Understanding Cement

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(3) Understanding Cement An introduction to cement production, cement hydration and deleterious processes in concrete. Nicholas B Winter WHD Microanalysis Consultants Ltd.

(4) Published by WHD Microanalysis Consultants Ltd Iken House, 8 Acer Road, Rendlesham, Woodbridge, Suffolk IP17 1PL United Kingdom Copyright © 2009 N B Winter. All rights reserved. This work is registered with the UK Copyright Service Registration No: 307063 NOTICE: This e-book is sold for individual use only and may be stored on a single computer. It MAY NOT be stored on any computer network or other retrieval system that allows access by more than one person. You MAY print a single copy for your own personal use. You DO NOT have the right to reprint or resell this e-book. You also MAY NOT give away, sell or share the content herein. Please contact us for details of CORPORATE or EDUCATIONAL multiple user licensing If you obtained this e-book from any source other than through http://www.understanding-cement.com, you have a pirated copy. Please help stop Internet crime by reporting this to: mailto:[email protected] THE PUBLISHER AND AUTHOR MAKE NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY, APPLICABILITY OR COMPLETENESS OF THE INFORMATION CONTAINED IN THIS WORK AND SPECIFICALLY DISCLAIM ALL WARRANTIES INCLUDING WITHOUT LIMITATION WARRANTIES OF FITNESS FOR A PARTICULAR PURPOSE. THE PUBLISHER AND THE AUTHOR DISCLAIM ANY AND ALL RESPONSIBILITY FOR THE APPLICATION OF ANY OF THE CONTENTS OF THIS WORK SINCE ANY SUCH APPLICATION IS OUTSIDE THEIR CONTROL. THE DESCRIPTIONS AND COMMENTS CONTAINED IN THIS WORK MAY NOT BE APPLICABLE TO EVERY SITUATION, REGARDLESS OF ANY APPARENT SIMILARITY TO THOSE DESCRIBED. ANYONE MAKING USE OF THE INFORMATION IN THIS WORK ASSUMES ALL LIABILITY ARISING FROM SUCH USE. IN ANY CRITICAL APPLICATION, THE SERVICES OF AN INDEPENDENT, COMPETENT, PROFESSIONAL PERSON SHOULD BE OBTAINED. THE PUBLISHER AND AUTHOR SHALL IN NO EVENT BE HELD LIABLE FOR ANY LOSS OR OTHER DAMAGES, INCLUDING BUT NOT LIMITED TO SPECIAL, INCIDENTAL, CONSEQUENTIAL OR OTHER DAMAGES. REFERENCE TO ANY ORGANISATION OR WEBSITE IN THIS PUBLICATION DOES NOT MEAN THAT THE PUBLISHER OR AUTHOR NECESSARILY ENDORSES ANY INFORMATION OR RECOMMENDATIONS THE ORGANISATION OR WEBSITE MAY PROVIDE. READERS SHOULD BE AWARE THAT THE CONTENT OF WEBSITES REFERRED TO IN THIS WORK MAY CHANGE, OR THE WEBSITE MAY DISAPPEAR, BETWEEN WHEN THIS WORK WAS WRITTEN AND WHEN IT IS READ. THIS WORK DOES NOT PURPORT TO ADDRESS ALL OF THE SAFETY CONCERNS, IF ANY, ASSOCIATED WITH ITS CONTENTS. IT IS THE RESPONSIBILITY OF THE READER TO ESTABLISH APPROPRIATE HEALTH AND SAFETY PROCEDURES..

(5) Contents. Introduction. vi. 1. A brief history of cement. 1. 2. Cement basics. 5. 2.1 2.2. 5 6. 3. Portland cement composition and microstructure 3.1 3.2 3.2.1 3.2.2 3.3 3.4 3.5 3.5.1 3.6 3.7. 4. Some definitions Some non-Portland cements. 9. Portland cement minerals Portland cement composition and cement notation Note on oxide analysis Cement chemistry notation Microstructure of clinker and cement Hydraulic properties of the main clinker minerals Proportions of the main clinker phases The Bogue calculation Common parameters used in cement manufacturing Other Portland cement types. 10 12 12 13 15 18 19 20 26 28. Portland cement manufacturing - the main components of a cement plant. 32. 4.1 4.2 4.3 4.4. 32 33 39 40. The The The The. quarry kiln cooler cement mill.

(6) 5. 6. 7. Portland cement manufacturing – from raw materials to cement. 43. 5.1 5.2 5.3 5.4 5.5 5.5.1 5.5.2 5.5.3 5.5.4 5.6 5.7 5.8 5.9. 43 43 45 49 50 50 51 52 53 54 57 59 61. Raw material blending Minor constituents Combinability temperatures Raw material proportioning Reactions in the kiln Reactions before the burning zone Reactions in the burning zone Reducing conditions Cooling of Clinker Clinker sulfate phases Clinker grinding and gypsum addition Other additions Calculation of clinker minerals in cement. Hydration of cement – chemical and physical properties of cementitious materials 6.1 Hydration of cement: heat evolution 6.2 Hydration of cement: main types of hydration product 6.3 Hydration of cement: further considerations 6.3.1 AFm and AFt phases 6.3.2 Flash set and false set 6.3.3 Hydration of cement: other hydration products 6.3.4 Description of cement hydration 6.4 Hydration of cement: paste microstructure and water/cement ratio 6.5 Hydration of cement: pore structure of cement paste and the Powers-Brownyard model 6.6 Some physical properties of cementitious materials 6.6.1 Concrete workability 6.6.2 Concrete strength 6.6.3 Concrete permeability 6.6.4 Other factors. 63. Composite Cements. 84. 7.1 7.1.1 7.1.2. 84 84. 7.1.3 7.1.4 7.1.5 7.2 7.2.1 7.2.2 7.2.3 7.3 7.3.1. Introduction Background Difference between pozzolanic and latently hydraulic mineral additions Effects of mineral additions on hydration products and paste microstructure Summary of benefits of using mineral additions Potential problems of using mineral additions Blastfurnace slag Blastfurnace slag composition Blastfurnace slag as a cementitious material Hydration products in mixes containing slag Low-lime fly ash Fly ash composition. 64 65 68 69 71 71 71 73 77 79 79 81 82 82. 85 85 85 87 88 88 88 91 93 93 93.

(7) 7.3.2 7.3.3 7.4 7.5 8. 9. 10. 11. Fly ash as a cementitious material Hydration products in mixes containing fly ash Microsilica (silica fume) Limestone. 94 96 98 99. Cement variability. 102. 8.1 8.2. 102 104. In defence of the cement producer Causes of cement variability. Deleterious processes in concrete. 108. 9.1 9.1.1 9.1.2 9.2 9.2.1 9.2.2 9.2.3 9.2.4 9.2.5 9.3 9.4 9.5 9.6 9.7. 108 108 115 116 118 119 122 124 124 125 131 132 133 134. Reactions between cement paste and aggregate Alkali-silica reaction Alkali-carbonate reaction Sulfate attack in concrete and mortar External sulfate attack Internal sulfate attack Thaumasite form of sulfate attack (TSA) Sulfate attack in mortar Identification of sulfate attack Carbonation Steel corrosion Leaching Frost damage (freeze-thaw action) Efflorescence on masonry. Standards for Portland Cement. 137. 10.1 10.2 10.2.1 10.2.2 10.3 10.4. 138 140 140 142 145 145. ASTM C 150-07 European Standard EN-197 European Standard EN-197, composition European Standard EN-197, strength classes Other specified cement properties Don’t ever mix the standards!. Cement concepts. 147. 11.1 11.1.1. 147. 11.1.2 11.2 11.3 11.4 11.4.1 11.4.2 11.5. Altering the properties of cement Altering the properties of cement: modify the Portland cement Altering the properties of cement: combine Portland cement with other materials A “mind’s eye image” of cement Cement clinker Hydrated cement Some useful principles Hydration of different cements Towards quantifying cement hydration products. 148 150 152 154 160 160 162 174.

(8) 12. Making cement greener. 176. 12.1 12.2. 176 177. Cutting back on burning fossil fuels Reducing CO2. Appendix 1 - further reading. 180.

