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An inevitable byproduct of industrialization and modernization has been environmental pollution. The construction industry is often considered as one of the most polluting industries. Both the construction process and building use contribute significantly to the pollution of air and water resources. One of the most important challenges facing the construction industry in the 21st century is the development of construction materials that are both environment-friendly and sustainable. This has led to the development of photocatalysts which are compounds which with the absorption of light, facilitates a chemical reaction and is regenerated in the process. Research on use of photocatalytic material synergistically with construction material is under progress.
HISTORY OF PHOTOCATALYTIC CONSTRUCTION MATERIAL
A new vista has been opened for photocatalytic building material with the discovery of the photocatalytic activity of TiO2 in 1972. Research has shown that these materials with special binders are able to absorb NOx ions on the surface, transform them into non-noxious ions and block them in the form of salts (nitrates). These can then be removed by rain. A similar process has also been demonstrated with SOx pollutants. Another important spin-off is that ozone production is also inhibited and since ozone is known to cause a number of respiratory diseases, this is also a welcome consequence. While the first developments in this field were made by Italcementi and Mitsubishi in the mid 1990s, rapid progress has been made in this area to develop commercially available photocatalytic cement based materials.
The self-cleaning and anti-bacterial surfaces were the initial impetus for the photocatalytic material production. The production of this material was dominated by Japan in 1990s. Since the first official publication in 1997 by LCassar et al, the photocatalytic material usage has come a long way with several innovative solutions being proposed. The most famous application was the "Dives of Misericordia" church in Rome. Since the requirement was prolonged maintenance of colour over time, a number of trials on concrete mixes were conducted from 1997 and the church was inaugurated in 2002. The trend of luminescence measured shows a slight decrease from the benchmark value in the year 2000 of average 91.8 to around 88.5 while the yellowness value shows an increase from 2.5(2000) to 7.5(2009). Two other important buildings built from photocatalytic building material are a school in Mortara, Italy which was completed in 1999 and a city hall, Cite de la Musique in Chambery, France which was completed in 2000.
Among the most active research groups in this field is the PICADA (Photocatalytic Innovation Coverings Applications for Depollution Assessment). Partially funded by the European Union and partnering with companies and Universtities across Europe, they have produced a number of photocatalytically active materials including the TxActive brand by the ItalCementi group. Also it is estimated that the photocatalytic surfaces in Europe will be around 1.5 million m2. With optimisation of the products in terms of composition and performance, it is expected that the price of the photocatalytic cements will also reduce.
Currently research is in progress to develop photocatalytic cement based material which has properties to satisfy the expected service life and durability.
METHODS TO PREPARE PHOTOCATALYTIC CONSTRUCTION MATERIALS
One of the common methods adopted include fixing TiO2 and other photocatalytic material on building materials is the spray-coating technology. In this method, aqueous or methalonic suspension is sprayed on the surface of the construction material. The amount of the photocatalyst which should cover the specific area can be easily regulated in this method. The solvent is removed after spraying by heating the sample to around 100oC.
Another method adopted is the sedimentation technology. In this method, the sample is kept in photocatalytic suspension for a defined time. The suspension is then slowly drained and similar to the spray-coating technology, the solvent can be removed by heating to around 100oC.
APPLICATIONS OF PHOTOCATALYTIC CONSTRUCTION MATERIAL IN AIR POLLUTION
DECOMPOSITION OF ADSORBED ORGANIC SUBSTRATES
Discolouration of building surfaces can be attributed to accumulation of organic atmospheric aerosol pollutants. The hydrocarbon and fatty acids groups present in these organic compounds help the particles adhere to the building surfaces. While the -COOH groups in fatty acids chemically combine with the Ca groups in the concrete and thus stick them to the surface of the buildings, the long chains in the fatty acids molecule stick link to the other hydrophobic surfaces resulting in long chains which can trap dust and particulate matter.
The red-ox reaction takes place
Eg palmitic acid
Synergistic effect of photocatalysis and super-hydrophilicity promotes self-cleaning. The hydroxyl radicals are important for decomposition of the organic matter. Since hydroxyl radicals are also produced due to the super-hydrophilic nature, this will result in increase in efficiency of degradation by photocatalysis. Also adsorption of the organic compounds will lead conversion from hydrophilic to hydrophobic surface, so degradation can lead to restoration of the super-hydrophilic property.
