It’s safe to say that without microbes, biotechnology would be an extremely limited science. Microbes are microscopic organisms such as fungi (which include yeasts), bacteria and viruses. They not only provide the foundation for much of the basic researchinvolved in biotechnology, they help to create durable building materials and structures. The early scientific study of microbes concentrated on their effects, such as causing disease. Eventually, scientists discovered microbes could be used for the study of processes which are common to all living organisms. An innovative alternative approach lies in the combined use of microorganisms, nutrients and biological processes naturally present in the subsurface soils to effectively improve their engineering properties. Considerable research on carbonate precipitation by bacteria has been performed using ureolytic bacteria. These bacteria are able to influence the precipitation of calcium carbonate by the production of an enzyme, urease (urea amidohydrolase, EC 3.5.1.5). Calcium carbonate precipitation occurs as a consequence of bacterial metabolic activity that raises the pH of the proximal environment.
Recently I discovered and improved few bacterial species which were able to precipitate calcite at higher rate and eventually this process lead to improved compressive strength, reduced permeability and low corrosion rate of reinforcement.
Biocement, a self-healing material to enhance durability of building structures and conservation of cultural heritages
Although hundreds of thousands of successful concrete and buildings are annually constructed worldwide, there are large numbers of concrete structures (including historical monuments) that deteriorate or become unsafe due to changes in loading, changes in use or changes in configuration. The constant developments in the field of civil engineering and the growth of industrial activity have created a growing demand for materials for the construction industry that do more and more to comply with structural requirements and meet stricter demands for working conditions and environment. Traditionally, mechanical strength has been the main criterion used when choosing building materials such as cement, concrete or bricks. Compressive strength, permeability and corrosion analysis are the most common used measures in designing of buildings structures. Considerable effort has been devoted to develop high-strength materials. However, with increasing volumes of constricted facilities that need to be maintained the focus is shifting towards durability.
Besides building materials preservation of the cultural heritage, socioeconomic growth and sustainable development is finding considerable resonance amongst specialists in the field. It calls for an innovative strategy for the maintenance of our cultural heritage. This strategy implies that the protection of historical buildings represents an important prerequisite for peace and stability and provides social and economic opportunities at the same time. The preservation of culture contributes to the identity of the citizens, creates jobs, supports the economy and promotes the responsible handling of societal resources. Although there is a great deal of knowledge and information on world heritage monuments, this is lacking in respect of standard monuments both at national level and international level. There is a need for research at this level into the number and quality of monuments and historical sites. Large sums of money are being spent worldwide on measures for the preservation of monuments and historical buildings. The economic and ecological commitment to the preservation of monuments and historical buildings requires, however, a prudent handling of the appropriate funds. This demands an optimization of damage analysis procedures and damage process controls as well as the development of monitoring and early warning systems for damage prevention. Therefore, the goal needs to be the implementation of permanent preservation measures, which requires long-term maintenance.
All building materials are porous. This porosity of building material along with ingress of moisture and other harmful chemicals such as acids, chlorides and sulfates affect the material and seriously reduce their strength and life. An additive that seals the pores and cracks and thus reduces the permeability of the structure would immensely improve its life. Conventionally, a variety of sealing agents such as latex emulsions and epoxies etc.; and surface treatments with water repellents such as silanes or siloxanes are used to enhance the durability of the concrete structures. However, they suffer from serious limitations of incompatible interfaces, susceptibility to ultraviolet radiations, unstable molecular structure and high cost. They also emanate toxic gases.
In order to overcome the shortcomings of conventional sealing agents, materials with self-healing capability can be used effectively. Use of urease producing microbes addresses these problems effectively, as these continue to survive and grow within the concrete structure after the initial use. Urease helps in mineralization of calcium carbonate, by hydrolyzing urea present in the environment. It releases carbon dioxide from urea that combines with calcium ions resulting in deposition of calcium carbonate in the form of calcite. Due to urease activity, bacteria are able to use urea as a sole nitrogen source and produce ammonia, which increases the pH in the proximal environment, causing Ca2+ and CO32- to precipitate as CaCO3. These unique properties make it particularly suitable for many applications in civil engineering (concrete structures, plasters, mortars, prefabricated elements, refractory elements, bricks, natural stones, etc.)
A microbial additive that helps in calcite precipitation with urease would enhance durability of building materials as well preserve the cultural heritage.
I am pleased to present herewith my preliminary findings of the effects of microbial additives (where I used Sporosarcina pasteurii, previously known as Bacillus pasteurii, a facultative anaerobic Gram-positive soil bacterium) to enhance the durability of building materials. The significant amount of data, some of which are attached hereto, accumulated to date leads us to the preliminary findings:
1. Microbial additive resulted in improvement in compressive strength of mortar by up to 38%.
2. Microbial additive can remediate cracks in building materials and monumental stones and regain strength within 28 days.
3. To make the process economic, microbial additive can be prepared by growing cells using industrial by products such as lactose mother liquor, corn steep liquor as nutrient sources.
4. Microbial additive can enhance the durability of bricks by reducing their permeability and increasing compressive strength.
5. The reduced permeability rates resulting from the microbial additive will increase the concrete structures’ useful life.
The data accumulated to date are, in my opinion, sufficient in quantity and trend to allow me to draw some preliminary conclusions with a reasonable confidence that in future it will further support the preliminary findings. As previously stated though the study period has not yet run the full course, the data and trends indicate the microbial additive is having the beneficial effect of enhancing the durability ofbuilding materials and preservation of cultural heritage.
