Self-cleaning Concrete:

Self-cleaning buildings and pollution-reducing roadways: these may sound like futuristic ideas but they are realities of some of today’s concrete. Recently introduced formulations of cement are able to neutralize pollution. Harmful smog can be turned into harmless compounds and washed away. The concrete can be imparted self-cleaning properties, using a special type of cement, the so called 'Photocatalytic Cement'. It uses photocatalytic process for the in situ degradation of polluting compounds. The cement is used in the same manner as regular Portland cement, gray or white. These products provide value through unique architectural and environmental performance capabilities. The cement gains the special properties incorporating fine particles of semiconductor material, titanium dioxide, during the process of manufacture.

There are two major cement companies distributing this type of cement in the United States. Italcementi, the Italian parent company of Essroc Cement, originally brought this product to market. They have recently licensed Lehigh White Cement Company to produce and market the material in Europe and North America. In the U.S., the photocatalytic cement is marketed under the brand name 'TX Active' available as gray or white Portland cement Type I, II, or III complying with ASTM C150. In Canada, TX Active is available as gray or white Portland Cement Type GU, MS, and HE, complying with CSA A3001. The product is offered in two formulations. The first, 'TX Arca', offers the self-cleaning properties against atmospheric compounds that stain concrete over time. The second, 'TX Aria', adds depolluting qualities to the self-cleaning properties of TX Arca.

Question:

Discuss the importance of imparting self-cleaning properties to concrete in the current scenario.

Self-cleaning surfaces:

TiO₂-containing photoactive materials represent an appealing way to create self-cleaning surfaces, thus limiting maintenance costs, and to promote the degradation of polluting agents. Several mortars containing anatase TiO₂, added as suspension or as powder, were characterized: among the photocatalytic mortars, the use of a combined additive (both powder and suspension) improved the material response. The best photoefficiency was exhibited by a mortar containing TiO₂ as surface covering; nevertheless, the adhesion problems shown by the surface layer open the way for future widening of investigations focused on the optimization of layer durability. Read more .. ..

Self-cleaning Concrete: Action

The self-cleaning action of concrete is due to photocatalytic reaction that occurs on the concrete surface. This promising technique utilizes nano-sized particles of semiconductor materials like TiO2, ZnO, WO3, FeTiO3 and SrTiO3 to carry out photo-induced oxidation process to disassemble volatile organic compounds (VOC) into CO2 and H2O and completely mineralize NOx and SOx. Due to its high efficiency and the generation of harmless byproducts, the concept of the photocatalytic process has therefore caught the attention of many researchers and environmentalists all over the world. When the semiconductor is exposed to UV light or even sunlight, the REDOX reaction occurs at the surface of the catalyst particle generating ●OH or hydroxyl radicals and O-2 or oxygen ions. These two powerful oxidizing agents will then disintegrate and rearrange the structure of the VOC and convert them into CO2 and H2O. As sunlight hits the surface, most organic and some inorganic pollutants are neutralized. They would otherwise lead to discolored concrete surfaces.

The primary ingredient of smog, ozone, is an extremely reactive gas molecule that reacts chemically with lung tissues, causing decreased lung function, respiratory infection, lung inflammation and aggravation of respiratory illnesses. The raw ingredients for ozone are nitrogen oxides (NOX)—produced primarily by internal combustion engines—and volatile organic compounds (VOCs)—
hydrocarbons that have evaporated from chemical plants, refineries, factories, gas stations and paints. Ozone is created when the two raw ingredients are
combined in the presence of heat and sunlight. Italcementi tests have demonstrated that a road paved with concrete made with the photocatalytic cement can reduce NOX levels by 20 to 80%, depending on atmospheric conditions. A building with photocatalytic precast concrete cladding can do the same. Because the proprietary compound oxidizes both NOX and VOCs, it combats ozone at the source. The other chemicals known to be oxidized using photocatalytic cement include:
* Inorganic compounds such as SOX, CO, NH3 and H2S
* Organic compounds such as alcohol, acids and aromatics
* Chlorinated organic compounds such as dioxins and chlorobenzene
* Pesticides such as diazinon and atrazine

The titanium-based catalyst is not spent as it breaks down pollution, but continues to work. Typical products are oxygen, water, carbon dioxide, nitrate and sulfate.

Question:

Mention three applications of semiconductor photocatalysis

Self-cleaning Concrete: Applications

The Photocatalytic Cement is already being used for sound barriers, concrete paver blocks, façade elements. Other applications include:

* Precast and architectural concrete panels
* Pavements, road surfacing and sidewalks
* Portland cement-based plaster for finish coat applications
* Concrete masonry units, roof tiles and cement-based tiles
* Cement-based restoration products

The best applications, for cost effectiveness, include any product used in thin layers or produced in a two-stage manufacturing process (with structural and face mixtures).

