Cement

• 1. Introduction

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  Cement is the invisible force holding the modern world together. It binds the sand and gravel in the concrete beneath your feet, the mortar between the bricks of your walls, and the lining of the water pipes running underground. Concrete, which cement makes possible, is the most widely used material in existence. Only water is consumed in greater quantities on this planet.

  Yet cement is almost never used alone. It is a binder, a chemical agent whose whole purpose is to grip other materials and fuse them into something stronger than any of its parts. When mixed with fine aggregate, cement becomes mortar. When mixed with sand and gravel, it becomes concrete. And the key to all of it is water, which triggers a set of chemical reactions inside the wet mix that gradually locks everything into a hard, durable mass.

  World production of cement runs to around 4.4 billion tonnes per year, as of a 2021 estimation. About half of that is made in China, with India and Vietnam trailing behind. And that same production process is responsible for nearly 8% of global carbon dioxide emissions as of 2018, making cement one of the most consequential industrial materials on Earth.

  How did a substance rooted in volcanic ash and burnt limestone come to underpin nearly every structure humans build? And what does its future look like when the planet it helped build is warming? Those are the questions this documentary will answer.

• 2. Fire and Stone

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  Perhaps the earliest known occurrence of cement happened twelve million years ago, when oil shale lying next to a bed of limestone caught fire by natural causes. The heat fused the limestone into a primitive cement deposit. Those ancient formations were not studied until investigations carried out in the 1960s and 1970s.

  Long before modern engineers appeared, ancient builders had already made the same basic discovery through trial and error. The Babylonians and Assyrians bound burnt brick and alabaster slabs with bitumen, the tarry substance also called asphalt or pitch. In Ancient Egypt, stone blocks were joined with a mortar made from sand and roughly burnt gypsum, a material equivalent to what we now call plaster of Paris, which often contained calcium carbonate.

  Lime made the next leap. It was used on Crete and by the Ancient Greeks, and there is evidence that the Minoans of Crete used crushed potsherds as an artificial pozzolan, a volcanic-ash-like additive that reacts with lime to create a hydraulic mixture. Nobody knows who first made that discovery. But the Greeks, and specifically the Ancient Macedonians, were using pozzolanic concrete. Roman engineers then scaled it up massively, three centuries later.

  The Romans drew on a natural material found near the town of Pozzuoli, west of Naples. An ancient description of it survives: it is found near Baiae and in the country around Mount Vesuvius, and when mixed with lime and rubble it lends strength to buildings and sets hard even underwater. That material was volcanic ash, which the Romans called pozzolana after the town where it was extracted. When they lacked pozzolanic ash, they substituted powdered brick or pottery.

  The results were extraordinary. The dome of the Pantheon in Rome, the massive Baths of Caracalla, and the vast system of Roman aqueducts all relied on this hydraulic concrete. Many of those structures still stand today. The typical Roman technique used brick facing as the outer form while the interior was filled with mortar mixed with broken stone, brick, potsherds, and recycled chunks of old concrete.

• 3. Lost and Rediscovered

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  After Rome fell, knowledge of hydraulic cement did not vanish entirely, but the written record thins out. What is known is that medieval masons and military engineers continued to use hydraulic cement in canals, fortresses, harbors, and shipbuilding facilities. In the German Rhineland, hydraulic mortar stayed in continuous use throughout the Middle Ages because local deposits of a volcanic material called trass were available.

  Formal, scientific understanding of hydraulic cement came back into focus during the 18th century, driven by two practical pressures. Good-quality building stone in Britain was becoming expensive during a period of rapid industrial growth, so builders turned to brick and then looked for a stucco that could imitate stone. And for harbor and marine construction, engineers needed a mortar that could set in water.

  John Smeaton made the next decisive contribution while planning the third Eddystone Lighthouse in the English Channel, a project that ran from 1755 to 1759. The structure is now known as Smeaton's Tower. He needed a hydraulic mortar that could gain some strength in the twelve-hour window between successive high tides. He ran systematic experiments with different limestones and additives, including trass and pozzolanas, and he documented his finding: the hydraulicity of a lime was directly related to the clay content of the limestone used to make it. Smeaton was a civil engineer, not a chemist, and he took the insight no further.

  James Parker followed a different path in the 1780s. He developed what he called Roman cement, and finally patented it in 1796. It had nothing to do with actual Roman materials. Parker made it by burning septaria, the clay-and-calcium-carbonate nodules found in certain clay deposits, and grinding the burnt nodules to a fine powder. Mixed with sand, it set in 5 to 15 minutes, which was strikingly fast. Roman cement became popular across Britain and beyond, but was largely displaced by Portland cement in the 1850s.

