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Concrete is one of the world's most-used building materials with around 6 billion cubic meters produced each year. Concrete is used to construct buildings, roads, surfaces and has even been used to construct boats! Concrete is made from three primary ingredients - Cement, Aggregate and water. When mixed to appropriate levels concrete is formed as a sludgy type material. This can then be moulded into any shape and once dry is set to the shape of the mould.
The cement used in modern concrete buildings is Portland cement. This is made from heating limestone and clay, which is then ground up and mixed with a sulphate such as gypsum. Cement manufacture is a carbon intensive process requiring a lot of heat to produce.
CO2 is released from three main areas of cement manufacture.
The de-carbonation of limestone as it is heated produces around 0.5 kg of CO2 per KG of cement produced.
CO2 production from kiln usage varies from site to site but figures of around 0.3KG of CO2 per kg of cement are normally achievable in UK plants.
The power a plant uses. This varies with nuclear power providing virtually zero carbon emissions (apart from large releases during construction and transport of raw materials) to coal fired power which produces vast amounts of CO2.
When all three as well as other areas (such as plant machinery, explosive use etc.) are combined, the average CO2 release for a kilogram of cement can range from 0.7 kg of CO2 per kg of cement in ultra efficient plants, to as high as 1.5kg of CO2 per kg in less efficient plants.
Water required by concrete must be as pure and free of impurities as possible to ensure that the concrete remains strong and durable. This means that potable (drinking) water is used. The ratio of water to cement changes its properties. A high amount of water makes concrete more workable with a lesser amount of water making it more durable.
Once the water and cement mixture is completed aggregate is added to provide the bulk of the mix. Sand, gravel and crushed stones are the most commonly used aggregates, however all three require quarrying of some degree. To reduce the amount of new materials needed, recycled materials can be used to make the aggregate. These can include concrete rubble, ground bricks, tiles and even glass. Using recycled aggregate reduces the need for fresh materials and will reduce the amount of carbon emitted during concrete production.
Additives are also added to concrete. These can include pigments to change its colour as well as materials such as fly-ash (which is produced in coal power stations) which help the cement to dry.
When used as a building material concrete provides some of the highest pressure resistance per square inch (up to 10,000 PSI) allowing concrete structures to bear large amounts of weight. Concrete has a high thermal mass, which can be utilised when green building using passive solar design. Concrete also has good weatherproof capabilities able to resist moisture without the need for coating. Concrete does not expand and contract under heat as much as other materials such as metal although some movement should be allowed for. Where extra strength is required concrete structures can be reinforced with steel bars to increase their tensile strength.
The carbon produced during the creation of cement accounts for the bulk of concrete's CO2 emissions, with transport being second. Cement manufacturers have long sought ways in which to make cement more environmentally friendly. The most common method is to use alternative fuels to fire the kilns required. These include car tyres (which are difficult to store in landfills taking up more room per weight due to their hollow construction), waste solvents and lubricants (such as used motor oil which would need to be disposed of carefully to prevent it from poisoning water tables) and even bone meal produced from slaughter houses.
Cement and concrete building materials are estimated to contribute 5-10% of the world's total CO2 emissions. The much criticised aviation industry contributes only 4%.
Where possible, keeping its use to a minimum will reduce the carbon footprint of a build substantially. Where concrete must be used it is essential to ensure it contains as many recycled materials as possible to reduce the impact a build will have on the environment.
However after the initial build, concrete is one of the more sustainable building materials, when both the energy consumed during its manufacture and its inherent properties in use are taken into account. Over the life of a home, the operational CO2 emissions of a house have far more environmental impact than the embodied CO2 of the material used to build it.
Concrete is very good at heat storage. Some calculations (Arup 2007) show that over the lifetime of a building - easily in excess of 100 years - the gains in insulation and heat retention, which reduce the use of active energy systems, balance out against the higher energy cost of production, and actually make concrete more eco than wood. This depends on the type of insulation used in the timber structure.
The surprising results were mainly due to adding energy used in air conditioning, which is not actually used much in UK residences. Since high thermal mass buildings stay cooler longer, they use less energy for air conditioning.
These results for housing are of relevance to other sectors such as offices where a major design challenge is keeping cool. Here, adequate ventilation, solar shading and use of thermal mass are the three accepted methods of avoiding over-heating through passive means.
The moderate to high cooling loads associated with office environments enables significant energy savings to be realised if thermal mass and night ventilation is used to help avoid or minimise the need for air conditioning. This will in turn lead to a significant reduction in operational CO2 emissions, enabling any additional embodied CO2 burden to be offset in a short space of time.
This life-cycle analysis should be applied to all green technologies. As examples compare a highly efficient small diesel car with a heavy high tech battery powered 'hybrid' car; or nuclear power stations with solar arrays or wind turbines. Once life-cycle issues such as disposal, embedded energy in manufacture, and long term savings or coats are factored in, many seemingly green technologies are far from beneficial.
For more information see the UK's Concrete Centre website.