Category Eco-efficient construction and building materials

Several European associations of the concrete industry (BIBM, ERMCO, UEPG, EUROFER, and CEMBUREAU EFCA), in collaboration with the Dutch envi­ronmental consultant INTRON BV studied the possibility of minimizing the environmental impacts of concrete elements. One of the objectives of this study, was to develop the tool EcoConcrete, in order to evaluate the environmental impact associated with a particular element of reinforced concrete (Schwartz – entruber 2005). Some authors (Gerrilla et al. 2007) compared the performance of houses built with wooden and concrete structures, reporting that the latter had an overall environmental impact only 21% higher than the former. Xing et al. (2008) compared the performance of two office buildings with different structures (reinforced concrete and steel) and found that the steel structure consumes 75% energy compared to the concrete structure and is responsible for half of the emissions GHGs, however, in operational terms the concrete structure exhibits a much lower energy consumption having an overall favorable environmental per­formance. Marinkovic et al. (2010) studied concretes with and without recycled aggregates and found that their environmental performance is dependent on the transportation distance, regardless of whether they are recycled or not.

Under the project Beddington Zero (Fossil) Energy Development (BedZED), 82 households and 3,000 m2 of commercial or live/work space with low envi­ronmental impact were built in South London. The choice for the construction and building materials in the BEDZED project was made using the BRE. Envest eco-points system (Figs. 11.5, 11.6).

Desarnaulds et al. (2005) also used the BRE. Envest eco-points system to compare different sound insulation materials mentioning that the best environ­mental performance is associated with recycled paper, followed by rock wool and finally by polystyrene. Nicoletti et al. (2002) showed that ceramic tiles have an environmental impact throughout its life cycle that is over 200% higher than the environmental impact of marble tiles. These results are confirmed by more recent investigations (Traverso et al. 2010). Jonsson (2000) assessed the environmental performance of three floor covering materials using six different approaches:

• An LCA

• An Eco-label (The Swan)

• Two eco-guides (EPM and the Folksam Guide)

• An EPD

• An environmental concept (Natural Step).

Waste disposal Water extraction H Minerals extraction Щ Fossil fuel depletion Eutrophication to water Щ Ecotoxicity to water

■ Human toxicity to water h Photochemical ozone m creation potential

Human toxicity to air

Ozone depletion

■ Acid deposition Climate change

□ Waste Disposal

Fig. 11.6 Comparison of the environmental profile of different framed windows (BEDZED2002)

The results showed that while the LCA considers all environmental impacts in a similar way, some forms of sustainability assessment allow prioritizing certain impacts, either during production the phase or during the application of the material in the building. The results also show that only the LCA and the eco­guides allow the development of product rankings. Regarding the aggregation of the results, the eco-label has the best performance and the EPD gets the worst, making it difficult to understand the performance of a particular product.

11.2 Conclusions

Although LCA is the most appropriate way to scientifically evaluate the envi­ronmental performance of a given material, it is very time consuming and has some uncertainties. Besides the success of LCA is dependent on the existence (in each country) of lists on the environmental impacts associated with the man­ufacture of different materials and of the different construction processes. Another drawback of LCA is the fact that it does not take into account possible and future environmental disasters associated with the extraction of raw materials. This means that for instance the LCA of the aluminum produced by the Magyar Aluminum factory, the one responsible in October 2010 for the sludge flood in the town of Kolontar in Hungary, should account for this environmental disaster. Similar considerations can be made about the construction materials that were processed or transported using oil extracted from the Deepwater Horizon well in the Gulf of Mexico. Or even about the materials that were processed using the electricity generated in the Fukushima nuclear power plant. Only then construction and building materials will be associated with their true environmental impact. As for eco-labels they allow a more expedient information for a particular envi­ronmental performance, although its value is dependent on the entity and the assumptions that were on the basis of its allocation. Although eco-labels exist for almost 30 years, its use is still neglected by the construction materials market. In fact only a tiny fraction of the current commercial construction materials already have eco-labels. The emphasis in the respect for environmental values will lead to an increase in the number of material producers using eco-labels as a means of differentiation. As regards EPDs they have disadvantages similar to LCA, so it is not expected that in the coming years there may be an accelerated growth of products with EPDs.

