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Nano-Engineered Concrete

INTRODUCTION

Nanotechnology is a powerful new technology for taking apart and reconstructing nature at the atomic and molecular level. It is being touted as the basis of the next industrial revolution and will be used to transform and construct a wide range of new materials, devices, technological systems and even living organisms. Nanotechnology deals with the production and application of physical, chemical and biological systems at scales ranging from a few nanometers to submicron dimensions. It also deals with the integration of the resulting nanostructures into larger systems. Nanotechnology also involves the investigation of matter to individual atoms.

One nanometre (nm) is one thousandth of a micrometer (μm), one millionth of a millimeter (mm) and one billionth of a meter (m).

Engineered nanoparticles are deliberately manufactured and can be distinguished from nanoparticles that ‘exist in nature’, or are by-products of other human activities. The properties of atoms and molecules are not governed by the same physical laws as larger objects or even larger particles, but by “quantum mechanics”. The physical and chemical properties of nanoparticles can therefore be quite different from those of larger particles of the same substance. Altered properties can include but are not limited to color, solubility, material strength, electrical conductivity, magnetic behavior, mobility (within the environment and within the human body), chemical reactivity and biological activity.

NANO-ENGINEERED CONCRETE

Nano-concrete is defined as a Concrete made with portland cement particles that are less than 500 nano-meters as the cementing agent. Currently cement particle sizes range from a few nano-meters to a maximum of about 100 micrometers. In the case of micro-cement the average particle size is reduced to 5 micrometers. An order of magnitude reduction is needed to produce nano-cement.

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Based on the available data, the beneficial action of the nano-particles on the Micro-structure and performance of cement based materials can be explained by the following factors.

• Well-dispersed nano-particles increase the viscosity of the liquid phase helping to suspend the cement grains and aggregates, improving the segregation resistance and workability of the system;
• Nano-particles fill the voids between cement grains, resulting in the immobilization of “free” water (“filler” effect).
• Well-dispersed nano-particles act as centers of crystallization of cement hydrates, therefore accelerating the hydration.
• Nano-particles favor the formation of small-sized crystals (such as Ca(OH)2 and AFm) and small-sized uniform clusters of C-S-H.
• Nano-particles improve the structure of the aggregates’ contact zone, resulting in a better bond between aggregates and cement paste.
• Crack arrest and interlocking effects between the slip planes provided by nanoparticles improve the toughness, shear, tensile and flexural strength of cement based materials.

NANO TECHNOLOGY AND SELF CLEANING CONCRETE Strong sunlight or ultraviolet light decomposes many organic materials in a slow, natural process. You have seen this process, for example, in the way the plastic dashboard of your truck fades and gets brittle over time. Photocatalysts speed up this process and stimulate a chemical transformation without being consumed or worn-out by the reaction. Photocatalysis is a reaction which uses light to activate a substance which modifies the rate of a chemical reaction without being involved itself. And the photocatalyst is the substance which can modify the rate of chemical reaction using light irradiation. Chlorophyll of plants is a typical natural photocatalyst. The difference between chlorophyll photocatalyst to man-made nano photocatalyst (here below mentioned as photocatalyst) is, usually chlorophyll captures sunlight to turn water and carbon dioxide into oxygen and glucose, but on the contrary photocatalyst creates strong oxidation agent and electronic holes to breakdown the organic matter to carbon dioxide and water in the presence of photocatalyst, light and water.

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The catalytic ingredient uses in the concrete is titanium dioxide (TiO2). Making it photocatalytic requires manipulating the material to create extremely fine nanotechnology-sized particles with a different atomic structure. At the nano-scale, this new type of titanium undergoes a quantum transformation and becomes a semiconductor. With reduction in size, more atoms are located on the surface of a particle and, in addition to a remarkable surface area of nano-powders; this imparts a considerable change in surface energies and surface morphologies. As a result, all these factors alter the basic properties and the chemical reactivity of nano-materials. The shift in properties helps to develop the improved catalytic ability, tunable wavelength sensing ability, as well as enables to make self-cleaning concrete.

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SELF CLEANING PRINCIPLE

The macro effect of self-cleaning is, in fact, a combined effect of self cleaning surface and degradation of organic deposits. Although the photo-induced super-hydrophilicity and degradation of organic contaminants are different processes, they may take effect simultaneously. It is difficult to distinguish which mechanism is more important for self-cleaning.