  Meanwhile, in France, Louis Vicat identified the same clay-content principle that Smeaton had found, apparently without knowing of Smeaton's work, in the first decade of the 19th century. Vicat then went further, devising a method of blending chalk and clay into an intimate mixture before burning, producing what he called an artificial cement in 1817. That material is considered the principal forerunner of Portland cement.

• 4. The Portland Story

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  Joseph Aspdin patented a material he called Portland cement in 1824, naming it after the prestigious Portland stone quarried on the Isle of Portland in Dorset, England, because the render made from it matched that stone's color. What Aspdin actually produced was nothing like the Portland cement used today. It was a proto-Portland cement, a first step.

  The critical advance came from his son. William Aspdin had left his father's company, and in his own cement manufacturing he apparently stumbled into the production of calcium silicates in the 1840s, a middle stage in Portland cement's development. The discovery was counterintuitive. It required more lime in the mix, a much higher kiln temperature that demanded more fuel, and the resulting clinker wore down the millstones used to grind it with unusual speed. Manufacturing costs rose sharply. But the product set at a reasonable pace and developed strength quickly, opening up the market for concrete construction.

  William Aspdin's work was shrouded in deliberate mystery. Because of that secrecy, others including Vicat and Isaac Charles Johnson also claimed credit for inventing Portland cement. Johnson further refined what he called meso-Portland cement and called himself its true father. Recent analysis of Aspdin's concrete and raw cement, specifically from his works at Northfleet in Kent, has shown that his product was a genuine alite-based cement. But Vicat established the underlying chemistry, and Johnson established the critical importance of sintering the mix properly in the kiln.

  The key mineral in all of this is alite. Earlier hydraulic limes and natural cements developed their strength through belite, a compound that works slowly. Because they were burned at temperatures below 1,250 degrees Celsius, they contained no alite at all. Alite is responsible for early strength in modern cements, and the first cement to consistently contain it was William Aspdin's product from the early 1840s. From 1850 onward, the use of concrete in construction grew rapidly, and Portland cement became dominant.

  In the United States, a very different cement was foundational for a time. Rosendale cement was a natural cement mined from a massive deposit of dolomite discovered near Rosendale, New York. It underpinned the Statue of Liberty, the Capitol Building, and the Brooklyn Bridge. But after World War One, its long curing time of at least a month made it impractical for highway and bridge construction, and states switched to Portland cement. By the end of the 1920s, only one of the original fifteen Rosendale cement companies had survived.

  The story did not end there. In the early 1930s, builders found that while Portland cement set faster, it was less durable on highways. Some states stopped using it for roads. An engineer named Bertrain H. Wait, who had helped build New York City's Catskill Aqueduct, proposed a blend of Rosendale and Portland cements. He convinced the New York Commissioner of Highways to test the mix on an experimental section of highway near New Paltz, New York, using one sack of Rosendale cement to every six sacks of Portland. The test worked, and the blend was used in highway and bridge construction for decades.

• 5. How Cement Actually Works

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  Portland cement is made by heating limestone, which is calcium carbonate, with other materials such as clay to 1,450 degrees Celsius inside a kiln. The heat drives off carbon dioxide from the calcium carbonate, producing calcium oxide, or quicklime. That quicklime then reacts chemically with the other materials to form calcium silicates and related compounds. The resulting hard substance is called clinker. Clinker is then ground with a small amount of gypsum to produce the familiar grey powder.

  Four main mineral phases form inside the clinker. The first two, alite and belite, are the silicates responsible for mechanical strength. The other two, tricalcium aluminate and calcium aluminoferrite, are essential for forming the liquid phase during the high-temperature sintering process in the kiln. The chemistry of all these reactions is not yet fully understood and remains an active area of research.

  When water is added to cement, the curing process begins. A widespread misconception is that hydraulic cement sets by drying out. It does not. Setting and hardening depend on hydration reactions, which require water. The mineral hydrates interlock as they form, and that interlocking is what gives cement its strength. If the cement dries out prematurely during curing, the hydration reactions stop, and the resulting material is insufficiently hardened and significantly weakened. Curing temperatures should stay between 5 and 30 degrees Celsius, and the young concrete must be shielded from direct sunlight, low humidity, high heat, and wind, all of which accelerate water loss.

  There is also a structural feature at the boundary between cement paste and aggregate particles called the interfacial transition zone. That zone can be up to 50 micrometers wide, and its properties differ from the bulk of the paste. As you move toward the aggregate surface, porosity changes, unreacted clinker decreases, and the mineral ettringite becomes more concentrated.

