Discovering Antique Concrete Production Strategies

Researchers Uncover Ancient Concrete Production Strategies The ancient Romans were engineering masters who built extensive road networks, aqueducts, harbors and massive buildings whose remains have stood for two thousand years. Most of these structures were built with concrete: Rome's famous Pantheon, which has the world's largest unreinforced concrete dome and was dedicated in 128 AD, still stands, and some ancient Roman aqueducts still carry water to Rome today. By contrast, many modern concrete structures have crumbled within decades. Researchers have spent decades trying to unlock the secret of this ultra-durable ancient building material, particularly in structures built to withstand especially harsh conditions such as docks, sewers and seawalls, or in seismically active locations. A research team from MIT, Harvard University and laboratories in Italy and Switzerland has made progress in this area by discovering ancient concrete production strategies that incorporated a number of important self-healing functions. The findings were published in Science Advances in an article by MIT civil and environmental engineering professor Admir Masic, former doctoral student Linda Seymour and four others. For many years, researchers assumed that the key to the durability of ancient concrete lay in a single component: pozzolanic material such as volcanic ash from the Pozzuoli region in the Bay of Naples. This special ash type was shipped throughout the vast Roman empire for use in construction and was identified by architects and historians of that time as an important component for concrete. Upon closer examination, these ancient samples also contained small, distinctive, millimeter-scale bright white mineral features that have long been accepted as a ubiquitous component of Roman concretes. These white particles, commonly called "lime clasts," derive from lime, another important component of the ancient concrete mix. Masic said, "Since I began working with ancient Roman concrete, these features have always fascinated me. They are not found in modern concrete formulations, so why are they present in these old materials?" Previously dismissed merely as evidence of careless mixing practices or poor-quality raw materials, the new study suggests that these small lime clasts imparted to the concrete a previously unrecognized self-healing ability. Masic says, "The idea that the presence of these lime clasts was simply attributable to poor quality control has always troubled me." "If the Romans expended so much effort following all the detailed recipes optimized over centuries to make an extraordinary building material, why would they expend so little effort to ensure the production of a well-mixed final product? There must be more to this story." Using high-resolution multiscale imaging and chemical mapping techniques pioneered in Masic's research laboratory, the researchers conducted further characterization of these lime clasts and gained new insights into their potential functionality. Historically, it was assumed that when lime was incorporated into Roman concrete, it was combined with water in a process known as slaking to create a highly reactive, paste-like material. However, this process alone cannot explain the presence of lime clasts. Masic wondered: "Is it possible that the Romans used lime directly in a more reactive form known as quicklime rather than slaked lime?" Examining samples of this ancient concrete, Masic and his team determined that the white residues were indeed made of various calcium carbonate forms. And spectroscopic examination provided clues that these formed at elevated temperatures, as would be expected from the exothermic reaction produced by using quicklime instead of or in addition to slaked lime in the mixture. The team concluded that hot mixing was key to the super-durable nature. Masic says, "The benefits of hot mixing are twofold. First, when the overall concrete is heated to elevated temperatures, it allows chemistry that is not possible when using only slaked lime, producing high-temperature-associated compounds that would otherwise not form. Second, this increased temperature significantly reduces curing and setting times, because all reactions are accelerated and provides much faster construction." During the hot mixing process, lime clasts, characteristically developing a brittle nanoparticulate architecture, can provide a critical self-healing function, as the team proposes, creating an easily fractured and reactive calcium source. As soon as small cracks begin to form in the concrete, they can preferentially pass through the high-surface-area lime clasts. This material can then react with water to form a calcium-saturated solution that can recrystallize as calcium carbonate and rapidly fill the crack, or it can react with pozzolanic materials to further strengthen the composite material. These reactions occur spontaneously and thus automatically heal cracks without propagating them. Previous support for this hypothesis was found through examination of other Roman concrete samples exhibiting calcite-filled cracks. To prove that this was indeed the mechanism responsible for the durability of Roman concrete, the team produced hot-mixed concrete samples containing both ancient and modern formulations, deliberately fractured them, and then ran water through the cracks. Within two weeks the cracks healed completely and water could no longer flow through. An identical piece of concrete made without quicklime never healed and water continued to flow through the sample. Following these successful tests, the team is working to commercialize this modified cement material. Masic says, "It is quite exciting to think not only about how these more durable concrete formulations could extend the service life of these materials, but also how they could improve the durability of 3D-printed concrete formulations." Through extended functional lifetime and development of lighter concrete forms, the team hopes these efforts could help reduce the environmental impact of cement production, which currently accounts for approximately 8 percent of global greenhouse gas emissions. Along with other new formulations, such as concrete that can actually absorb carbon dioxide from the air, which is another current research focus of Masic's laboratory, these improvements could help reduce the global climate impact of concrete. The research team included Janille Maragh from MIT, Paolo Sabatini from DMAT in Italy, Michel Di Tommaso from Instituto Meccanica dei Materiali in Switzerland and James Weaver from the Wyss Institute for Biologically Inspired Engineering at Harvard University. The work was carried out with the assistance of the Priverno archaeology museum in Italy. Academic reference: Linda M. Seymour, Janille Maragh, Paolo Sabatini, Michel Di Tommaso, James C. Weaver, and Admir Masic. Hot mixing: Mechanistic insights into the durability of ancient Roman concrete. Science Advances, 2023 DOI: 10.1126/sciadv.add1602 Source: https://www.sciencedaily.com/releases/2023/01/230106144441.htm