Construction materials to reduce urban air pollution Environment

Air pollution is still a burning issue in Spain and the rest of Europe. Air quality or, conversely, its degradation or contamination, is the result of a complex set of factors involving many different causes and effects, generally associated with human activity and the emission of pollutants into the air. Bearing in mind the serious health effects and environmental impact of polluted air, clean air has become a top-priority target of environmental policies and sustainable development strategies. Clean air is now increasingly being seen by society as a prime factor in the quality of life.

One of the stiffest challenges faced by urban sustainability is the undesired quantitative and qualitative impacts and changes produced within cities by our current urban lifestyles. The great increase of industrial activities in our cities and also city sprawl itself have produced wholesale changes in the atmosphere we breathe. The burning of fossil fuels and emissions from industry and transport systems are the main emission sources of harmful substances. The upshot is the contamination of the urban atmosphere, directly affecting the health of individual human beings and whole ecosystems. Breathing in clean air without health risks is an inalienable right of all human beings. To safeguard this right there is now a need for a paradigmatic shift in city-dweller’s lifestyles, in urban planning systems, structural elements and transport arrangements, plus systematic policies to bring all this about.

In built-up environments transport is one of the principal sources of pollutants and gases. The exhaust tube of any average vehicle pumps out a combination of polluting substances; the most harmful are the following^[1]:

• CO₂, a gas that exacerbates the greenhouse effect and drives climate change on our planet.
• Nitrogen oxides (NOx), gases that affect human health, contribute to acid rain and also increase the greenhouse effect and climate change.
• Hydrocarbons and volatile organic compounds (VOCs) stemming from incomplete fuel combustion. Polycyclic aromatic hydrocarbons (PAHs) are carcinogenic while others irritate the eyes and the upper respiratory tract.
• Reaction of the organic compounds with nitrogen oxides generates ozone in the lower layers of the atmosphere, especially in a region like the Mediterranean with very high summer temperatures. This photochemical cocktail causes headaches and irritation of the respiratory system as well as contributing to the degradation of woodland.
• Pb and other heavy metals like Cd, Cu, Cr, Ni, Se and Zn.

European Union studies have estimated the cost of the noise and pollution emitted by the transport system to be about 0.6% of the GDP. Over 90% of this figure is related to road transport [1]. The gases CO, CO₂ and NOx account for the lion’s share of transport-emitted pollutants. Figure 1 shows, by way of example, the emission levels of these gases from diverse means of transport during 2007 in Andalusia^[2].

Toxic effects of nitrogen oxides

Means of transport nowadays issue huge amounts of nitrogen oxide into the air, at levels only slightly below carbon gas emissions (figure 1). Nitrogen oxides are highly toxic, even more so than carbon monoxide. Nitrogen dioxide has been shown to be four times as toxic as monoxide; its toxicity is increased in combination with ozone, forming complex toxic organic compounds called peroxyacyl nitrates from sunlight-triggered photochemical reactions^[3].

In general the inhalation of nitrogen oxides causes bronchitis and pulmonary edema; concentrations above 2 ppm deplete bronchial-tree cilia. Ultramicroscopic investigation has shown up structural changes in the pulmonary collagen as well as alveolar lesions, interstitial infiltration and petechiae. These harmful health effects increase in direct proportion to airborne concentrations. Ten-minute exposure to concentrations of 10 mg/m3 causes intense respiratory upsets, still reversible at that level. But if the concentration rises to 150 mg/m3 this may give rise to pulmonary edemas and a definitive reduction of the respiratory function; extreme cases of exposure to concentrations of 600-900 mg/m3 causes death by asphyxia in a few days^[3].

«THERE IS NOW A NEED FOR A PARADIGMATIC SHIFT IN URBAN PLANNING PROCEDURES, CITY STRUCTURAL ELEMENTS AND TRANSPORT SYSTEMS»

Another problem deriving from airborne NOx gases, in this particular case N2O, is related to their atmosphere-heating capacity (greenhouse effect), 207 times higher than CO₂^[4]. Their contribution to the planet’s climate change should not be underestimated, therefore, even though the emission levels from means of transport are lower than those for CO₂.

