According to ancient Chinese manuscripts, lime mortars used in those structures also contained sticky rice, although the formula for preparing the latter was not clear. However, there are archaeological evidence that show that they were also used during the South-North Dynasty (386–589 AD). The earliest record of sticky rice–lime mortar was found in the ancient building book Tian Gong Kai Wu which was written during the Ming Dynasty (1368–1644 AD). The sticky rice mortars were used in many historical buildings completed during the Ming (1368–1644) and the Qing (1644–1911) dynasties, such as the dyke at the Hangzhou Bay where the Quiantan river drains or the Jingzhou Historic Town Wall, one of the best preserved and largest historic town walls in Southern China that survived to date.
Amylopectin is a branched-chain polysaccharide composed of glucose units linked primarily by α − 1,4-glycosidic bonds but with occasional α − 1,6-glycosidic bonds, while amylose is a linear polysaccharide composed entirely of d-glucose units joined by the α − 1,4-glycosidic linkages. Rice contains two types of starch, amylopectin that makes rice sticky, and amylose that does not gelatinize approximately 70% w/w of the starch is amylopectin, while amylose is only 5% w/w, and protein is also found, between 6.8 and 9.6% w/w, that could apparently influence the carbonation of the lime. Thus, each region developed its own specific tradition in China, sticky rice was the preferred additive, this being the water in which sticky rice was cooked. The selection of additives depended on the availability of the product, so olive oil was used in Mediterranean countries, such as Portugal and Italy, sugar in Brazil, and tallow in countries where sheep or cattle were abundant. For example, oil from various sources, i.e., olive, linseed, or tallow, to make them water-repellent especially for limewashes casein or blood as plasticizers sugar, as setting retardants egg white to improve workability, and hair to increase their mechanical resistance. Lime mortars have been used since historic times and to improve their texture and durability organic additives were included. Results showed that the use of sticky rice in combination with nanolime yields a higher degree of consolidation increasing drilling resistance and delivering hydrophobic properties although prolonged exposure to high temperature and moisture can compromise treatment durability.
The durability of the treatments was investigated by exposing samples to accelerated weathering.
The resulting consolidation was evaluated by measuring changes of superficial cohesion, porosity, contact angle, drilling resistance, water absorption by capillarity, drying rate and aesthetic properties. This study aimed to test a novel approach consisting of applying a pre-treatment of sticky rice and subsequently the nanolime. However, in-depth consolidation still needs to be achieved, and research needs to focus on ways to attain this objective. Nanolime products have resulted in successful superficial consolidations. In recent decades, nanolime products based on Ca(OH) 2 nanoparticles suspended in alcohol or hydro-alcoholic medium have been extensively used for the consolidation of calcareous substrates mainly due to their chemical affinity and absence of side effects. In recent years, these mortars have been methodically studied leading to the conclusion that amylopectin, a polysaccharide in the sticky rice, plays a crucial role in regulating calcite crystals growth, creating a denser microstructure and providing the mortar with hydrophobic properties which contributed to their survival.
The main reason for this long-term durability is the addition of sticky rice water to the standard mortar ingredients (lime and sand) following traditional recipes. For almost 1500 years, many ancient Chinese mortars have remained unaltered despite exposure to atmospheric agents.
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