Eco-labels were created to favor the choice of products with enhanced environ­mental performance and provide a guarantee for a certain environmental perfor­mance certified by an independent entity. Since they are quite simple and their meaning is unambiguous these labels have obvious advantages when compared to LCA. Although the advantages of eco-labels are clear, it is important to understand the specifics of the environmental performance in which they are based. Some authors warn that the validity of eco-labels could be in jeopardy if their envi­ronmental requirements could be influenced by producer lobbies (West 1995; Ball

2002) . On the other hand since the environmental performance of a product or material must include their transportation impacts, there is no way the eco-label can include this impact. So using a particular construction or building material with an eco-label, produced thousands of miles away from the location site, could

Fig. 11.1 Symbol of the German eco-label ‘‘Blue Angel’’

Fig. 11.2 Symbol of the Canadian ‘‘EcoLogo’’

be less preferable than the use of local materials, even without that eco-label. Most eco-labels are based on an assessment of the environmental impacts throughout the lifecycle of the product or material in the version ‘‘cradle to grave’’. Germany was the first country to establish in 1978 a labeling system based on environmental criteria with the designation of ‘‘Blue Angel’’ (Fig. 11.1).

Currently, the eco-label ‘‘Blue Angel’’ is applied on 11,500 products covering 90 different categories. This classification means the efficient use of fossil fuels, the reduction of GHG emissions and the reduction of the consumption of non-renewable raw materials, being reviewed every 3 years. The contruction and building materials that already received this label are the following:

• Bituminous coatings

• Bituminous adhesives

• Materials based on glass wastes

• Materials based on paper wastes

• Plywood panels

• External thermal insulation composite systems—ETIC’s

• Thermal and acoustic insulation materials

• Wood panels with low VOC emissions

• In 1988 Canada established the label EcoLogoTM (Fig. 11.2), and currently almost 7,000 products are certified by it, including the following construction and building materials:

• Adhesives

• Paints

Fig. 11.3 Symbol of the Nordic eco-label ‘‘The Swan’’

• Varnishes

• Corrosion inhibitors

• Floor coverings

• Gypsum plaster boards

• Recycled plastic plumbing

• Thermal insulation materials

• Steel for construction.

The use of the EcoLogo implies the respect for a set of environmental proce­dures dependent on each product. For instance, gypsum boards certified with this label must contain a certain percentage of synthetic gypsum and 100% of recycled paper. In the case of construction steel with the EcoLogo it must contain 50% recycled materials, less than 0.025% of heavy metals and has even to meet a series of environmental requirements during the extraction and production phases. In 1989 the countries of Northern Europe (Finland, Iceland, Norway and Sweden, Denmark only in 1998), created the eco-label ‘‘The Swan’’ (Fig. 11.3). ‘‘The Swan’’ covers 5,000 products of 50 different areas, as below with regard to the construction and building materials area:

• Wood

• Wood panels

• Filling materials

• Materials for floor covering

• Paints and varnishes

• Adhesives

• Windows and doors.

The European ‘‘Eco-Label’’ was created in 1992 (Fig. 11.4), is a system for a voluntary environmental classification for products with low environmental impact throughout its life cycle. The Eco-label applies to a large variety of products with the exception of food, pharmaceutical, medical and hazardous products and like ‘‘Blue Angel’’, involves a periodic review after 3 years. Concerning the construction and building materials, only paints, varnishes and hard floor covering

Fig. 11.4 European Eco-label

materials (tiles, natural stones, concrete, ceramic and clay) are already covered under this label:

• Interior paints and varnishes (2009/544/EC)

• Exterior paints and varnishes (2009/543/EC)

• Hard floor coverings (2002/272/EC; Baldo et al. 2002).

The documents related to the certification of paints and varnishes (Ecobilan 1993) show that its LCA, assessed the following environmental impacts:

• Global warming potential (COeq)

• Potential for atmospheric acidification (increase acidic substances in the lower layers of the atmosphere)

• Eutrophication potential (excess of nutrients from agricultural fertilization)

• Non-renewable resource depletion.

