1. INTRODUCTION
A construction material remove pollutants from the air as it keeps
its surface clean. This new astonishing concrete that not only keeps itself
clean but also removes pollutants from the air is called Self Cleaning
Concrete. The key to such properties are photo catalytic components that use
the energy from ultraviolet rays to oxidize most organic and some inorganic
compounds. Air pollutants that would normally result in discoloration of
exposed surfaces are removed from the atmosphere by the components, and their
residues are washed off by rain. So, this new cement can be used to produce
concrete and plaster products that save on maintenance costs while they ensure
a cleaner environment. Air inside buildings can be more polluted than outdoor
because there are various sources of pollution in some big cities.
For decades,
scientists have recognized two unique effects of titanium dioxide, a common
compound that is used in products as diverse as quick-setting concrete, tile
grout and even suntan lotion. When exposed to sunlight, titanium dioxide acts
as a catalyst to break down organic matter, while also creating a super
hydrophilic (water-loving) surface. The versatile function of TiO2, which can
both serve as photocatalytic materials and structural materials, has
facilitated its application in exterior construction materials and interior
furnishing materials, such as cement mortar, exterior tiles, paving blocks,
glass and PVC fabric.
As such the use of the special additive promotes
self-cleaning of large concrete structure and at the same time promotes
reactions that help in cleansing the environment as well. The properties of
photocatalyst including photocatalytic water and air purifications,
self-cleaning and photocatalytic anti-bacterial effect. Its application is limited because of chemical engineering
limitations such as support of photocatalysts or separation of the
photocatalysts from the effluent. This is why, the most largely applicationof
photocatalysis is the self-cleaning materials. Self cleaning
concrete serves to fulfill the following objectives.
· Depollute air by means of oxidation of
(common) inorganic pollutants, such as nitrogen oxides (NOx)
· Convert the pollutants to less harmful
substances
· Improve building aesthetic durability
through enhanced self-cleaning properties of building facades
Fig
2.1:Effect of pollution
Buildings are exposed to many organic contaminants. From bird
residue to diesel fumes, all urban buildings are constantly exposed to organic
material that makes their surfaces appear dirty. Yet there’s another kind of
organic material constantly bombarding buildings that is harder to see: NOx
(nitrogen oxides). As the primary component of smog, NOx not only makes
buildings dirty, but it also threatens the quality of the air we breathe.
A
variety of air pollutants have known or suspected harmful effects on human
health and environment. In big cities with dense population the pollutant
concentration at street level is quite high because the dispersion of the
exhaust generated by a large number of vehicles is hindered by the surrounding
tall buildings. In most areas of Europe, these pollutants are principally the
products of combustion from space heating, power generation or from motor
vehicle traffic. Pollutants from these sources may not only prove a problem in
the immediate vicinity of these sources but can travel long distances,
chemically reacting in the atmosphere to produce secondary pollutants such as
acid rain or ozone. The principle pollutants emitted by vehicles are carbon
monoxide, oxides of nitrogen (NOx), volatile organic compounds (VOC’s) and particulates. These pollutants have an increasing impact
on the urban air quality. In addition, photochemical reactions resulting from
the action of sunlight on NO2 and VOC’s lead to the formation of ozone, a
secondary longrange pollutant, which impacts in rural areas often far from the
original emission site. Acid rain is another long-range pollutant influenced by
vehicle NOx emissions and resulting from the transport of NOx, oxidation in the
air into NO3 and finally precipitation of nitrogen acid with harmful
consequences for building materials (corrosion of the surface) and vegetation.
Indoor air quality has received immense attention in the early 1990s. The
indoor air quality in any building can be compromised by microbial contaminants
(mold, bacteria, and gases), chemicals (such as, carbon dioxide, formaldehyde),
allergens, or any mass or energy stressor that can induce health effects.
Indoor air pollutants mainly include nitrogen oxides, VOCs and particulates.
These pollutants are emitted from different sources such as combustion,
construction materials and consumer products. Many VOCs are known to be toxic
and carcinogenic. Photocatalytic oxidation (PCO) is one of the most feasible
options to improve indoor air quality Additionally, the construction industry
is the major source of air pollution; transportation for a high population
density of people and the numerous tall buildings hinder and prevent the
dispersion of air pollutants generated by a high concentration of vehicles at
the street level.
