Beijing, China - Oral tradition has it that old Chinese building mortars contained rice broth and bittern, instead of the hydraulic lime used in the West, due to the shortage of naturally-occurring hydraulic material. Among the 9 limewashes and 18 lime mortars one used sticky rice, and it was this one which was extensively used in important buildings, such as tombs, pagodas, city walls, and public works.
In 2010, a study of sticky rice-lime mortar with varying sticky rice content. The mechanical strength of the mortar was found to be significantly improved by the introduction of sticky rice. Amylopectin from the sticky rice was found to act as an inhibitor to the growth of the calcium carbonate crystals and a more compact structure results.
Ancient mortar samples were analysed by iodine starch, acid attack, thermogravimetric differential scanning calorimetry, X-ray diffraction, Fourier transform infrared, and scanning electron microscopy. These showed that the inorganic component is calcium carbonate, and the organic component is amylopectin, which is presumably derived from the sticky rice soup added to the mortar.
Amylopectin, a type of polysaccharide and highly branched polymer of glucose, found in rice and other starchy foods, is the secret ingredient responsible for the mortar's legendary strength, including its durability, and ability to withstand earthquakes.
[sources:
Chang Liu, Study of the Traditional Method of Lime Slaking and Lime Products in Beijing, 2000
Yang, Zhang and Ma, Study of Stick Rice-Lime Mortar for the Restoration of Historical Masonry Construction, May 2010. Their findings appear in an article published in the American Chemical Society (ACS) monthly journal, Accounts of Chemical Research.