Dives of Misericordia - surface has remained white after 6 years with only a slight difference between the internal and external values of lightness observed. Furthermore, washing with water can eliminate this colour variation also.
Another important photocatalytic building material based structure is the city hall at Chambery. The primary colour of this structure has also remained constant for 5 years.
SELF-CLEANING CEMENTITIOUS MATERIALS BASED ON TiO2
White ceramic tiles by TOTO Inc. for exterior surfaces - Liquid suspension containing TiO2 powder or gel is sprayed and heated to temperature between 600-800OC. The heat treatment sinters and completely attaches the TiO2 to the surface of the tiles resulting in micrometer thick layers. The performance was proved by hanging them exposed to the environment for 6 months.
Interior tiles - The fatty acids from soap form bond with Ca and Mg from hard water and stick to the tiles. The TiO2 can break these bonds thereby making the washing process easier.
SELF-CLEANING GLASS - steam can cool on the surface of the glass resulting in the fogging on the surface of the glass. The hydrophilic property results in the formation of a water sheet (so the view is not obstructed) also during drying, it dries without leaving droplet marks on the surface of the glass. It is therefore necessary to have a TiO2 coating which is not just highly active but also should not affect the optical properties of the glass. Thus the photocatalytic activity of such a TiO2 film depends on the calcination temperature, flow rate of the carrier gas and the partial pressure of the starting materials during the fabrication process.
DURABILITY - High temperature is necessary for sintering and stabilising the TiO2 however PVC is not tolerant to high temperatures.
The organic building materials are decomposed because of the photocatalytic activity, resulting in disruption of the strength and structure. Therefore, it may be necessary to place an intermediate layer between the photocatalytic coating and the organic building material leading to increase in costs and difficulty in manufacturing.
TiO2 coated paints directly on the surface of buildings have poor durability. A 5.5 years outdoor exposure tests have shown that most paints made with organics lost their photocatalytic ability.
Chalking problem due to decomposition of organic binders.
For concrete, the use of organic admixtures and other cementitious organic materials should be minimized as it can affect photocatalytic activity. The dust can anchor to the large pore spaces available in the concrete making self-cleaning difficult.
SELF-DISINFECTING BUILDING MATERIALS
Two mechanisms have been proposed for the biological inactivation of the illuminated TiO2. One is the death of microorganisms are caused due to attack by the chemical species.
Studies containing TiO2 photocatalysts have higher efficiency and are capable of responding to visible light when TaON, In1-xNixTaO4 and quantum yield of TiO2 was improved with La-doped NaTaO3.
TIO2 PHOTOCATATLYST COATING FOR CONCRETE PAVEMENT
By integration of nano-particles of TiO2 in the concrete pavement surface, the self-cleaning concrete pavement surface can be got. TiO2 can be blended with cementitious mortar mixture and applied as an ultra-thin coating on the concrete pavement surface. While the environmental friendly nature is ensured, the durability and abrasion and wear resistance of such surfaces needs evaluation. So accelerated loading test and rotary abrasion were measured using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) techniques. Delamination and adhesion failure through tires action were used to measure the abrasion properties.
Nitrogen oxides by heterogeneous photocatalytic into water soluble nitrates
Sulfur dioxide by heterogeneous photocatalytic into water soluble sulfates
So can be washed away by rainfall
TiO2 with hydrophobic properties are found to be better for pavement applications. In this fashion, the contaminants can adhere to the water droplets during rain and can get removed from the surface.
TiO2 as an air purifier in urban and metropolitan areas are preferred and can be used in thin film form for coating and slurry. Tiocem, white cement, facades for buildings and tiles are other common applications.
TiO2 as a cementitious mixture improves SO2 removal efficiency through the action of the alkaline substratum.
