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.
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.
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.
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.
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.
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).
About the author: G K Suraishkumar is Professor in the Department of Biotechnology, IIT - Madras.