Legislation lays down limit values for the various pollutants. These are becoming ever stricter, in terms not only of the exposure threshold but also the number of times this threshold can be exceeded. Since 1 January 2010, for example, an average annual limit of 40 μg/m3 has been set for NO₂^[5]. Despite these restrictions it is estimated that the European pollution-reduction strategy will cost over 7 billion euros a year as from 2020, by which time all the measures will have been phased in. The saving in health improvement costs is put at 42 billion euros a year, six times higher than the outlay, avoiding 140,000 early deaths a year in Europe due to exposure to these polluting gases and also reducing sick leave and the pharmaceutical costs of treating respiratory disorders.

In general many people living in Spanish cities are exposed to high levels of air pollution due to the increase in private transport and the existence of factories near the built-up areas.

Construction materials as urban elements to reduce air pollution

Even with the efforts to improve the energy- and environmental-efficiency of the means of transport and the capping of industrial emissions, our society is still a long way from avoiding Nox gas emissions. There is now a pressing need for air-pollution reduction measures in built-up areas. One way of reducing cities’ levels of these gases is the creation of large decontaminating areas to «cleanse» these gases from the environment or, more specifically, using buildings for this purpose.

«THE AMOUNT OF NITROGEN DIOXIDE EMITTED INTO THE ATMOSPHERE BY MEANS OF TRANSPORT IS HUGE NOWADAYS, ONLY JUST BELOW THE LEVEL OF CARBON GASES»

Despite the decades-long trend of using glass and steel to cover building facades (mainly public buildings), the majority of buildings, especially family dwellings, are still rendered in cement, concrete or mortar. New functions have recently been proposed for these materials, apart from their basic structural, insulation and soundproofing purpose. Besides the work carried out by L. Cassar and collaborators^[6], the Italian group Italcementi has recently patented the use of photocatalytic additives in cement and derivative products. With the addition of these components construction materials can be made to act as decontaminants of NOx gases and of the BTEX fraction (benzene, toluene, ethylbenzene, and xylenes) of the volatile organic compounds, both being the main toxic gases given out by combustion engines. This is possible thanks to the photochemical reaction of additives like TiO₂, which, on exposure to sunlight, break down the abovementioned pollutants chemically and therefore remove them from the air. In the specific case of NOx gases, the breakdown occurs according to the reaction sequence shown in figure 2^[7]. By means of this mechanism the gases are oxidised and retained as nitrate species in the construction material or, more likely, are flushed from the concrete surface as a weak nitric acid.

«NITROGEN OXIDES (NOX) HARM HUMAN HEALTH, CONTRIBUTE TO THE FORMATION OF ACID RAIN AND ALSO DRIVE THE GREENHOUSE EFFECT AND CLIMATE CHANGE»

TiO₂ has been chosen as photocatalytic additive on the strength of its inherent properties, such as high stability, low toxicity, compatibility with traditional construction materials and high photocatalytic activity in comparison with other metallic oxides. These properties make it the most suitable photocatalyst for inclusion in photocatalytic construction materials.

The downside, however, is the high cost of this additive, dissuading widespread use. Its promising air-cleaning potential in construction materials has therefore been little taken up in cities. In fact there is no clear policy of implementing air-cleaning construction products in public works projects, neither at home nor abroad. There are now a few civil buildings made from TiO₂-containing cement and currently in use. The Jubilee Church, Dives in Misericordia, in Rome, Italy, and the public building Cité de la Musique et des Beaux-Arts in Chambéry, France, tend to hog the limelight. But there are also other finished buildings with concrete of this type in Belgium, France, Italy, Monaco, Morocco, Japan and China. In Spain it has been used only in one-off projects, due to the high cost of its application. These include the building housing the Polo de Innovación Audiovisual (PIA) and the church called Iglesia de Riberas de Loiola, both in Donostia-San Sebastián, and another in the square called Plaza Conresa de Mislata in Valencia.

New breakthroughs in the preparation of photocatalytic construction materials

Although this technological development has been well studied in cement [8,9], concrete [10,11] and granite blocks [12], it has hardly been implemented for dry mortar mixes, a construction material widely used in Spain for the rendering of buildings. This material has a high porosity, favouring contact with atmospheric air and hence ensuring a good air-cleaning performance. As already pointed out, however, we also have to factor in the high cost of the TiO₂ photocatalytic additive, which is a drawback for the marketing of these products. Working from all these premises and factors, the Research Group FQM-175 of the Universidad de Córdoba has conducted a study with the prime aim of developing traditional construction materials with a decontaminating capacity, varying the dosing components to favour its NOx gases breakdown properties.

