Giving infrastructure the bio-cement advantage!
The basic function of cement is to ‘cement’ or to ‘bind’ the relevant components/parts. For example, it could be used to bind bricks together to make a strong wall when mixed with sand in, say, a 1:4 ratio of cement and sand and made fluid enough with water (mortar) to evenly lay the bricks. With differences in the process, it could be used to create structures such as reinforced concrete columns, slabs, and others. Owing to the reactions that take place at the microscopic level during the setting and curing process, cement molecules adhere strongly to other surrounding components (bricks, stones, sand, etc) and form a strong, consolidated, stone-like mass by binding the various components together.
The basic function of cement is to ‘cement’ or to ‘bind’ the relevant components/parts. For example, it could be used to bind bricks together to make a strong wall when mixed with sand in, say, a 1:4 ratio of cement and sand and made fluid enough with water (mortar) to evenly lay the bricks. With differences in the process, it could be used to create structures such as reinforced concrete columns, slabs, and others. Owing to the reactions that take place at the microscopic level during the setting and curing process, cement molecules adhere strongly to other surrounding components (bricks, stones, sand, etc) and form a strong, consolidated, stone-like mass by binding the various components together. Different materials have been used as cement for at least 2,000 years or more. The downsides of cement Given the immense demand for various construction purposes, the worldwide cement industry is huge. It is estimated that the annual cement production across the world is 4 billion tonne. Unfortunately, the process of cement production releases a large amount of a greenhouse gas, carbon dioxide (CO2), into the atmosphere – about 2.8 billion tonne a year. That is more than the CO2 emitted by all airplanes and ships, combined. We all know that CO2 in the atmosphere is bad for the planet and serious global efforts have been made to reduce CO2 emissions. About 140 countries have pledged to reduce CO2 emissions toward achieving net-zero emission targets at the COP 26 UN Climate Change Conference held in November 2021 in Glasgow, Scotland. In addition, some parts of the cement production process need to operate at temperatures higher than, say, 900oC. Thus, the cement production process consumes tremendous power. Imagine how advantageous it would be if cement could be made with negligible CO2 emission and low power expenditure! This is indeed possible – with bio-cement. What is bio-cement? Bio-cement is a material made at normal tropical day temperatures with negligible CO2 emission using a microorganism, a type of bacteria. Bacteria are used to produce many useful materials from everyday yogurt to lifesaving medicines. They can also be used to produce cement. In addition to the significant advantages of environment-friendliness and energy-efficiency, bio-cement production is faster and has the potential to be much cheaper through the use of industrial waste materials such as lactose mother liquor or corn steep liquor as food for bacteria. Its properties such as shear strength and durability are comparable to conventional cement, whereas its water absorption capacity and permeability are lower than conventional cement, which are desirable characteristics. Bio-cement basics and our work As mentioned earlier, many chemicals can serve to bind/cement things together at the microscopic level. The relevant cementing molecules of the most common, Portland cement, are silicates of calcium, whereas calcium oxide was the relevant cementing molecule in old cements. When the silicates in Portland cement react with water, the hydration reaction leads to addition of hydrogen to these cementing molecules, which binds the components and hardens the whole material (setting) into a solid, strong mass. One of the important raw materials for producing Portland cement is naturally occurring limestone, which is mined. An important constituent of limestone is calcium carbonate (CaCO3), which is a good cementing material by itself. It so happens that some bacteria, under certain conditions, can produce CaCO3 outside the cell. The CaCO3 crystals formed can act as a cement to bind the surrounding sand/gravel/other parts into a solid structure with good mechanical properties. Let us simplistically look at some molecular details of this process in one of the common bio-cement relevant bacteria, Sporosarcina pasteurii (Figure 1). Urea (NH2-CO-NH2) enters the bacterial cell and gets broken down to ammonium and carbonate. Ammonium, in a suitable form, can get out of the cell to create alkaline conditions that are preferred for this process. Carbonate also gets out of the cell in a suitable form and combines with calcium to form CaCO3. The work in our lab done by a team consisting of Subasree Sridhar, Dr Nirav Bhatt and myself has focused on better understanding the quantitative details such as rates of the various related cellular sub-processes through formulation and analysis of a mathematical model. Any dynamic process can be manipulated toward desirable engineering goals only by understanding the rates of the various sub-processes at the micro-level. Understanding things at this level significantly widens the scope of manipulating the relevant parameters with confidence to yield desirable results.Details on bio-cement production and application As of now, world-wide, bio-cement bricks, and self-healing cementing mixtures are the most common products from bio-cement, which are commercially available. To make bio-cement bricks, appropriate sand and the bacteria are mixed together and poured into brick moulds. Then, the moulds are set aside under conditions suitable for CaCO3 production by bacteria. The CaCO3 thus formed binds the soil particles into a solid brick that can be used for construction. To make self-healing cement mixtures, a suitable cementing material is mixed with ‘inactive’ bacterial spores, packaged and sold. Much later, when cracks develop in the cemented parts of a construction, water and oxygen seep through the cracks and reach the spores. When appropriate conditions are reached with water and oxygen in their environments, the inactive bacterial spores spring to life and form CaCO3 around them in the cracks to seal them as they form. The spores can remain in an inactive form for many decades together. To seal cracks in existing construction with cements that do not have the bacterial spores, an appropriate self-healing cement mixture with the bacteria is sprayed into the cracks. The bacteria in this mixture form CaCO3, which seal the existing cracks and significantly extend the life of the construction. Current sparse use A preliminary web search for worldwide companies that produce bio-cement shows at least three companies. A company called BioMASON in North Carolina, USA, has been working in this area from 2012. In February 2022, it received Series C funding of $ 65 million to scale up. It makes precast bio-cement bricks, Biolith®. Two other companies – Costain, Maidenhead BRK, UK, and Green Basilisk, Delft, Netherlands – seem to be involved in self-healing bio-cement products. Self-healing bio-cement related products are available in India. A preliminary web search led to a commercially available product by Prions Bio Tech, Belgaum, Karnataka, called Aquatic White Bactaheal-PR Self Healing Concrete Bacteria (www.prionsbiotech.in/bacillus-subtilis.html). In the future Once people start accepting bio-cement, it can be expected that its production and use will rapidly grow. There could be futuristic scenarios in which cement is no longer made in factories located at a distance from the construction site. Instead, bio-cement could be generated, on a need basis, at the place of use by merely mixing the right ingredients (brick, sand, gravel, etc) with the relevant bacteria, setting it aside, and waiting for some hours or a few days. This is similar to the current process for reinforced concrete structures. However, the ingredients mixed need not have Portland cement at all because CaCO3 is the cementing molecule! About the author: G K Suraishkumar is Professor in the Department of Biotechnology, IIT - Madras.