Regarding the hard floor coverings the European Eco-label means that:

• The environmental impacts during the extraction of raw materials were minimized

• During the production phase there is a reduction in overall pollution

• Possible recycled materials were used

• The ceramic tiles are burned with a reduction in the firing temperature.

Eco-labels are advantageous to the final consumer (Kirchoff 2000), however its effectiveness is dependent on the knowledge that consumers may have about their existence and some surveys made in the European Union, indicate that the European eco-label is not well known. In addition to eco-labeling there is another form of environmental certification for construction and building materials known as environmental product declarations (EPDs). They are prepared in accordance with ISO14025 and contain the results of LCA (performed according to ISO14040), of the material or product for the following indicators (Braune et al. 2007):

• Consumption of non-renewable energy

• Consumption of renewable energy

• Global warming potential

• Potential degradation of the ozone layer

• Acidification potential

• Eutrophication potential.

Some authors present information for the development of EPDs for concrete (Askham 2006) and for aluminum (Leroy and Gilmont 2006). An evident disad­vantage of EPDs relates to the fact that they do not guarantee a certain level of environmental performance, instead they provide a set of information about it, which only an expert in the field can assess (Manzini et al. 2006; Lim and Park 2009).

The LCA ‘‘includes the complete life cycle of the product, process or activity,

i. e., the extraction and processing of raw materials, manufacturing, transportation and distribution, use, maintenance, recycling, reuse and final disposal’’ (Setac 1993). The application of LCA has been regulated internationally since 1996 under ISO14040, ISO14041, ISO14042 and ISO14043. Some of the biggest drawbacks of the LCA, rely on the fact of being very time consuming, implying vast amounts of data on the environmental impacts of materials for all the phases of the life

F. P. Torgal and S. Jalali, Eco-efficient Construction and Building Materials, DOI: 10.1007/978-0-85729-892-8_11, © Springer-Verlag London Limited 2011

cycle. The categories of environmental impacts commonly used in the LCA, may include the following:

• Consumption of non-renewable resources

• Water consumption

• Global warming potential

• Potential reduction of the ozone layer

• Eutrophication potential

• Acidification potential

• Smog formation potential

• Human toxicity

• Ecological toxicity

• Waste production

• Land use

• Air pollution

• Alteration of habitats.

However, it is understandable that the importance of each category is not the same for each country, being dependent on its environmental specifics. For example a product that consumes a large amount of water, poses a high environmental impact in a very arid country, but that’s not the case if the product is produced in a country located in Northern Europe, so it makes perfect sense that the category of environmental impact on drinking water, has a different weight depending on the country where a product or material is produced.

Lippiatt (2002) refers to the case of assigning different categories of environ­mental impacts by different institutions (Table 11.1).

There are several tools that use LCA and to make an evaluation of the envi­ronmental impacts of construction materials such as: BEES (US); BRE. Envest (UK); ATHENA (CANADA); ECOQUANTUM and Simapro (The Netherlands). The software Building for Environmental and Economic Sustainability (BEES), is produced by the US Environmental Protection Agency and is available free of charge to any potential user. BEES has the following impact categories:

• Global warming potential

• Acidification potential

• Eutrophication potential

• Fossil fuel consumption

• Indoor air quality

• Alteration of habitats

• Water consumption

• Air pollutants

• Public health

• Smog formation potential

• Potential reduction of the ozone layer

• Eco-toxicity.

The material performance assessment is made by carbon dioxide units and its contribution for global warming. BEES has a limitation arising from the databases related to US processes, so this tool is recommended only for experimental and educational purposes. The BRE, Envest tool (Anderson and Shiers 2002) uses a notation based on eco-points normalized to the environmental impacts caused by a citizen in the UK during 1 year (100 eco-points). One must bear in mind that the methodologies related to LCA suffer from some uncertainties. In fact it is not possible to tell whether the emission of 1 ton of sulfur dioxide is more polluting than the emission of 3 tons of carbon dioxide or if water pollution is more serious than air pollution, or even if it is possible to know which is the most polluting, the electricity produced by a power plant or by a nuclear power plant. Ekvall et al. (2007) present a more detailed analysis of the LCA limitations. The widespread application of LCA to construction and building materials needs previous surveys on the environmental impacts of these materials throughout their life cycle, something that cannot be extrapolated from studies conducted in other countries due to the different technological and economic contexts.

