And if we succeed in transforming harmful gases into O2 with waste from the steel industry?
Photocatalytic Construction Materials through the Valorisation of Waste from the Steel Production Process.
[COMFUTURO Researcher from the Spanish National Research Council (CSIC)]
The steel industry generates a substantial volume of waste (slag, blast furnace dust, etc.), which is primarily managed by depositing it in landfills, causing a negative impact on the environment. Moreover, a certain amount of this waste is classed as hazardous and may suppose a cost for the steel industry, with the added problem that in many places landfills are reaching their capacity limit. Currently, a large quantity of the waste generated is being reused as aggregate in the construction industry, leading to a significant reduction in the use of natural resources. However, new efforts must be made and these materials must be given the highest possible value added. In this connection, the development of new technologies aimed at boosting the circular economy of this waste based on the 4 “Rs”: reduce, reuse, recycle and energy recovery (valorisation), has become a necessity.
The majority of the methodologies aimed at promoting alternative uses of the waste generated are focused on the recovery of metals, such as zinc and iron . An additional field of application for waste is its reuse in the construction industry as a substitute for raw materials, in order to minimise significantly the use of natural resources and/or stabilisation to minimise the environmental risk associated with this waste. This is one of the best ways to reuse this waste given the consumption of large quantities of raw materials and the large volume of finished products.
The construction industry’s main alternatives include: the introduction of waste as a supply of iron for the Clinker manufacturing process , aggregate for concrete , use in the wearing course layer of road surfaces with bituminous paving mixtures or as a material for road sub-grades and bases , and in ceramic materials .
Conversely, in large cities, despite the improvements made in recent years poor air quality persists, which has led to significant problems for human health with approximately 16,000 premature deaths per year in Spain attributed to this problem. Moreover, a contaminated environment causes urban architecture to gradually deteriorate and become dirty, which implies significant maintenance costs in the regular budget. To address these problems, in recent years the use of photocatalytic construction materials has emerged as a new technology to contribute to reducing urban air pollution and maintaining the aesthetics and functionality of architecture. Experimental tests allow us to assert that, in sunlight or artificial light, a building constructed or covered with photocatalytic material not only reduces significantly the amount of harmful substances in the air, which gives rise to an increase in the quality of life and savings in public health costs, but can also maintain the aesthetic appearance of buildings for a long period of time, which also leads to a reduction in maintenance costs.
The photocatalytic process commences when a photon, with sufficient energy, reaches the surface of the photocatalyst, resulting in a molecular excitation that leads to the formation of an electron-hole pair (e-/h+) in the photocatalyst. This e-/h+ pair may give rise to a series of redox chemical reactions, which may cause the degradation/mineralisation of organic and inorganic contaminants that come into contact with the surface of the photocatalyst.
The growing interest in this technology and the successful results obtained have encouraged many construction material manufacturers to develop this type of materials containing photocatalytic nanocomposites, mainly TiO2, with the aim of marketing new self-cleaning and decontaminating materials. Pavements and the vertical surfaces of infrastructures provide optimal substrates for the application of photocatalytic solutions due to the large exposed surface and the relatively flat configuration that facilitates the photocatalyst’s exposure to sunlight. However, TiO2 only uses 4-5% of the total solar energy in its activation, which, together with its price, makes its extensive use in our cities impossible. Therefore, developing low-cost, preferably sustainable, photocatalytic materials that are light-activated in the solar spectrum is a major challenge. An alternative to mitigate this deficiency could be to prepare a semiconductor photocatalyst from industrial waste. Of the metal oxides, which often appear in two of the most prominent waste products of the steel industry (slag and electric arc furnace dusts), those based on Fe and Zn are the most interesting since they could be transformed into photocatalytic materials with visible-light absorption capacity [6-8]. Thus, this waste could be converted into new sustainable and low-cost photocatalytic products activated in the visible spectrum (from waste to business opportunity).
According to our level of understanding, research into the development of photocatalytic materials from waste has been carried out in very few experiments:  describes the preparation of a TiO2-polystyrene photocatalyst from waste material,  describes the synthesis and characterisation of magnetite nanoparticles using iron ore waste, and  uses slags from a local industry as a catalyst in photo-fenton processes.
Moreover, previous experiments on the valorisation of industrial waste for the production of construction materials with photocatalytic properties have also been developed on very few occasions: with industrial wastes from the sandblasting cleaning operation , with waste from the granite industry , with steel slag for the degradation of organic pollutants from waste water , and a publication that demonstrated the viability of using electric arc furnace dust as a photocatalytic material and developed photocatalytic mortars with degrading and self-cleaning properties . In this project, cement-based materials with electric arc furnace dust as an additive were obtained, [electric arc furnace dusts / (cement + electric arc furnace dusts)] = 35.5%, which degraded up to 27% of the NOx concentration in the air.
However, the viability of the results obtained in previous projects indicates that photocatalytic behaviour depends on the origin of the raw materials and the processing variables, fundamental parameters on which the resulting physical-chemical characteristics of these wastes depend. Therefore, the viability of the photocatalytic behaviour demonstrated in the previous applications cannot be extended to other wastes, even those of the same type.
In order to respond to some of the dispersion variables obtained in previous projects, the project "Valorisation of steel slag: from waste to intelligent construction material, 4R-photoslag” is being developed. The ComFuturo Programme (www.comfuturo.es) of the CSIC General Foundation is financing this project, and Acerinox, S.A., a Spanish multinational group that manufactures stainless steels, is collaborating, in addition to other companies. The aim of this project is to attribute photocatalytic activity to the intrinsic characteristics of waste from the steel industry. To this end, the pre-treatments necessary for conditioning and optimising the waste are evaluated and construction materials with photocatalytic properties with additives from different wastes are designed and manufactured. Finally, the environmental, technical and economic viability associated with each of the potential applications is validated. In this context, the project goes beyond the experiments carried out so far: a better understanding is expected to be gained and technology is expected to be developed that ensures the replicability of the proposed solution for the valorisation of waste from the steel industry by assigning photocatalytic properties according to their physical-chemical composition and processing variables. Thus, this project aims to generate generalised results to bridge the gap in scientific knowledge that limits the replicability of this technological application of slag, which may even be transferable to other types of waste. In doing so, this waste would become an eco-innovative and highly profitable business opportunity.
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