(9) Introduction This e-book is an informal introduction to cement. Perhaps you are just starting a cement or concrete-related career; maybe you are a student, or perhaps you already know quite a bit about cement but want to refresh your mind on some cement chemistry. Whatever your interest in cement, if you are looking for a basic guide to cement science, I believe that you will find this e-book helpful. No one fully understands cement; the title of this e-book should really be something like “Understanding Cement a Bit Better.” Cement is like any other fascinating subject; the more you find out, the more you realise just how huge the gap really is between what you know and what there is to be known. I’ve worked with cement for nearly thirty years, first at the research division of a major cement producer, then as an independent consultant and I’m only too happy to talk about cement with anyone who is interested, so thank you for reading this e-book. Much of my work has been to do with cement and concrete microscopy and as a result, this e-book is very “visual”. They say that a picture is worth a thousand words, and in the following pages there are a lot of images taken using optical and scanning electron microscopes; pictures really do help in gaining an understanding of how cement is made and how it works. Until recently, “cement” generally meant Portland cement, the normal grey powder, and Portland cement forms the main subject of this e-book as it is still the principal cement used. However, increasingly, Portland cement is used in conjunction with other cementitious materials such as fly ash and slag. The extent to which these “mineral additions” are used varies in different parts of the world, but looks set to increase everywhere. The reasons are varied, but include enhanced cement performance, conservation of virgin raw materials and a reduction in carbon dioxide emissions. We will look first at Portland cement production, then at Portland cement hydration. With this firmly established, we then widen the scope of the term “cement” to include composite (ie: blended) cements, looking at why they are used and how they alter the cement hydration products. After a quick look at some physical properties of hydrated cement, we move swiftly on to consider why variations in Portland cement might occur between one day and the next before considering the basics of the principal deleterious processes that can affect concrete such as sulfate attack and alkali-silica reaction. The penultimate chapter draws everything together in the form of “thought experiments” that consolidate the main subjects from the earlier chapters. Finally, we look briefly at how cement is becoming “greener”. The material presented here is either mainstream cement science that has undergone peer review before publication in journals or textbooks, or is industry data regarded as non-controversial. Of course, if you should find something you disagree with, please contact me via the Understanding Cement web site and I would be delighted to discuss it..

(10) “Understanding Cement” has its origins in a one-day seminar on cement that Arthur Harrisson and I first gave back in 1991. Since then, the seminar content and cement science have both evolved, and the technology for presenting it has progressed hugely, from handwritten overhead projector slides to the internet. This e-book is intended to be a fairly gentle introduction into the slightly arcane world of cement science and I very much hope that the basic grounding it provides will enable you to go on and read with confidence some of the many other textbooks available on cement and concrete. Excellent though they mostly are, they can be a little daunting at first, so “Understanding Cement” is here to get you started. References are provided at the end of each chapter; many are references to more detailed treatments in some of the standard works on cement, particularly Taylor’s “Cement Chemistry” and Lea’s “The Chemistry of Cement”. There are also some references to individual papers where these are especially relevant. However, I have limited the range of external references to some extent because not all readers will have easy access to a technical library where these often slightly obscure references may be obtained. It is the purpose of this e-book to be helpful and encouraging, not to handicap anyone who does not have a university library next door. In Appendix 1 are my suggestions for the nucleus of a small “cement library” that will be of value for many years to anyone interested in cement science. Together, these books contain more references than anyone is likely to want to read in a lifetime of cement study. I am very grateful to the many people who have helped me with this e-book, especially the two groups of reviewers who so kindly read the draft version. The “expert reviewers” were people who have spent their working lives in cement manufacturing, concrete production, masonry, education or related fields; they contributed greatly to the technical content. The “readability reviewers” had a technical background but no specific knowledge of cement; they identified many areas where I had not explained the subject matter sufficiently clearly. I am also very grateful to those who have helped in other ways too numerous and diverse to describe in detail. I particularly thank Don Ashcroft, Geoff Bowler, Tom Burnham, Mike Burton, Josette Camilleri, Mike Connell, Ian Ferguson, Ron Green, Arthur Harrisson, Paul Livesey, Robert Matthews, Kelly Park, Lindon Sear, Anthony Tidder and Anna Wright..

(11) NOTES Spelling The spelling used is standard “British English,” except for “sulphur” and related words where I have adopted the International Union of Pure and Applied Chemistry (IUPAC) standard spelling of “sulfur”. How to use this book Some people can happily read a book from a computer screen. If you can, that has the advantage that you can use the “search” function of Acrobat Reader. Many people, however, including me, prefer to read from the printed page and if that is you, then print this e-book out and bind it with a comb binder or similar, or (even better) get someone to do it for you. The margins are set so that the document should print on both A4 and US letter paper without adjustment. Printing on both sides of the paper will make the book more manageable. Microscope images This e-book contains many images taken using either a scanning electron microscope (SEM) or petrographic microscope. SEM images are black-and-white images as electrons don’t have colour; petrographic microscope images are in colour. Microscope images have scale bars to indicate the size of features. These are shown in microns, or micrometres (µm). A micron is 10-6 metre, so there are 1,000 microns in a millimetre and 1,000,000 microns in a metre. The microscope images do not show the magnification (eg: x1000) since this will vary depending how the image is displayed. An image that is x100 displayed fullscreen on a 15-inch computer monitor will be x160 on a 24-inch monitor, so indicating a fixed magnification is meaningless unless you know the size of the original image. It is the scale bar that is important, not the magnification. If you want to work out the magnification of an image, measure the scale bar as displayed on your monitor, or on the page if you have printed it, and work it out. For example, if a 1000 µm (=1 mm) scale bar measures 10 mm on your monitor, the magnification is x10. If a 100 µm scale bar measures 10 mm (=10,000 µm), the magnification is 10,000/100 = x100. If a 50 µm scale bar measures 20 mm, the magnification is 20,000/50 = x400. Glossary You may find it helpful to download the cement glossary, available if you sign up to the free Newsletter, from www.understanding-cement.com..

(12) Contact details. Feedback Comments, suggestions, bouquets, brickbats – all feedback will be gratefully received. Please do tell me what you think of “Understanding Cement”. Your comments will be very useful in improving future editions of this e-book. The best way to send feedback is to use the feedback form at: www.understanding-cement.com/ucebookfeedback.html. E-book technical problems If you have any technical problems with the e-book (eg: printing it out), contact us using the Contact Form on the Understanding Cement web site and we will try to help. http://www.understanding-cement.com/contact.html. Consultancy If you have any queries about the consultancy services offered by WHD Microanalysis Consultants Ltd., I can be reached at: [email protected]. Nick Winter December 2009. Cover picture courtesy Castle Cement.

(13) IMPORTANT! I hope you know this already, but the first thing anyone should know about cement, is that it is HIGHLY ALKALINE. If you get wet cement, or dry cement powder, on your skin you will get ALKALI BURNS. These can be severe and in extreme cases can result in the amputation of limbs, or even death. You should therefore avoid contact with cement by taking appropriate precautions, including wearing suitable protective clothing and equipment. If you do get cement on your skin, or in your eyes, wash it off immediately and seek medical advice if necessary.. On that happy note, read on!.

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(15) 1 A brief history of cement. Cementing materials were used widely in the ancient world. The Egyptians used calcined gypsum as a cement. The Greeks and Romans used lime made by heating limestone, and then added sand to make mortar, with coarser gravel for concrete. This process of heating limestone so that it decomposes to lime (calcium oxide) and carbon dioxide gas is called “calcination”. Before the lime is used in building, it is normally mixed with water (“slaking”) to convert the calcium oxide to calcium hydroxide. The Romans found that a cement could be made which set under water and this was used for the construction of harbours. The cement was made by adding crushed volcanic ash to lime, and was later called a ‘pozzolanic’ cement named after the village of Pozzuoli near Vesuvius. Where volcanic ash was scarce, crushed brick or tile was used instead. The Romans were therefore probably the first to manipulate the properties of cementitious materials for specific applications and situations. Marcus Vitruvius Pollio, a Roman architect and engineer in the 1st century BCE wrote in his ‘Ten books of Architecture,’ a revealing insight into ancient technology (1): “There is also a kind of powder from which natural causes produces astonishing results… This substance, when mixed with lime and rubble, not only lends strength to buildings of other kinds, but even when piers are constructed of it in the sea, they set hard under water.” The Romans used pozzolanic material more widely than just in marine construction. Vitruvious says: “First I shall begin with the concrete flooring, which is the most important of the polished finishings, observing that great pains and the utmost precaution must be taken to ensure its durability….”.

