Weathering can be chemical, such as by oxidation (rust), or mechanical, such as by water, ice, or wind.
Citations
Journal: L.M. Seymour et al. Hot mixing: Mechanistic insights into the durability of ancient Roman concretes. Science Advances. Published online January 6, 2023. doi: 10.1126/sciadv.add1602.
About Carolyn Gramling
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Carolyn Gramling is the earth & climate writer at Science News. She has bachelor’s degrees in geology and European history and a Ph.D. in marine geochemistry from MIT and the Woods Hole Oceanographic Institution.
