Scientists create cement entirely from waste

Researchers at Nanyang Technological University in Singapore have found a new way to create cement from waste.

Creating renewable biocement entirely from waste

Cement is a binder, a substance used in construction that hardens, sets and adheres to other materials to bind them together. When sand and gravel are combined with cement, concrete is produced. Cement is classified as hydraulic or non-hydraulic, with non-hydraulic cement not setting in the presence of water, while hydraulic cement requires a chemical reaction between dry materials and water.

Cement is one of the most widely used materials on the planet. Cement consumption in the United States was estimated at 109 million metric tons in 2021.

The manufacture of cement has an impact on the environment at all levels of the process. Some examples include air pollutants in the form of dust, fumes, noise and vibration during the operation of equipment and blasting in quarries, as well as damage to the landscape caused by quarrying.

Scientists at Nanyang Technological University in Singapore (NTU Singapore) have discovered a method to produce biocement from waste, making the alternative to traditional cement greener and more sustainable.

Biocement is a type of renewable cement that uses bacteria to create a hardening reaction that binds soil into a solid block.

NTU scientists have now created biocement from two common wastes: industrial carbide sludge and urea (from mammalian urine).

They devised a method to form a hard solid, or precipitate, from the interaction of urea with calcium ions in industrial carbide slurries. When this reaction occurs in the soil, the precipitate binds the soil particles and fills the spaces between them, resulting in a compact mass of soil. This produces a strong, durable and less permeable block of biocement.

The research team, led by Professor Chu Jian, President of the School of Civil and Environmental Engineering, showed in a proof-of-concept research paper published on February 22, 2022 in the Journal of Environmental Chemical Engineering that their biocement could potentially become a sustainable and cost-effective method of soil improvement, such as strengthening soil for use in construction or excavation, controlling beach erosion, reducing dust or wind erosion in the desert, or the construction of freshwater reservoirs on beaches or in the desert.

Wu Shifan Chu Jian

(left to right) Dr. Wu Shifan, Senior Researcher, Center for Urban Solutions, School of Civil and Environmental Engineering, NTU, and Professor Chu Jian, Chairman, School of Civil and Environmental Engineering, NTU holding blocks of biocement at based on urea and carbide sludge. Credit: Nanyang Technological University, Singapore

It can also be used as a biological grout to seal cracks in rock to control seepage and even to touch up and repair monuments such as rock carvings and statues.

“Biocement is a sustainable and renewable alternative to traditional cement and has great potential for use in construction projects that require soil treatment,” said Professor Chu, who is also director of NTU’s Center for Urban Solutions. . “Our research makes biocement even more sustainable by using two types of waste as raw materials. In the long term, this will not only make it cheaper to manufacture biocement, but it will also reduce waste disposal costs.

The research of NTU scientists supports the NTU 2025 strategic plan which aims to address some of humanity’s grand challenges, including mitigating human impact on the environment by advancing research and development in the field of durability.

Urine, bacteria and calcium: a simple recipe for biocement

The biocement manufacturing process requires less energy and generates fewer carbon emissions compared to traditional cement production methods.

The NTU team’s biocement is created from two types of waste: industrial carbide sludge – waste from acetylene gas production, from factories in Singapore – and urea found in urine.

First, the team treats the carbide sludge with a acid produce soluble calcium. The urea is then added to the soluble calcium to form a cementation solution. The team then adds a bacterial culture to this cementing solution. The bacteria in the culture then break down the urea in the solution to form carbonate ions.

These ions react with soluble calcium ions in a process called microbe-induced calcite precipitation (MICP). This reaction forms calcium carbonate, a hard, solid material found naturally in chalk, limestone and marble.

Biocement test sample

The test specimen of a Buddha hand was provided by Dazu Rock Carvings, a UNESCO World Heritage Site in China. Repair work using biocement was carried out at Chongqing University, China, by Dr. Yang Yang. The biocement solution is colorless, allowing restoration work to retain the original color of the sculpture. Credit: Nanyang Technological University, Singapore

When this reaction occurs in soil or sand, the calcium carbonate generated binds the soil or sand particles together to increase their strength and fills the pores between them to reduce water seepage through the material. The same process can also be used on rock joints, making it possible to repair rock carvings and statues.

Soil reinforced with biocement has an unconfined compressive strength of up to 1.7 megapascals (MPa), which is greater than that of the same soil treated with an equivalent amount of cement.

This makes the team’s biocement suitable for use in soil improvement projects such as soil reinforcement or reducing water seepage for use in construction or excavation or soil control. erosion of beaches along the coasts.

The first author of the article, Dr. Yang Yang, a former NTU Ph.D. student and research associate at the Center for Urban Solutions, currently a postdoctoral fellow at Chongqing University, China, said: “The precipitation of calcium carbonate at different levels of cementation strengthens the soil or sand by gradually filling the pores between the particles. Biocement could also be used to seal cracks in soil or rock to reduce water infiltration.

A sustainable alternative to cement

Biocement production is greener and more sustainable than the methods used to produce traditional cement.

“Part of the cement manufacturing process involves burning the raw materials at very high temperatures, over 1,000 degrees. Celsius to form clinkers – the binder of cement. This process produces a lot of carbon dioxide,” Professor Chu said. “However, our biocement is produced at room temperature without burning anything, so it’s a greener, less energy-intensive and carbon-neutral process.”

Dr Yang Yang said: “In Singapore, carbide sludge is considered waste. However, it is a good raw material for the production of biocement. By extracting calcium from carbide sludge, we make production more sustainable because we don’t need to use materials like limestone that has to be mined from a mountain.

Professor Chu added: “Limestone is a finite resource – once it’s gone, it’s gone. Limestone mining also affects our natural environment and ecosystem.

The research team claims that if biocement production could be scaled to the levels of traditional cement manufacturing, the overall cost of its production compared to conventional cement would be lower, making biocement an alternative both greener and cheaper than cement.

Restore monuments and strengthen shorelines

Another advantage of the NTU team’s method in formulating biocement is that the bacterial culture and cementation solution are colorless. When applied to soil, sand or rock, their original color is preserved.

This makes it useful for restoring ancient rock monuments and artifacts. For example, Dr. Yang Yang used biocement to repair ancient Buddha monuments in China. Biocement can be used to seal gaps in cracked monuments and has been used to restore broken pieces, such as the fingers of a Buddha’s hands. As the solution is colorless, the monuments retain their original color, keeping the restoration work true to history.

Working with relevant national agencies in Singapore, the team is currently testing its new biocement at East Coast Park, where it is being used to reinforce sand on the beach. By spraying the biocement solutions on the sand, a hard crust forms, preventing the sand from being washed out to sea.

The team is also exploring other large-scale applications of their biocement in Singapore, such as repairing roads by sealing cracks in roads, sealing gaps in underground tunnels to prevent water seepage, or even as a breeding ground for coral reefs as carol larvae like to grow. on calcium carbonate.

Reference: “Using Carbide Sludge and Urine for Sustainable Biocement Production” by Yang Yang, Jian Chu, Liang Cheng, Hanlong Liu, February 22, 2022, Journal of Environmental Chemical Engineering.
DOI: 10.1016/j.jece.2022.107443

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