(16) A brief history of cement “…On this, lay the nucleus, consisting of pounded tile mixed with lime in the proportions of three parts to one, and forming a layer not less than six digits thick.” Vitruvious’ ‘Ten books of architecture’ is a real historical gem bringing together history and technology, although anyone wishing to follow his instructions might first need to find a thousand or so slaves to dig, saw, pound and polish... After the end of the Roman Empire, much of their building technology was lost certainly in Europe with regard to cement. Mortars hardened mainly by carbonation of lime, a slow process, and it wasn’t until the late Middle Ages that the benefits of pozzolanic material mixed with lime were rediscovered. The great European mediaeval cathedrals were clearly built by highly skilled masons but it would probably be fair to say they did not have the technology to manipulate the properties of cementitious materials in the way the Romans had done a thousand years earlier. The Renaissance and Age of Enlightenment brought new ways of thinking and compelling new reasons to develop technology, including cement technology. For example, maritime nations needed to build lighthouses on exposed rocks to reduce shipping losses. John Smeaton, the “father of civil engineering” in England, found that a mix of hydraulic lime, produced by burning an impure limestone and then mixing the lime with a natural pozzolan, produced a mortar that hardened under water (2). Smeaton used his mortar with interlocking stone blocks in the construction of the third Eddystone lighthouse (1759) off the coast of Cornwall in Southwestern England. This mixture for a mortar of lime and pozzolan was specified for government contracts until 1867, 43 years after Aspdin’s patent for Portland cement.. Figure 1.1 Ardnamurchan lighthouse, Scotland, built by Alan Stevenson and completed in 1849. The Stevensons were a Scottish family of engineer lighthouse builders who used similar construction technology to Smeaton’s. Alan Stevenson was the uncle of author Robert Louis Stevenson, who wrote “Treasure Island” and many other books.. www.whd.co.uk. 2. www.understanding-cement.com.

(17) A brief history of cement Joseph Aspdin took out a patent in 1824 for “Portland Cement,” a material he produced by firing a mixture of finely-ground clay and limestone. What Aspdin did that was different was to calcine the limestone, then mix the lime with clay and fire it again. He called the product “Portland Cement” because the concrete made from it looked like Portland stone, a widely-used building stone in England. While Aspdin is usually regarded as the inventor of Portland cement, Aspdin’s cement was not produced at a high-enough temperature to be the real forerunner of modern Portland Cement. His son, William, found that a higher temperature (around 1400 C) was beneficial. At this increased temperature, a small proportion of the material melts but the bulk of it remains solid. This process is known as “sintering” and it produces a material called clinker. A ship carrying barrels of William’s cement went aground off the Isle of Sheppey in Kent in 1848 and the barrels of set cement, minus the wooden staves, were later incorporated into the wall of an inn in Sheerness. They are still there now and the landlord occasionally receives requests from cement enthusiasts for small pieces. In 1845 William’s main competitor, Isaac Johnson, had also made an improved cement, by firing a mixture of chalk and clay at higher temperatures (1400 C– 1500 C), similar to those used today. Because both William Aspdin and Johnson burnt at higher temperatures than had been used previously, minerals were produced which were very reactive and more strongly cementitious. While they both used similar materials and temperatures to make Portland cement as we use now, three more important developments in the manufacturing process lead to modern Portland cement:. . Development of rotary kilns (Ransome, 1885; Hurry and Seaman, 1895). . Addition of gypsum to control setting. . Use of ball mills to grind clinker and raw materials. A rotary kiln is a long, rotating, tube slightly tilted from the horizontal and will described more fully in Chapter 3. Ransome’s kiln did not work properly in trials in 1887. Later, Hurry and Seaman in the USA resolved the problems and produced the first modern rotary kiln. These gradually replaced the original vertical shaft kilns from the early 1900s. Rotary kilns heat the clinker mainly by radiative heat transfer and this is more efficient at higher temperatures, enabling higher burning temperatures to be achieved. Also, rotary kilns give a more consistent product because the clinker is constantly moving inside the kiln; this gives a more uniform temperature in the burning zone, the hottest part of the kiln. The two other principal technical developments, gypsum addition to control setting and the use of ball mills to grind the clinker, were also introduced at around the end of the 19th century.. www.whd.co.uk. 3. www.understanding-cement.com.

(18) A brief history of cement This brief and selective whirlwind tour of cement development sets Portland cement in its historical context. If you’re particularly interested in cement history, the Blezard paper and his chapter in Lea referred to below, contain more information with numerous references.. References, Chapter 1 1. Vitruvius, “The Ten Books of Architecture,” Dover Publications, 1960. 2. “Reflections on the history of the chemistry of cement,” R G Blezard, Society of Chemical Industry, 1998. (www.soci.org/SCI/publications/2001/pdf/pb72.pdf). Further reading Lea, Chapter 1, “The History of Calcareous Cements,” R G Blezard. This first chapter in Lea contains a lot of information on cement history, plus 90 references.. www.whd.co.uk. 4. www.understanding-cement.com.

(19) 2 Cement basics. 2.1. Some definitions. There are many different types of cement used in the construction industry. By far the most important in terms of volume is Portland cement and cement that is based on Portland cement. Cement may be “pure” Portland cement, or, alternatively, it may be made from Portland cement mixed with other materials that also have cementitious properties, such as blastfurnace slag from iron smelting or fly ash from coal-fired electricity power stations. Cements composed of mixtures of Portland cement with these other materials can enhance concrete properties; these mixtures are widely used and are called “composite cements” or “blended cements”; the exact terminology varies according to local custom. We’ll focus initially on Portland cement and return to composite cements later. “Pure” Portland cement was the most widely used cement in most parts of the world until recently. In some parts of the world, it still is but technical developments as well as environmental concerns have led to the increased use of other cements, particularly composite cements. The meanings of the words “cement” and “concrete” are rather blurred in general use, so let’s start by defining these words more carefully, along with a few others..

(20) Cement basics. Portland cement. Cement. Aggregate Concrete Mortar Grout. Material made by heating a mixture of limestone and clay in a kiln at about 1450 C, then grinding the resulting clinker to a fine powder with a small addition of gypsum. Usually taken to mean either Portland cement or, more recently, a cement with Portland cement as one of the main constituents but also containing other materials. It could also mean any other type of cement, depending on the context. Cobbles, pebbles, crushed rock, gravel, sand and silt – the ‘rock’ component of all particle sizes in concrete. Synthetic rock made using cement mixed with water and aggregate. Mixture of cement and fine aggregate, mainly sand. Used typically to bond bricks, blocks and building stone. Mixture of cement (possibly of various types) and other fine material such as fine sand. Used in a wide range of applications from filling the gaps between bathroom tiles to oil wells.. Portland cement itself can be divided into a number of different types, each cement having different characteristics. “Normal,” grey, cement for general-purpose use will be referred to in this e-book as “ordinary” Portland cement. Other types of Portland cement include white Portland cement and sulfate-resisting Portland cement, of which more later.. 2.2. Some non-Portland cements. As a brief aside, there are many other types of cement that are not based on Portland cement. However, the quantities of these other cements used are small compared with Portland cement and composite cements based on Portland cement; here, we will briefly mention three and then get back to the main subject of Portland cement:. . Calcium aluminate cements. . Lime concrete/mortar. . Expansive cements. Calcium aluminate cements (CACs) These cements used to be called ‘high alumina cements.’ They are made from lime or limestone mixed with bauxite (aluminium ore) or other high-alumina material heated in a furnace until the raw materials have completely melted, then cooled and the solid material is then ground to produce cement.. www.whd.co.uk. 6. www.understanding-cement.com.

