In contrast to their modern counterparts, ancient Roman mortars and concretes have remained durable in a variety of climates, seismic zones, and even in direct contact with seawater. Because of this proven longevity on the order of millennia, these ancient construction materials have been a model for the design of sustainable, durable cementitious composites for modern engineering applications.
In this blog post, we will examine the Roman-inspired cementitious mixture prepared by the hot mixing method, which has been observed to effectively self-heal concrete cracks up to 0.5 mm in width.
Why is self-healing concrete important?
One method to reduce the carbon footprint of cement is to improve the longevity of concrete through self-healing concrete designs. The resulting extended use life, combined with a reduction in the need for extensive repair, could thus reduce the environmental impact and improve the economic life cycle of modern cementitious constructs.
Ancient Roman concretes survived for thousands of years.
The ancient Romans were masters of engineering, with their roads, aqueducts, ports, and massive buildings, whose remains have survived for two millennia. Many of these structures were built with concrete. The famous Pantheon in Rome, which has the world’s largest unreinforced concrete dome and was dedicated in 128 C.E., (Image 1) is still intact. Some ancient Roman aqueducts still deliver water to Rome today.
Image 1. Rome’s Pantheon Temple
Throughout the entire ancient Roman Empire, architectural elements, such as walls and foundations, and infrastructure systems, including aqueducts, roads, and bridges, were constructed with unreinforced concrete. This concrete was typically bound by a mortar based on lime and pozzolanic materials such as volcanic ash. It was composed of volcanic tuff and other coarse aggregates.
Researchers have spent decades trying to figure out the secret of this ultradurable ancient construction material, particularly in structures that endured especially harsh conditions, such as docks, sewers, and seawalls, or those constructed in seismically active locations.
For many years, researchers have assumed that the key to the ancient concrete’s durability was based on one ingredient: pozzolanic material such as volcanic ash from the area of Pozzuoli, on the Bay of Naples. This specific kind of ash was even shipped all across the vast Roman empire to be used in construction and was described as a key ingredient for concrete in accounts by architects and historians at the time.
However, the significant part was chunks of lime, a ubiquitous component of Roman concretes.
An ancient manufacturing strategy may hold the key to a concrete design that will survive millennia.
Now, a research team from MIT (Massachusetts Institute of Technology), Harvard University, and laboratories in Italy and Switzerland, has discovered an ancient concrete-manufacturing strategy that included self-healing functionalities. The findings are published in the journal Science Advances, in a paper by MIT professor of civil and environmental engineering Admir Masic, Linda Seymour, and four others.
The new study suggests that these tiny lime clasts, previously disregarded as poor-quality raw materials, gave the concrete a previously unrecognized self-healing capability. Prof. Admir Masic stated, “If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, there has to be more to this story.”
To address these yet unresolved questions, the study reported on the chemical characterization of relict lime clasts found in 2000-year-old Roman concrete samples obtained from the archaeological site of Privernum, Italy (Image.2). The investigated samples are compositionally consistent with other architectural mortars encountered throughout the Roman Empire. They were taken from the masonry mortar of the city wall, an open-air structure.
The researchers used high-resolution multiscale imaging and chemical mapping techniques and gained new insights into the potential functionality of these lime clasts.
These spectroscopic analyses provided new insights into mortar preparation methodologies. They proved that the Romans employed hot mixing, using quicklime in conjunction with, or instead of, slaked lime. The analyses provided clues that these had been formed at extreme temperatures, as would be expected from the exothermic reaction produced by using quicklime. The team has concluded that hot mixing was actually the key to the super-durable nature.
Image 2. Ancient Roman concrete fragment collected from the archaeological site of Privernum, Italy
On the right: Elemental map (Calcium: red, Silicon: blue, Aluminum: green)
A calcium-rich limestone clast (in red), which is responsible for the self-healing properties in this ancient material, is clearly visible in the lower region of the image.
The benefits of hot mixing are twofold.
- First, during the hot mixing process, the lime clasts develop a characteristically brittle nanoparticulate architecture. They create an easily fractured and reactive calcium source, which could provide a self-healing functionality. As soon as tiny cracks start to form within the concrete, this material can then react with water, creating a calcium-saturated solution, which can recrystallize as calcium carbonate and quickly fill the crack, or react with pozzolanic materials to further strengthen the composite material. These reactions take place spontaneously and therefore automatically heal the cracks before they spread (Figure 1).
- Second, this increased temperature significantly reduces curing and setting times since all the reactions are accelerated, allowing for much faster construction.
Figure 1. Scheme of suggested mechanism for the ancient Roman self-healing concrete design.
Repairing concrete cracks:
To prove that this was indeed the mechanism responsible for the durability of the Roman concrete, the team produced samples of hot-mixed concrete that incorporated both ancient and modern formulations, deliberately cracked them, and then ran water through the cracks. Sure enough: Within two weeks the cracks had completely healed, and the water could no longer flow (Figure.2). An identical chunk of concrete made without quicklime never healed, and the water just kept flowing through the sample. As a result of these successful tests, the team is working to commercialize this modified cement material.
Figure 2. Self-healing concrete experiments.
- After casting, hot-mixed concrete samples were mechanically fractured and then re-mated (with a gap of 0.5 ± 0.1 mm) (Figure 2.A).
- Water flow through the sample over the course of 30 days was documented with a flowmeter (Figure 2.B).
- The initial values of flow rates through open cracks ranged between 10 and 30 liters/hour. Then flow rates reduced over the course of 1 to 3 weeks depending on the geometry of the concrete cracks, to almost zero when the crack was eventually sealed (Figure 2.C).
- Compared to the lime clast–free control (orange line), after 30 days, water flow through the limestone clast–containing sample (blue line) ceased (Figure 2.C).
- Examination of the cracked surface revealed that the concrete cracks had been completely filled. When the flow of water eventually stopped (and the flow rate was zero or at a negligible flow), the test was stopped.
- The nature and distribution of secondary products formed in the crack (Figure 2, D to F) during the process were evaluated using optical microscopy (Figure 2, D and E) and Raman spectroscopy (Figure 2.F).
What is the production process for Roman concrete or mortar?
Detailed research of ancient mortars and concretes allowed for the identification of some of the key chemical and mineralogical processes associated with interactions between pozzolanic materials and hydrated lime and provided insights into the mechanical performance of these materials.
The production process for Roman mortar began with the calcining of lime from a source such as limestone, marble, or travertine (all predominantly calcite, CaCO3) to form quicklime [calcium oxide (CaO)]. This lime-based material, which can be hydrated using water (a process known as slaking) or added directly (a process known as hot mixing), was then mixed with volcanic ash, ceramic fragments, or other pozzolana, sand, and water to form the hydraulic mortar.
Strict specifications for the limestone, which was to be pure white, were detailed by the ancient scholars Vitruvius and Pliny. Previous studies on Roman architectural mortars determined that the calcined lime in these samples frequently contained <5 weight % oxides other than CaO.
The work was carried out with the assistance of the Archeological Museum of Priverno in Italy.
References:
Riddle solved: Why was Roman concrete so durable?
Hot mixing: Mechanistic insights into the durability of ancient Roman concrete
