The high strength of the ancient walls of China was provided by rice broth, which the builders added to the lime mortar. A mixture containing the carbohydrate amylopectin may have been the world’s first organic-inorganic composite material.
Composite materials, or composites – multi-component solid materials that allow you to combine the useful properties of their components, have already become indispensable for the infrastructure of human communities. The peculiarity of composites is that they combine reinforcing elements that provide the necessary mechanical characteristics of the material, and a binder matrix that ensures the joint operation of the reinforcing elements. Composite materials are used in construction (reinforced concrete) and in internal combustion engines (coatings on friction surfaces and pistons), in aviation and astronautics, in the manufacture of armor and rods.
But how old are composites and how quickly have they become effective? The first thing that comes to mind is primitive bricks made of clay, but mixed with straw (which is just the “bonding matrix”), used in ancient Egypt.
However, although these designs were better than modern non-composite counterparts, they were still very imperfect and therefore short-lived. However, the family of “ancient composites” is not limited to this. Chinese scientists managed to find out that the secret of the ancient mortar, which ensures the strength of the Great Wall of China against the pressure of centuries, also lies in the field of composite materials science.
The ancient technology was very expensive, but effective.
Mortar was made using sweet rice, a staple of modern Asian dishes. A group of professor of physical chemistry Bingjiang Zhang found that builders used a sticky mortar made from rice as early as 1,5 years ago. To do this, rice broth was mixed with the usual ingredients for the solution – slaked lime (calcium hydroxide), obtained by calcining limestone (calcium carbonate) at a high temperature, followed by slaking the resulting calcium oxide (quicklime) with water.
Perhaps rice mortar was the world’s first complete composite material that combined organic and inorganic components.
It was stronger and more resistant to rain than ordinary lime mortar and was certainly the greatest technological breakthrough of its time. It was used only in the construction of particularly important structures: tombs, pagodas and city walls, some of which have survived to this day and withstood several powerful earthquakes and demolition attempts by modern bulldozers.
Scientists managed to find out the “active substance” of the rice solution. It turned out to be amylopectin, a polysaccharide consisting of branched chains of glucose molecules, one of the main components of starch.
“An analytical study has shown that the mortar in ancient masonry is an organic-inorganic composite material. The composition was determined by thermogravimetric differential scanning calorimetry (DSC), X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. It has been established that amylopectin forms the microstructure of a mixture with an inorganic component, which provides valuable building properties of the solution, ”says the Chinese researchers in an article.
In Europe, they note, since the time of the ancient Romans, volcanic dust has been used to add strength to mortar. Thus, they achieved the stability of the solution to water – it did not dissolve in it, but, on the contrary, only hardened. This technology was widespread in Europe and Western Asia, but was not used in China, since there simply were not the necessary natural materials. Therefore, Chinese builders got out of the situation by developing an organic rice-based supplement.
In addition to historical value, the discovery is also important in practical terms. The preparation of test quantities of the mortar showed that it remains the most effective means for the restoration of ancient buildings, where it is often necessary to replace the connecting material in brick or masonry.