"THE CITY WALL OF NANJING, built 600 years ago, was the first line of defense for the founding capital of the Ming dynasty. Originally 22 miles long, it was built with 350 million bricks, most of which have survived centuries of weathering. In 2010, intrigued by the wall’s sturdy composition, a team of Chinese researchers analyzed mortar samples from one section. The secret ingredient turned out to be humble sticky rice, a staple of Chinese cuisine.
This use of gummy grains as an adhesive is not entirely surprising. For thousands of years, Chinese builders mixed sticky rice, or glutinous rice, with lime mortar to assemble structures across the country, including city walls, pagodas, bridges, and tombs. Cooked rice was first boiled into a paste, then blended with sand and lime, a substance produced by heating limestone. According to researchers Yan-Bing Luo and Yu-Jie Zhang of Sichuan University, this starchy concoction “holds important status and value in Chinese architectural history.” Because of its strength and low porosity, they refer to it as “Chinese concrete.”
Scientists have long been fascinated with this unusual formula, and in recent years, different teams have conducted studies to better understand it. Researchers Jiajia Li and Bingjian Zhang spent six years collecting 378 samples of ancient mortar from 159 sites throughout China, dating from the Taosi phase (2300-1900 BC) all the way to the late Qing dynasty (1644-1911). Their numerous chemical tests found that 219 mortars from 96 locations had “organic components”—that is, small traces of starch, protein, brown sugar, blood, and oil. These mixtures have helped preserve much of China’s built landscape. As the researchers write, “the quality of mortar used in construction has played an important role in determining monument durability.” ...
Many other organic additives favored by the Chinese helped repel water. Li and Zhang found oil samples from 87 sites, which they believe to be tung oil, a common waterproof seal for wooden ships. Another, egg white, is not only water resistant but also improves the viscosity of mortar. (Eggs whites were also used as a paint binder to color the famous Terracotta Army.) Researchers have found that brown sugar, too, reduces water content in mortars, enhancing their strength. According to ancient literature, sucrose was often used to build forts and homes in eastern and southeastern China.
These mortars were also likely invented out of necessity. In distant Rome, the secret ingredient of concrete was volcanic ash, which improved the durability of lime mortar and enabled it to set underwater. Similar mortars made with volcanic ash were adopted throughout Europe and western Asia; however, volcanic ash was not available in ancient China. Instead, engineers would have used their own regional ingredients to create distinctive building materials. Other innovative mortars have similarly developed out of convenience, from a church in the Philippines made of egg whites to a Brazilian chapel held together by wine."