Fig 2.2: Figure shows that transport sector is responsible for 54% NOx
release.
The second figure shows exhaust of NOx during peak hours in Paris.
3.CONCEPT
Fig
3.1: Process of cleaning
Photo catalytic effect of TiO2 is the underlying
principle behind the selfcleaning concrete. Using this additive concrete
structure will ideally behave like a tree absorbing gaseous pollutants from
atmosphere and releasing relatively harmless compound back into the
environment. Also structure is able to replenish its external finish by virtue
of the additives ability to carry out self cleaning by its reaction with
compounds present in the environment that are responsible for surface finish
degradation.
It is a natural phenomenon whereby a substance, called a photo
catalyst, alters the speed of a chemical reaction through the action of light.
By exploiting the energy of light, photo catalysts induce the formation of
strongly oxidizing reagents which can decompose some organic and inorganic
substances present in the atmosphere. Photo catalysis is, therefore, an
accelerator for oxidization processes that already exist in nature. Indeed, it
promotes faster decomposition of pollutants and prevents them from accumulating
on the surfaces. The worsening of the level of pollution in urban areas has
recently driven research towards the application of the capability of removing harmful substances present in the atmosphere. Photo catalysis, therefore,
makes an effective contribution to improving air quality.
TiO2 is a metal, which is multiply present in nature. The oxygen TiO2 has three different molecule structures: rutile, anatase and
brookiet (Fujishima et al. 1999). Rutile is known as pigment in white paints,
but shows up till now low photocatalytic reactivity. Anatase is preferable if
used as photocatalytic cell. To use anatase in heterogenic photocatalysis,
UV-light with a wave length lower than 387 mm has to be present. Also the
intensity of the light is important to optimize the photocatalytic activity.
Normal daylight can be used for the photocatalytic reaction. Research is
focusing now on the application of nano-particles of TiO2, active in the visible light range.
Existing applications may be
found in water purification, air conditioning (air purification), self-
cleaning glazing, ceramic tiles (self-cleaning, antibacterial,…), textile
(anti-odour), mirrors (anti- condensation), tunnel lightning, white tents,…
Besides the air purifying and antiseptical action, where the pollutants are
oxidized or reduced due to the presence of the photocatalyst, TiO2 is also used to obtain a self cleaning material. This is due to a
very high hydrophilicity of the surface when TiO2 is activated by
UV-light. The water layer is attracted between the dirt and the surface
resulting in the washing off of the dirt particles. This effect is more
pronounced with smooth surfaces like glass and ceramic tiles. In the case of
concrete surfaces, the selfcleaning effect will be more limited due to the
physical anchoring of the dirt in the larger pores. In addition, due to
photocatalytic working, a decomposition of the dirt particles, especially of the
organic particles takes place, followed by the washing of the surface resulting
in a cleaner surface.
Since then, some of the major applications have been the
degradation of organic pollutants in water, the purification of air and the
photo catalytic antibacterial effect in so-called “self cleaning” building
materials . Photo catalytic oxidization takes place on the surface of the
photcatalyst under ultra violet (UV) light. A photon of light is absorbed by
the TiO2, which starts a chemical reaction by producing an electron hole
pair. This electron hole pair can then further produce hydroxyl radicals. These
hydroxyl radicals are the key behind the self cleaning of the concrete. Two phenomena occur as a result of these factors; one is the photo-induced redox
reaction of absorbed substances, the other is the photo-induced
superhydrophilicity. The photo induced redox reaction is able to break down
organic substances where the super-hyrdohilicity cleans away the substances
from the surface.
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 a car fades and
becomes brittle over time. Photo catalysts accelerate this process and, like
other types of catalysts, stimulate a chemical transformation without being
consumed or worn out by the reaction. When used on or in a concrete structure,
photocatalysts decompose organic materials such as dirt, including soot, grime,
oil and particulates; biological organisms, including mold,algae, bacteria and
allergens; airborne pollutants, including volatile organic compounds, including
formaldehyde and benzene, tobacco smoke, and the nitrous oxides [NOx] and
sulfuric oxides [SOx] that are significant factors in smog; and even the
chemicals that cause odors. The catalyzed compounds break down into oxygen,
carbon dioxide, water, sulphate, nitrate and other molecules that are either
beneficial to or, at worst, have a relatively benign impact on the environment.
Most inorganic pollutants and stains, including rust, are not catalyzed.