(21) Cement basics. Concrete made with CAC typically develops strength quickly and is resistant to chemical attack. CACs have a wide range of compositions, mainly with different ratios of lime to alumina, and they are generally brown, grey or black in colour. One common type of CAC, ‘Ciment Fondu’, is grey but CACs can also be white if made from pure alumina. As well as being used in concrete, CACs are also used in grouts and other specialised applications, often mixed with Portland cement and other materials such as gypsum. Lime mortar and concrete Lime mortar and concrete was used for thousands of years until eclipsed by other cements particularly Portland cement. Its use declined to a very low level towards the end of the 20th century, being used mainly in the rebuilding or repair of historic or ancient buildings. More recently, the use of lime mortar and concrete in the construction of new buildings has increased in some countries, the UK being an example. Although it is not as strong, there are benefits in using lime mortar instead of a mortar based on Portland cement, in particular:. . Cracks that develop in lime mortar tend to heal themselves, unlike conventional mortar made with Portland Cement.. . Lime mortar is usually weaker than mortars made with Portland cement and so can be removed from the brick or stone at the end of the useful life of the building. Particularly in the case of bricks, this means that they can be recycled, saving energy otherwise needed to make new bricks. If mortar made with Portland cement is used, bricks generally can’t be reused as it is almost impossible to remove the mortar.. . Lime is produced at a lower temperature than Portland cement, so other things being equal, it takes less energy to produce a lime mortar compared with a mortar made with Portland cement.. . Lime mortar and concrete gain strength largely by carbonation, the process of re-absorbing carbon dioxide from the atmosphere. This converts calcium hydroxide to calcium carbonate, removing an equivalent amount of carbon dioxide from the atmosphere as was released when the limestone was calcined. (This, of course, neglects the CO2 emissions from the fuel used to heat the lime but the CO2 reabsorbed still represents a substantial part of the total CO2 emitted during manufacture).. . Lime mortars and plasters allow a building to “breathe” more than if gypsum plaster and mortar based on Portland cement is used. This results in fewer problems with condensation.. www.whd.co.uk. 7. www.understanding-cement.com.

(22) Cement basics. Expansive cements These are special cements designed to exert an expansive force on their surroundings after the cement has set. (With most cements, manufacturers go to a lot of trouble to make sure the cement is not expansive). Expansive cements are used mainly in demolition and also in mining.. www.whd.co.uk. 8. www.understanding-cement.com.

(23) 3 Portland cement composition and microstructure. Portland Cement is made by heating suitable raw materials, typically ground limestone and clay, at a temperature of about 1450 C to produce a dark grey nodular material called clinker. At this temperature, much of the clinker remains solid but perhaps 20%-30% of the clinker is liquid, thinly-dispersed within the nodules. When cool, the clinker is ground up to a fine powder and a small amount of gypsum is added to control the setting properties of the cement. Here are a few familiar, and perhaps less familiar, views of cement.. Figure 3.1 Bulk cement road tankers on a Figure 3.2 Cement bagging plant. quay. (Photo courtesy Rugby Cement.) Bagged cement is sold mainly to small builders and the do-it-yourself market. (Photo courtesy Rugby Cement.).

(24) Portland cement composition and microstructure. Figure 3.3 The familiar grey powder. The coin is 23mm in diameter.. Figure 3.4 Cement particles viewed in a scanning electron microscope.. A more complete description of the cement production process follows.. 3.1. Portland cement minerals. A more precise definition of Portland Cement is that it is the ground product of Portland cement clinker, usually interground with a small amount of calcium sulfate to bring the total sulfate content of the cement to about 2.5%-4% by weight. In turn, Portland cement clinker is produced from a mixture of finely-ground calcareous and siliceous components, together with a proportion of alumina and iron, as well as some impurities, fired in a kiln at a temperature of about 1450 C. Clinker is composed of rounded, dark grey or grey-green nodules, ranging in size from less than 1 mm to 30 mm or more. The clinker in Figure 3.5 is a typical example of Portland cement clinker. It isn’t always as clean or as coarse as this, the nodules can be smaller with a lot of fine sub-millimetre dust.. www.whd.co.uk. 10. www.understanding-cement.com.

(25) Portland cement composition and microstructure. Figure 3.5 Typical Portland cement clinker nodules: these nodules are 10mm – 15mm across.. Portland cement clinker contains four principal minerals, or ‘phases’:.    . Alite Belite A calcium aluminate phase A calcium alumino-ferrite phase. A “phase” in cement chemistry or petrology may be defined as: “A part or parts of a system occupying a specific volume and having uniform physical and chemical characteristics which distinguish it from all other parts of the system.” (1) Note that “system” includes temperature. Cooled clinker contains four main phases – the four principal clinker minerals – but in the burning zone of the kiln, it contains mainly alite, belite, free lime and the liquid phase. (“Free lime” is lime that has not yet combined to form alite or belite or other minerals). The compositions of the four main clinker minerals in any particular cement clinker vary a little, depending on the compositions of the raw materials. Their compositions are often simplified by approximating them to the following pure compounds:. www.whd.co.uk. 11. www.understanding-cement.com.

(26) Portland cement composition and microstructure. Table 3.1 Mineral names and approximate compositions of the four principal clinker minerals. Mineral name Approximate composition Formula Alite: tricalcium silicate (Ca3SiO5) Belite: dicalcium silicate (Ca2SiO4) Calcium aluminate phase: tricalcium aluminate: (Ca3Al2O6) Calcium alumino-ferrite phase: tetracalcium aluminoferrite: (Ca4Al2Fe2O10) or (Ca2AlFeO5) These simplifications are useful but need to be used carefully.. 3.2. Portland cement composition and cement notation. A certain amount of chemical notation is unavoidable from now on. However, it won’t be anything difficult – nothing beyond second or third year at High School. A typical analysis of a Portland cement is given below (Table 3.2). Table 3.2 Typical analysis of a Portland cement, expressed as oxides in weight %. SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 LOI IR Total 20.7 5.7 2.5 64.0 1.0 0.6 0.2 2.7 1.5 0.5 99.5 Balance is due to minor oxides, typically P2O5, Mn2O3, TiO2, Cr2O3. LOI=loss on ignition (due to water from traces of hydrates, carbon dioxide). IR=insoluble residue (ie: acid-insoluble residue, mainly comprising unreacted silica or feldspar).. 3.2.1. Note on oxide analyses. Cement analyses are usually shown as oxides, eg: CaO or SiO2. This is a standard form of analysis in physical chemistry and geology. For anyone unfamiliar with it, the following example may help: Consider the mineral quartz, composed of silicon dioxide (SiO2). We could represent the analysis of quartz as in Table 3.3.. Table 3.3 Quartz composition shown as individual elements. Percentage of element Si 46.67 O 53.33. www.whd.co.uk. Table 3.4 Quartz composition shown as an oxide. Percentage of oxide SiO2 100.00. 12. www.understanding-cement.com.

(27) Portland cement composition and microstructure. However, the proportion of silicon to oxygen in quartz has a fixed ratio of 2:1, determined by the valency of silicon and oxygen. Silicon has a valency of +4 and oxygen -2; this means that a silicon atom can share four electrons with neighbouring atoms and oxygen can share two. Since quartz is not electrically charged, it follows that each silicon atom will bond with two oxygen atoms: +4 + (2 x -2) = 0 This means that it isn’t necessary to show the oxygen separately; the data in Table 3.4 gives the same information as that in Table 3.3, but in a more convenient form. For more information, see Reference 2 at the end of this chapter, or try a basic chemistry textbook.. 3.2.2. Cement chemistry notation. Chemical compositions of cement clinker phases are expressed as oxides and this is convenient in many ways but can lead to rather long-winded formulae. To simplify them, cement chemists have adopted a form of notation that seems strange to chemists not familiar with cement. Using cement chemistry notation, the formulae are abbreviated. Remember that these compositions are approximate because the minerals contain impurities. Alite: Ca3SiO5 in terms of its oxides is 3CaO.SiO2. The CaO term is shortened to C and the SiO2 to S. In cement chemistry notation, the compound becomes C3S. Belite: Similarly, Ca2SiO4 is 2CaO.SiO2, which is shortened to C2S. Tricalcium aluminate: Ca3Al2O6 is 3CaO.Al2O3. The Al2O3 term is shortened to A and the compound becomes C3A. Tetracalcium aluminoferrite: Ca2AlFeO5 can be written as (Ca4Al2Fe2O10) or 4CaO.Al2O3.Fe2O3. Fe2O3 is shortened to F and the compound becomes C4AF. (With names like “tetracalcium aluminoferrite”, the need for snappier names is all-too clear.) In other words, for each of the clinker main minerals, we now have at least three possible descriptions, as below:. www.whd.co.uk. 13. www.understanding-cement.com.

