española del Pediatric Evaluation Disability Inventory (PEDI)

Until very recently, the majority of Portland cement was produced using fossil fuels as the primary, or only, fuel. Coal was the most common fuel but oil and gas were also used, and still are. Fossil fuel use in many cement plants has now been reduced principally by one or more of the following:

 Improvements in plant efficiency, mainly by phasing out old wet process kilns in favour of dry process kilns.

 Using alternative fuels, such as waste solvents, waste paper, old car tyres and bonemeal.

 Producing mineralised clinker.

and a smaller proportion of the fuel that is used is fossil fuel.

The use of alternative fuels to partly replace fossil fuels means that a scarce resource is conserved and also that nuisance wastes are disposed of productively.

Some hazardous wastes can be disposed of more effectively by burning in a cement kiln than by other means, due to the high burning temperature and longer residence time compared with an incinerator.

At least one UK cement works now produces clinker using 70% alternative fuels and only 30% coal. This significant achievement is all the more impressive considering that some of the non-combustible residues in these fuels affect clinker composition. Car tyres contain steel, so the iron content of the clinker will change unless the raw feed composition is adjusted; they also contain zinc, a retarder of cement hydration. Bonemeal contains calcium phosphate which is likely to inhibit alite formation unless well-dispersed throughout the clinker;

bonemeal is therefore only used in limited quantities in order control the clinker phosphate content.

Mineralised clinker is produced by the addition of small amounts of mineraliser, typically calcium fluoride, to the raw feed; the alite produced contains aluminium and fluorine substituting for a small proportion of the silicon. Not only can this alite give improved early strengths compared with “normal” alite, it forms at a lower temperature. Instead of burning temperatures of about 1450 C, clinker can be produced between about 1050 C – 1200 C. Clearly, this will require

significantly less energy. Not all raw materials are suitable for mineralising.

12.2 Reducing CO

About 5% of the carbon dioxide produced by man’s activities is due to cement production and, worldwide, roughly 900 kg CO2 is produced per tonne of cement (1), a figure dating from 2003 that will be somewhat less now; this is an industry average that includes cement plants with less efficient wet-process kilns and more efficient dry process kilns. In the UK, CO2 emissions directly from cement plants per tonne manufactured were 777 kg per tonne (2) in 2008. Evidently, this is significantly less, although without knowing just how the data were collected, it is not possible to be sure that the two figures were calculated on a like-for-like basis.

As a back-of-envelope calculation, assuming cement to contain 65% CaO and assuming that all the CaO comes from pure limestone, to make 1000 kg cement requires 650 kg CaO which in turn would be produced by 1161 kg of pure

limestone. Therefore, 1161-650=511 kg CO2 is produced from the calcination of the limestone, or 57% of the total, assuming 900 kg CO2 emitted per tonne cement.

The bulk of the other 43% of the total CO2 will be due to burning fuel in the kiln, with the remainder due to CO2 from electricity generation used in grinding the raw materials, milling the clinker and transporting raw materials and the finished product.

The bulk of the CO2 emitted by the burning of alternative fuels (paper, tyres, bonemeal) is recycled CO2 that was only recently taken from the atmosphere;

unlike the burning of fossil fuels, continued burning of these fuels should not produce a cumulative increase in atmospheric CO2.

Alternative fuels can therefore make a significant reduction in the net CO2

emissions related to cement production. However, the bulk of the CO2 emitted is still due to limestone calcination. Since this is an integral part of the process of Portland cement production, it seems there are only three ways in which this can be significantly reduced:

 Use less Portland cement

 Capture the emitted CO2

 Develop alternative cements not based on the calcination of carbonate raw materials

Some experimental alternative cements are indeed being developed. It will be interesting to see how this progresses, but at present they seem unlikely to offer a viable and widely-available alternative in the short or medium term to cements based on Portland cement.

Capturing CO2 from cement plants is also being examined but will be expensive and seems unlikely to happen to any great extent in the short term.

Using less Portland cement is a very viable option and is already being done. For example, for a 50/50 mix of Portland cement and slag compared with 100%

Portland cement, CO2 emissions are reduced considerably. Using a figure of 52 kg CO2 per tonne slag (Table 7.1), instead of 900 kg CO2 emitted per tonne of

Portland cement, we have 450+26=476 kg CO2 per tonne cement.

If we factor in that perhaps only one-third of the fuel used to make the Portland cement need be fossil fuel, the total CO2 produced from limestone calcination and the burning of fossil fuel could be as low as about 350 kg tonne^-1 cement. This is only about 40% of the initial 900 kg tonne^-1 cement - quite a saving.