11.1 General

As already stated in Chap. 1, eco-efficient construction and building materials present less environmental impact than common materials. However, it is difficult to say if for instance concrete is more environmentally friendly than steel. Because it is truth that the former is responsible for some CO2 emissions (0.8 tonnes emitted per tonne produced), on the other hand it uses local raw materials, and may even allow the incorporation of several industrial wastes. The second has the advantage of being recycled indefinitely, but its production involves a higher energy consumption and higher CO2 emissions (3 tonnes emitted per tonne pro­duced) and is prone to degradation by corrosion. It is then necessary to assess all the environmental impacts of a given material from cradle to grave. This meth­odology known as ‘‘Life Cycle Assesment’’ (LCA) was used for the first time in the US in 1990. One of the first studies using LCA assess the the resource requirements, emissions and waste caused by different packages of drinks and was conducted by the Midwest Research Institute for the Coca-Cola Company in 1969 (Hunt and Franklin 1996).

One of the most important applications of materials with photocatalytical prop­erties concerns the destruction of fungi and bacteria. Indoor fungi and bacteria proliferation are one of the main causes responsible for construction materials degradation and also for health problems (Zyska 2001; Santucci et al. 2007; Wiszniewska et al. 2009; Bolashikov and Melikov 2009) because fungi are responsible for mycotoxins growth (Reboux et al. 2010). Saito et al. (1992) studied the addition of TiO2 powder with an average size of 21 nm (30% rutile and 70% anatase) to a bacterial colony. The results showed that 60 to 120 min were suffi­cient to destroy all the bacteria. Those authors state that using bigger TiO2 par­ticles reduces the bactericidal capacity and that the best results are obtained for a TiO2 concentration between 0.01 to 10 mg/ml. Huang et al. (2000) also confirmed that using lower dimension TiO2 particles leads to a faster bacterial destruction. Those authors noticed that bacterial destruction begins after 20 min of UV radi­ation exposition, being that after 60 min all the bacteria have been destroyed. They also reported that after the destruction has been initiated the fact that UV radiation is stopped does not reduce the bactericidal effect (Fig. 10.10).

Some authors (Kuhn et al. 2003) believe that the bactericidal capacity associ­ated with TiO2 photocatalysis is dependent on the use of UV-A radiation with a wavelength between 320 and 400 nm, being that UV-C type is only effective if the light is applied in a direct manner, thus preventing the treatment of less illuminated areas. Seven et al. (2004) found that zinc-based photocatalysis is as bactericidal as effective as TiO2. Cho et al. (2004) confirmed that hydroxyl radicals are mainly responsible for the bactericidal capacity of semiconductors photocatalysts. Those authors mentioned that hydroxyl radicals have a destruction capacity of E. coli bacteria which is 1000 to 10000 times more effective than the chemical disin­fection products. Vohra et al. (2006) used silver-doped TiO2 noticing a 100% bacteria destruction just after 2 min, this compares in a most favorable manner with current TiO2 which took 2 h to achieve the same destruction level. The bactericidal capacity is reduced over time because of the accumulation of dead bacteria and viruses (Bolashikov and Melikov 2009). Other authors (Chen et al. 2009) used wood specimens coated with a TiO2 thin film (1.5 mg/cm2) noticing that the photocatalytic reaction prevented fungi growth. Calabria et al. (2010) analyzed the application of TiO2 thin films (20 to 50 nm thickness) by the sol-gel process in adobe blocks as a way to increase their water absorption and the bactericidal capacity. Those authors mentioned that TiO2 thin films could be more cost-effective than current commercial paints. One of the main disadvantages of the bactericidal effect associated with photocatalysis relates to the need of UV radiation with a wavelength between 200 to 400 nm, however, recent findings show some possibilities in the development of composite materials with photo­catalytic properties even when exposed to visible light (Dunnill et al. 2009; Chen et al. 2010). The use of titanium and trioxide tungsten-based films showed high- photocatalytic capacity under visible light above 400 nm (Song et al. 2006;

Saepurahman and Chong 2010). Herrmann et al. (2007) mentioned several ques­tions that should be addressed in a near future:

• Use of semiconductors which are not TiO2 based;

• Photocatalysis activation using visible light;

• Development of semiconductors with improved bactericidal capacity.