Titanium oxide (TiO2), the primary
catalytic ingredient, is widely used as a white pigment in paint, plastics,
cosmetics and a host of other products. Making it capable of photocatalysis
requires manipulating the material to create extremely fine nano-sized
particles with a different atomic structure than that of the ordinary pigment.
At the nano scale, this type of titanium undergoes a quantum transformation and
becomes a semiconductor. Activated by the energy in light, the TiO2 creates a
charge separation of electrons and electron holes.
The electrons disperse on
the surface of the photocatalyst and react with external substances, causing
chemical reductions and oxidations and forming hydroxyl radicals that act as
powerful oxidants to decompose organic compounds. Due to this oxidation
reactions the pollutants are converted to less harmful substances.
Fig 4.1:Cleaning mechanism
5. SUPER-HYDROPHILICITY
Super-hydrophilicity is a phenomenon that occurs when a TiO2 film is subjected to UV irradiation a very small water contact
angle appears. On this surface, water tends to spread out flat instead of
beading up. It has been shown that the reciprocal of the contact angle
corresponds to the density of the surface hydroxyl groups reconstructed by UV
irradiation . The binding energy betweenTi atom and the lattice oxygen atom is
weakened bythe hole generated after UV irradiation. Therefore, the adsorbed
water molecules can break a Ti–O–Ti bond to form two new Ti–OH bonds resulting
in super-hydrophilicity . In fact, TiO2 film is not only
hydrophilic but also amphiphilic after UV irradiation. The surface may adsorb
both polar and nonpolar liquids. When water is rinsed over the surface,
contaminations like oil can be washed away.
The macro effect of self-cleaning
is, in fact, a combined effect of superhydrophilicity 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. . Also worth noting is the
interesting fact that, to some extent, there might be synergetic effect of
photocatalysis and superhydrophilicity promoting self-cleaning.
Fig 5.1: Photo-induced hydrophilic TiO2 surface.
Hydroxyl radical
plays an important role in the decomposition of organic compounds. If more
hydroxyl groups can occur on the surface of TiO2 due to
superhydrophilicity, the efficiency of degradation of organics may also be
improved . On the other hand, the adsorption of organic compounds on the film
surface may lead to a conversion of hydrophilic surface to hydrophobic surface.
The photocatalytic decomposition of these organic contaminants can restore the
super-hydrophilic property. Thus the synergetic effect of photocatalysis and
super-hydrophilicity ensure the self-cleaning character of TiO2 film can be maintained continuously.
Fig
6.1:Chemical reactions
The photocatalytic efficiency of a system can be assessed by a
number of criteria again being influenced by various circumstances and factors.
Therefore, a model pollutant was selected and a standard measurement procedure
was defined as being explained in the following sections. From the relevant
literature it becomes clear that the degradation of nitric oxide (NO) or more
general of nitrogen oxide (NOx), also referred to as DeNOx- process
(denitrogenization), delivers a suitable model to assess the ability of
surfaces for air purification. This denitrogenization process can roughly be
described as a two-stage reaction on the surface of a photocatalyst, which in
most of the cases is titanium dioxide in the anatase modification or variants
of it. For this purpose a certain amount of water molecules, supplied by the
relative humidity, and electromagnetic radiation are required to start a
degradation process. The electro- magnetic radiation (E) is expressed by the
product of Planck’s constant (h) and the frequency (ν). Herewith, the two steps
can be summarized as follows:
The free hydroxyl radicals (OH) originate from the photo generated water
electrolysis on the anatase surface. These two reactions describe the processes
on the surface of the sample and therefore define the compounds which have to
be measured in order to evaluate degradation ability. With the help of the
deployed chemiluminescence NOx analyzer the amounts of NOx and NO can be
quantified. Subsequently, the amount of NO2 can be calculated by difference
formation. Hence, a quantitative analysis can be conducted.
7. APPLICATIONS
Photocatalytic
cement is already being used for sound barriers, concrete paver blocks, and
façade elements. The progress in academic research significantly promotes its
practical applications, including the field of photocatalytic construction and
building materials. TiO2 modified building materials are most popular because
TiO2 has been traditionally used as a white pigment. The major applications of
TiO2 based photocatalytic building materials include environmental pollution
remediation, selfcleaning and self-disinfecting. The advantage of using solar
light and rainwater as driving force has opened a new domain for
environmentally friendly building materials. 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; and
· Cement-based restoration products.