  Safety is a practical concern in handling cement. Wet cement is strongly caustic, with a pH of 13.5, and can cause severe skin burns if it is not promptly washed off. Dry cement powder can severely irritate the eyes and respiratory tract. Some batches contain trace amounts of chromium from natural raw materials, and that chromium can cause allergic dermatitis. To counter this, ferrous sulfate is commonly added to cement to convert the carcinogenic hexavalent form of chromate into a less toxic trivalent chromium species.

• 6. A Taxonomy of Cements

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  Portland cement is by far the dominant form, but the family of cements is wider than most people realize. Blast-furnace cement replaces up to 95% of Portland clinker with ground granulated blast furnace slag, which reduces heat generation, improves sulfate resistance, and cuts cost. Portland-fly ash cement incorporates up to 40% fly ash under American standards, and because fly ash is pozzolanic it maintains the ultimate strength of the concrete while allowing a lower water content.

  Calcium aluminate cements, patented in France in 1908 by Jules Bied, are made primarily from limestone and bauxite. Their specialty is heat resistance, making them the preferred choice for refractory concrete in furnace linings. Calcium sulfoaluminate cements, pioneered in China where several million tonnes per year are now produced, require lower kiln temperatures and less limestone, resulting in roughly half the carbon dioxide emissions of Portland clinker.

  Expansive cements are designed to counteract the drying shrinkage that normally occurs in hydraulic cements, and can be used for floor slabs up to 60 meters square without the need for contraction joints. Geopolymer cements are made from water-soluble alkali metal silicates combined with aluminosilicate powders such as fly ash and metakaolin, incorporating recycled materials to reduce demand on raw inputs.

  Tabby, which blends oyster shell lime, sand, and whole oyster shells into a concrete, has a different history. The Spanish introduced it to the Americas and the Philippines in the sixteenth century. Along the South Atlantic seaboard of the United States, tabby construction relying on Native American oyster-shell middens was used in houses from the 1730s through the 1860s.

  Very finely ground cements offer another direction. When cement is interground with sand, slag, or other pozzolanic minerals to an extremely fine powder, the greater surface area accelerates the chemical reaction. Such cements can achieve the same physical characteristics as standard cement while using up to 50% less cement by mass, and can require up to 50% less energy to fabricate. At the far experimental edge, researchers at the University of Edinburgh have developed a process using a bacterium called Sporosarcina pasteurii, which precipitates calcium carbonate. When mixed with sand and urine, it can produce mortar blocks reaching 70% of the compressive strength of concrete.

• 7. Carbon and the Future

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  Cement manufacturing releases carbon dioxide in two distinct ways. The first is direct: heating calcium carbonate in a kiln liberates carbon dioxide through calcination, and this chemical decomposition accounts for approximately 60% of the industry's emissions. The second is indirect: the fuel burned to run the kilns contributes the remaining 40%. Taken together, the cement industry produces roughly 10% of all human-made carbon dioxide emissions, and a 2018 Chatham House study put the annual production of around 4 billion tonnes of cement at 8% of worldwide emissions. For every 1,000 kilograms of Portland cement produced, nearly 900 kilograms of carbon dioxide are emitted.

  The European Union has made measurable progress. Since the 1970s, specific energy consumption for producing cement clinker in the EU has fallen by approximately 30%, a reduction equivalent to roughly 11 million tonnes of coal per year. EU cement producers already draw more than 40% of their thermal energy from waste and biomass fuels rather than fossil fuels alone.

  There is a partial offset built into the material itself. Concrete and mortar slowly reabsorb atmospheric carbon dioxide over the course of their lives through a process called carbonation, which is essentially the reverse of calcination. It is estimated that this reabsorption recovers approximately 30% of the carbon dioxide released during cement production across the full life cycle of the concrete product. Carbonation has a complication, though: it gradually lowers the pH of concrete, which can promote corrosion of the steel reinforcement embedded in it. On the other side of that ledger, the calcium carbonate formed during carbonation occupies more volume than the compound it replaces, which reduces the porosity of concrete and increases its strength and hardness.

  Carbon capture and storage is entering the picture. The French company Air Liquide has been granted European Union funding for two projects: one at Kujawy in Poland and one called the K6 Program in Lumbres, France, which is aimed at producing the first carbon-neutral cement in Europe. Both projects are expected to begin operating by 2028 and together are projected to capture 18.1 million tonnes of carbon dioxide over a decade.

  Cementitious materials also have a role in nuclear waste management, having been used as an immobilizing matrix for radioactive waste for more than half a century, with technologies deployed at industrial scale across many countries.