10.4 Conclusions

Nanotechnology has the potential to be the key to a brand new world in the field of construction and building materials. Although the replication of natural systems is one of the most promising areas of this technology, scientists are still trying to grasp their astonishing complexities. Recent years showed an intensive use of the potential of some photocatalytic nanomaterials, by the development of products with self-cleaning ability, products capable of reducing air pollution and with bactericidal capacity. The results of these investigations show that the titanium dioxide is the most widely used semiconductor in the photocatalytic reaction due to its low toxicity and stability. They also show that the efficiency of the photo­catalytic reaction is dependent on the type of TiO2, being that a mixture of rutile (30%) and anatase (70%) seems to be the most reactive. The use of TiO2 with a high-specific surface area also shows a higher reactivity. As to the use of TiO2 dispersed in cement matrix it is less effective than as thin films.

The subject of air pollutants such as VOCs released from building materials have already been addressed in. Chap. 2. In the last years several investigations have been carried out in order to use photocatalysis to reduce air pollution. The reaction

Table 10.1 Patent for paving tile comprising an hydraulic binder and photocatalyst particles

(Cassar and Pepe 1997)

Field of the invention Hydraulic binder, dry premix, cement composition having

improved property to maintain the brilliance and color quantity and to prevent aesthetic degradation

Working principle/ Use of photocatalyst particles able to oxidize air and

product requirements environmental pollutants;

Use of a photocatalyst which is able to oxidize in the presence of light air and environmental polluting substances for the preparation of an hydraulic binder for manufacturing paving tiles that maintain after installation for a longer time brilliance and color quantity;

Use of a dry premix containing a hydraulic binder and a

photocatalyst that is able to oxidize in the presence of light air and environmental polluting substances for manufacturing paving tiles that maintain after installation for a longer time brilliance and color quantity

Binder Hydraulic binder

Cement (white, gray or pigmented)

Cement used for debris dams Hydraulic lime

Photocatalyst TiO2 without further requirements-TiO2 or a precursor thereof,

mainly in the form of anatase TiO2 with anatase structure for at least 25, 50 and 70%

Blend of anatase and rutile TiO2 having a ratio 70:30 TiO2 doped with one or more atoms different from Ti TiO2 doped with one or more atoms selected from Fe(III), Mo(V), Ru(III)

Os(III), Re(V), V(V), Rh(III)

Photocatalyst selected from the group consisting of tungstic oxide (WO3), strontium titanate (SrTiO3) and calcium titanate (CaTiO3)

Amount of photocatalyst 0.01-10% by weight

0.1% by weight with respect to the binder 0.5% by weight with respect to the binder

of photocatalytic oxidation of pollutants, generates water and carbon dioxide as by-products. Murata et al. (1997) patented a paving block for the reduction of air pollution (Table 10.2):

Zhao and Yang (2003) mentioned a high-photocatalytic capacity for indoor air pollution reduction when using P25 TiO2 (70% anatase? 30% rutile) with 300 nm diameter and a specific surface of 50 m2/g. Yu (2003) studied cementi­tious paving blocks for NOx reduction, noticing that the photocatalytic capacity is reduced by the presence of dust, grease or plastic gum, thus suggesting that these blocks should not be placed in pedestrian areas. Maier et al. (2005) reported a fast pollution reduction in indoor air by the use of gypsum plasters containing 10% TiO2 (Fig. 10.6). Those authors mentioned that although air pollution reduction is dependent on the UV intensity, nevertheless, visible light still allows acceptable degradation rates. Those plasters were used to cover some bedrooms in Sweden, being responsible for a reduction on VOC of about 1/3 (to 26 pg/m3).