When placed close to the source of pollution, the removal efficiencies of TiO2 against NOx, SOx, VOCs in the atmosphere is good. The efficiency depends on the size of the surface exposed, concentration of the pollutants, air humidity and the ambient temperature. Porosity of the surface is also important because with the increase in porosity, the NOx removal ability also increases. Deposition of pollutants on the surface decreases the efficiency of removal while self cleaning can restore the efficiency. The technology however is confined to non-structural elements because investigation is needed before it is applied to large-scale projects.
The aim of the experiment was to test the abrasion and wear resistance of different TiO2 coatings used for environmental pollution. Standard concrete mix design with compressive strength of 41 MPa was prepared. 3 samples for each test with dimension 305 mm x 381 mm x 25.4 mm were prepared and cured with curing compound for 7 days. The surface mixture consisted of ultrafine titanium with sand, water and cement. The filler particles were maintained above 300microm, because previous studies showed that less fine particles result in higher porosity, which improves NO removal efficiency. The water cement ratio was 0.6 and was applied as a coating of 10 mm thickness. 2 concentrations of 3% and 5% of TiO2 nanomaterial were used. The Japanese equivalent of ISO standard was followed. A Teflon reactor was used inorder to accommodate sample for durability testing. Reactors' dimensions were kept constant to ensure laminar flow. The light intensity and air humidity could be varied and the pollutants were introduced through an inlet jet stream. The average temperature maintained was 26.5 +1.3oC. Fluorescent lamps were used to simulate radiation similar to that of sunlight. Chemiluminscent NOx analyser was used to measure the recovered air before and after the photocatalytic device. The results obtained correspond to temperature of 23oC, relative humidity 50%, with nitrogen monoxide concentration being 410ppb and flow rate of 9L/min. The testing time was 5 hrs with photocatalytic process being started only after first 30 min to ensure that the concentration reached a steady concentration in the environmental chamber. The NOx removal efficiency was calculated using formula given below:
Wear and abrasion tests were done using accelerated loading test and rotary abrasion. A Hamsburg-type-load-tester with load of 702N with frequency of 56 passes per min, which passes back and forth on the specimen was used. After 20,000 passes, the experiments were stopped and Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) techniques were used to study the samples. The surface rutting was monitored with a maximum of 6mm according to LADOTD. Only macro damage was measured. Rockwell freestanding drill was used to study the abrasion resistance and the weight loss measured. A constant load of 98N for 2 min with cutter rotating at 200rpm was used for the purpose.
The SEM and EDS was used to compare the distribution of TiO2 before and after the tests and samples of 25mm x 25mm were cut for the purpose.
The results obtained are tabulated below:
Rut depth measurement
< 1 mm
Maximum rut depth is 6mm after 20,000 cycles on asphalt surface, so use of coating does not affect wear resistance of surface
Coating with 3% TiO2
Coating with 5% TiO2
Loaded Wheel Tester (LWT)
Wearing of surface and loss of particles associated with loss of mortar or nano- TiO2 particles
Coating with 3% TiO2
Coating with 5% TiO2
Rotary abrasion test (RA)
Coating with 3% TiO2
Coating with 5% TiO2
The NO removal efficiency was tested for the normal samples, samples subjected to LWT and RA. The table below summarizes the removal efficiencies which is the average for 3 samples:
NO REMOVAL EFFICIENCY (%)
Coating with 3% TiO2
Coating with 5% TiO2
It can be seen that wearing resulted in a small percentage decrease for the 5% TiO2 while it increased the removal efficiency for the 3% TiO2 concentration. The SEM analysis showed an uniform distribution of the TiO2 particles for a coating of 5% TiO2. This is good since it ensures maximum exposure of the nano-particles on the surface for maximum NO removal efficiency. From the results, it can be seen that relative concentration of TiO2 on the worn surfaces did not substantially change compared to original samples.
PHOTOCATALYSIS OF CEMENTITIOUS MATERIALS: CLEAN BUILDINGS AND CLEAN AIR
TiO2 surface exposed to near-UV light has been found to have amphiteric property, i.e., it exhibits alternate distribution of hydrophilic and oleophilic submicrometer-sized regions which makes it extremely useful in anti-fogging as well as other self-cleaning applications.
The main reason for discolouration of buildings is due to accumulation of organic compounds on their surfaces with a system consisting of TiO2 and cement, this problem could be alleviated.
