Cemented in - challenges to the cement industry

Materials World magazine
,
5 Jun 2012
cement plant

Michael Forrest talks to Dwight Demorais and Dr Richard Leese of MPA Cement about the challenges facing the industry.

Cement is ubiquitous, familiar to all from DIY stores to large construction sites. Its principal use is in concrete, which has the advantage over other construction materials in that it can be readily poured into the desired shape, creating the flexibility in building design that makes our modern world. Cement is not new and is well known in history. The ancient Egyptians used burnt gypsum (calcium sulphate), while the Greeks and Romans used lime created by calcining limestone and adding sand to make a mortar. The Romans also discovered that by adding fine-grained, silica-rich volcanic ash from the village of Pozzuoli near Vesuvius, cement could be made that would set underwater and so facilitate harbour construction.

Today the cement-making process is universal and there is little difference in technique across the world. In Europe during the 1990s there was a consolidation in the cement industry, leaving five major producers in the UK: Tarmac (Anglo American), Lafarge (France) Hanson (Germany), CEMEX (Mexico) and Quinn Cement (Northern Ireland). All these companies have limestone quarries and linked cement plants. Together they account for the entire UK production and around 90% of UK consumption. They are members of the UK Mineral Products Association (MPA), a trade body that represents the producers of cement, aggregates and concrete in the UK.

In cement making, improvements and refinements are mainly related to materials handling and furnace efficiencies. The key resource is limestone – calcium carbonate – which occurs in a number of time periods throughout geological history. In the UK, the most prominent are the Carboniferous Period limestones that occur in the Pennines, Mendips and Northern Ireland, and the chalk of Cretaceous age that extends from southern England to Yorkshire. There are three essential ingredients: limestone, argillaceous clay or shale, and gypsum. Together they provide the essential elements of calcium, silicon, aluminium and iron. As limestone accounts for more than 80% of the input materials, cement plants are found adjacent to those quarries. Initially the rock is crushed into pebble-sized pieces to which the other minerals are added and then milled to a fine powder. The amounts of additional minerals determine the cement type and characteristics. The next stage is to calcine the mineral mixture at 1,450ºC in long, tubular, rotating furnaces that result in the formation of crystalline solids known as clinker. These are ground again with the addition of a small amount of gypsum to form CEM I Portland cement, formerly known as ordinary Portland cement (OPC).

The process is carbon-emission intensive, first in the fuel required to calcine the limestone – mainly to calcium oxide (CaO) by the liberation of carbon dioxide – and secondly in the chemical reaction to form clinker. CEM I Portland cement is hydraulic cement. It requires the addition of water to hydrate through an exothermic reaction, causing the mixture to set, usually in less than an hour. However, the hardening process continues indefinitely if free moisture is present, but standardised strength is determined at one month.

‘The CO2 liberated in calcining limestone is stoichiometric and cannot be avoided,’ says Dwight Demorais, Special Advisor at MPA Cement and responsible for the industry’s performance reports. However, the industry has been working hard to reduce these emissions and particularly those of carbon dioxide, which, in turn, means a reduction in the use of fossil fuels in calcining the limestone. ‘This can be achieved in two ways,’ says Demorais. First is in the use of alternative raw materials (ARM) to reduce the amount of limestone and improvement in the thermal efficiencies of the rotary kilns. These are up to 50 metres long and angled to allow the materials to flow through by gravity. Second is in the substitution of fossil fuels by combustible waste.

Demorais explains, ‘In the UK there was a signifi cant downturn in demand during the 1990s, at which time the profitable producers took out old, inefficient plant, replacing it with more modern equipment.’ This marked the end of the energy-intensive wet method of producing clinker that required significant heat input to dry the raw materials before formation. It is then ground with the addition of gypsum to complete the cement product before packaging and distribution. Today all UK plants use the dry or semi-dry method of producing clinker and, therefore, use less energy.

However, the major problem facing the UK industry is the European Union Emission Trading System (EU ETS), which requires an overall reduction of 21% below 2005-verified emissions by 2020. These reductions are consistent with the EU’s longer-term aim to deliver an 80% cut in emissions by 2050, a stringent target that may force cement production to those non-EU countries where ETS trading is not enforced. According to Dr Richard Leese, Energy and Climate Change Director at the MPA, ‘It is not just the cost of carbon emission trading, but the cumulative burden of environmental, energy and carbon costs’. Research by Boston Consulting Group commissioned by the European cement industry’s trade body, CEMBUREAU, determined that at a CO2 cost of around £10 per tonne, 36% of UK clinker production is at risk of being replaced by imports. This would mean that production within 50 kilometres of ports would be in danger of closure. If the cost of emitting CO2 were to rise to £19 per tonne, production would be at risk in the whole of the UK – imports would be more cost-effective than local production at a distance of 250 kilometres from the ports. The cement industry is regarded as vulnerable to carbon leakage, where local production is replaced by imports. In all, there are 164 sectors considered at risk on a list that will be reviewed by the EU Commission by 2014.

Carbon leakage can take place when companies move their production to countries where the ETS does not apply, such as Turkey or China. Of course, this will not reduce global cement CO2 emissions but it will result in EU countries suffering unintended consequences of loss of jobs and increased imports. The MPA intends to fight its corner to protect the UK cement industry as a special case.

Overall, the UK cement industry’s environmental record is good. In comparison to a 1998 baseline, it has reduced its nitrous oxide emissions by 59%, sulphur dioxide by 87%, cement kiln dust (CKD) by 83% and CO2 by 22%. It is also a net consumer of waste – 1,320,000 tonnes against 14,021 tonnes produced. These have not come cheaply, says Demorais, ‘The industry has made a multi-million pound investment to reduce emissions and meet a series of environmental performance objectives’. One of the most successful has been the use of waste raw materials in the product. In 1998, 1,498 kilogrammes of natural raw materials were used per tonne of OPC. The largest substitutions are alternative raw materials, granulated blast-furnace slag and pulverised fly ash together with construction waste, CKD, ceramic moulds and recycled road-sweepings, which together have reduced this to 1,378 kilogrammes per tonne, less than the 1,420 kilogrammes per tonne target for 2010. These are introduced into the cement mill. The recycling of CKD has been one of the most significant achievements – today only 1.8 kilogrammes per tonne require disposal, against a 2010 target of 7.5 kilogrammes. This has been possible through re-injection into the clinker grinding process. Work with the Environment Agency has also enabled the re-introduction of CKD as an agricultural fertiliser that adds potassium and corrects soil acidity.

The UK industry has reduced CO2 emissions by 22% (199 kilogrammes per tonne of cement) from its 1998 baseline of 924 kilogrammes, more than meeting the 2015 target of 775 kilogrammes per tonne by achieving 725 kilogrammes in 2010. Some of this can be attributed to waste materials replacing limestone, but the majority is down to the use of waste combustible materials, some of which posed serious environmental disposal challenges due to lack of landfill capacity in the UK. Scrap vehicle tyres, waste oils and solvents, packing and refuse-derived materials, meat and bone meal, and wood are all used. Leese explains, ‘There is an investment cost in the use of alternative waste fuels as they need to be locally sourced to prevent negation by transport costs’. They also have to be a reliable resource to warrant the plant modifications for their use.

If these levels of progress can be maintained, the presence of cement in everything from high-rise buildings to sub-sea level harbour walls looks set to continue into 2050 and beyond.