Strini et al. (2005) mentioned that TiO2 thin films have a photocatalytic capacity which is 3 to 10 times higher than for TiO2 based cementitious com­posites. In 2006 the results of the PICADA (2006) project ‘‘Photo-catalytic innovative coverings applications for de-pollution assessment’’ aiming to the development of TiO2-based coatings for self-cleaning and air pollution reduction coatings were disclosed. This consortium gathered 8 partners (Italcementi, Millenium Chemicals, AUT, NCSRD, CNR ITC, CSTB, Dansk Beton Teknik and GMT) and was financed in 2.3 million euro by the EU (Gurol, 2006). Aside from the study of small specimens in laboratory the PICADA project also covered pilot tests at macro-scale (1:5) in order to reply the effect of a street by using (18 x 5.18 m2) ‘‘walls’’ and an artificial NOx pollution source (Fig. 10.7). The results showed a reduction in the NOx emissions between 40% to 80%. However, results published in a scientific journal mentioned NOx reductions between 36.7%

to 42% (Maggos et al. 2008). The use of a three-dimensional numerical model (MIMO) based on the data generated in the pilot test allowed the insertion of win velocity and temperature in order to predict the reductions in air pollution by the photocatalytic activity of the facade coatings containing TiO2.

In another macro-scale test carried out in the PICADA project, the ceiling of an underground car park (322 m2) was painted with TiO2-based paint. Then the park was sealed and polluted by the exhaust gas from a single car. The results showed a 20% reduction on NOx emissions, due to the photocatalytic capacity of the paint in the ceiling. Wang et al. (2007) confirmed that in the last few years a lot of investigations has been made about the reduction of indoor air pollution when using UV radiation but very few have analyzed the possibility of using photo­catalysts active under visible light. Poon and Cheung (2007) mentioned that TiO2 cementitious composites with increased porosity show a high NOx emissions reduction. Those authors compared the performance of several TiO2 forms, con­cluding that although P25 is much more reactive it does not have a very high performance/cost ratio. Guerrini and Peccati (2007) mentioned a case of a street in Bergamo, Italy, paved with blocks (12,000 m2) of photocatalytic properties where high reductions of NOx emissions (45%) have been reported. In Antwerp a park with 10,000 m2 of semiconductors paving blocks also showed a reduction in NOx emissions (Beeldens 2007). In Tokyo cement mixtures containing TiO2 colloidal solutions were used to coated several road areas (Fig. 10.8). The results obtained in an area of 300 m2 show a 50 mg to 60 mg/day NO emissions degradation (Fujishima et al. 2008).

Auvinen and Wirtanen (2008) studied the reduction of VOCs in indoor air when using paints with TiO2 applied in several substrates (glass, gypsum and polymer) noticing that the substrate does not influences the photocatalytic reaction. Those authors mentioned that organic additives must not be used for this paints because they will be damaged by the radical hydroxyls. Also that the photocatalytic reactions generates not only water and CO2 but also other pollutants that are harmful for human health. Other authors (Demeestere et al. 2008) confirm the reductions of NOx emissions between 23% and 63% when using TiO2-based tiles. They also reported that the accumulation of reaction products generated in the oxidation process reduces the photocatalytic activity. For the production of TiO2- based cementitious composites some authors (Husken et al. 2009) recommend the use of TiO2 as a solution with the mixing water because it allows for a better

dispersion than if it is mixed with the cement. Also that a semiconductor with a high-specific surface gives better results than the use of a superior volume of low-specific surface semiconductor. These authors used a specific surface TiO2 (between 0.7 m2/g to 1.5 m2/g) which is much lower than the P25 form. Chen and Poon (2009b) found out that when using grounded glass for partial sand replacement the photocatalytic capacity, increases as much as 3 times for lighter glass (Fig. 10.9). Those authors suggest that glass particles can allow light to enter more deeply in the mortar leading to a higher oxidation rate.