White cement containing TiO2 photocatalysts
Construction of school in Mortara, Italy - completed in 1999
Cite de la Musique in Chambery, France - completed in 2000
Cementitious paints containing photocatalysts have also been developed and applied in residential buildings in Italy.
The most important aspect of TiO2/cement is that there is an excellent synergy between the two. Many photo-oxidising compounds like NO2 and SO2 are acidic while the cement matrix is suitable for fixing the pollutant and photo-oxidation products at its surface. Also nanometric TiO2 increases the photocatalytic activity of the TiO2/cement system.
In Segrate, Italy, photocatalytic mortar was applied to 220m road of surface area around 6000m2 and vehicular traffic rate of 1200veh/hr. This is one of the largest experimental tests and the NOx reduction rate is around 50% on a sunny summer day when wind speed recorded was around 0.7m/s and brightness greater than 90,000lux. While it was proved that photocatalytic properties of mortar were maintained for atleast 1 year, the longer duration tests are currently under progress.
PHOTOCATALYTIC CEMENTITIOUS MATERIALS - SITUATION, CHALLENGES AND PERSPECTIVES
HISTORY OF PHOTOCATALYTIC CEMENT BASED MATERIALS
With many advantages for the integration of photocatalysts in building materials, many cement based construction materials like concrete, cement, paints etc. have been developed. The use of TiO2 in its anatase form is mainly due to its
Strong oxidising power
Chemical stability when exposed to acidic and basic compounds
Chemical inertness in the absence of UV light
Low cost in comparison with other photocatalysts
NOx, aromatics, aldehydes and ammonia
In comparison with other photocatalytic building materials, incorporation of TiO2 are able to absorb NOx on the surfaces and transform them into non-noxious ions and block them as nitrates and remove them by means of rain or washing. A similar mechanism is observed for SOx also. In addition, ozone production is also contained and this is also an advantage.
With growing awareness of green technology and importance given to sustainability, these building materials are being widely recognized because of their aesthetic qualities and environmental benefits. Products like the TXActive have an enormous growth in the near future. LEED (Leadership in Energy and Environmental Design) is a worldwide certification which gives points for using environmental friendly techniques during various stages of design, construction and during use. Use of photocatalytic building materials, like usage of TXActive will earn points upto 9-15 against a maximum possible of 69 under different categories like sustainable site, innovation and design process, materials and atmosphere and energy and resources.
URBAN HEAT ISLAND MITIGATION
Urban heat island effect, where in a metropolitan area is significantly warmer than its surrounding rural areas is a growing concern among scientists. Research has shown that increase in temperature is due to black or dark surfaces and this condition could be limited by use of light roofs and pavements. The photocatalytic properties of these materials can futher help reduce the intensity of environmental pollution. For instance, it is found that pavements, parking areas, sidewalks also represent 30-60% of the developed urban areas, if they can be transformed into environmental friendly surfaces during their periodical maintanence it would be a good trend. The pavement type should take into account the albedo or the surface reflectance since it is directly related to heat generation. The use of white photocatlytic building material in alleviation of the urban heat island effect is apparent but it also has secondary effects like reduction in smog produced since the kinetics is dependent on the temperature and therefore production of noxious substances is also reduced.
Both qualitative and quantitative standards have to be established in order to test these new photocatalytic building material. In development of indoor systems, suitable lighting should also be developed. Doped TiO2 (C doped, N doped) seems to have achieved some progress in this direction but still some doubts on their efficiency in cementitious products in long term applications persists. "Clear-up", an European funded project, is being carried out by Italcementi to improve the indoor air quality. Also the antibacterial properties with its many potential applications also is under research.
PHOTOCATALYTIC CONSTRUCTION AND BUILDING MATERIALS FROM FUNDAMENTALS TO APPLICATIONS
USE OF TiO2
-rel inexpensive, safe and chemically stable
-high photocatalytic activity compared to other metal oxide photocatalysts
-compatible with traditional construction materials without change in photocatalytic activity
-eff under weak solar radiation in ambient atmospheric environment
The versatility of TiO2 facilitates its use in exterior construction (tiles, glass, tents, etc), interior furnishing (tiles, wall paper, internal blinds, etc) and as road construction materials (sound proof walls, tunnel walls, concrete pavements etc) because of exhibiting properties like self-cleaning, air-cleaning and anti-bacterial cleaning among others.