In one of the more innovative applications, Italcementi has successfully partnered with lighting system makers to develop artificial lighting systems that generate enough ultraviolet light to activate the photocatalytic process. The lighting system and photocatalytic cement concrete have been used in the renovation of a heavily trafficked tunnel in Rome.

The Photocatalytic cement has already been used in North America. A white precast concrete carillon tower was constructed at Dalton State College in Georgia. The 23 m tall tower is the centerpiece of a new quadrangle project and is visible from a nearby freeway, so it is important that it remains pristine.
Hyacinth Place, an affordable “green” housing complex in Highland Park, IL, has courtyard areas with concrete pavers incorporating photocatalytic cement. The pavers help clean the air as well as store and filter storm water that would have normally been nuisance runoff. The photocatalytic cement was used to
produce two 9 m tall gateway elements at the entrances to the new I-35 W bridge in Minneapolis, MN. These gleaming white concrete sculptures represent the
international symbol for water and serve as markers to remind travelers they’re crossing the Mississippi River. With the help of advanced technology and energy from the sun, they will remain proud symbols for decades to come.

The rain washes away the pollution from the concrete surface, hence buildings stay cleaner and do not require chemical applications that are potentially harmful to the environment. Maintenance costs are reduced. This is true even for buildings in highly polluted locations. One noted application is the Air France headquarters at Roissy-Charles de Gaulle International Airport near Paris, a white concrete building that has remained white. Another is the Church of the Year 2000 in Rome.

Clean buildings are great. Perhaps even more astounding environmental benefit is the potential for cleaner air. Concrete products that are exposed to sunlight throughout their life, like precast building panels, pavers and roof tiles, are especially suited to manufacture with photocatalytic cement. For instance, city streets made with special pavers are capable of reducing the pollution at its source—where it comes out of the tailpipe.

Question:

Discuss techno-economic feasibility of using Photocatalytic Cement in a building project

Reference:

(1) Barbesta M. and Schaffer D, "Concrete that cleans itself and the air", Concrete international, February 2009, pp 49-51

Bacterial Concrete

Concrete is the most used, strong and durable building material in the world. It is known that Concrete has a large load bearing capacity for compression load but it is weak in tension. The steel reinforcement bars (rebars) are embedded in concrete to impart tension bearing capacity. The reinforcement takes over the load when concrete structure is subjected to tension and prevents deformation, deterioration (i.e. crack development) and failure. The concrete, on the other hand, prevents corrosion of steel bars, protecting them from attacks of deteriorating agents from the environment, such as chlorides, sulphates, carbon dioxide. These deteriorating agents are responsible for causing cracks, make concrete more permeable and ultimately result in corrosion of reinforcement. The lower the permeability the higher is the durability of concrete. Thus to make the structures durable, either the cracks that have formed are repaired later or in the design phase extra reinforcement is placed in the structure to ensure minimum deformation under tension so that crack widths stay within a certain limit. This extra reinforcement is then only needed for durability reasons (to keep the crack widths small) and not for structural capacity. The extra steel is not desirable, keeping in view the current steel prices. It is not desirable from the sustainability considerations also. Thus a reliable method to automatically repair cracks in concrete will increase and ensure durability and functionality of concrete structures substantially. On the other hand, it will reduce the cost extra reinforcement. The bacterial concrete helps in doing that.

Bacterial Concrete: Biomineralisation

The bacterial concrete makes use of calcite precipitation by bacteria. The phenomenon is called microbiologically induced calcite precipitation (MICP). The pioneering work on repairing concrete with MICP is reported by the research group of Prof. Ramakrishnan V. and others (see bibliography) at the South Dakota School of Mines & Technology, USA.

The MICP is a technique that comes under a broader category of science called biomineralization. It is a process by which living organisms or bacteria form inorganic solids. Bacillus Pasteruii, a common soil bacterium, can induce the precipitation of calcite. Under favorable conditions Bacillus Pasteruii, when used in concrete, can continuously precipitate a new highly impermeable calcite layer over the surface of the already existing concrete layer. The precipitated calcite has a coarse crystalline structure that readily adheres to the concrete surface in the form of scales. In addition to the ability to continuously grow upon itself, it is highly insoluble in water. It resists the penetration of harmful agents (chlorides, sulphates, carbon dioxide) into the concrete thereby decreasing the deleterious effects they cause. Due to its inherent ability to precipitate calcite continuously, bacterial concrete can be called as a “Smart Bio Material” for repairing concrete. The MICP comprises of a series of complex biochemical reactions. It is selective and its efficiency is affected by the porosity of the medium, the number of cells present and the total volume of nutrient added. The phosphate buffer or urea-CaCl2 have been found effective as nutrients. The bacteria precipitate calcite in the presence of nutrients. The optimum pH for growth of B. Pasteurii is around 9. The alkaline environment of concrete with pH around 12 is the major hindering factor for growth of bacteria. However, B. pasteurii has the ability to produce endospores to endure an extreme environment, as observed by Prof. Ramakrishnan and the research team.