Fig 7.1:Applications
7.1. PAVING TILES
Fig
7.2:Paving tiles
Evaluation of concrete pavements treated with titanium dioxide
provided promising results as recent research shows that a thin surface coating
is able to remove a significant portion of NOx, SOx, and VOC pollutants from
the atmosphere when placed as close as possible to the source of pollution. It
was reported that each square meter of titanium dioxide coating, subject to
sunlight, can remove nitrogen oxides and VOCs from about 200m3 and 60m3 of air
per day, respectively. The efficiency of this technology depends on the size of
the surface exposed, the concentration of pollutants, the humidity, and the
ambient temperature. Porosity of the surface is also important as the NOx
removal ability is improved as the porosity is increased. Photocatalytic
activity decreased by approximately 8% with aging of the surface but stabilized
at the age of 90 days. The deposition of pollutants on the surface was reported
to decrease efficiency of removal but it can be regained through the
self-cleaning mechanism.
Results of the experimental program showed that the
three contaminant types had a strong negative impact on the photocatalytic NOx
removal efficiency. The impact of contaminants’ coverage was largely dependent
on the soil type with oil having the largest negative impact. An increase in
the flow rate and air relative humidity also resulted in lower NOx
efficiencies. Hassan and co-workers also evaluated the environmental
effectiveness of a TiO2 coating to photo-degrade mixed NO2 and NO gases from
the atmosphere Results of the experimental program determined that an increase in the flow rate and NO2/NOx ratio negatively
affects the effectiveness of the photocatalytic process. However, the extent of
this impact depends on many other factors including flow rate.
A few studies
attempted to use TiO2 in asphalt pavements TiO2 was incorporated into asphalt
pavements as a thin surface layer to be sprayed on existing pavements. The
water-based emulsion was applied by two different methods (hot and cold)
distinguished by the spraying of the emulsion either during asphalt pavement
laying operations when the pavement temperature is over 100°C, or on existing
pavements at ambient temperatures.
The study results showed that reduction
efficiencies were highly dependent on the type of TiO2 nanoparticles used with
NOx reduction, with efficiency ranging from 20 to 57%. Meanwhile, researchers
in China mixed TiO2 with an asphalt binder, applying 2.5% content of the binder
weight to an emulsified asphalt . This study achieved a maximum efficiency in
removing nitrogen oxide near 40%. A more efficient approach may be achieved by
concentrating the photocatalytic compound at the pavement surface.
An important
issue is the conversion of the results, obtained in the laboratory to real applications.
The construction of a test section of 10.000 m² photocatalytic pavement blocks
as pilot project on the parking lanes of a main axe in Antwerp allows us to
search for answers on frequently asked question, like the amount of reduction
of NOx, the durability of the material not only for mechanical and aesthetical
properties, but also for the photocatalytic efficiency, the minimum surface
needed related to traffic density and the frequency of maintenance, i.e.
washing off of the surface by rain or by artificial spraying.
To obtain results
towards the reduction of e.g. NOx and on the durability of this reduction,
different measurement techniques may be applied. The use of the global
measurement sites is not appropriate, due to the limited surface covered by the
photocatalytic pavement blocks, compared to the overall surface. Local
continuous measurement of the NO and NO2 concentration demands a long period
during which the measurement has to be conducted, to eliminate the influence of
parameters such as wind, temperature, light intensity, traffic intensity and so
on. Although the presence of a reference lane allows comparing measurements,
the time needed to come to reliable results will be too long. Another method, which seems to be
more suitable, is the measurement of NO deposite on the surface of the blocks.
The NO and NO2 which is oxidized into NO3 is deposited on the surface. By washing
off the stones with distilled water and determining the amount of N in the
water, an idea of the minimum amount of reduced NOx can be obtained.
Furthermore, a regular measurement in the laboratory will be executed on blocks
from the surface to measure the possible reduction of efficiency under
controlled conditions.
First results indicate a reduction over time of the
efficiency of air purification of 20% after 1 year. The measurements will
continue over 2 more years to see the influence of aging, dirt, season.