Para la formulación del mortero de cemento objeto de estudio se utilizaron los siguientes materiales: cemento blanco, calcita como relleno (filler), arena tipo dolomita como agregado y aditivos orgánicos. Se han preparado seis tipos diferentes de morteros siguiendo la distribución de materias primas que aparece en la tabla 1. Al ir variando la cantidad de arenas o filler utilizados, se obtiene un producto seco con diferente distribución de tamaño de partícula. En la figura 3 se pueden distinguir hasta tres tipos diferentes de mortero que varían en la proporción mayoritaria del tamaño de partícula de sus materias primas, desde el menor (tipo I) al mayor tamaño (tipo III).

«Una manera de disminuir la concentración de estos gases en la atmósfera de una ciudad sería mediante la creación de grandes superficies descontaminantes que permitan la ‘limpieza’ de los mismos en su entorno, utilizando los edificios para este propósito»

The cement mortar used in the study was made up from the following elements: white cement, calcite as filler, dolomite sand as aggregate and organic additives. Six different types of mortar were prepared with the distribution of raw materials shown in table 1. Variation of the amount of sand or filler used produced a dry mix product with a different distribution of particle size. Figure 3 shows three different types of mortar that vary primarily in terms of particle size, from the smallest (type I) up to the biggest (type III).

The observed differences in the physical, mechanical and chemical properties of the hardened mortar, after mixing the dry mortar with water, were brought into relation with its various types. As regards the mechanical properties, the compression strength readings for each mixture at different setting times are shown in figure 4. There is a logical increase with setting time, as a result of the continuous hydration of the cement phases present in the mortar (setting reaction). For these mortar types a hierarchical resistance-strength order was established, i.e., samples type I > type II > type III (or mortars D,F > A,B > C,E).

Construction and decontamination

This hierarchical relation also shows up in the mortar microstructure. Cement mortar microstructure is the result of the cavities formed among the various sands of its composition and also the pores that appear in products of the set cement, which in turn fill some of the aforementioned cavities. Obviously the mechanical strength of any mortar will depend on its porosity [13]. A study of the type and volume fraction of the pores formed tells us the post-setting microstructure; this can then be brought into relation with the mechanical strength readings. The technique used to gauge the pore volume of set cement is the mercury intrusion porosimetry test. As its name suggests this involves forcing mercury under pressure into the pores. Figure 5 shows, by way of example, the increment of the mercury intrusion volume in terms of the pore diameter for two of the mortars (B and D) set for 28, 90 and 150 days. The main pore size (that of the biggest intrusion volume) was seen to bear a close relationship with the aforementioned particle size distribution type. Thus, for samples type I, type II and type III, the observed pore diameter is 0.4-0.55, 1.0-1.5 and 1.5-2.0 m, chiming in well with the hierarchical relation established for the resistance readings. The type I samples, therefore, with the smallest pore diameter, are those that show the highest mechanical strength readings. The pore volume distribution hardly changes with setting time, showing once again that the internal microstructure develops during the first setting stages and depends on the initial formulation established for each mortar (table 1).

«ONE WAY OF REDUCING THE CONCENTRATION OF THESE GASES IN A CITY’S ATMOSPHERE WOULD BE BY CREATING LARGE DECONTAMINATING AREAS THAT ‘CLEANSE’ THE GASES FROM THE ENVIRONMENT, USING BUILDINGS FOR THIS PURPOSE»

The detailed study of specific cement mortar formulations therefore tells us the direct influence of the particle size distribution on the mechanical and physical properties. The study also showed that the porosity of each mortar is influenced by the water/cement ratio used in the mixture and that this porosity impinges on the setting reaction, a parameter that will finally have consequences for the mortar’s NO and NOx gas decontamination capacity. To gauge the nitrogen oxide breakdown on the surface of these materials, three mortars with a different particle size distribution type (B, C and D) were chosen. To each of these formulations (table 1) 1% of the compound TiO₂ P25 (photocatalytic additive) was added, using a different water/cement ratio for each one. Figure 6 shows how the NO and NOx gas degradation capacity differs. Mortar D (type I) shows the best nitrogen oxide photocatalytic degradation performance, while B (type II) shows the worst. This gas degradation is assumed to occur according to the reaction sequence shown in figure 1. The degradation will therefore be greater when the necessary reagents – TiO₂, H2O and O2 – are present in the mortar surface. It should be borne in mind here that the formation of the hydrated cement products in gel form might trap some TiO₂ particles inside and therefore withdraw them from the surface. This explains why it is the B mortar, which we know has the quickest setting reaction, that shows the worst NOx gases degradation performance. Furthermore, accessibility to the water and oxygen molecules will be proportional to the specific surface of the sample exposed to the gases (varying directly with porosity); for this reason mortar D shows the best degradation performance.