The microbial modified mortar or concrete has become an important area of research for high-performance construction materials. Ghosh et. al. investigated the effects of incorporating a facultative anaerobic hot spring bacterium on the microstructure of a cement–sand mortar. Environmental scanning electron
microscopic (ESEM) views and image analysis (IA) of the bacteria modified mortar (thin-section) showed significant textural differences with respect to the control (without bacteria) samples. X-ray diffraction (XRD) study confirmed the formation of new phases of silicates (Gehlenite) within the matrix of such mortar material, which cause an improvement in the strength of the material. Electron probe microstructure analysis (EPMA) suggested that the bacterial treatment promoted uniform distribution of silicate phases and increased the calcium/silicon (C/S)ratio within CSH gel of the matrices. The bacterium is found to leach a novel protein, which is capable of isolating silica from its source. The addition of such isolated protein, instead of the bacteria, into mortar also improves the strength of mortar.

Reference:

Ghosh S., Biswas M., Chattopadhyay B. D., Mandal S., "Microbial activity on the microstructure of bacteria modified mortar", Cement & Concrete Composites, Vol. 31, 2009, pp 93–98

Bacterial Concrete: Applications

Giancarlo Ranalli, an Italian researcher in Pesche, Italy, has already used bacteria to clean the base of Michelangelo's Pietà Rondanini in Milan and another kind of bacteria to remove harmful animal glue from frescos in Pisa. Ramirez also describes the use of forensic DNA techniques to identify burrowing insects in wooden pieces from just minuscule droppings or a tiny body part so that the precise species can be identified and properly eradicated, as well as the use of biomineralization process(same as that used in making the bacterial concrete) in which microbes, introduced to a crack in a stone sculpture, will deposit a calcium carbonate that picks up the color of the original while filling the gap.

"Science has finally set a solid foot in the art world," says José Luis Ramirez, co-author of a 2005 study of the use of biotechnology in art preservation and director of the United Nations University's Program for Biotechnology for Lain America and the Caribbean (BIOLAC), which is an interdisciplinary school that promotes the use of biotechnology in fields from agriculture to manufacturing.

Bibliography:

1. Bang, S.S., Galinat, J.K. & Ramakrishnan, V. Calcite precipitation induced by polyurethaneimmobilized Bacillus pasteurii, Enzyme and Microbial Technology 28 (2001) 404–409
2. Rodriguez-Navarro, C, Rodriguez-Gallego, M., Chekroun, K.B. and Gonzalez-Munoz, M.T. Conservation of Ornamental Stone by Myxococcus xanthus-Induced Carbonate Biomineralization, Applied and Environmental Microbiology, Apr. 2003, p. 2182–2193.
3. Ramikrishnan, V., Panchalan, R.K. and Bang, S.S. Improvement of concrete durability by bacterial mineral precipitation, in Proc. ICF 11, Torino, Italy, 2005.
4. Ramakrishnan V., Ramesh K. P., Bang S. S., “Bacterial concrete”, Proceedings of the Society of Photo-Optical Instrumentation Engineers (SPIE), The International Society for Optical Engineering, Bellingham, WA, Vol. 4234, 2001, pp. 168-176
5. Stocks-Fischer, S., Galinat, J. K., and Bang, S.S., “Microbiological precipitation of CaCO3”, Soil Biology and Biochemistry, Vol. 31, 1999, pp. 1563-1571
6. Gollapudi, U.K., Knutson, C.L., Bang, S.S., and Islam, M.R., “A New method for Controlling Leaching through Permeable Channels”, Chemosphere, Vol. 30, No. 4, 1995, pp. 695-705
7. Zhong, L., And Islam, M.R., “A New Microbial Process and its Impact on Fracture Remediation”, 70th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, Dallas, Texas, Oct 22-25, 1995.
8. Harmon K.,"The Science of Saving Art: Can Microbes Protect Masterpieces?",
Scientific American, February 9, 2009

Question:

Discuss possible applications of bacterial concrete