Volcanic ash is secret ingredients for preserving Roman buildings

They brought us roads and a clever sewer system. But it seems the Romans also knew a thing or two about making concrete.

While Britain's 1960s concrete tower blocks and flyovers are crumbling, Roman monuments such as the Pantheon and the Colosseum are still standing.

Now scientists have discovered the secret ingredient behind the longevity of Roman concrete - volcanic ash.

The study by the University of California, in Berkeley, found Roman concrete showed no corrosion, with a smooth surface suggesting long-term stability.

Manufacturing modern cement requires heating a mix of limestone and clay at 1,450C.

But Roman architectural mortar uses 85 per cent volcanic ash, fresh water and lime, which is cast at a much lower temperature, significantly reducing its carbon footprint.

Now the researchers want to take the Roman techniques and apply them to modern cements.

Dr Marie Jackson said: 'If we can find ways to incorporate a substantial component of volcanic rock in the production of specialty concretes, we could greatly reduce the carbon emissions associated with their production. We could also improve their durability and mechanical resistance over time.'

The study, published in the Proceedings of the National Academy of Sciences, involved using beams of X-rays on a reproduction of Roman volcanic ash-lime mortar.

In the concrete walls of Trajan's Markets in Rome, constructed around 110 AD, this mortar binds cobble-sized fragments of tuff - consolidated volcanic ash - and brick.

The team observed the mineral changes that took place in the curing of the mortar over a period of 180 days and compared the results to 1,900-year-old samples of the original.

They discovered that the concrete contained particular water-based elements that prevented it from cracking.