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.”