Kolarik et al. (2010) confirmed that the photocatalytic reaction is a good way to reduce VOCs in indoor air. Ramirez et al. (2010) mentioned that the substrate porosity influences the photocatalytic reaction when TiO2 thin films are being used and that a high-porosity surface leads to a high photocatalytic reaction. Ballari et al. (2010) presented a model that can predict NOx emissions reduction using concrete with TiO2 particles. Hassan et al. (2010) used 41 MPa concrete blocks covered with 1 cm TiO2-based mortar layer in order to evaluate nanoparticles removal by abrasion tests and thus reducing the photocatalytic capacity in NOx emissions reduction. Those authors mentioned that even after 20,000 abrasion cycles the NOx emissions degradation and reduction remained stable.

Although self-cleaning properties of photocatalysts materials are known since the 60s (Fujishima and Honda 1972), only recently they start to be used in a wide – scale (Fujishima et al. 1999). Cassar and Pepe (1997) patented a concrete block with self-clening ability (Table 10.1).

The first application of self-cleaning concrete took place in the church ‘‘Dives in Misericordia’’ in Rome (Fig. 10.4). This building was designed by the Arq° Richard Meyer and officially opened in 2003. It is composed of 346 pre-stressed concrete blocks made with white cement and TiO2 (binder 380 kg/m3 and W/ B = 0.38) (Cassar et al. 2003).

Visual observations carried out six years after its construction revealed only slight differences between the white color of the outside concrete surfaces and the inside block surfaces (Chen and Poon 2009b). Diamanti et al. (2008) studied mortars containing TiO2 having noticing reductions in the contact angle between water and solid surface of almost 80%. Ruot et al. (2009) mentioned that the photocatalytic activity is dependent on the matrix properties. Increasing the TiO2 content in cement pastes above 1% leads to a proportional increase in the pho­tocatalytic activity, as for mortars a TiO2 content increase just lead to a very small increase in the photocatalytic activity (Fig. 10.5). Those authors suggest that most TiO2 particles in mortars are not reached by UV radiation.

The use of TiO2 thin films on tiles or glasses has significant potential in terms of self-cleaning ability. According to Fujishima et al. (2008) Japan buildings must be cleaned at least every five years to maintain a good appearance, while that covered with self-cleaning tiles should remain clean over a span of 20 years without any maintenance.

The most known application of nanomaterials in the construction industry relates to the photocatalytic capacity of semiconductor materials. Several semiconductors materials, such as TiO2, ZnO, Fe2O3, WO3 and CdSe, possess photocatalytic

capacity (Makowski and Wardas 2001). However, TiO2 is the most used of all because of its low toxicity and stability (Djebbar and Sehili, 1998). Titanium dioxide can crystallize as rutile, anatase and brookite, being the first form the most stable (thermodynamically speaking), it is also the most available form (it is the 9th most abundant element in the Earth crust), being currently used as additive in the painting industry.

The anatase and brookite forms are meta-stable and can be transformed into rutile by thermal treatment. Being a semiconductor with photocatalytic capacity, when TiO2 is submitted to UV rays (320-400 nm), in the presence of water molecules (Husken et al. 2009), it leads to the formation of hydroxyl radicals (OH) and superoxide ions (O2-). Those highly-oxidative compounds react with dirt and inorganic substances promoting their disintegration. The photocatalysis of TiO2 is also responsible for the reduction of the contact angle between water droplets and a given surface, leading to super-hydrofobic or super-hydrophilic surfaces increasing their self-cleansing capacity. Water repellent surfaces are one of the features of natural systems as it happens in the leaves of the lotus plant, whose microstruture allows self-cleansing ability (Fig. 10.2). According to Fujishima et al. (2008), the potential of photocatalysis can be perceived by the high number of citations (almost 3700) of a related paper published on Nature in 1972, as well as by the number of papers concerning photocatalysis investigations that increased in an exponential pattern between 1997 and 2007. Another form to evaluate the potential of this technology is by knowing that the Japanese Corporation TOTO Ltd has already issued 1200 international patent requests in this field. So far 500 have been approved. The applications related to photacatalysis cover five different groups (Fig. 10.3).

Considering the cost to clean graffiti paintings (in Los Angeles city this could amount to 100 million euro/year (Castano and Rodriguez 2003) )one can realize the huge potential of the photocatalytic capacity of nanomaterials.

