Archaeologists have known for a long time that sticky rice was mixed with lime to make a strong, long-lasting mortar during the Ming dynasty (1368-1644) because it was mentioned in a Ming-era construction techniques book. Archeological investigations indicate that it was in use long before then, that in fact it was a mature technology during the South-North Dynasty (386-589).
Now a study published in Accounts of Chemical Research, the American Chemical Society’s journal, explains exactly how the rice works to produce a mortar that gets stronger and stronger with time.
The super-strong mixture is made by combining sticky rice soup with a standard mortar ingredient called slaked lime, or limestone that has been heated to a high temperature and exposed to water, said study researcher Bingjian Zhang, a professor at the Department of Chemistry at Zhejiang University in China.
“Analytical study shows that the ancient masonry mortar is a kind of special organic-inorganic composite material,” Zhang said. “The inorganic component is calcium carbonate, and the organic component is amylopectin, which comes from the sticky rice soup added to the mortar.”
The secret ingredient that makes the mortar so strong and durable is amylopectin, a type of polysaccharide, or complex carbohydrate, found in rice and other starchy foods, the scientists determined. The mortar’s potency is so impressive that it can still be used today as a suitable restoration mortar for ancient masonry.
The team experimented with several formulas of lime mortar combined with sticky rice and performance tested the end-results compared to traditional lime mortar. They found that the rice-lime mixtures was more able to withstand environmental stresses, less permeable to water, and far more compatible with historic structures than lime alone.
If you use contemporary mortar with historic masonry, the hardness of the mortar will destroy historic bricks which can be much softer than bricks made today. The mortar has to be softer than the brick, so recreating the original mortar used in the construction of the building allows you to preserve as much of the original structure as possible.
Ancient buildings made with rice mortar are known to have withstood centuries of powerful earthquakes and even modern heavy machinery. A bulldozer wasn’t able to budge a Ming dynasty tomb made from rice mortar.