The Paper's reporter He Liping
Concrete is already one of the most important civil engineering materials, it is inexpensive and simple, but the flaw is that it will start to crack and break after decades of use. But scientists have noticed that many Roman buildings are not like this.
The environment destroys modern concrete, but the structures of these Roman buildings still stand tall and exhibit extraordinary durability. The large cylindrical tomb of Caecilia Metella, an ancient Roman aristocratic woman, is typical of one.
Four researchers from the Massachusetts Institute of Technology, Lawrence Berkeley National Laboratory (LBNL), and the University of Utah recently conducted a study in the authoritative journal of the international ceramics field and the Journal of the American Ceramic Society, the journal of the American Ceramic Society, showing that the quality of the concrete of Caecilia Metella's grave may exceed that of the men of the same period. They argue that this high quality stems from volcanic aggregates chosen by the builders and unusual chemical interactions with rainwater and groundwater that have accumulated over two thousand years.
The study's corresponding authors were Admiral Masic, associate professor of civil and environmental engineering at the Massachusetts Institute of Technology, and Marie Jackson, associate professor of geology and geophysics studies at the University of Utah, whose collaboration aimed to understand the mineral composition of ancient concrete structures.
"Understanding the formation and processes of ancient materials could provide researchers with new ways to create durable, sustainable building materials in the future." Masic said, "The Caecilia Metella Tomb is one of the oldest buildings that still exist, providing inspiration for modern architecture. ”
The Catacombs of Caecilia Metella are important sights and landmarks on the most famous Roman avenue, appian Way. It has a cylindrical tower that sits on a square base and is about 70 feet (21 m) high in total and 100 feet (29 m) in diameter. The tomb was built around 30 BC at the time of the transformation of the Roman Republic into the Roman Empire. Caecilia Metella herself came from an aristocratic family, and she married the Marcus Crassus, who formed the famous "first three-headed alliance" with Julius Caesar and Pompey.
The researchers mention that this tomb is a model of concrete construction techniques in the late Roman Republic. The technique, described by the ancient Roman architect Marcus Vitruvius Pollio, built thick walls of coarse brick or volcanic rock aggregates combined with ash and lava (porous fragments of glass and crystals produced by volcanic eruptions) would allow the building to "not become ruins after a long time."
What the ancient Romans did not know, however, was how potassium-rich white garnet crystals in volcanic aggregate dissolved over time, beneficially reshaping and recombining the interface between volcanic aggregate and cementitious matrix, improving the cohesion of concrete.
"Focusing on designing modern concrete that continuously enhances interface areas may provide us with an alternative strategy to improve the durability of modern building materials." "Doing this by integrating time-proven 'Roman wisdom' provides a sustainable strategy that can increase the lifespan of our modern solutions by several orders of magnitude," Masic said. ”
Linda Seymour, a doctoral student in Masic's lab, studied the microstructure of concrete. "Every tool we use provides clues to the process of making mortar." Seymour mentioned that scanning electron microscopy (SEM) shows the microstructure of mortar blocks at the micron level, and energy dispersive X-ray spectroscopy (EDS) shows the elements that make up these components.
The analysis revealed that in the thick concrete walls of caecilia Metella's tomb, a mortar containing volcanic lava combined large bricks with lava aggregates. It is similar to the mortar used in Trajan Market, another Roman ruin, built 120 years later. The binder of Trajan mortar consists of a block called C-A-S-H binding phase (calcium-aluminum-silicate-hydrate) and a mineral crystal called strätlingite.
But the volcanic lava of the mortar used by the Romans in the catacombs of Caecilia Metella was rich in potassium. Over the centuries, rainwater and groundwater seeped into the walls of tombs, dissolving white garnets and releasing potassium into the mortar. In modern concrete, large amounts of potassium produce an expanding gel, which can lead to microcracking and eventually destroy the structure. However, in the Caecilia Metella catacombs, potassium dissolved and reconfigured the C-A-S-H binding phase.
Jackson said that after the above-mentioned remodeling, "it is clear that a solid cohesive component has been created in the concrete." In these structures, unlike the Trajan market, almost no strätlingite is formed. Stefano Roascio, the archaeologist in charge of the tomb, notes that the study has great implications for understanding other ancient concrete structures that use Pozzolane Rosse aggregates.
"The interface between aggregate and mortar in any concrete is the basis for structural durability." Masic said, "In modern concrete, the alkali-silicon reaction that forms an expanding gel can disrupt the interface of even the hardenest concrete. "The so-called alkali silicon reaction, also known as alkali aggregate reaction (ASR), refers to when the alkalinity of cement is too strong, the active silicon component (SiO2) in the aggregate reacts with the alkali to generate silicate, causing uneven expansion of concrete, resulting in cracking and destruction.
"It turns out that the ancient Roman concrete interface of the caecilia Metella tomb has evolved through long-term remodeling." Masic said, "These modification processes enhance the interface area and may help improve mechanical properties and resist the destruction of ancient materials." ”
Editor-in-Charge: Li Yuequn
Proofreader: Yan Zhang