First use of TiO2 in pollution abatement was with the discovery of its ability to decompose cyanide in wastewater.
The different pure or doped metal oxide semiconductors can be used as photocatalysts. These photocatalysts generate strong oxidising (h+) and reducing agents (e-) which are capable of destroying many organic contaminants completely. Sometimes intermediates are detected in the process. These intermediates can occupy the active sites of the catalysts and hence decrease the activity of the catalyst or even introduce new pollutants in the system. It is found that the reaction depends on a number of factors such as characteristics of the photocatalyst, humidity, reactor type, light source etc.
Japan has applied coatings of TiO2 in cities such as Osaka, Chiba, Chigasaki etc. Atleast 50,000m2 of surface area in Japan has been coated with such surfaces and the removal rate ranges from 0.5 to 1.5mmol per m2. Observation has shown very little performance detioration. Also the coating has been used in highway sound barriers to reduce NOx pollutants and provide a self-cleaning effect. (1)
Highway case study : From BOOK
Among the most active research groups in this field is the PICADA. Partially funded by the European Union and partnering with companies and Universtities across Europe, they have produced a number of photocatalytically active materials including the TxActive brand by the ItalCementi group.
They compared a 10mm thick mortar rendering (mixture of cement lime and sand) and a 1mm thick mineral paint (mixture of cement and fillers) which were compared with a standard concrete mix, CEN mortar. It is reported that both samples contained the same amount of titanium dioxide, in the anatase form, with an average crystallite size of 20nm. They were exposed to 300ng/L nominal concentration of Benzene-Toluene-Ethylbenzene-O -Xylene (BTEX). While the 10mm-thick mortar rendering showed high photocatalytic activity of 2.2 m/h for toluene and o-xylene and was better compared to the mineral paint also. Another set of experiments were conducted for 2 photocatalytic paints which were to be used for cementitious materials. The first paint contained 3% TiO2 used for aesthetic surfaces while the second was a transclucent paint Millenium Chemical Co. It was a water based product using siloxanes as binders with 10% TiO2. From macro experiments in chamber of 30m3, at 30oC and RH 50% and radiation 2.1W/m2, degradation rate was 0.21g/m2/s for mineral and 0.06g/m2/s for transclucent paints.
The paints were also tested in the real world scenario at a closed car park of volume 917m3. The ceiling was painted and the surface area was 322m2. A car was used to provide the pollutants while UV lamps were provided. The paint contained 10% TiO2. The NO and NO2 concentrations were measured. While labtests at the same irradiance and same paint showed complete destruction of NO, the real life sceanario showed different results. This was attributed to the fact that the car produced not only NO but other volatile organic compounds (VOC) and since TiO2 can degrade those also, some reactive sites in the TiO2 were occupied by the other pollutants. The average NO and NO2 removal were around 19% and 20% respectively.
The Houston Advanced Research Centre is also actively working on using TiO2 surfaces for degrading NOx and VOC gases. In the experiments, concrete slabs with titanium powder were used. NO was pumped into the reactive chamber and the outlet concentration of NO was measured, this allowed only a small interval for it to degrade NO gas.
SELF CLEANING EFFECT OF TIO2 SURFACES
Cite de la musique, Chambery, France.
TiO2 performs under direct sunlight but it is only a small portion of the light with which it reacts. It requires light with wavelength less than 388nm, which is only 5% of the visible light spectrum. It is therefore not enough to increase the effectiveness of TiO2 but also expand the range of light with which it reacts. The manner in which the surfaces are produced and the pollutants against which they are tested also varies greatly so it is difficult to judge the performance and the modifications necessary. Chromium and vandanium doping has proved that significant improvement in the properties while recently co-doping, including more than 1 ion doping using bromine and chlorine has been done.
