Concrete - it’s greener than you think
While “greenwash” (the practice of making an unsubstantiated or misleading claim about the environmental benefits of a product) has become common practice in the marketing world, the Cement & Concrete Institute recognised the importance of validating claims concerning the greenness of concrete with integrity and scientific evidence.
To be able to quantify the environmental advantages of concrete, the Institute commissioned InEnergy Africa to determine the CO2 emissions of the most commonly used constituent materials in concrete. i.e. cement, aggregate, water, fly ash, ground granulated blast furnace slag (ggbs) and admixtures, and the processes of production of both precast, and readymixed concrete. InEnergy was also requested to develop a model to allow users to determine the CO2e (equivalent) from the production of a cubic meter of concrete using various materials.
The development of the model was the next logical step in the South African cement manufacturers commitment to responsible manufacturing that can be traced back to the 1990’s when the first reports about the burning of scrap tyres in kilns were published in the local press. The use of waste tyres in kilns reduces the percentage of CO2per GJ of energy consumed by approximately 11%, while the burning of tyres results in curbs on ash emissions. The cement producers have managed to boost production while decreasing the use of finite raw materials, while the use of cement extenders has significantly reduced the clinker portion in certain cement products. The industry has travelled an impressive road since then with the introduction of modern technology such as the fitting of bag house filters and electrostatic precipitators, to further reduce particulate emissions. Other emissions are being reduced by the use of pre-calciners and pre-heaters. With this world class technology in place, the producers have already reached 50% of the target set by the Department of Minerals and Energy which calls for a 15% reduction of energy consumption by 2015.
The rehabilitation programmes of mines and quarries, together with other Corporate Social Investment programmes, consolidate the industry’s buy-in to the Bruntland definition of sustainability: Development that meets the needs of the present without compromising the ability of future generations to meet their own needs.
Codes and Standards
It was also during the 90’s that SA adopted the European standards for cement, in line with development overseas to concentrate on more environmentally friendly practices. The increased use of more extended cements over the past years is testimony to the fact that manufacturers and consumers have already inadvertently bought into the concept of using environmentally friendly products long before the media and environmentalists intensified their campaigns.
Chart showing increase of extended cement sales compared to other cements over a 4-year period.
Currently large clients such as SANRAL and ESKOM are moving towards durability specifications and away from only specifying strength in an attempt to achieve durable concrete. Even though the materials used during construction only account for between 10 and 20% of a structure’s whole life energy consumption (the rest coming from the energy consumption or human interaction such as lighting and heating and cooling), materials specifications now play a critical role to reduce the embodied energy in a building. With the adoption of the Green Star rating in SA, materials manufacture needs to be factored in terms of measurable emissions, energy and finite material consumption. Buildings must now be constructed with longer life spans in mind with the emphasis on durability, and then retrofitting rather than demolishing.
The use of admixtures which result in a reduction of cement and water content in mixes, together with the use of cement extenders such fly ash, ggbs, and silica fume have for many years been standard practice in the concrete industry, initially only to reduce costs. The use of these materials have however also a pronounced effect on improving the durability and sustainability of concrete.
Precast products such as hollow-core slabs also reduce the volume of in-situ concrete substantially, while the use of the new generation of permeable concrete pavers form part of a responsible water management and safety programme by getting water off the surface of a road and letting it get back into the groundwater. Self-compacting concrete in sustainable developments allows for architectural shapes and forms previously regarded as impossible.
Moreover, new research is producing exciting data on the re-absorption of carbon dioxide by hardened concrete. A Danish study has found that 50% of the volume of concrete will be ‘carbonated’ over 70 years of any building’s service life. This sponge effect makes concrete a more green choice than previously thought, emphasising how global sustainability can be achieved with concrete.
The InEnergy study was done on a cradle to gate basis and had two goals: to quantify emissions of concreting materials, and to manage these emissions. The model that was used took into account Scope 1, 2 and 3 emissions, which included transport of materials. Average emission numbers expressed in kg CO2/ tonne of material produced are available in the report. A model was then designed enabling the designer to input specific materials and calculate the Carbon footprint of a given mix, thereby making it possible to find the optimum mix to minimise the environmental impact of a the concrete in a proposed project or structure.
Being able to calculate the CO2 for materials and concrete does however not tell the full story.
Sustainability is not only about embodied energy and CO2 emissions. Rather, a balance needs to be achieved between economic, social and environmental factors. This concept is often referred to as the Triple Bottom Line.
The Triple Bottom Line
Concrete has a number of inherent characteristics that contribute towards achieving sustainability, whether it is for the benefit of the owner, the developer or the designer.
Modern civilisation is built on concrete and as such the positive social impacts are immense. The concrete industry supports and uplifts social infrastructure and provides dignified and durable shelter that all human beings have a right to. To address the economical impact, construction and maintenance costs are monitored to ensure positive life cycle costs. On the environmental side, the energy efficiency and thermal mass of concrete increase sustainability. Case studies can already prove that the recyclability of concrete make it a very attractive option as a construction material.
All things considered, concrete is virtually unequalled in combining the many unique qualities and attributes to make it the responsible choice in construction materials, balancing all the elements for increased sustainability.
In fact, concrete is much greener than you think.