7.2. BUILDING FACADES
Fig
7.3:Hydrophilic property of photocatalytic concrete
White cement is a
key ingredient in architectural and decorative concrete. By using it, in
particular, the resulting concrete not only becomes an expressive material that
having an infinite range of colour tones, intensifies one of its aesthetic
qualities, but could also gain remarkable validity in terms of structural
qualities due to its high mechanical strength.Indeed, a new type of white
cement is here proposed, containing TiO2, possessing photocatalytic properties
which allow to maintain the aesthetic characteristics of concrete over time and
contribute to eliminate dangerous pollutants from the urban environment. A
remarkable application is also described, concerning
14 the innovative construction of a church in Rome, named “Dives in
Misericordia” whose sails were built using white High Performance Concrete,
based on this new cement. Main physical and mechanical properties of this HPC
are described. Hydrophilic property of the photocatalytic concrete is the
underlying principle of the self-cleaning action. The super-hydrophilic reaction
has three processes. First is the absorption of a photon to produce an
electron/hole pair. The second is that instead of the hydroxyl and superoxide
being produced, the TiO2 surface is reduced and an oxygen vacancy is created.
The third is when oxygen in the air immediately oxidizes Ti3+. However, the oxygen bonds with a water molecule to form a
hydroxyl group on the surface. The creation of the hydroxyl group on the
surface acts as a chemisorbed water layer. When pollutants such as dirt, grit
and organics come into contact with the super-hydrophilic layer, they can then
be washed away with rain. There are two phenomenon involved in the degradation
process. One is the photo induced redox reaction and the other is the
super-hydrophilic conversion of TiO2 that takes place on the surface of TiO2
modified cement products, which make it a favorable method for the degradation
of organic pollutants. This phenomenon is also being used to reduce the
temperature of buildings which are being cooled by the latent heat flux caused
by the evaporation of the water on their surface This cooling effect was
observed in the trials.
7.3. RESTORATION OF MONUMENTS
TiO2-based suspension was
applied on travertine samples by spray coating obtaining two different
treatments: single layer (C1) and three layers (C2) coatings. Application
through spray coating was chosen due the simplicity and quickness of this
method and its compatibility with stone surfaces and with other restoration
techniques, an important prerequisite for application on Cultural Heritage
7.4. WATER PURIFICATION
Photocatalytic permeable concrete could be an ideal alternative
to low strength traditional pavement systems in urban environments, for the
incorporation with waste water management systems. A baseline was established
to determine the effect that the addition of TiO2 had on the permeable
concrete. It was demonstrated during the laboratory trials due to its particle size and density, that the
addition of TiO2 significantly reduced the workability and therefore compaction
of the Photocatalytic Permeable Concrete. This can be overcome by increasing
the water cement ratio with no discernible reduction in compressive strength.
An addition of 5% TiO2 has shown to have the least effect on the mechanical and
hydraulic properties of the concrete. Moreover, it also appears to be the most
effective dose rate for the degradation of organic pollutants and has with
minimal cost additions to the overall mix design. From the research it can be
concluded that a 20% void ratio design is most effective mix design for
mechanical and hydraulic properties but most importantly for the degredation of
poly-aromatic hydrocarbons present in simulated road runoff.The hydroxyl
radical and the electron holes have sufficient energy to oxidize organic
pollutants such as aliphatic, aromatics, detergents, dyes, pesticides and
herbicides.
According to the American Lung
Association, one out of every three members of the U.S. population lives in an
area with unhealthful levels of ozone (O3).1 The primary ingredient of smog,
ozone is an extremely reactive gas molecule that reacts chemically with lung
tissue, 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).
Ozone is created when the two
raw ingredients are combined in the presence of heat and sunlight. 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. 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 chloro benzene; and Pesticides such as diazinon and atrazine. The
final products of the reactions include harmless quantities of nitrates and
sulfates.
Fig 7.4:Atmospheric pollution
Studies have shown that these by-products
are negligible and do not contribute significantly to soil and ground water nitrification. Interestingly, it’s
expected that the best results from photocatalytic cements will be obtained in
the worst pollution conditions. It’s estimated that in Milan, Italy, where air quality standards
sometimes force local administrators to shut down automobile traffic for a full
day at a time, could become 50% cleaner if just 15% of the buildings and roads
were resurfaced with photocatalytic cement products. The product promises to
provide significant improve- ments in urban air quality, as it also can reduce
micro- organisms such as bacteria and fungi and is capable of eliminating odors
associated with pollutants. Residents of urban areas in Italy have reported
that some unpleasant odors have disappeared after the product was installed
nearby.
oad runoff waters are exposed to vehicle pollution. The main
pollutants are polycyclic aromatic hydrocarbons (PAH), mineral oils, and heavy
metals (Schipper et al 2007). Schipper et al (2007) conducted field trials for
13 months, examining road runoff and vehicle spray on two motorways. Their
findings showed that pollutants affected the top soils, ground water and
surface water, and that the concentration of contaminants over time exceeded
standard limits.Over the past decade there has been significant research and
development in the area of photocatalysts. One of the major applications has been photo-induced redox reaction and super-hydrophilic
conversion of TiO2 for its degradation of organic pollutants).
