Many ancient civilizations came up with lime-based cementlike materials to use as mortar in construction, from the gypsum-based plaster used on the pyramids in Egypt to sticky rice/lime mortar employed in parts of the Great Wall of China, but it was the Greeks and Romans who got lucky by adding locally abundant volcanic ash to their cement mix. This formula resulted in a mortar that not only set up quickly and with great strength but could also harden under water and then survive 2,000 years of crashing seas, as the seawall in Italy’s Pozzuoli Bay has. When the Romans mixed in bits of brick and gravel, they got opus caementicium, which we now know as concrete from the Latin, concretus, which means “to grow together.” Ever visited the Pantheon in Rome? That amazing freestanding dome with a 142-foot diameter is made of the stuff. Unreinforced. It has survived 1,890 years and multiple earthquakes with little or no maintenance. So why can’t our roads and bridges survive a tiny fraction as long?
Research just released in February by MIT, Georgetown University, the French National Center for Scientific Research, and the Concrete Sustainability Hub aims to provide some answers. For all these millennia, nobody has fully understood the atomic-level crystal grain structure of this miraculous material that upon adding water becomes a paste that can flow into complex forms and then harden into a strong solid in a matter of hours or weeks depending on the formula used.
When the water is added to dry cement and aggregate materials, it forms a cement paste. Chemical reactions then begin to take place, resulting in the formation of calcium silicate hydrates (calcium aluminum silicate hydrates when volcanic ash is added) of varying grain sizes and chemistries and the release of heat. These interlocking grains and the sizes of the pores and voids between them give concrete its strength. It turns out that this structure is neither a continuous solid (like metal or stone) nor an aggregate of small particles. It behaves like a mixture of both, the amount and distribution of the pore spaces between grains suggesting the material’s susceptibility to water intrusion and subsequent cracking. “These pores are the fingerprints of the water you put in initially,” senior research scientist Roland Pellenq says. Hence it is vital to minimize the amount of water used to form the paste. Big pores can form pathways for chorine ions to reach and corrode reinforcing steel, and this porosity also has an effect on creep and fracturing that can lead to structural failure.
For all these millennia, nobody has fully understood this miraculous material.
Pellenq’s team is developing computer models for predicting the crystal structures that result from particular cement formulations. They’re also working to model mechanical loading of concrete structures formed by these mixes—say, for example, the impact of heavy trucks traversing a stretch of roadway. (The sooner academia can scientifically prove the idiocy of Michigan laws permitting 164,000-pound, 11-axle trucks, the better our roads will be regardless of cement formulation.)
Optimizing concrete formulations to suit particular applications has the promise of greatly extending the material’s useful life. This will pay huge environmental dividends, as global manufacture of today’s ubiquitous portland cement (which involves de-carbonating calcium carbonate in a kiln at 2,642 degrees F) is currently the largest industrial source of CO2 production. Devising formulations that incorporate various polymers or ash from volcanos or coal power plants promises to reduce the CO2 required to produce the cement, as does the use of a new experimental cement called Celitement, which uses one-third as much limestone. And if the Pantheon’s any indication, I’ll bet Pallenq’s simulation models will award high marks for strength, durability, and eco-friendliness to the volcanic ash mixes.
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