Of course, this assumes that there is sufficient slag (or fly ash) available, without the need for transporting it over long distances, and that, from a global

perspective CO2 reductions are accounted for consistently. The steel and electricity industries may want to include these savings in calculations for their own activities and the savings can’t be counted twice.

Nevertheless, it is clear that by using alternative fuels and composite cements, the cement industry can make major savings in its use of raw materials and fossil fuels, make effective use of rubbish that would otherwise have gone to landfill as well as reduce its CO2 emissions. In coming years, climate science will doubtless evolve, and global temperatures will continue to vary; meanwhile, these savings enable us all to be better custodians of our planet.

References, Chapter 12

1. "The Cement Industry and Global Climate Change: Current and Potential Future Cement Industry CO2 Emissions". M Natesan; S Smith, K Humphreys, (2003). Greenhouse Gas Control Technologies - 6th International Conference.

Oxford: Pergamon. pp. 995–1000. ISBN 9780080442761.

2. http://www.cementindustry.co.uk/PDF/MPAC_Performance_2008.pdf

Having now got to grips with some of the basics, I very much hope that you will continue to explore cement science with some of the excellent textbooks

available.

If you have access to a technical library, the following books will doubtless be available. However, if your work involves cement in any significant aspect, you should probably get your own copy of some of the more important books anyway.

If you are an engineer rather than a chemist, the first of the books on the short list below would be an excellent purchase, especially if you can wait for the new edition to come out. However, if you are involved in cement or concrete

production from a technical or scientific perspective, or if you are an academic with an interest in either cement or concrete, you really need the first three books: Bye, Taylor and Lea. These three are the essentials of any cement reference library.

“Portland Cement: Composition, Production and Properties,” G C Bye, pub. Thomas Telford Ltd., 2^nd edition, 1999. ISBN-13: 978-0727727664.

This is a superb little book, the small size of which belies the huge quantity of concentrated information that it contains. A third edition is in preparation.

“Cement Chemistry”, H F W Taylor, pub. Thomas Telford, 2nd edition 1997. Language: English. ISBN: 07277 2592 0

This book is the distillation of an eminent scientist’s lifetime’s work in cement chemistry. It contains about 460 pages on cement composition and production, high temperature chemistry, cement hydration and the hydration products, composite cements, calcium aluminate and other cements and concrete chemistry. There are also 44 pages of references. The assumed level of knowledge on the part of the reader is quite high in some parts of the book.

There are still some places in it that I don’t go to on a dark night on my own, but like great works of literature, over the years more is revealed as you re-read it.

Some proficiency in chemistry as well as in crystallography and various

techniques of instrumental analysis, would undoubtedly be of benefit in getting the best from the book. However, at least two-thirds of the text is quite

accessible with only a basic level of chemistry and anyone who has absorbed the content of “Understanding Cement” should have no hesitation in getting Taylor’s book. A well-used copy should certainly be on the bookshelf of every works chemist, concrete plant technical manager, engineer, academic and anyone else with a detailed interest in cement science. I admit to having two, one at work and one at home; some people may consider this excessive, but I can’t think why.

“Lea’s Chemistry of Cement and Concrete”, 4th edition, edited by Peter Hewlett, pub. Elsevier, 1998. Language: English. ISBN-13: 978-0-7506-6256-7

First published in 1935, this updated splendid work of reference contains 16 detailed chapters, each written by experts in that niche of cement or concrete.

This is a weighty book, both physically and in content, and runs to over 1000 pages. Generally, the assumed level of the reader’s knowledge is perhaps slightly less than that assumed by Taylor. This is another “must have” for anyone with a serious interest in cement and concrete chemistry.

“Properties of Concrete”, A M Neville, pub. Prentice Hall, 4th edition 1995. Language English: ISBN-13: 978-0582230705

This book is focussed more closely on concrete from an engineering viewpoint but it contains some accessible cement and concrete chemistry. It is widely regarded as a standard text on concrete.

“Concrete Petrography”, D A St John, A W Poole and I Sims, pub Arnold, 1998, Language English: ISBN-13: 978-0340692660

The standard text for anyone interested in concrete petrography, this book contains a mass of information on petrographic techniques and also some very accessible cement and concrete chemistry.

“Chemical Fundamentals of Geology”, Robin Gill, pub. Chapman and Hall, 2^nd edition 1996. Language: English. ISBN-13: 978-0412549304

This is an excellent introduction to geochemistry, and by extension, to some of the basic tools of cement chemistry. The book isn’t about cement - cement isn't even mentioned - but I have found it to be invaluable.

If everything goes wrong and your concrete breaks up beyond all hope of repair, you can still put it to good use and grow strawberries…