This famous aqueduct and bridge, visible from airplanes heading to Marseilles-Provence Airport, is between Remoulins and Vers-Pont du Gardon, 13 miles from Avignon. The Pont-du-Gard was built in just five years using 51,000 tons of stone.
No mortar was used. The structure is on three levels – the first two are the arches of the bridge and third and smallest level is the aqueduct carrying the water 160 feet above the Gardon river. The whole structure is the second tallest Roman building in the world, just six feet lower than the Coliseum in Rome.
The Pont du Gard was the most important element in the water system taking nine-million gallons of water daily from the spring at Uzes a distance of 30 miles to the castellum at Nimes.
The drop in height was only 40 feet over the whole distance, an average of one inch for every 330 feet, a quite remarkable achievement given that the majority of the system was underground. The water took one day to make the trip and the system worked for 150 years. This engineering marvel was created in the 1st Century AD.
The water was carried in a four-foot-wide, six-foot-high channel that was lined with waterproof mortar. This mortar contained a quite remarkable substance called pozzolan. Vitruvius, an engineer and architect for the Emperor Augustus, wrote 10 books on architecture and engineering.
He devotes an entire chapter in his second book to pozzolan, stating that “there is also a kind of powder which from natural causes produces astonishing results. It is found in the neighborhood of Baiae and in the country belonging to the towns round about Mount Vesuvius. This substance, when mixed with lime and rubble, not only lends strength to buildings of other kinds, but even when piers of it are constructed in the sea, they set hard underwater.”
In other words, here was a lime mortar from the slopes of Mt Vesuvius that would set hard underwater and ensure that the aqueduct wouldn’t leak. Roman water channels in use today used pozzolan in their construction.