Sticky rice: an ancient Chinese construction secret

This will make sense to anyone in Hawai'i who has ever cleaned a rice pot that has stood overnight.

The ancient Chinese used the sticky qualities of rice in mortar. And many of the buildings they erected with this stuff still stand today.

Romans are credited with developing cement, which was often mixed with volcanic ash, brick dust and other materials to make concrete. The Chinese in the same era also had a very similar compund. But as the Roman era ended, the Chinese were also making a unique mortar of a combination of organic and inorganic compounds.

A study was performed by Chinese researchers Fuwei Yang, Bingjian Zhang and Qinglin Ma, a team of chemists and cultural researchers. Their “Study of Sticky Rice-Lime Mortar Technology for the Restoration of Historical Masonry Construction” was aimed at finding ways to repair some of China's cultural treasures, but in the process, they identified the surprising recipe for Chinese sticky mortar.

Here is a press release on a sticky rice research program, published in the American Chemical Society's publication, Accounts of Chemical Research.

Early Chinese builders experimented with a number of organic compounds in developing construction materials: “Via addition of natural organic compounds like sticky rice soup, the juice of vegetable leaves, egg white, tung oil, fish oil, or animal blood, the performance of lime mortars can be greatly improved, and these mortars are believed to play an important role in the longevity of ancient Chinese buildings,” the paper says.

The sticky rice mortar was made by adding a rice soup to mortar materials. The resulting material was used in tombs, bridges, temples and many other structures.

“In 1978, the tomb of Xu Pu and his wife was found. This lime compartment tomb built during the Ming Dynasty is so firm that a bulldozer could do nothing about it,” the paper says.

The technology is dated to about 1,500 years ago.

Studies on sticky rice mortar showed that an addition of 3 percent rice soup to the compound significantly increases its compressive and flexural strength. And not surprisingly, perhaps, it doubles the adhesive strength.

© Jan TenBruggencate 2010