The Great Wall of China stands firm because it was built with 'living mortar'

It seems fragile, but a mysterious "biological shell" is what helps the Great Wall stand the test of time.

A study recently published in the scientific journal Science Advances reveals the astonishing "living mortar" technology used by the Ming Dynasty to build the Great Wall.

The team of authors led by Professor Bo Xiao from the College of Land Science and Technology of China Agricultural University found the secret through analyzing samples from eight different sections of this wonder.

A section of the Great Wall built with unburnt mortar - (Photo: SCIENCE ADVANCES).

The eight Great Wall sections studied were all built between 1368 and 1644 during the Ming Dynasty.

67% of these samples were identified as containing biological material .

Comparing the mechanical strength of the wall in the area made of "living mortar" with that of pure rammed earth sections using on-site mechanical equipment as well as in the laboratory, the authors found that the "living mortar" sections had three times the strength.

Detailed analysis of the composition revealed that this raw mortar contained cyanobacteria, mosses and lichens.

They were intentionally incorporated into the mortar by workers during construction.

Over centuries, the growth of these tiny organisms and the substances they secrete, such as polymers, has formed an extremely durable biological shell that enhances the structural stability of these sections of the Great Wall, making the rammed earth as strong as concrete.

This helps explain the great mystery surrounding the wall's enigmatic durability: Although rammed earth mixed with gravel - the main material of the Great Wall - is much more easily eroded than stone, it has remained stable despite the passage of time.

According to Live Science, although less durable, it is the rammed earth that helps promote the formation of a "bio-shell" from the living mortar . This shell becomes more and more durable over time, preventing erosion.

In addition, this Ming Dynasty "living mortar" technique is an amazing discovery, showing that the spectacular development of science and technology - including seemingly modern things such as the use of biological materials - has been applied by the Chinese for a long time.