Recycling of waste for the needs of the construction industry
In Slovenia, a total of about 4.5 million tons of industrial and construction & demolition waste are produced every year. Most of these materials are just dumped, some illegally. The recycling rate is low, and does not exceed 30%. For this reason it will be necessary to make changes in the way we think and behave. Such changes are also required by the new European strategies, according to which waste must no longer be considered to be just an environmental problem, but should be unequivocally recognized as an important potential source of raw materials for industry. In this way a circular economy could be established, and the materials loop could be closed, so that the amount of waste which needs to be disposed of could be reduced. The greatest advantages of the use of such an approach in a Europe that is dependent on the import of energy and raw materials are the following: more efficient use of resources, less pollution, and improved economic competitiveness, while at the same time striving towards a global priority: ethical growth taking into account the realistic limits of our planet.
The construction industry is an ideal field for the use of recycled waste. This is because in this industry large quantities of materials are consumed, and even in cases where such materials are not environmentally inert it is possible, by means of various substances or processes, to permanently immobilize hazardous components. The characteristics of new products can be equal to or even better than those which are conventionally produced. The present legislation also permits such an approach: materials are not discriminated against, regardless of their origin or denomination; what matters is simply their characteristics, their potential uses, and their environmental footprint. A new and additional essential requirement (No. 7) was introduced in 2013 in the revised version of the Construction Products Act, which promotes the efficient and sustainable use of resources. Construction products made from recycled waste can be put on the market on the basis of harmonized standards or Slovenian Technical Approvals.
In construction composites, natural aggregate and / or traditional binders (cement, lime, bitumen) can, to a considerable extent, be replaced by recycled construction & demolition and industrial waste. An excellent alternative to natural aggregate is the steel slag which is obtained in the production of carbon steels. After recycling, in which high toughness and arough microstructure is achieved, it can be counted as one of the world's most high-quality aggregates. Waste foundry sand, too, has a high potential for use, after recycling, as an artificial aggregate, as well as other sands obtained from industry, and from construction & demolition waste,. Of exceptional importance for recycling are wastes in powder form with a binding capacity, either pozzolanic or hydraulic (e.g. ash obtained from coal-fired and biomass power plants, from municipal incineration plants, and from the incineration of paper sludge, as well as some types of steel slag, and gypsum waste from the chemical industry). These materials are an excellent replacement for part of the cement which is used in concrete. Concrete is the most important building material used in the construction industry, but from the perspective of sustainability it is controversial due to the fact that, in the production of 1 ton of cement, a total of 750 kg of CO2 is produced (according to recent data for the EU-28). If the cement content in concrete can be reduced by using binders made from recycled waste, then the carbon footprint of such production can be reduced by as much as 20%. The development of so-called "green concretes" using recycled waste (including hazardous waste, such as galvanic sludge and tar-contaminated asphalt) and their ethical use is therefore a need and challenge for the concrete production industry, as well as a great opportunity.
Many other types of waste can also be used as a potential source of raw materials. Municipal sludge which is produced as a solid waste in the cleaning of water is an excellent material for use in geotechnical composites. The use of different types of recycled plastics, waste cellulose, and waste plastic fibres is also feasible. An important source of raw materials, both in terms of quantity and quality, is the sediment which occurs in lakes, reservoirs, estuaries and port areas. Particularly promising are "a la carte" building composites, which consist of several different types of recycled waste, in which tailor-made combinations can provide the best possible synergistic effect. Contaminated soil, which can be remediated in situ, now appears to be a promising material for some geotechnical applications, especially for the construction of embankments. Such soil thus changes into a construction product, and can be incorporated back into the ground using standard procedures for earthworks.
On the behalf of owners of waste, ZAG can perform comprehensive research into the various possibilities for its use in the construction industry. Such research is always designed and performed in such a way that several different solutions are investigated in order to find the most realistic and balanced option, in terms of both the nature and quantity of the waste, taking into account the location of potential end users, the current market demand for the product, its carbon footprint, the price of recycling processes, and, in particular, the feasibility of the application for standard civil engineering technology, so that new materials can be produced, transported and installed with existing civil engineering machinery and technology. Higher added value of a new product or environmental technology is always welcome, but is not an absolute priority. It is also important that the final product is not harmful to the environment, and that the solutions used are robust − in this way variations in the quality of the recycled waste can be kept under control, which are larger than in the case of conventional materials. The research which is usually most successfully transferred into practice is such that the final end user of the construction product is, from the very start, included in the design process, so that a circular materials loop can be achieved.