THE imposing sight of the Hagia Sophia church has wowed visitors to Istanbul for 15 centuries. But aside from its towering beauty, the Byzantine building is also remarkable because it has somehow withstood earthquakes that have felled many nearby edifices. At last we know why: Hagia Sophia owes its durability to flexible cement.
Built in AD 532 under the orders of the Emperor Justinian, Hagia Sophia is regarded as the finest example of Byzantine architecture in the world. The architects commissioned for the project 鈥� Anthemius of Tralles and Isidore of Miletus 鈥� were not renowned builders, but they were excellent engineers and well versed in mathematics, statistics and kinetics.
Now a team headed by Antonia Moropoulou from the National Technical University of Athens has discovered their engineering solution. To make the church earthquake-proof they relied not on making the walls as rigid as possible, as was usual for the time, but on the principle of letting buildings 鈥済ive鈥� a little. Modern engineers know that this is one of the keys to earthquake-proofing.
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Moropoulou鈥檚 team analysed the chemical composition of the Byzantine cement using an electron microscope. They found it contained a calcium silicate matrix similar to that found in Portland cement, a mixture that was not commercially developed until 1824 (Construction and Building Materials, vol 16, p 543). This was no accident though, according to team member Ahmet Cakmak of Princeton University in New Jersey. 鈥淭he Byzantians knew exactly what they were doing,鈥� he believes. 鈥淭hey were very advanced scientists.鈥�
Cakmak thinks the ancient engineers deliberately added volcanic ash or other silica-rich materials to their mortar based on limestone and crushed brick. This reacted with the limestone and water to produce a silica matrix that can absorb seismic energy that would otherwise fracture the building. So far, Hagia Sophia, which has been used as both a Christian church and a mosque over the years, has withstood quakes of up to magnitude 7.5 on the Richter scale.
The team hopes that conservators will now be able to recreate the historic mortar so they can faithfully restore deteriorated walls and ensure that this ancient building lasts even longer. 鈥淲e have much to learn about lifetime engineering from the durability of this building,鈥� says Moropoulou.