Nanoparticles have a high-surface area to volume ratio providing high-chemical reactivity. They act as nucleation centers, contributing to the development of the hydration of Portland cement. Most investigations use nanosilica while some already used nano-Fe2O3. The production of nanoparticles can be obtained either through a high-milling energy (Sobolev and Ferrada-Gutierrez 2005) or by chemical synthesis (Lee and Kriven 2005). Porro et al. (2005) mentioned that the use of nanosilica particles increases the compression strength of cement pastes. The same authors state that the phenomenon is not due to the pozzolanic reaction, because calcium hydroxide consumption was very low but, instead, due to the increase use of silica compounds that contributes to a denser microstructure. According to Lin et al. (2008), the use of nanosilica on sludge/fly ash mortars, compensates the negative effects associated to the sludge incorporation in terms of setting time and initial strength. Sobolev et al. (2008) reported that nanosilica addition led to an increase of strength by 15% to 20%. Other authors (Gaitero 2008; Gaitero et al. 2009) believe that nanosilica leads to an increase of C-S-H chain dimension and also to an increase of C-S-H stiffness. Chen and Lin (2009) used nanosilica particles to improve the performance of sludge/clay mixtures for tile production. The results show that nanoparticles improved the reduction of water absorption and led to an increase of abrasion and impact strength. Others (Vera-Agullo et al. 2009) also confirm that the use of nanoparticles (nanotubes, nanofibers, nanosilica or nanoclay) is responsible for a higher hydration degree of cementitious compounds, as long as a higher nanoparticle dispersion can be achieved. Nasibulin et al. (2009) reported an increase in strength by 2 to 40 times for electric conductivity, which means a high potential for sensing ability. Several authors confirm the suitability of mortars with Fe2O3 nanoparticles to act as sensing materials (Li et al. 2004; Qing et al. 2008; Lin et al. 2008). Chaipanich et al. (2010) mentioned that 1% of carbon nanofibres (by binder mass) can com­pensate the strength reduction associated with the replacement of 20% fly ash. Gdoutos-Konsta et al. (2010) also studied the effect of carbon nanofibres on cement pastes (0.08% by binder mass) observing an increase in the mechanical strength. Those authors used ultra-sounds to achieve a high-nanofibre dispersion stating that this is a crucial step in order to obtain a high performance of nanotubes in the cement matrixes. Nevertheless, the fact that carbon nanotubes are not cost-efficient prevents the increase of its use in commercial applications in a near future.

10.2.1 Investigation of Portland Cement Hydration Products

Concrete is the most used construction material on Planet Earth and presents a higher permeability that allows water and other aggressive elements to enter, leading to carbonation and chloride ion attack, resulting in steel corrosion

problems. Therefore, the nanoscale study of the hydration products (C-S-H, calcium hydroxide, ettringite, monosulfate, unhydrated particles and air voids), as a form to overcome durability issues, is a crucial step in the concrete eco­efficiency. Investigations in this field have already been carried out in recent years (Porro and Dolado 2005; Balaguru and Chong 2006). Mojumdar and Raki (2006) have already analyzed calcium silicate nanophase composites which will allow the future development of anti-corrosion and fire-retardant coatings. Until very recently electronic microscopy has allowed the understanding of the morphology, as well as the composition of hydration products. However, the use of nanotech­nology currently allows the possibility of the knowledge of the elastic modulus by nanoindentation techniques (AFM). In nanoindentation a material with known characteristics is used to make a mark in another material with unknown properties and through the specific nature of this mark it is possible to infer the properties of the marked material. Recently Mondal (2008) used nanoindentation in cementi­tious phases and obtained the following elastic modulus: 35 MPa for the Ca(OH)2 phase; 26 and 16 MPa for high – and low-stiffness C-S-H and 10 MPa for the porous phase. Other authors (Constantinides et al. 2003; Dejong and Ulm 2007; Constantinides and Ulm 2007) have already confirmed the existence of different types of CSH, low density, high density and ultra-high density. More recently some authors (Pellenq et al. 2009) from MIT have used nanotechnology to develop a molecular model for the hydration products of Portland cement. These authors confirm that the molecular model is in excellent agreement with experimental values obtained by nanoindentation techniques.