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INTRODUCTION

Concrete is likely to continue to be the primary volume construction material for most structural applications and its use is likely to grow. Portland cement (PC) and blended Portland cements are currently the only economic binders for concrete that meet the performance and durability requirements under the wide range of conditions to which it is exposed.

Global cement manufacture (including both ‘pure' and ‘blended' Portland cements) rose from 594 million tonnes in 1970 to 2284 million tonnes in 2005 with virtually all of this growth occurring in developing countries. The UK produces approximately 12 million tonnes of Portland cement per annum (11.55 tonnes of CEM 1 cement was produced in 2005). Global production is likely to increase significantly over the coming decades as the global population increases. Recent forecasts have indicated that global cement production could exceed 5 billion tonnes per annum by 2050.

Cement manufacture is energy intensive. The raw materials are finely ground and homogenised, and then heated to about 1450°C to form Portland cement clinker. On cooling, the hard nodules of clinker must be finely ground with small amounts of other ingredients such as gypsum to make the finished cement. Energy consumption as fuel and electricity consequently represents about 65–70% of the variable costs associated with Portland cement manufacture.

Cement manufacture also produces large amounts of CO₂ due to energy use and the calcination of CaCO₃ (about 1.2 tonnes of CaCO₃ are required to produce a tonne of a typical modern Portland cement clinker). The amount of CO₂ produced per tonne of Portland cement clinker manufactured depends on a number of factors such as clinker, fuel and raw materials compositions and the energy efficiency of the specific kiln system. The amount of CO₂ produced per tonne of finished cement varies, depending on cement composition (e.g. content of gypsum and other non-clinker ingredients), manufacturing efficiency etc. and is therefore difficult to quantify. However, recent data on global CO₂ emissions from cement manufacture (discussed in Section 2) give an overall average of 0.88 tonnes per tonne of cement produced (the same data indicate a value for Europe of about 0.63 tonnes/tonne). Data for the UK indicates that 0.82 tonnes of CO₂ is produced per tonne of CEM 1 cement¹. These data imply that about 2 billion tonnes of CO₂ per annum are produced from cement manufacture globally. Cement manufacture therefore accounts for about 7.5% of total^v global anthropogenic CO₂ emissions, and about 1.7% of anthropogenic emissions in the UK². It is reported to be the third highest readily-identifiable source of anthropogenic CO₂ emissions after fossil fuel combustion (e.g. for electricity and transportation) and deforestation.

The pressure to reduce energy consumption and CO₂ emissions during cement manufacture has led the industry to increase the extent to which Portland clinker is substituted in conventional cements by other ingredients that are currently approved in the existing norms, such as granulated blastfurnace slag (ggbs), pulverized fuel ash (pfa), natural pozzolans and limestone. However, there have until recently been few serious attempts to develop novel cements based on alternative clinkers with intrinsically lower energy requirements and CO₂ emissions during manufacture than conventional Portland cement clinkers. Possible alternative cements have been reviewed elsewhere.

This report summarises work carried out to support the development of cements which contain high proportions of the minerals di-calcium silicate (belite) and calcium sulfoaluminate (ye'elimite). Production of the clinkers for such cements requires a lower proportion of limestone and they can also can be manufactured at lower temperatures than conventional Portland cement clinkers. Consequently less energy is required in their manufacture and CO₂ emissions are reduced. The report provides background information on these cements, and summarises the results of practical work carried out to assess the performance of these cements in concrete.

¹ British Cement Association data

² DTI data indicate that total UK net CO₂ emissions were 561 million tonnes in 2005. Assuming UK CO₂ emissions are 0.82 tonnes/tonne CEM 1 cement and that 11.5 million tonnes per annum are produced then about 9.4 million tonnes of CO₂ would be produced.