Workers built the Ming dynasty sections of the Great Wall about 600 years ago by mixing together a paste of sticky rice flour and slaked lime, the standard ingredient in mortar, said Dr Zhang Bingjian.
The sticky rice mortar bound the bricks together so tightly that in many places weeds still cannot grow. However, there was widespread resentment against the Wall in the south of China because the Ming emperors requisitioned the southern rice harvest both to feed the workers on the Wall and to make the mortar.
“The ancient mortar is a special kind of organic and inorganic mixture,” said Dr Zhang, a professor of chemistry at Zhejiang university in the city of Hangzhou in eastern China.
“The organic component is amylopectin, which comes from the porridge of sticky rice that was added to the mortar,” he said.
“The inorganic component is calcium carbonate, and the organic component is amylopectin, which comes from the sticky rice soup added to the mortar. This amylopectin helped create a compact microstructure, [giving the Great Wall] more stable physical properties and greater mechanical strength,” he reported in the journal of the American Chemical Society.
Dr Zhang said the use of sticky rice, a staple in East Asian food, was one of the greatest technical innovations of the time, and helped Ming dynasty tombs, pagodas and walls weather earthquakes and other disasters.