(28) Portland cement composition and microstructure. . Alite or tricalcium silicate or C3S. . Belite, or dicalcium silicate or C2S. . Tricalcium aluminate (or the ‘aluminate phase’) or C3A. . Calcium alumino-ferrite (or the ‘ferrite’ phase) or tetracalcium aluminoferrite or C4AF. There are also the full chemical formulae for the pure compounds, eg: C3S or Ca3SiO5. Although strictly, these do not mean the same thing, they are frequently used indiscriminately. This lax use means that names like ‘C3A’ should not usually be taken to signify a definite composition in the sense of a pure compound, unless this is indicated by the context. To take another example, strictly speaking tricalcium silicate is a pure compound, while alite is a mineral composed largely of tricalcium silicate but also with a significant quantity of impurities, mainly magnesium, iron and aluminium. All the oxides commonly found in cement are abbreviated as follows: Table 3.5 Cement chemistry notation for the principal oxides in Portland cement clinker. C=CaO T=TiO2 S=SiO2 H=H2O A=Al2O3 F=Fe2O3 and K=K2O _ N=Na2O S=SO3 M=MgO _ P=P2O5 C=CO2. _ _ S and C are spoken as ‘S-bar’ and ‘C-bar’ respectively, but are less used now than they once were. (Probably, they are a victim of technology - fine when notes were hand-written or typed on a typewriter, but just too complicated to do on a word-processor. Have a try and see why it isn’t used so much these days!) People unfamiliar with cement notation sometimes have trouble adjusting to the notion of S representing SiO2, for example, but do stick with it – it is usually a great time saver and it does work.. www.whd.co.uk. 14. www.understanding-cement.com.

(29) Portland cement composition and microstructure. 3.3. Microstructure of clinker and cement. Consider an individual nodule of cement clinker (Figure 3.6). If we take that nodule, or pieces of it, we can make a polished section (Figure 3.7) and view the individual minerals in the nodule using a microscope.. Figure 3.6 Single clinker nodule and a coin, 23mm in diameter, for scale.. Figure 3.7 Polished section containing pieces of clinker nodules. Section diameter is 30 mm.. The polished section could be examined using either an optical microscope or a scanning electron microscope (SEM); the techniques for preparing polished sections are broadly the same for either. Clinker nodules are porous and it is desirable to fill these pores with epoxy resin; nodules are embedded in the resin using vacuum impregnation to force the resin into as many small pores as possible. In Figure 3.7 the nodules have been separated into different size fractions by sieving and the fragments held in place during specimen preparation by card divisions, which remain visible. When the epoxy resin had set, the hardened resin block was sawn to reveal the nodules in section, then polished using diamond polishing compound in successively finer grades. For examination using an optical microscope, various etches – liquids that react with the surfaces of the crystals - may be used to highlight particular minerals. Etches are not normally used for SEM examination. Clinker appears quite different when examined using a petrographic microscope compared with when using a scanning electron microscope. The obvious main difference is that the petrographic microscope image is a colour image while the SEM image is black-and-white. Both have their advantages and disadvantages. Examples of a petrographic microscope image of clinker and an SEM image are shown below.. www.whd.co.uk. 15. www.understanding-cement.com.

(30) Portland cement composition and microstructure. Figure 3.8 Polished section of Portland cement clinker, examined in reflected light using a petrographic microscope, showing crystals of individual minerals. Brown crystals are alite and blue crystals are belite. The bright material between the alite and belite is a mixture of aluminate and ferrite. Grey areas are pores filled with epoxy resin. The colours are the product of etching; the section was etched with hydrofluoric acid vapour in order to distinguish between the different minerals. Belite is not actually blue and alite is not brown.. Figure 3.9 Polished section of Portland cement clinker, backscattered SEM image. Alite (‘a’) and belite (‘b’) can be clearly distinguished, alite being light grey and belite darker grey –examples arrowed. Black areas are epoxy resin. Small bright, nearly white, crystals are ferrite and tiny darker crystals close to the ferrite are mainly aluminate phase, but these are hard to distinguish at this magnification. SEM images are always black and white, unless they have had false colours added. This is a different clinker to that in Figure 3.8 but the magnification is similar.. Alite crystals are elongated, typically 20 µm-60 µm in length and hexagonal in shape, as can be seen in Figures 3.8 and 3.9. Belite crystals are generally rounded in shape and 10 µm-30 µm across. In both images, since the crystals are randomly orientated, many of them will not appear in polished sections showing their full length. Some nodules are highly porous and others much denser. The small nodule in Figure 3.10 is of fairly typical density. In this low-magnification view of most of a nodule, alite is the mid-grey mineral comprising the bulk of the nodule. The darker ‘patches’ are belite clusters.. www.whd.co.uk. 16. www.understanding-cement.com.

(31) Portland cement composition and microstructure. Figure 3.10 SEM image of polished section of a whole small nodule, approximately 2mm across. The pores in the nodule structure are the black areas which are filled with epoxy resin. In this clinker, most of the belite occurs in large clusters (arrowed, dark grey); ideally, the belite would be distributed more uniformly. The uneven distribution suggests that some of the siliceous particles in the raw feed were too coarse. Despite relict structures of coarse raw feed particles, this nodule is well-combined with no free lime visible.. Ferrite and aluminate phase vary in appearance from one clinker to another and within the same clinker. Sometimes ferrite and aluminate crystals are small and intergrown but they can also be coarse separate ‘blocky’ crystals (Figure 3.11).. Figure 3.11 SEM image of polished section, showing coarse blocky aluminate (dark grey, examples shown by white arrows) and ferrite (bright, examples shown by black arrows). Most of the material in this image is alite, with a small elongated belite cluster towards the centre. Again, black areas are pores in the clinker filled with epoxy resin used in specimen preparation.. www.whd.co.uk. 17. www.understanding-cement.com.

(32) Portland cement composition and microstructure. When the clinker is ground up to produce cement, it is obvious that much of the microstructure of the nodules is lost (Figure 3.12).. Figure 3.12 SEM image of polished section of cement particles set in epoxy resin. The four main clinker minerals are visible (aluminate only just visible). Much information that would be visible in a clinker nodule is lost on grinding, such as alite crystal sizes, belite cluster sizes and nodule porosity.. 3.4. Hydraulic properties of the main clinker minerals. Alite (C3S) Alite is very reactive in the presence of water. It is the main constituent of Portland cement, typically between 45% and 70% of the clinker by weight. It is the main strength-giving component of cement and is thus strongly hydraulic. Belite (C2S) Belite is reactive and hydraulic, but less so than alite. The proportion of belite in clinker typically varies between 5% and 30%. Aluminate (C3A) The aluminate phase is the most reactive constituent of clinker. However, it is not a major contributor to strength in Portland cement, in other words it is only very weakly hydraulic. Although only weakly hydraulic, because it is very reactive it has a strong influence on early setting properties of cement and it produces a lot of heat. In order to control this reactivity, gypsum is added to cement to control the aluminate phase hydration.. www.whd.co.uk. 18. www.understanding-cement.com.

