Perfect for scooping with chopsticks, sticky rice is also helping centuries-old bridges, temples, tombs, and city walls stand firm in China, according to a new study that unlocks the secrets of the mixture's "legendary strength."

An analysis of mortar from the 600-year-old city wall in Nanjing (map) confirmed its mortar is a mix of powdered limestone and sticky-rice soup. The tip-off was the presence of amylopectin, a carbohydrate found in the rice, according to the study, led by Bingjian Zhang of Zhejiang University in China.

Historical-mortar expert Sedat Akkurt said via email, "Over time people have added many different materials to mortars (such as urine, blood, and eggs) so the addition of rice in China, where rice is such a big thing, does not surprise me."

Used in both sweet and savory Chinese recipes, sticky rice, or glutinous rice, is a short-grained Asian rice that becomes gummy when cooked.

Sticky ... and Bulldozer Proof?

The presence of sticky rice didn't surprise Zhang's team either.

Thanks to age-old writings, it's long been known that sticky-rice mortar was used in China as far back as 1,500 years ago, and probably longer.

In many cases, the sticky stuff is still going strong, having survived major earthquakes and even human-made menaces. Part of a tomb built during the Ming Dynasty (1368 to 1644), for example, is "so firm that a bulldozer could do nothing about it," the authors write.

Exactly what accounts for the mortar's staying power, though, has been a mystery.

Sticky-Rice Mortar's Secret Recipe

Using chemical analysis, scanning electron microscopy, and other methods, the team discovered that the addition of the rice's amylopectin to the lime's calcium carbonate results in smaller calcium carbonate crystals than in regular mortar.

The result is a more tightly bonded mixture that's especially water resistant and more likely to hold its shape over time, according to the study, released Tuesday by the journal Accounts of Chemical Research.

"Compared to other mortars of the world, its strength is average, but its effect on controlling shrinkage is interesting," said Akkurt, of the Izmir Institute of Technology.

What's more, the results suggest sticky-rice mortars become stronger over the years, because the key chemical reactions in the mortar continue to occur.

Sticky rice, the study authors note, has already been successfully used in recent conservation projects, such as at the 800-year-old Shouchang Bridge in eastern China.