A team of scientists from South Ural State University and Zhejiang University, China, have developed a technology of calcareous sand reinforcement using enzymes, coconut fiber, and calcium carbonate. Such a material is up to five times stronger in case of dynamic loads and is environmentally friendly. The results have been published in the Sustainability international journal.
In island and reef construction, engineers come across a problem: local calcareous sand is porous and fragile, and easily crumbles even under light loads. Shipping common sand from the continent is difficult and expensive. Traditional methods of soil stabilization – cementation or chemical resins – are harmful for the environment. Scientists from Chelyabinsk and Zhoushan have proposed an alternative: adding calcium carbonate (EICP) obtained by enzyme-induced precipitation of fine grade building material (produced by rock fracturing and screening) to common sand.
How does this technology work? Urease (enzyme) is extracted from soybeans and mixed with the urea and calcium chloride solution. The enzyme decomposes urea, and calcium ions bind to carboxylate ions thus generating calcium carbonate (also known as calcareous rock), which glues sand grains together. The process is carried out at room temperature and generates no hazardous emissions. To better strengthen the structure, coconut fiber is added to the mixture – it works as micro-reinforcement and impedes the deterioration of material in case of impacts.
At their laboratory, the researchers produced samples with different ratios of fine and coarse reinforced sand (0.5-1 mm and 1-2 mm), different content of coconut fiber (0%, 0.2%, and 0.4% of the sample mass), and different number of processing cycles (2-4). The best result was demonstrated by a mixture with equal content of fine and coarse sand and 0.4% of coconut fiber.
The static (compressive) strength and dynamic strength of the samples were tested using the Split-Hopkinson Pressure Bar, which simulates the impact load. It has been revealed that the dynamic strength is always higher than the static one at the same concentrations of calcium carbonate, and the difference may reach 2.5 MPa (megapascals). That is, the higher the content of calcium carbonate, the stronger the material. The optimum mixture demonstrated high stability even at strong impacts.
“Coconut fiber plays a key role: it strengthens the material. It is like reinforcement in concrete,” explains Professor of the SUSU Department of Building Materials and Products Tamara Chernykh. “In case of dynamic loads (for instance, in case of wave impact or a plane landing), it impedes the disintegration of the reinforced concrete. The samples without fiber deteriorated completely, and those with fiber preserved their integrity. This is of fundamental importance for island airfields and mooring berths.”
Comparing with the analogues showed that this innovation exceeds the known solutions in its strength at the same content of calcium carbonate. The authors stress that this is the first scientific systemic comparison of the static and dynamic strength for EICP-stabilized calcareous sand with various grain size compositions and fiber contents.
This technology may be used for soil stabilization at air strips, mooring berths, breakwaters, and other facilities in coastal and island areas. It is environmentally friendly, uses local materials, and does not require hard firing, like cement, which means that it generates next to none carbon dioxide emissions.
This research study has been conducted with the support from the National Natural Science Foundation of China.