(33) Portland cement composition and microstructure. Ferrite (C4AF) Ferrite phase is moderately reactive, but only weakly hydraulic. It reacts quickly initially when mixed with water but the rate of reaction slows and unhydrated ferrite can be found in concrete over a hundred years old. Ferrite is black and gives cement its characteristic grey colour. Aluminate and ferrite are often referred to as “flux phases”. Why are aluminate and ferrite present in cement? If alite and belite are the main hydraulic minerals in cement, the minerals that give strength to concrete, what do the aluminate and ferrite phases do? Why not just make cement containing alite and belite? The short answer is that it would be very difficult to make cement that did not contain at least some aluminate or ferrite. We’ll be looking at the reactions in the cement kiln later, but broadly cement-making is a clinkering (sintering) process. This means not all the material in the kiln melts – at the hottest part of the kiln, the burning zone, about three-quarters of the clinker remains solid and only a quarter is liquid. Remember that what we are trying to do in the kiln is make calcium silicates, particularly alite. The solid material entering the burning zone of the kiln is, roughly speaking, belite and free lime that we want to combine to make alite. The liquid is mainly composed of oxides of calcium, iron and aluminium; when cooled, this liquid crystallises into the aluminate and ferrite phases. The rôle of the liquid is to accelerate the reactions in the clinker. Ions are transferred through the liquid much more easily than through a solid. All other things being equal, the higher the proportion of liquid, the easier it is to combine the belite and free lime to make alite. In cement-making terminology, the liquid is also called the flux. The liquid from which aluminate and ferrite crystallise is therefore critical to the process of making cement. Without the liquid, ion transfer would be much slower; belite and free lime would not combine adequately. The liquid is essential and the aluminate and ferrite form from the liquid is it cools, even though they do not contribute greatly to the strength of concrete.. 3.5. Proportions of the main clinker phases. As we’ll see, the proportions of each of the main minerals (that is, the quantity of alite, belite, etc.) are of major importance in determining the properties of the cement produced from the clinker. The term “phase composition” is often used – it means the proportions of the clinker minerals in a cement or clinker.. www.whd.co.uk. 19. www.understanding-cement.com.

(34) Portland cement composition and microstructure. The “actual” phase composition is dependent on:. . The quantities of each of the main oxides (CaO, SiO2, Al2O3 and Fe2O3) in the raw materials.. . The extent to which they have combined to form the main clinker phases.. . The compositions of the phases (including impurities).. The actual phase composition is not easy to calculate without additional information, in particular, the true compositions of the minerals. (These can be determined by SEM and X-ray microanalysis.) The “potential” phase composition simplifies things by ignoring the extent to which the oxides have actually combined. It means: “these are the mineral proportions you would expect if everything were to combine and all the chemical reactions reached equilibrium as the clinker cooled”.. 3.5.1. The Bogue calculation. The Bogue calculation is a very useful method of calculating the approximate quantities of the four main clinker minerals in a clinker or cement. In essence, it is a calculation of the potential phase composition based on some simplifying assumptions. If we know the composition of the clinker from its oxide analysis, and if we also know the compositions of the four main clinker minerals, we can calculate how much of each mineral is present. The Bogue calculation can be used in two slightly different ways:. . It can be used prescriptively – a good example is the ASTM C-150 standard for Portland cement, in which the Bogue calculation is used to specify limits for the proportions of the different clinker minerals in different types of cement.. . It can be used for troubleshooting; as an example, suppose you had two cements that were nominally similar but one gave better strengths than the other and you wanted to know why. A good starting point would be to use the Bogue calculation to calculate the proportions of clinker minerals in either the cement or the clinker from which the cement was made.. The calculation used will vary slightly, depending whether you are applying it to cement or to clinker, and on what you are trying to achieve:. www.whd.co.uk. 20. www.understanding-cement.com.

(35) Portland cement composition and microstructure. . To apply the calculation to cement, you need to take into account any material that was added to the clinker, such as gypsum and fine limestone.. . To use the calculation to compare the phase proportions of two particular cements or clinkers, you would want to correct for uncombined lime (free lime).. The following version of the Bogue calculation is applied to clinker and corrects for free lime; it does not allow for added gypsum or fine limestone and so should not be used for cement in this form. We’ll look at applying the calculation to cement later (Chapter 5.9). The calculation assumes that the four main clinker minerals have the actual compositions given by:. . Alite, C3S, or tricalcium silicate. . Belite, C2S, or dicalcium silicate. . Aluminate phase, C3A, or tricalcium aluminate. . Ferrite phase, C4AF, or tetracalcium aluminoferrite. (At the risk of becoming repetitive, these are only approximations of the true compositions of the minerals.) To make clinker, we are combining lime and silica and also lime with alumina and iron. If some of the lime remains uncombined, we need to subtract this from the total lime content before we do the calculation, or we will overestimate the actual alite content. For this reason, a clinker analysis normally gives a figure for free lime. The calculation is simple in principle:. . Firstly, according to the assumed mineral compositions, ferrite phase (C4AF) is the only mineral to contain iron. The iron content of the clinker therefore fixes the ferrite content.. . Secondly, the aluminate content is fixed by the total alumina content of the clinker, minus the alumina in the ferrite phase. We can now calculate this, since we have a figure for the amount of ferrite.. . Thirdly, we assume all the silica is present as belite and calculate how much lime is needed to form belite from the total silica content of the clinker. There will be a surplus of lime.. www.whd.co.uk. 21. www.understanding-cement.com.

(36) Portland cement composition and microstructure . Fourthly, we allocate the lime surplus to the belite, converting some of it to alite.. In practice, the above process of allocating the oxides can be reduced to the following equations: C3S. =. 4.0710CaO-7.6024SiO2-1.4297Fe2O3-6.7187Al2O3. C2S. =. 8.6024SiO2+1.0785Fe2O3+5.0683Al2O3-3.0710CaO. C3A. =. 2.6504Al2O3-1.6920Fe2O3. C4AF. =. 3.0432Fe2O3. Here’s a worked example. First, we need a clinker analysis (Table 3.6). Table 3.6 An example of a typical clinker analysis (note that the analysis in Table 3.2 was of a cement, not a clinker). SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 LOI IR Total 21.5 5.2 2.8 66.6 1.0 0.6 0.2 1.0 1.5 0.5 98.9 Free lime = 1.0% CaO. Example of a Bogue calculation using the clinker data in Table 3.6: Combined CaO = (Total CaO – Free CaO) = (66.6% - 1.0%) = 65.6% This is the figure we use for CaO in the calculation. So, for the four oxides we have: CaO=65.6%; SiO2=21.5%; Al2O3=5.2% and Fe2O3=2.8% The Bogue calculation is: C3S C2S C3A C4AF. = = = =. 4.0710CaO-7.6024SiO2-1.4297Fe2O3-6.7187Al2O3 8.6024SiO2+1.1Fe2O3+5.0683Al2O3-3.0710CaO 2.6504Al2O3-1.6920Fe2O3 3.0432Fe2O3. Therefore: C3S C2S C3A. = = =. www.whd.co.uk. (4.0710 x 65.6)-(7.6024 x 21.5)-(1.4297 x 2.8)-(6.718 x 5.2) (8.6024 x 21.5)+(1.0785 x 2.8)+(5.0683 x 5.2)-(3.0710 x 65.6) (2.6504 x 5.2)-(1.6920 x 2.8). 22. www.understanding-cement.com.