SUPER HYDROPHILICITY

The wetting of a solid with water, where air is the surrounding medium, is dependent on the relation between the interfacial tensions (water/air, water/solid and solid/air). The ratio between these tensions determines the contact angle between water droplets on a given surface. A contact angle of 0o means complete wetting, and a contact angle of 180o corresponds to complete non-wetting.

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Surfaces with low wet-ability and contact angles of about 100o are known as hydrophobic surface. The higher this angle the lower is the value of the adhesion work. The water repellency of plant surfaces has been known for many years. Those water-repellent surfaces also indicate self-cleaning properties. For example, lotus plant’s surface which has micro rough surfaces show contact angles higher than 130o. That means, the adhesion of water, as well as particles is extremely reduced. Water which contacts such surfaces will be immediately contracted to droplets. The particles of contaminants adhere to the droplet surfaces and are removed from the rough surface when the droplets roll of (fig. 1).

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Cleaning procedures based on low contact angles are known since the discovery of soap (3rd millennium BC). Generally, detergents reduce the surface tension of water and the contact angle will lowered. Decreasing of the contact angle leads to enlarged values of the adhesion work. The surfaces which have low contact angle and low wet-ability know as hydrophilic surfaces

Another possibility to cause low contact angles without detergents is the use of active thin films on the material surface. For the preparation of these thin layers mainly photocatalytic active metal oxides have been applied.

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In our case Super-hydrophilicity is a phenomenon that occurs when a TiO2 film is subject to UV irradiation a very small water contact angle appears. On this surface, water tends to spread out flat instead of beading up. The binding energy between Ti atom and the lattice oxygen atom is weakened by the hole generated after UV irradiation. Therefore, the adsorbed water molecules can break a Ti-O-Ti band to form two new Ti-OH bands resulting in super-hydrophilicity. In fact, TiO2 film is not only hydrophilic but also amphiphilic after UV irradiation. When water is rinsed over the surface, contaminations like oil can be washed away.

DEGRADATION OF ORGANIC DEPOSITS

The reaction begins with the irradiation of light over TiO2. When TiO2 absorbs a photon containing the energy equal to or larger than the band gap, an electron will be promoted from the valence band to the conduction band. The activation of the electrons results in the generation of “holes” (electron vacancy) in the valence band. In this reaction, h+ and e- are powerful oxidizing and reducing agents respectively. The electron-hole pairs may recombine in a short time or take part in chemical reactions depending on reaction conditions and molecular structures of the semiconductors. The strong oxidation power of h+ enables it to react with water to generate the highly active hydroxyl radical (.OH) which is also a powerful oxidant.

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Most organic air pollutants can be degraded completely by either the hydroxyl radicals or the holes themselves to innocuous final products (e.g. CO2 and H2O). In addition, the reducing power of the electrons can induce the reduction of molecular oxygen (O2) to superoxide (.O2 -). It has been confirmed that the superoxide is almost as effective as the holes and hydroxyl radicals in the chain reactions for the breaking down of organic compounds.

1. After irradiation of UV light ranging from 300nm to 400 nm, the photocatalytic reaction begins with the generation of electron-hole pairs.

2. The h+ reacts with OH- dissociated from water to form the hydroxyl radical

3. The e- reacts with molecular oxygen to form the superoxide anion

4. The superoxide anion further reacts with H+ dissociated from water to produce HO2 • radicals

5. NO diffuses to the surface of TiO2 and is oxided to NO2 by 2 HO • radicals

6. Finally NO2 reacts with hydroxyl radicals to form nitric acid

ADVANTAGES OF SELF CLEANING CONCRETE

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• Self-cleaning concrete will not require the use of the solvents now used to clean buildings, eliminating another source of pollutants.
• Clean concrete will reflect more light, reducing the heat buildup associated with “urban heat islands.” This may help keep our cities cooler during hot seasons.
• It may also reduce the formation of smog since the chemical reaction that creates smog increases as air temperatures increase
• Keep the building in new and clean view
• Protect the surface from dust, acid rain and air pollutant damage
• Purify the air pollutant near and on the surface (e.g. car exhausts NOx, Formaldehyde, Benzene, VOCs)
• Restrain the dust electrostatic adsorption
• Reduce the energy consumption for cooling the building in summer
• Restrain mildew or alga growing
• Make the surface without water stain after raining
• Kill the bacteria and virus on the surface and in the air near the coated building
• Absorb the UV from sun and then protect the surface from UV damage
• Decompose the organic pollutant on the surface (e.g. oil, graffiti)