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Concrete In Australia : March 2012
34 Concrete in Australia Vol 38 No 1 1. INTRODUCTION It is estimated 15 billion tonnes of concrete is used worldwide per annum, and this figure is increasing rapidly with the current worldwide infrastructure boom. Concrete is the highest consumed material, far exceeding the coal, oil and steel which are used in quantities that are unsustainable in the long term. Almost all the materials used to manufacture concrete are virgin materials mined from the earth. Recycled concrete is a term used for concrete containing crushed concrete aggregates, but the recycled concrete making still requires virgin cement and admixtures. In fact, crushed concrete is often weaker than conventional aggregate and so requires higher cement content to compensate for strength development, resulting in concretes with higher carbon emissions. Crushed concretes are better utilised as road base or other aggregate substitutes where it does not lead to increased carbon emissions. Figure 1 shows the most highly consumed materials by humans (these figures are compiled by the author from various sources). Consumption of virgin materials at these levels is unsustainable and any discussion on the sustainable future should include sustainable alternatives to these materials. Carbon emissions due to concrete manufacture are the fourth largest contributor to man-made global carbon emissions, only falling behind oil, coal and natural gas. Carbon emissions due to concrete manufacture range between 0.3 to 0.4 tonne of CO2 per cubic metre of concrete, depending on the type of concrete as estimated by Flower et al (2005) shown in Figure 2. e figure shows the carbon emissions of various grades of concrete and relative contributions of each of constituents of concrete for the carbon emissions. However, this will not directly translate into increased price due to carbon tax. Due to very generous allowances for untaxed emissions, only a fraction of the full cost of the carbon tax of $23 will be passed on to the price of concrete, which would be negligible. Portland cement is the dominant source of carbon emissions in all of the concrete mixes. A major part of carbon emission generation in Portland cement manufacturing comes from limestone releasing CO2 which would otherwise be buried in the ground; this is similar to CO2 emissions from coal and oil. It can be seen in Figure 2 (Flower et al, 2005) that the percentage of the total carbon emissions associated with Portland cement increases considerably in the higher strength concrete mixes. However, the other processes and materials in concrete still generate substantial amounts of carbon emissions and must be considered in the calculation of total emissions estimates. It is also clear that if the emissions of a concrete are to be reduced significantly, Portland cement is the main component that should be targeted. Carbon emissions due to Portland cement in concrete can be reduced by partially replacing it with supplementary cementitious materials (SCMs). SCMs are industrial byproducts with little carbon emissions. Most commonly used SCMs are fly ash, ground granulated blast furnace slag (GGBFS) and condensed silica fume. ese SCMs have long track records in construction in many countries, typically well over three decades. To investigate two of the methods by which the amount of CO2 generated by concrete can be reduced, four mixes are considered with binders including SCMs. e first Non-Portland based cements and concretes Jay Sanjayan Professor of Concrete Structures, Centre for Sustainable Infrastructure Swinburne University of Technology, Hawthorn, Victoria Discussions on sustainable use of resources should include concrete which is the most highly consumed material without a major option for re-use or recycling method. e amount of virgin materials consumed for concrete making is unprecedented and rising very rapidly with the worldwide infrastructure boom. Carbon emissions from Portland cement manufacture are second only to the emissions from the use of fossil fuels. With the looming introduction of a price on carbon, there may be more incentives to re-visit the alternative low carbon cements. Many of the technologies of producing cements from waste materials, such as fly ash and slag as resource materials have been known for several decades. However, commercial implementations of these novel cements are only taking place in recent years. e paper outlines the major technological advances in Portland cement free concretes and discusses the future directions. Figure 1. Highest consumed materials. 2/4 3/6 4/8 7/8 35/5 2 6 32 36 #)%-.(& $.'') // // "() !+%) !+*&,'.' Billion Tonnes / Year