(37) Portland cement composition and microstructure C4AF. =. 3.0432 x 2.8. = = = = =. 64.7% 12.9% 9.0% 8.5% 95.1%. So: C3S C2S C3A C4AF Total. To the total, we can add the 1% free lime we deducted at the start to give 96.1%; note that the total still does not add up to 100%. This is because the calculation neglects impurities in the four clinker minerals, and because of other minor constituents in the clinker not accounted for, such as clinker sulfate and, possibly, periclase (MgO). It should be stressed that the Bogue calculation does not give the exact amounts of the four main clinker phases present, although this is sometimes forgotten. These differ from the ‘true’ amounts mainly because the actual mineral compositions differ a little (occasionally more than a little) from those assumed in the calculation, as shown in Table 3.7.. Table 3.7 Comparison of clinker mineral compositions calculation, with typical compositions. SiO2 Al2O3 Fe2O3 CaO MgO Bogue C3S 26.3 0.0 0.0 73.7 0.0 Bogue C2S 34.9 0.0 0.0 65.1 0.0 Bogue C3A 0.0 37.7 0.0 62.3 0.0 Bogue C4AF 0.0 21.0 32.9 46.1 0.0. assumed by the standard Bogue K2O 0.0 0.0 0.0 0.0. Typical 25.2 1.0 0.7 71.6 1.1 0.1 Alite Typical 31.5 2.1 0.9 63.5 0.5 0.9 Belite Typical 3.7 31.3 5.1 56.6 1.4 0.7 ‘Aluminate’ Typical 3.6 21.9 21.4 47.5 3.0 0.2 Ferrite *Balance is mainly P2O5, Mn2O3, TiO2, Cr2O3 Typical mineral compositions are from Reference 3 at the end of. Na2O 0.0 0.0 0.0 0.0. SO3 0.0 0.0 0.0 0.0. Total 100 100 100 100. 0.1. 0.1. 99.9*. 0.1. 0.2. 99.7*. 1.0. 0.0. 99.8*. 0.1. 0.0. 97.7*. this chapter.. It is clear from Table 3.7 that there are appreciable differences between the compositions assumed by the Bogue calculation and those typical of minerals in cement clinker. For example, alite and belite both contain aluminium and iron, and the aluminate phase (C3A) is not strictly tricalcium aluminate as it contains some iron and silicon. That said, the Bogue calculation gives a useful approximation and is widely used within the cement industry. It provides a standardised format for estimating the clinker mineral proportions and the approximations made are not normally. www.whd.co.uk. 23. www.understanding-cement.com.

(38) Portland cement composition and microstructure important. Of much greater interest are relative changes over time in the indicated mineral content rather than the absolute values. If concrete early strengths suddenly drop and if alite content as calculated using the Bogue calculation also shows a drop, the lower strength may well be due to the lower alite content. The calculation can be expressed in other ways, of which perhaps the most useful is as a set of four simultaneous equations. In the following, (A)= alite; (B)=belite; (Al)=aluminate; (F)=ferrite and (bulk) refers to the oxide composition of the clinker. S(A) + S(B) + S(Al) + S(F) A(A) + A(B) + A(Al) + A(F) F(A) + F(B) + F(Al) + F(F) C(A) + C(B) + C(Al) + C(F). = = = =. S(bulk) A(bulk) F(bulk) C(bulk). For convenience, these equations can be solved using a spreadsheet by the method of inversion of matrices. If the true mineral compositions, as distinct from the assumed ideal compositions, are known (eg: from SEM/X-ray microanalysis) the actual phase proportions can be calculated more accurately. Note that C(bulk) should have the free lime subtracted before the calculation for the best estimate of the alite content. Similar calculations can be performed for cement, provided that gypsum and any other added material, such as fine limestone, is taken into account (see Chapter 5.9).. Free lime: to subtract or not to subtract In the above calculation, we subtracted the free lime from the total CaO content in order to obtain the “best estimate” of the alite content. However, as mentioned above, if used prescriptively in a standard specification the form of the calculation specified may not require free lime to be deducted. In the ASTM standard for Portland cement, ASTM C150-07, the free lime content is not deducted from the total lime in calculating the phase composition; it isn’t even mentioned. This is because the objective of the calculation is different. The specification defines limits to the phase composition of certain cement types to ensure that the cements are suitable for specific purposes. Differences in free lime content in individual cements are not an issue and would be an unnecessary complication. In summary, the form of calculation to use depends on the context. If the calculation relates to a standard specification, you would obviously use the calculation given in that specification. If you are a cement producer trying to work out why one clinker is making better cement than another, you would want the best estimate of the alite content; that would mean deducting the free lime. www.whd.co.uk. 24. www.understanding-cement.com.

(39) Portland cement composition and microstructure from the total CaO first. Free lime is lost alite To see how alite content and free lime content are related, look at the effect of free lime on the mineral content of a typical clinker, as shown by the Bogue calculation (Table 3.8). Table 3.8 Effect of uncombined lime on alite and belite contents. SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 LOI IR Total 21.5 5.2 2.8 66.6 1.0 0.6 0.2 1.0 1.5 0.5 98.9 LSF=97.6; SR=2.69; AR=1.86 (see Chapter 3.6). LOI=Loss on ignition (weight loss when heated to approx. 1000 C), due to small amounts of carbon dioxide or water) IR=Insoluble residue - insoluble material which has not combined, usually silica or feldspar. Clinker A Clinker B Clinker C 0% Free CaO 1% Free CaO 2% Free CaO Alite 68.7 Alite 64.7 Alite 60.6 Belite 9.8 Belite 12.9 Belite 15.9 Aluminate 9.0 Aluminate 9.0 Aluminate 9.0 Ferrite 8.5 Ferrite 8.5 Ferrite 8.5 Total 96.1 Total 95.1 Total 94.1. Applying the Bogue calculation gives an alite content of about 69% if no free lime is assumed to be present (Clinker A) but only about 61% if 2% free lime is assumed (Clinker C). This illustrates why a full clinker analysis should also show the free lime content if the purpose of the analysis is to indicate the actual phase proportions of the clinker. Cements made from Clinkers A, B and C in Table 3.8 would probably perform differently. If early strengths were dependent solely on alite content, Clinker A with the highest alite content should give the best early strengths (from about 1 day to 7 days). With 2% free lime (Clinker C) the lower alite content should result in the lowest early strengths. At later ages, the higher belite content of Clinker C would start to partly close the gap with Clinker A, although this may take several weeks or months. However, the mineral proportions are not the only factors controlling the performance of a cement, although they are very important. For example, as mentioned previously a hard-burned clinker may contain calcium silicates which are less reactive than those in a clinker burned with a lighter touch. In other words, Clinker A with little or no free lime, was likely to have been very hard-burned in order to get all the lime to combine. Clinker C would be much lighter-burned (all other things being equal) and Clinker B somewhere inbetween. Clinker B might well have the best compromise between alite content and silicate reactivity and cement made from it may out-perform cements made from Clinkers A and C in terms of both early and late strengths. Comparing Figure 3.10, a normally-burned clinker nodule, with the underburned nodule in Figure 3.13, the underburned nodule is clearly more porous and of. www.whd.co.uk. 25. www.understanding-cement.com.

(40) Portland cement composition and microstructure lower density. Harder burning would cause the nodule to coalesce. Nodule density is therefore, very approximately, a measure of how hard the clinker was burned. Indeed, the ‘litre weight’ (weight of a known volume, ie: density - more strictly the bulk density) of a clinker is often used as a rough measure of burning. Usually, the clinker sample for calculating the litre weight has been sieved, so that the nodule sizes are within defined upper and lower limits.. Figure 3.13a This clinker nodule is underburned; it was not at burning temperature for a sufficient time for adequate combination to take place. This image shows much belite (dark grey) in large, arcuate, clusters. Some alite is present (light grey) and much free lime (“FL” - white). This nodule has an open, porous, structure and contains more belite than alite. Compare with the wellcombined nodule in Figures 3.10-3.11 shown at a similar magnification.. 3.6. Figure 3.13b Detail of Figure 3.13a, showing free lime (“FL”), alite (“a”) and belite “b”). Harder burning would have resulted in a densification of the microstructure and enabled more lime to combine with belite to produce more alite.. Common parameters used in cement manufacturing. Cement clinker is frequently characterised in terms of three compositional parameters, based on the bulk oxide analysis of the clinker. The three parameters are:. . Lime Saturation Factor (LSF). . Silica Ratio (SR, or S/R). . Alumina Ratio (AR). www.whd.co.uk. 26. www.understanding-cement.com.