Chen & Poon’s (2009) paper on photocatalytic construction materials
discussed the application of titanium dioxide (TiO2) being added to
construction material products such as tiles, glass, concrete, and paints. It
was found that the products could be used for water and air purification, self
cleaning, and selfdisinfecting.
TiO2 has been used in the construction industry
traditionally as a white pigment. Although it is approximately 10 times more
expensive than cement, it is a similar cost to coloured concrete, per metre cube. Chen & Poon’s (2009)
research detailed how photocatalytic TiO2 affects organic pollutants and oxides
such as NO, NO2 and SO2 by the photo-induced redox reaction. When sunlight
strikes a particle of titanium dioxide, electrons within the particle become
excited, creating a higher state of energy within the electrons. The energized
electrons transfer energy to water in the air and form free radicals, •OH
(hydroxyl radicals) and O2- (superoxide anions). These free radicals are both powerful oxidizers that can attack any organic material either on the panel’s
surface or floating near it.
These cause ozone depletion, ozone is gradually
being destroyed by manmade chemicals referred to as ozone-depleting substances,
including chlorofluorocarbons, hydrochlorofluorocarbons, and halons. These
substances were formerly used and sometimes still are used in coolants, foaming
agents, fire extinguishers, solvents, pesticides, and aerosol propellants. Thinning
of the protective ozone layer can cause increased amounts of UV radiation to
reach the Earth, which can lead to more cases of skin cancer, cataracts, and
impaired immune systems. UV can also damage sensitive crops, such as soybeans,
and reduce crop yields.
Air pollution is a problem for all of us these causes
heart or lung diseases, long-term exposure to air pollution can cause cancer
and damage to the immune, neurological, reproductive, and respiratory systems.
In extreme cases, it can even cause death we must be careful about this.
Fig
8.1:Cleansing power
Of course, buildings are exposed to many organic contaminants.
From bird residue to diesel fumes, architectural building panels are constantly
exposed to organic material that makes their surfaces appear dirty. Yet there’s
another kind of organic material constantly bombarding buildings that is harder
to see: NOx (nitrogen oxides).
As the primary component of smog, NOx not only
makes buildings dirty, but it also threatens the quality of the air we breathe.
But when NOx molecules float near the surface of self cleaning concrete, they
are attacked by free radicals generated from the titanium dioxide reacting with
water and oxygen in the air. The free radicals oxidize the NOx molecules,
converting them to a harmless nitrate. In this way, self cleaning concrete
constantly works to remove pollutants by using sunlight and the water vapor and
oxygen in the air to clean the air itself. The factors which affect on the self
cleaning is environmental conditions.
A solution for the air pollution by traffic can be found in the treatment of
the pollutants as close to the source as possible. Therefore, photocatalytic
materials can be added to the surface of pavement and building materials. In
combination with light, the pollutants are oxidized, due to the presence of the
photocatalyst and precipitated on the surface of the material. Consequently,
they are removed from the surface by the rain.
Heterogeneous photocatalysis with
TiO2 as catalyst is a rapidly developing field in environmental engineering. It
has a great potential to cope with the increasing pollution. The impulse of the
use of TiO2 as photocatalyst was given by Fujishima and Honda in 1972. They
discovered the hydrolysis of water in oxygen and hydrogen in the presence of
light, by means of a TiO2-anode in a photochemical cell. In the eighties,
organic pollution in water was decomposed by adding TiO2 under influence of
UV-light. The application of TiO2 as air purifier originated in Japan in 1996.
