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.
The strength and longevity of Roman 'marine' concrete is understood to benefit from a reaction of seawater with a mixture of volcanic ash and quicklime to create a rare crystal called tobermorite, which may resist fracturing.
Old concrete is stronger than new concrete because it was made from volcanic ash. The ash contained rare minerals such as Al-tobermorite and Phillipsite. These minerals cure the concrete continuously which makes its chemical structure grow denser and stronger over time.
Modern concrete—used in everything from roads to buildings to bridges—can break down in as few as 50 years. But more than a thousand years after the western Roman Empire crumbled to dust, its concrete structures are still standing.
The research team discovered that while modern concrete is made to be inert, the Roman version interacts with the environment. When seawater interacts with the mixture, it forms rare minerals aluminous tobermorite and phillipsite which are believed to strengthen the material.
This analysis suggests that the materials undergo a rare chemical reaction. The concrete is made of quicklime, or calcium oxide, and volcanic ash. When seawater gets into its cracks, it causes a chemical reaction that actually strengthens the concrete.
However, concrete reaches its full strength after approximately 25-28 days. You should be able to walk and perform normal tasks on concrete one week after its application. However, the concrete will only be at around 70-70% of its full strength, so driving heavy machinery on it is not advised during this time.
Roman Concrete didn't exactly follow a formula, and they don't appear to have done any controlled testing of how the material performed. That means the strength of a block of “Roman Concrete” was really up to a die roll, and ultimately wasn't as reliable as a modern approach.
It turns out the ancient Romans had the perfect recipe for water-resistant concrete. The material, called opus caementicium by the Romans, is made from a hydraulic cement, meaning it can set underwater or in wet conditions.
The answer is that concrete never cures completely. It is always hardening a little bit more each day. The way concrete hardens is a function of the cement particles reacting with the water it is mixed with.
For large scale projects like buildings, concrete should last up to 100 years if it's properly cared for. Concrete projects that experience more wear-and-tear like sidewalks and driveways have an expected lifespan of about half that—50 years.
Carbonation is a slowly occurring process whereby concrete (in the presence of moisture) reacts with carbon dioxide in the air, thereby reducing the pH of the concrete. Over a century, the carbonation depth may be on the order of several inches depending on the quality of the concrete.
The roads they built were made from aggregates – lots of different sized stones that compacted down to create a strong, stable and long-lasting surface. These roads could stand up to the marching of hundreds of soldiers, and carts laden with supplies.
It is shown that crushed and unprocessed volcanic ash could be utilized as a natural pozzolan in cement production, surpassing mortars manufactured with limestone fillers.
Based on the team's spectroscopic examination, it seems like Roman concrete was probably made by mixing the calcium carbonate with the pozzolanic material and water at very high temperatures, a process called 'hot mixing'. The team had now concluded that 'hot mixing' was the key to the concrete's super-strong nature.
As it turns out, not only is Roman concrete more durable than what we can make today, but it actually gets stronger over time.
Cons of Roman Concrete
A significant setback on Roman concrete is that it takes longer to set and cure, which can be problematic for construction projects. Due to the unavailability of materials around the world like volcanic ash, producing Roman concrete is difficult.
It was a formidable tool of Roman engineering know-how. Yet, for all its advantages, concrete had one major defect: it was unsightly. Once the wooden formwork was removed, it showed an ugly surface. In the beginning, its use was mainly restricted to substructures where noone would see it.
When the ancient Romans made mortar, they heated up the lime to turn it into a substance called "quicklime" – a very reactive chemical sibling to limestone. And, because they introduced water to the quicklime during mixing, the heat it produced set up a chemical foundation that could strengthen the concrete later.
300 BC - 476 AD Romans
Pliny reported a mortar mixture of 1 part lime to 4 parts sand. Vitruvius reported a 2 parts pozzolana to 1 part lime. Animal fat, milk, and blood were used as admixtures (substances added to cement to increase the properties.) These structures still exist today!
So, even with the tremendous pressure of the water behind the structure, there are no tensile stresses in the concrete, and thus no need for reinforcement. But lack of steel reinforcement isn't the potential only reason Roman concrete structures have lasted for so long.
The research team found the ancient Romans made their concrete with quicklime, which is lime in its pure state, rather than the more typical slaked lime, and that this gave it "self-healing" properties.
Technically, concrete never stops curing. In fact, concrete gets stronger and stronger as time goes on. But, as far as we're concerned, to reach a practical strength, most industrial concrete mixes have a 28 day curing period.
Because concrete compressive strength is at 99% in 28 days, it is very close to its final strength which could be in 1 or 2 years time. So engineers rely on the results of compressive strength test after just 28 days and use this number in any design calculations.
What is the maximum life of modern concrete? Generally, for large infrastructures, the lifespan of modern concrete is about 100 years if properly maintained.