Totaling more than 13,000 miles in length, the Great Wall of China continues to astound and awe visitors with its ancient defensive architecture. Boasting a 2,300-year-old history, the wall was built to prevent invasion from the Mongols and protect the Silk Road Trade. Thousands died during its construction; many remain interred within. The best-known and best-preserved portions of the Great Wall date to the Ming Dynasty (1368-1644). They comprise 5,500 miles of the overall structure.
Despite their age, these centuries-old portions of the wall still tower powerfully above the landscape. In fact, the mortar bound the bricks so tightly that weeds still haven’t sprouted in many locations. So, what was the secret of Ming Dynasty construction?
Sticky rice!
In one of the greatest technological innovations of the Ming dynasty, workers developed sticky rice mortar. They crafted it from a mixture of slaked lime—a standard ingredient in mortar—and sweet rice flour. The result? According to Chinese researchers, the first composite mortar in history, a potent admixture of inorganic and organic ingredients.
The architects and engineers of China’s Ming Dynasty used their updated mortar recipe in the construction of many important state structures. Besides the Great Wall, these included lesser city walls, pagodas, and tombs.
Many of these structures remain intact. Buildings constructed with sticky rice mortar have withstood natural disasters including earthquakes. Heck, one Ming-era tomb remained stock-still against the advances of a modern-day bulldozer! These structures prove that sticky rice mortar is stronger and lasts longer than pure lime mortar.
So, why is sticky rice mortar so darn strong?
Bingjian Zhand, Ph.D., and a team of researchers from Zhejiang University investigated the chemical composition of Ming-era mortar to find out. They relied on chemical analysis and scanning electron microscopy to come to a fascinating conclusion. The legendary strength of rice-lime mortar comes from amylopectin.
A type of complex carbohydrate or polysaccharide, amylopectin is found in many starchy foods including rice. This “secret ingredient” endowed many Ming Dynasty-era structures with a kind of immortality. But how exactly does it work?
When amylopectin—the organic portion of the mortar recipe—comes into contact with calcium carbonate—the inorganic part—a complex interaction occurs. Acting as an inhibitor, the amylopectin controls the growth of the calcium carbonate crystal.
The result? A more tightly bonded mixture. There’s a caveat, though. Compared to other mortars around the world, this denser microstructure only rates average in terms of strength. Deeper inspection reveals three key advantages sticky rice mortar has over other mortar types.
First, it’s highly water resistant. Second, it shrinks less and holds its shape. Third, the key chemical reaction in the mortar continues over time. Put another way, the mortar gets stronger as the years go by!
Zhang and his colleagues believe it’s time to resurrect the recipe for sticky rice mortar, especially to aid in the restoration and preservation of historic buildings. Besides its renowned strength, rice-lime mortar boasts greater overall physical stability and compatibility. These factors make it a viable (and more authentic) alternative for repairing ancient masonry.
The sticky rice mortar recipe has already been successfully used to repair the 800-year-old Shouchang Bridge in eastern China. By using Zhang and his colleague’s analysis techniques moving forward, conservators hope to identify each historic building’s specific mortar “recipe.” Then, they can whip up a fresh batch based on each structure’s unique formulation.
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