Roman concrete has often been revered for its durability but why exactly is it so much better
than modern concrete? It appears to have magical abilities that allow it to self-heal cracks,
make it resistant to sea water and allow it to become stronger over time. A series of
recent studies have shed light on this and reveals that Roman concrete's durability
is connected with its ingredients and how the environment reacted to it over time.
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Modern Concrete
To the average person, concrete may seem like an indestructible material. It is used to build huge
structures, is nearly completely weatherproof. But how long does modern concrete actually last?
There are a number of factors that can affect the lifespan of concrete. The primary ones are:
ingredients in the mix, the proportions of those ingredients, the process and curing
techniques used and lastly the environment and weather the concrete is exposed to.
For large-scale projects like buildings concrete should last up to 100 years if properly cared for.
If it is exposed to high weathering or use this could be as little as 50 years.
And yet there are concrete structures which are thousands of years old that
still stand. Why is our concrete so poor when compared to Roman concrete?
The ancient Romans were master engineers, constructing vast networks of roads,
aqueducts, ports, and massive buildings which have survived for two millennia.
Many of these structures were built using concrete.
Roman concrete was used in the construction of ancient Rome. Similar to modern equivalents,
it was based on a hydraulic setting cement added to an aggregate but differs in that
the aggregates often included larger components meaning it was laid rather than poured. It was
also able to set underwater which made it useful for bridges and other waterside constructions.
Many buildings, bridges, aqueducts and reservoirs were built by the ancient Romans are still
standing today. Rome's famed Pantheon is the world's largest unreinforced concrete dome was
built in 128CE and is still intact today. Some ancient Roman aqueducts still deliver water to
Rome today. This attests to both its versatility and its durability. Roman concrete was in
widespread use from about 150 BC and some scholars believe it was developed a century before that.
Researchers have spent decades trying to understand the secrets
of this ultradurable ancient construction material, particularly in structures,
like docks, sewers, and seawalls, that endured especially harsh conditions.
For many years researchers had assumed that the key to ancient concrete's durability was
based on one ingredient, volcanic ash from an area near the Bay of Naples.
They know that this specific kind of ash was shipped all across the vast Roman empire and
was described as a key ingredient for concrete in accounts by architects and historians at the time.
When ancient Roman concrete is examined in detail it revealed that it contains small, indistinctive,
millimeter-scale bright white mineral features. These are referred to as 'lime clasts' and
originate from lime which is a key component of the ancient mix. These features are not found in
modern concrete. In the past, they had disregarded this feature as simply poor quality raw materials
or sloppy mixing practices. Why would the Romans have put so much effort into making
an outstanding construction material, perfected and optimized over many centuries, with detailed
recipes then put so little effort into ensuring the production of a well-mixed final product?
A new study performed by MIT is revealing that these imperfections may in fact be the
key to the longevity of ancient Roman concrete.
It appears that these lime clasts give the concrete a self-healing capability.
Historically it was thought that the Romans incorporated lime into the concrete by first
combining it with water to form a highly reactive past-like material in a process called slaking.
The problem was that this process cannot account for the presence of the lime clasts. By studying
the samples of the ancient concrete they were able to determine that the white inclusions were
made out of various forms of calcium carbonate. This is formed by a more reactive form of lime
known as quicklime. Their analysis revealed it had been formed at extreme temperatures
which are associated with the exothermic reaction produced by using quicklime rather than the slaked
lime in the mixture. The process of hot mixing was how they created a super-durable material.
By using hot mixing it allows the concrete to be heated to a high temperature,
this allows chemistries that are not possible if you used slaked lime. The second benefit
is that the increased temperature increases the rate of reactions
leading to a quicker curing and setting times allowing for faster construction.
The lime clasts which occur through the hot mixing process develop a brittle
nanoparticulate architecture, creating an easily fractured and reactive calcium source.
When a tiny crack develops within the concrete this will preferentially travel
through the high-surface-area lime clasts. If this then comes into contact with water
it will react creating a calcium-saturated solution, this recrystallizes as calcium
carbonate and quickly fills the cracks. Alternatively, it can also react with the
ash materials in the concrete which would further strengthen the composite material.
These reactions occur spontaneously and heal any cracks before they can spread.
Roman concrete also has an incredible resistance to seawater and the science
behind how this works was also a mystery up until recently. Modern concrete is made to be inert,
whereas Roman concrete is designed to interact with the environment.
In modern concrete, there are chemical reactions that can affect the integrity of the concrete.
These include sulphate attack, lime leaching and alkali-aggregate expansion. This changes
the microstructure and leads to an increase in volume within the concrete, resulting in physical
stress and potential cracking. Lime leaching is a simple process of water passing through
the concrete and dissolving calcium hydroxide from the concrete. This is often seen as white
patches on the exterior of concrete and leads to a reduction in the strength of the material.
Alkali-aggregate expansion occurs when aggregates in the concrete decrease the
alkalinity of the cement paste resulting in the expansion of minerals and cracking of cement.
Roman concrete is not susceptible to any of these processes. When seawater interacts with
the mixture, it forms rare minerals which are believed to strengthen the material.
Long-term exposure to seawater causes tobermorite to crystalise from another mineral present called
phillipsite as it becomes more alkali. Tobermorite has long plate-like crystals that allow the
material to bend rather than crack under stress. This was written about in ancient Roman accounts:
"As soon as it comes into contact with the waves of the sea and is submerged it
becomes a single stone mass, impregnable to the waves and every day stronger"
Not only did it have these self-healing properties but it required less energy to create in the first
place. Portland cement is the most common modern concrete blend and lacks the lime-volcanic ash
combination compared to Roman concrete. This means it doesn't bind as well. To manufacture Portland
cement they need to heat the mix of limestone and clays to 1450C by comparison the Romans
would bake their limestone at 900C. So not only does the Roman concrete last longer meaning you
require less of it as it rarely decays but it also takes less fuel to make it in the first place.
Roman concrete is not only solid but durable due to the way it sets and
cures. It is salt resistant, more environmentally friendly. Less
likely to crack under stress with no reinforcement of steels.
Most modern concrete is reinforced which increases its strength significantly compared
to normal concrete. Around 10-15x the compressive strength and more than 100x the tensile strength.
This allows for the creation of shallow concrete elements that can still span long
distances which wouldn't be possible if the concrete were not reinforced.
Concrete is brittle and steel is ductile. If a concrete element fails it is likely to
fail suddenly without warning. If there is a steel element present it will sag and stretch
significantly before failure. This makes reinforced concrete fundamentally safer.
There is however a major drawback with using steel to reinforce concrete. Over time the
steel corrodes and expands. Placing internal pressure on the concrete which eventually
results in cracking and the loss of chunks of material. One drawback of modern concrete is
that the quicker set and cure time introduces many more cracks that allow water ingress.
A significant drawback of Roman concrete is that it takes longer to set and cure.
Obtaining large quantities of volcanic ash is also a limiting factor at the moment but scientists
are looking at how they can incorporate this knowledge to improve modern concrete's properties.
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