«A PATENT HAS RECENTLY BEEN TAKEN OUT FOR THE USE OF PHOTOCATALYTIC ADDITIVES IN CEMENT AND DERIVATIVE PRODUCTS BASED ON SUNLIGHT-TRIGGERED PHOTOCHEMICAL REACTIONS THAT CHEMICALLY BREAK DOWN THE POLLUTANTS»

The study shows that modulation of the mortar composition and microstructure produces variations of up to 52% in NO/NOx gas degradation performance and up to 38% in mechanical strength readings. The main conclusion that can be drawn from the study is therefore that correctly modulated formulation of the cement mortar with photocatalytic function will enable us to greatly reduce the amount of cement and photocatalytic additives to be used – these being the dearest raw material in products of this type – without thereby forfeiting too much of its desired properties in the mortar. Furthermore, the best NOx gas decontamination performance is shown by low porosity mortar with a high specific surface. The preparation of new state-of-the-art, cheaper dry mortar mixes, with the optimum formulation, will therefore favour the takeup of this material by the construction sector, converting the new building items into air-cleaning areas in the major thoroughfares, squares and streets of our cities.

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ACKNOWLEDGEMENTS

The authors of this study would like to express their gratitude for the 2010 research grant awarded by FUNDACIÓN MAPFRE and the co-financing received from the Junta de Andalucía through the research group FQM-175 and the project P09-FQM-4764, as well as the technical support of the company Grupo Puma S.L.

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TO FIND OUT MORE

1. Plan de Infraestructuras para la Sostenibilidad del Transporte en Andalucía 2007-2013 (PISTA 2007- 2013). Consejería de Obras Públicas, Junta de Andalucía.
2. Emisiones de las Fuentes de Área Móviles. En: Inventario de Emisiones a la Atmósfera en la Comunidad Autónoma Andaluza. Consejería de Medio Ambiente, Junta de Andalucía, 2007. p 4.2-4.11.
3. Capó Martí, Miguel. Principios de ecotoxicología. Diagnóstico, tratamiento y gestión del medio ambiente. McGraw-Hill, 2002.
4. Domenech, Xavier. Química atmosférica. El origen y los efectos de la contaminación. Mirabueno Ediciones, 1995.
5. Evaluación y gestión de la calidad del aire ambiente en relación con el dióxido de azufre, dióxido de nitrógeno, óxidos de nitrógeno, partículas, plomo, benceno y monóxido de carbono. Real Decreto 1073/2002, 2002.
6. Cassar, L. Photocatalysis of cementitious materials: Clean building and clean air. Materials Research Society Bulletin 2004; 328-31.
7. Chen, J.; Poon, C.S. Photocatalytic construction and building materials: From fundamentals to applications. Building and Environment 2009; 44: 1899-1906.
8. Lackhoff, M.; Prieto, X.; Nestle, N.; Dehn, F.; Niessner, R. Photocatalytic activity of semiconductormodified cement - Influence of semiconductor type and cement ageing. Applied Catalysis B: Environmental 2003; 43: 205-16.
9. Ramírez, A.M.; Demeestere, K.; De Belie, N.; Mäntyla, T.; Levänen, E. Titanium dioxide coated cementitious materials for air purifying purposes: Preparation, characterization and toluene removal potential. Building and Environment 2010; 45: 832-38.
10. Hüsken, G.; Hunger, M.; Brouwers, H.J.H. Experimental study of photocatalytic concrete products for air purification. Building and Environment 2009; 44: 2463-74.
11. Chen, J.; Poon, C.S. Photocatalytic activity of titanium dioxide modified concrete materials-Influence of utilizing recycled glass cullets as aggregates. Journal of Environmental Management 2009; 90: 3436-42.
12. Poon, C.S.; Cheung, E. NO removal efficiency of photocatalytic paving blocks prepared with recycled materials. Construction and Building Materials 2007; 21: 1746-53.
13. Lanas, J.; Álvarez, J.I. Masonry repair lime-based mortars: Factors affecting the mechanical behaviour. Cement and Concrete Research 2003; 33: 1867–76.

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