A broad spectrum of products appeared on the market for indoor use as well as
for outdoor use. Heterogeneous photocatalysis with TiO2 as catalyst results in
a total mineralization of a broad gamma of organic compounds (alkanes, alkenes,
alcohol, pesticides) Further, it is possible to reduce NOx, bacteria, viruses,
The speed at which these reactions take place depends on the intensity of the
light, the environmental conditions (temperature, relative humidity), the
amount of TiO2 present at the surface and the adhesion of the pollutants to the
surface. In the case of traffic, it is important that the exhaust gases stay in
contact with the surface during a certain period. The geometrical situation,
the speed of the traffic, the speed and direction of the wind, the temperature,
influence the final reduction rate of pollutants in situ.
TiO2 is a metal,
which is multiply present in nature. The oxygen TiO2 has three different
molecule structures: rutile, anatase and brookiet. Rutile is known as pigment
in white paints, but shows up till now low photocatalytic reactivity. Anatase
is preferable if used as photocatalytic cell. To use anatase in heterogenic
photocatalysis, UV-light with a wave length lower than 387 mm has to be
present.
20 Also the intensity of the light is important to optimize the photocatalytic
activity. Normal daylight can be used for the photocatalytic reaction. Research
is focusing now on the application of nano-particles of TiO2, active in the
visible light range. Existing applications may be found in water purification,
air conditioning (air purification), self- cleaning glazing, ceramic tiles
(self-cleaning, antibacterial,…), textile (anti-odour), mirrors (anti-
condensation), tunnel lightning, white tents,… Besides the air purifying and
antiseptical action, where the pollutants are oxidized or reduced due to the
presence of the photocatalyst, TiO2 is also used to obtain a selfcleaning
material. This is due to a very high hydrophilicity of the surface when TiO2 is
activated by UV-light. The water layer is attracted between the dirt and the
surface resulting in the washing off of the dirt particles. This effect is more
pronounced with smooth surfaces like glass and ceramic tiles. In the case of
concrete surfaces, the selfcleaning effect will be more limited due to the
physical anchoring of the dirt in the larger pores. In addition, due to
photocatalytic working, a decomposition of the dirt particles, especially of
the organic particles takes place, followed by the washing of the surface
resulting in a cleaner surface.
PHOTOCATALYST
The increase in patents during the last decade indicates a huge
interest, especially from Japan and Europe, in the application of TiO2 as
photocatalyst in building materials. Regarding the reduction of air pollution
due to traffic in urban areas, the application on pavement surfaces or on the
building surfaces in cementitious materials gives optimal solutions. To increase
the efficiency of the photocatalyst, its presence at the surface of the
material is crucial. It has to be accessible by sunlight to be activated.
Consequently, the pollutant has to be absorbed on the surface and oxidized or
reduced to a less harmful element.
The goal is to have as much TiO2 as possible
at the surface of the material, without the risk of loosing it by abrasion or
weathering. Up till now, the most efficient way to apply the TiO2 is in a thin
layer cementitious material, which is placed on the surface. Application in
concrete tiles is therefore very suitable: the TiO2 can be added to the weathering layer. If the layer is slightly
used, new TiO2- particles will be present at the surface. Other applications can be found in
architectural concrete. The use of white cement with TiO2 at the surface of buildings and construction attribute to the
durability of the visual aspect of the building. Due to the photocatalytic
action, the whiteness of the building will remain and dirt will be washed away more
easily due to the hydrophilic properties or will be decomposed. The application
of photocatalytic panels at the facades of buildings is investigated in the
European PICADA project
11. DIFFERENT CONSTRUCTIONS USING PHOTOCATYLYST
CONCRETE
Fig
11.1:Dives in mescodia church Rome.
Fig 11.2: Minneapolis bridge
Fig 11.3:Air France Headquarters- Paris
Photocatalytic cement was
recently used to produce two 9 m (30 ft) 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’ll remain
proud symbols for decades to come.
12. CONCLUSION
This paper focuses on the application of TiO2 as photocatalytic material in building materials concrete pavement
blocks. The addition of TiO2 in building materials
adds an additional property to the road. Purification of the air, which is in
contact with the surface, is obtained when the surface is exposed to UV-light
(present in daylight). The measurements in the laboratory on photocatalytic
pavement blocks gave good results towards air purification, measured as NOx
reduction. The best results were obtained by high temperature (> 25°C), low
relative humidity, high light intensities and long contact times. This
situation is obtained on hot sunny days, without any wind, when the risk on
smog formation due to the high rate of pollution is the biggest.
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