A number of byproducts are formed during the photocatalytic reaction using TiO2. The oxidation of NO to NO2 and formation of ozone and carbon monoxide is of importance. NO2 if oxidised to nitrate salts are mostly harmless but O3 is a highly toxic gas. NO photocatalysis yields NO2 and some ozone. NO2 photocatalysis also yields O3 and some NO. NOx and VOC are responsible for tropospheric ozone so even if the reaction yields ozone, it could still result in an overall lower ozone content.
The typical grey colour of ordinary Portland cement is due to presence of ferrous and other heavy metal composites. By ensuring that they do not exceed 0.15%, white clinker can be produced. With a lot of research, white cement produced is not only aesthetically appealing but has high performance characteristics.
Among the efforts made to use photocatalytic materials for mitigating environmental pollutions, the first ones were to maintain the aesthetic quality of structures using photocatalyts. Addition of suitable amounts of TiO2 not just maintained the appearance over time but was able to do so even in urban environments with significantly high pollutant concentrations. This is especially true for white cements. The main reason for colour change is the organic pollutants that remains on their surface. The efficiency of the TiO2 photocatalyst on the surface of white cement with phenanthroquinone was studied. For this, white cement of 2mm thickness was placed supports in the form on discoid of diameter 3.2cm and thickness 7mm and containing 5% TiO2 (P-25 Degussa form). The phenanthroquinone in methanol solution was deposited by aerograph technique and amount was 0.1g/cm2 and showed a uniform surface of yellow colour. A Perkin Elmer spectrometer was used to measure the reflectance both before and after the phenanthroquinone deposition. An integrating sphere was fitted to eliminate scattered light for light anisotrophy and surface irregularities while solar flow simulator provided light with wavelength greater than 290 nm with lights being set up such that simultaneous irradiation of many samples with same amount of photons/unit time was possible. The device allowed accelerated ageing with 100 hours of irradiation equal to 1 year of sunlight. The percentage reflectance as a function of wavelength was obtained at 3 different times. One before the deposition, second was after deposition of phenanthroquinone while the third was after 8 hours of exposure. The results showed that the situation after 8 hours of irradiation was similar to one before application of the pollutant.
PHOTOCATALYSIS AND REMOVAL OF NOX
Reductions by hydrocarbons or ammonia
Two step adsorption-reduction or thermal destruction process
Mixed catalysis using TiO2 mixed with absorbents, like zeolites, have very high efficiency for NOx elimination because of the high absorption capacity of zeolites, this compensates the low adsorption capacity of TiO2. Another development is TiO2 dispersed in the cementitious matrix, this composite system has important practical implications. Research on concrete containing TiO2 has shown that it has an excellent potential in the area of pollution control. The mechanism of NO oxidation on exposure to light is not simple, because nitrogen dioxide is formed as an intermediate which is finally converted to nitrate. NO2 can escape from the photocatalytic surface. However, the cementitious matrix entraps the gas.
Tests containing 5% TiO2 by cement weight with cement and cement matrix without photocatalyst were conducted. The tests were carried out in the dark and in UV light. The results are summarized in the table shown below:
NOX RESIDUE (%)
Cement + 5% TiO2
UV Light for 7 hour exposure
Cement + 5% TiO2
Under dark, the decrease is due to adsorption alone, while UV light initiates the photocatalytic reaction. The removal by photocatalysis with cement is higher than cement alone. However, under light, the cement itself displays some photocatalytic behaviour due to the presence of some photocatalytic matrix inside the cementitious matrix.
MIX DESIGN OF WHITE CEMENT
The mix design takes into account both the aesthetics or surface finish and strength or structural aspects. While the components such as cement, aggregates and water, for high performance concrete the microstructure changes to give low porosity, high compacity, good interfacial binding such that strength and durability are good.
Aggregates chosen should be clean, free from impuritites, dust etc. Contamination with other materials should be avoided. The coarse aggregates should be chosen to be uniform in colour, while for fine sand should be extremely light sand for perfect white colour while coloured sand is sufficient for other purposes. Use of white cement makes the concrete far more brilliant than the colour of aggregate. The colour tones depend on the brilliance of the sand, the tone of the sand (can range from pink to blue) and colour percentage. From experiments, it has been proved that white sands may not be necessary to make white cement concrete.
