Concrete

As possibly the most extensively used building material, concrete attracts criticism from environmentalists on account of its carbon-intensive production techniques and its use of a once-only natural resource, limestone. Cement is formed by heating clay and lime in a rotary kiln to a temperature of about 1450°C which produces some 3000kg per tonne of CO2. In addition, the heating process produces a chemical reaction through the conversion of calcium carbonate into calcium oxide which releases about 2200 kg of CO2. Add to this the carbon miles in transportation, the impacts caused by mining etc., and concrete gains few points on the sustainability scale.

The UK Climate Change Levy is providing a powerful incentive for manufacturers to reduce CO2 emissions since an 80% abatement of the tax can be granted in return for specific CO2 abatement strategies. One method of reducing concrete's carbon impact is to use pulverized fuel ash (PVA) to reduce the proportion of cement in the concrete mix. These new blended cements contain up to 30% of PVA, a by-product of coal-fired electricity generation. This use of PVA has the further advantage of avoiding landfill costs whilst also reducing the need to quarry natural aggregates such as gravel. However, it must be remembered that it is the waste product of carbon-intensive coal-fired power stations. It also contains some toxic chemicals. Its availability will decline as coal-fired power stations are phased out.

The mountains of slate waste which encircle Blaenau Ffestioniog in north Wales are destined to become a powdered aggregate in the near future. This will offer the double benefit of avoiding depleting a natural resource and also drastically improving the amenity value of an area scarred by slate mining over the centuries.

The development of the technology of geopolymers offers the prospect of a more eco-friendly concrete. Geopolymerization is a geosynthesis which is a reaction that chemically bonds minerals to form molecules that are structurally comparable to the molecules which provide the strength to rocks. In the opinion of Jean Davidovits of the French Geopolymer Institute at St Quentin, these 'geopolymeric' concretes would reduce CO2 emissions associated with conventional concrete by 80-90%. This is said to be due to the avoidance of calcination from calcium carbonate and the lower kiln temperature of 750°C. The market availability of this material is said to be at least five years away.1

Recycled crushed concrete has been used for some time for low-grade applications like road construction. It is now being heralded as being suitable for less demanding structural elements. One attraction of this material is that it gains a BREEAM (Building Research Establishment Environmental Assessment Method) credit if used in sufficient quantity. The main disadvantage of the material concerns quality control. A consignment may contain concrete from numerous sites, which means that each batch must be tested by being sieved and chemically analysed to check its ingredients and quality.2,3

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