Many ancient Chinese buildings have stood the test of time thanks to a secret ingredient in the mortar that binds their stones. The ingredient? Rice.
Chinese builders incorporated sticky rice soup into their mortars to add strength. This ancient recipe is the best choice for restoring historic buildings, said researchers, who recreated the rice-containing mortar in a new study.
"Because of its good performance, sticky rice-lime mortar was extensively used in many important buildings, such as tombs, city walls and water resource facilities," wrote the authors in their paper published in Accounts of Chemical Research.
Archaeological evidence shows that such mortar was in use perhaps as much as 1,600 years ago.
One tomb built during the Ming dynasty (1368-1644) "was so firm that a bulldozer could do nothing about it," the authors said. Some religious structures and bridges built with sticky-rice-containing mortar even survived a magnitude-7.5 earthquake in 1604.
The team, led by Bingjian Zhang of Zhejiang University in Hangzhou, analyzed ancient mortar from the Nanjing city wall — also built during the Ming dynasty using sticky-rice mortar -- and compared it to mixtures they made containing different amounts of sticky rice in standard mortar, which is derived from limestone.
They found that mortar with sticky rice had smaller calcium carbonate crystals than mortar without it, creating a more compact structure and causing the crystals to stick together.
Mortar with sticky rice is less permeable to water and more resistant to the stresses of changing weather than standard mortar, the authors said. This makes it more compatible with the bricks used in old buildings, and therefore the best option for conservation and restoration.
Indeed, a bridge from the Song Dynasty (960-1279) was restored in 2006 using sticky rice-lime mortar, the authors reported.
"When you're conserving historic buildings, it's very important to produce a conservation mortar that has the same physical properties as what was originally used," said Belinda Colston a chemist of the University of Lincoln in Lincoln, U.K., whose research focuses on historical buildings and objects.
"If you put in modern mortar, it's too strong for the building material," she added. "The bricks would have been quite soft. If your mortar is too strong, you end up destroying the brick. When you build a building, the mortar is supposed to be softer than your brick."
Sticky rice was not the only thing that ancient masons added to mortar. Vegetable leaves, egg whites, tung oil, fish oil and animal blood were all used to improve the performance of mortars in China, the authors said.
This technique wasn't limited to ancient China, however. In the Mediterranean, eggs, pig's milk and straw were added to improve mortars, said Antonia Moropoulou of the National Technical University of Athens.
"People in history have used whatever was lying around," Colston said.