Mineral admixtures are added to concrete to improve their strength, durability and ensure good workability. Fine pozzolanic material are preferred for white cement concrete and should be white so generally metakaolin are only considered since white silica fume or other compounds may be very expensive. It accelerates the hydration greatly in the first 24 hours, because of its small size (1.5 micron), it acts as filler. Also exhibits pozzolanic reaction between 7th and 28th day. Shrinkage, water permeability, bleeding and efflorescence are also controlled apart from improving paste/aggregate bond.
Water reducing admixtures are added since water/cement ratio for high performance cements should be maintained between 0.20-0.35.
PHOTOCATALYTIC CEMENTITIOUS MATERIALS: INFLUENCE OF THE MICROSTRUCTURE OF CEMENT PASTE ON PHOTOCATALYTIC POLLUTION DEGRADATION
EFFECT OF CEMENT TYPE
Samples prepared with white cement showed higher photocatalytic activities than those prepared with normal Ordinary Portland Cement (OPC). This could be due to the variation in the light absorption characteristics of the different cements. The gray colour of OPC is due to the presence of transition metals such as iron and manganese. Two possible explanations have been proposed for this phenomenon. One is that transition metals could have absorbed or blocked part of the radiation that can otherwise be used for photocatalytic reaction. The second one is that the transition metals contributed to rapid electron-hole recombination thereby resulting in reduction in the photocatalytic activity. Research has shown that metals can create acceptor and donor surface centres that behave as recombination sites for electrons and holes and therefore the photocatalytic rate is reduced for surface modified TiO2.
A decrease in the NOx removal is observed with increase in the curing age. As the curing age increases, the microstructure becomes denser. During the hydration process, the dense gel structure filled the available voids. The increase in porosity corresponded to the capillary pore range (10-50 nm) and this was the reason for decrease in NOx absorptivity. Fine TiO2 acts as nucleation sites for reaction products, however CSH coats around the TiO2 particles. This TiO2 is similar to unreacted cement and is surrounded by CH, CSH and capillary pores.
Cement hydration occupies the sites on the reactive sites on the TiO2 crystal surface. Comparing Anatase and P25, P25 loses more reactive sites than Anatase because of its large surface area. The cement paste containing P25 lost its photocatalytic activity more than Anatase as the hydration progressed.
The photocatalytic reaction depends on a very complex interaction between the photocatalysts, reagents and photons. The controlling factor could be the kinetics of the reaction, the transport of the reagents or the transport of the radiant energy or combination of all these. The photocatalytic rate also varies with the light intensity gradients caused by variations in the light absorption concentration gradients and diffusion resistance. With time, CSH coatings thickened and formed diffusion barriers to both reactants and photons and continuous accumulation blocked the active sites in TiO2.
TiO2 IN CEMENT PASTE
MDI Jade 5.0 was used for identification of chemical composition and integration of corresponding intensities. It is observed that there is no significant mass change of TiO2 which means that TiO2 was stable and chemically inert during the reaction. Also from TG studies, it can be seen that TiO2 does not influence the amount of Calcium hydroxide produced so it is evident that TiO2 does not exhibit pozzolanic activity.
EFFECT OF CARBONATION ON NOx PHOTODEGRADATION
When compared to normal samples, samples subjected to accelerated carbonation showed a significant loss of photocatalytic activity. While Test on 56 day normal samples showed around 355Âµmolm-2h-1 NOx removal while those subjected to accelerated carbonation showed around 310Âµmolm-2h-1. Also while the normal samples exhibited almost the same removal rate after 56, 90 and 120 days, the carbonated samples showed progressive decrease. The removal rate after 90 days was while removal rate after 120 days was 300Âµmolm-2h-1 and 250Âµmolm-2h-1 respectively. Conversion of Ca(OH)2 to CaCO3 which is accompanied by an increase in volume of 11% is the reason attributed to this decrease in photocatalytic activity. There is a reduction in the total porosity because of the shift of pore size distribution among smaller ones. Also CaCO3 deposition in the pores results in reduction in the diffusivity of NOx and therefore reduction in the photocatalytic rate.
The photocatalytic process depends not only on the photocatalytic activity of TiO2 but also on the components of the cement paste and its hydration process which influences the removal rate of NOx.