Sticky Mortar

May 30, 2010

One of the things that makes many monuments from the ancient world, from the Pyramids, to Stonehenge, to the Great Wall of China, so fascinating is that they were accomplished with so little in the way of technology.  One of the minor mysteries of Chinese construction has been how the builders managed to produce a very strong mortar to hold masonry structures together — structures that in many cases have withstood strong earthquakes and modern bulldozers.

According to an article at the Web site, scientists have discovered that the secret ingredient comes from sticky rice, a staple food in much of Asian cuisine.   In a paper [abstract] published in the American Chemical Society’s Accounts of Chemical Research, a group of Chinese researchers showed that the mixing of the standard mortar material, slaked lime, with sticky rice soup produced the extra-strong mortar.  The key ingredient in the rice is amylopectin, one of the two key components of starch.  Amylopectin is a polysaccharide, a highly-branched sugar polymer that typically contains 2,000 – 200,000 glucose units.  Sticky rice contains it in abundance; other foods, like waxy corn and waxy potatoes, also contain large amounts.

The addition of amylopectin to the calcium carbonate from the slaked lime results in an organic / inorganic composite material that has exceptional properties, apparently because the amylopectin causes the calcium carbonate to crystallize in a more compact and orderly way:

…  we found that amylopectin in the mortar acted as an inhibitor: The growth of the calcium carbonate crystal was controlled, and a compact microstructure was produced, which should be the cause of the good performance of this kind of organic-organic mortar

The researchers also concluded that using the same mixture was the best mortar to be used in restoring old structures.

And here you thought that rice was just important in Chinese cooking!

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