Sticky rice mortar

The ingenuity of incorporating sticky rice, a readily available food source, into a building material showcases advanced traditional Chinese engineering knowledge. This practice was a closely guarded secret among master masons for centuries.^[3] Sticky rice soup was mixed with slaked lime to make an inorganic−organic composite mortar that had more strength and water resistance than lime mortar.^[4][5] The sticky rice was typically boiled to a thick soup or gruel, which was then mashed and added to the lime.^[1] Modern chemists identified amylopectin, a type of polysaccharide, or complex carbohydrate, found in rice and other starchy foods that appeared to be responsible for the sticky rice mortar's strength and durability.^[6][7]

Sticky rice mortar had high adhesive strength, sturdiness, waterproofing capability, and prevented weeds from growing, as crude mortar made of sticky rice and burnt lime created a seal between bricks that would rival modern cement in strength. It played a major role in maintaining the durability of the Great Wall, as well as tombs, pagodas, and city walls.^[8] Sections of the Great wall of China were widely built with bricks, lime mortar and sticky rice used to reinforce the bricks strongly enough to resist earthquakes and modern bulldozers while keeping the building intact.^[9] Other structures built with sticky rice mortar, such as the walls of Xi'an and Nanjing, have been standing for a significant amount of time against the natural elements and other human activities.^[2] The amylopectin enhances the cohesive forces within the mortar, leading to significantly higher compressive and flexural strength.^[2] It has higher water resistance than traditional mortar.^[1]