BRE IP7/08 PDF

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 (Mt) in 1970 to 2284 Mt in 2005, with virtually all of this growth occurring in developing countries⁽¹⁾. The UK produces approximately 12 Mt of Portland cement per annum. However, much of this material is used in concrete into which ground granulated blastfurnace slag (ggbs) or pulverized fuel ash (pfa) is added separately. Recent forecasts are that global cement production could exceed 5 Bt per annum by 2050.

Cement manufacture is energy intensive. The raw materials (usually limestone and clay) are finely ground and homogenised, and then heated to about 1450°C in a rotary kiln to form Portland cement clinker. On cooling, the clinker is finely ground with small amounts of ingredients such as gypsum. 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 limestone (CaCO₃) (about 1.2 t 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 the composition of the fuel and raw materials, and the energy efficiency of the kiln system. The amount of CO₂ produced per tonne of finished cement varies, depending on cement composition, manufacturing efficiency etc. and is therefore difficult to quantify. However, recent data on global CO2 emissions from cement manufacture give an overall average of 0.88 t/t of cement produced and a value for Europe of about 0.63 t/t. (This figure includes all cement types, not just PC.) UK data indicates that 0.82 t of CO₂ is produced per tonne of CEM 1 cement. These data imply that about 2 Bt of CO₂ per annum are produced from cement manufacture globally, which therefore accounts for about 7.5% of total⁽²⁾ global anthropogenic CO₂ emissions, and about 1.7% of anthropogenic emissions in the UK.

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, 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. Low energy cements have been reviewed by BRE⁽³⁾.

The main mineral components of PC are alite (tricalcium silicate), about 60%, and belite (dicalcium silicate), about 20%, which are formed at about 1450°C and 1200°C, respectively. This Information Paper gives the background to work carried out by BRE to support the development of cements with high proportions of the minerals belite and calcium sulfoaluminate. Production of the clinkers for such cements requires a lower proportion of limestone and they can also be manufactured at lower temperatures than conventional PC clinkers. Consequently less energy is required in their manufacture and CO₂ emissions are reduced.

The Paper is based on BRE Report 496 Calcium sulfoaluminate cements: CO₂ reduction, concrete properties and applications⁽⁴⁾, which provides background information on these cements, and summarises the results of extensive practical work carried out to assess their performance in concrete.