Hot mixing: Mechanistic insights into the durability of ancient Roman concrete

6 January 2023


Ancient Roman concretes have survived millennia, but mechanistic insights into their durability remain an enigma. Here, we use a multiscale correlative elemental and chemical mapping approach to investigating relict lime clasts, a ubiquitous and conspicuous mineral component associated with ancient Roman mortars. Together, these analyses provide new insights into mortar preparation methodologies and provide evidence that the Romans employed hot mixing, using quicklime in conjunction with, or instead of, slaked lime, to create an environment where high surface area aggregate-scale lime clasts are retained within the mortar matrix. Inspired by these findings, we propose that these macroscopic inclusions might serve as critical sources of reactive calcium for long-term pore and crack-filling or post-pozzolanic reactivity within the cementitious constructs. The subsequent development and testing of modern lime clast–containing cementitious mixtures demonstrate their self-healing potential, thus paving the way for the development of more durable, resilient, and sustainable concrete formulations.

Ordinary Portland cement (OPC) is a key component of concrete, the most ubiquitous construction material in the world, but its production has serious environmental consequences. The manufacture of OPC releases up to 1 metric ton of CO2e emissions per metric ton of produced material, and current strategies to reduce this impact are insufficient as demand continues to rise (1). These emissions are primarily generated when limestone and clay are calcined to form clinker (mainly tricalcium and dicalcium silicates, also known as alite and belite, respectively), which is then finely ground. One method to reduce cement’s carbon footprint (which accounts for up to 8% of total global greenhouse gas emissions), is to improve the longevity of concrete through the incorporation of self-healing functionalities. 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 (2).
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, as in the case for maritime concrete. Because of this proven longevity on the order of millennia, these ancient construction materials are attractive model systems for the design of sustainable, durable cementitious composites for modern engineering applications.

For many centuries, and throughout the entire ancient Roman Empire, architectural elements, such as walls and foundations, and infrastructure systems, including aqueducts, roads, and bridges, were created from unreinforced concrete. This concrete was typically composed of volcanic tuff and other coarse aggregates (caementa), and bound by a mortar based on lime and pozzolanic materials such as volcanic ash (pulvis), the detailed formulations of which were tailored to their specific intended applications. Whereas aerial lime mortars relied on the uptake of CO2 from the air to harden, hydraulic mortars combined lime and water with reactive silicates and aluminosilicates (pozzolanic materials) to form cementitious hydrates [e.g., calcium alumino-silicate hydrates (C-A-S-H)] (913).
This reaction, also known as the pozzolanic reaction, can be written as

Any excess unreacted lime slowly carbonates in air via the reaction

By developing these hydraulic mortars, the Romans were able to create a stronger, more durable material that allowed them to build larger, more complex-shaped architectural structures for purposes that were not previously possible, including constructions in the sea.

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 (cocciopesto), or other pozzolana, sand, and water to form the hydraulic mortar.

There is more to the above paper...


While I was a project manager on large projects in Boston, I took a class on concrete. I learned that steel rebar and red-mix concrete work so well together because each has the same coefficient of expansion vis a vis temperature. This article finds that Roman concrete is so durable because of their addition of quicklime to the concrete mix. This material appears to be the reason why Roman concrete is able to heal itself through the action of rainwater with the lumps of quicklime. It is amazing to view the aqueducts, the Coliseum, the amphitheaters and the other remains of this great culture whose buildings awed anyone seeing them for the first time, then, and still do.


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