(41) Portland cement composition and microstructure Lime Saturation Factor The LSF is a ratio of CaO to the other three main oxides. Applied to clinker, it is calculated as: LSF=CaO/(2.8SiO2 + 1.2Al2O3 + 0.65Fe2O3) Often, the LSF is referred to as a percentage, and therefore multiplied by 100. The LSF controls the ratio of alite to belite in the clinker. A clinker with a higher LSF will have a higher proportion of alite to belite than will a clinker with a low LSF. LSF values in clinkers typically range between 92%-99%. Values above 1.0 (100%) indicate that free lime is likely to be present in the clinker. This is because, in principle, at LSF=1.0 all the free lime should have combined with belite to form alite. If the LSF is higher than 1.0, there will be excess lime that has nothing with which to combine. In practice, there are always regions within the clinker where the LSF is locally a little above, or a little below, the average for the clinker as a whole. This is partly due to imperfect mixing and partly to particle size; a large particle will cause the composition locally to deviate from the average composition. This means that there is almost always some residual free lime, even where the LSF is considerably below 1.0. It also means that to convert all the belite to alite, an LSF slightly above 1.0 is needed; there will inevitably be free lime remaining. Silica Ratio (SR) The silica ratio is defined as: SR = SiO2/(Al2O3 + Fe2O3) A high silica ratio means that more calcium silicates are present in the clinker and less aluminate and ferrite. SR is typically between 2.0 and 3.0. The silica ratio is also called the ‘silica modulus.’ Alumina Ratio (AR) The alumina ratio is defined as: AR=(Al2O3/(Fe2O3) This determines the potential relative proportions of aluminate and ferrite phase in the clinker. An increase in clinker AR (also sometimes written as A/F) means there will be proportionally more aluminate and less ferrite in the clinker. In ordinary Portland cement clinker, the AR is usually between 1 and 3.. www.whd.co.uk. 27. www.understanding-cement.com.

(42) Portland cement composition and microstructure. What’s the best clinker composition? A simple question, you may think, but there isn’t a simple answer. It depends on the requirements of the concrete to be made from the cement. For example, if high early strengths were required, we would need:  A high SR – this means a high proportion of the clinker will be in the form of calcium silicates.  A high LSF – this means the bulk of the calcium silicates will be in the form of alite. But, there is a “down side” to this… Reactions in the kiln are harder to achieve with high LSF and SR mixes - they are more difficult to “burn,” or to combine. This results in higher costs, and also in increased CO2 emissions due to the higher CaO content of the cement and an increased fuel requirement. To take another example, if we wanted a Portland cement with a low heat of hydration (perhaps for use in large concrete pours) we should limit the alite and also the aluminate, as these minerals give the strongest exothermic reactions on hydration. We would be looking for a cement containing a lot of belite and not too much aluminate; this would be given by a clinker of low LSF and low-medium AR. (In practice, in many parts of the world, a concrete mix containing Portland cement with a high proportion of slag or fly ash would probably be used rather than a mix with Portland cement only. We will be discussing slag and fly ash in Chapter 7.). 3.7. Other Portland cement types. We have just considered very brief examples of how changing the clinker composition can change the characteristics of the cement made from that clinker. The majority of Portland cement produced is “general purpose” cement, but other types of Portland cement are manufactured in some countries for specific applications. Broadly, these are:. . Sulfate-resisting Portland cement. . White Portland cement. . Rapid-hardening Portland cement. . Low heat of hydration Portland cement. www.whd.co.uk. 28. www.understanding-cement.com.

(43) Portland cement composition and microstructure. These different Portland cement variations are treated differently by the various national standards; ASTM C 150 lists five different types of Portland cement, plus three air-entrained sub-types, not all of which have equivalents in other standards. Chapter 10 has more on standard specifications. Sulfate-resisting Portland cement Sulfate-resisting cement (Type V cement in the ASTM C 150 specification) is used where concrete may be exposed to high levels of sulfate in solution. An example might be use in foundations where the groundwater is unusually high in sulfates. Reactions between hydration products of the aluminate phase and sufficient sulfate cause expansive reactions within the concrete – a condition known as ‘sulfate attack.’ Concrete made with sulfate-resisting cement remains sounder for longer in more extreme conditions of exposure to sulfate solution compared with ordinary Portland cement. It can do this because it contains a minimal amount of aluminate phase. There are several different forms of sulfate attack and these will be considered later (Chapter 9.2). So, how can we make cement that contains less aluminate phase?. . In theory, we could have less flux overall by increasing the silica ratio.. . We could make ferrite phase as a high a proportion of the flux as possible, by lowering the alumina ratio.. In practice, if we increase the silica ratio too much, there will be insufficient liquid at the burning temperature and good combination will be more difficult to achieve, so this isn’t a good option. If we can’t have much (or any) aluminate phase in the clinker, we will need to lower the alumina ratio. This may mean that harder burning is necessary to achieve combination (we’ll see why in Chapter 5.3) but we can offset this by having more liquid. In other words, we can make sulfate-resisting cement by adjusting the silica ratio downwards, as well as the alumina ratio. This means more liquid is formed overall, but almost all of it will crystallise as ferrite. We can’t take alumina out of the raw materials, but we can add iron. The ferrite that crystallises from the liquid in sulfate-resisting cement is more iron-rich than the ferrite in ordinary Portland cement. Because of this, the normal assumption made by the Bogue calculation that ferrite has the composition C4AF is even more wrong than it is for normal clinker. ASTM C150 provides an alternative calculation for use with Type V (sulfate-resisting) cement. Composite cements, containing ordinary Portland cement and granulated. www.whd.co.uk. 29. www.understanding-cement.com.

(44) Portland cement composition and microstructure blastfurnace slag (gbs), or ordinary Portland cement and fly ash also provide good resistance to sulfate solutions. These composite cements are increasingly used to make sulfate-resisting concrete. In Europe, composite cements have largely taken over from sulfate-resisting cement where sulfate resistance is required; in the UK, for example, the demise of sulfate-resisting Portland cement is almost complete, as it is no longer produced. White Portland Cement White Portland Cement is used for architectural purposes where a white concrete finish is wanted. It is not treated as a separate type of cement in national standards but is produced to meet the same criteria as normal grey cement. Normal cement contains ferrite phase. Ferrite is dark grey or black, giving cement its characteristic grey colour. So, to make white cement, we need to make cement that contains little or no ferrite phase. The principal element that gives the dark colour to ferrite is iron, at least in terms of quantity. Other elements, particularly chromium and manganese are also strong colorants. Titanium, copper and vanadium are also potential sources of colour. In other words, we want as little of these elements as possible in the clinker, achieved by careful selection of raw materials. Magnesium may also affect colour. Typical raw materials used for making white cement are pure limestone, such as chalk, china clay and silica sand. Of course, there is still likely to be a little iron, and other undesirable elements, present in the raw materials. To prevent this from forming ferrite, white clinker is usually burnt under slightly reducing conditions, converting Fe(III) to Fe(II). Fe(II) substitutes for calcium and then resides in the clinker minerals, thus avoiding ferrite formation and discolouration. The Bogue calculation will overestimate the true ferrite content of a white clinker, as it assumes that all iron is present in ferrite. White cement clinker is commonly water-quenched; the rapid temperature decrease helps to prevent Fe(II) oxidising to Fe(III). Rapid-hardening Portland cement Rapid-hardening Portland cement (Type III cement in the ASTM C 150 specification) is basically ordinary Portland cement that has been ground more finely so that it reacts more quickly with water. It is used where fast strength growth is required (eg: precast concrete products) or special applications such as sprayed concrete. As well as being more finely ground, the cement also may have a higher alite content than typical ordinary Portland cement. Typically, rapid-hardening cement at 3 days has a similar strength to ordinary Portland cement at 7 days. Since it reacts more quickly than ordinary Portland cement, it is unsuitable for use in large pours as the heat evolution may be too rapid.. www.whd.co.uk. 30. www.understanding-cement.com.

(45) Portland cement composition and microstructure. References, Chapter 3 1. “Chemical Fundamentals of Geology,” Robin Gill, pub. Chapman and Hall, 1996. 2. http://www.understanding-cement.com/basic-chemistry.html 3. “Cement Chemistry,” H F W Taylor, Thomas Telford, 2nd ed. 1997, Table 1.2.. www.whd.co.uk. 31. www.understanding-cement.com.

(46) 4 Portland cement manufacturing - the main components of a cement plant. 4.1. The quarry. We’ll follow a logical path looking at the main pieces of equipment at a cement plant as the raw materials go from the quarry to the kiln and then to the cement mill. The main basic components of a cement works are:  Crushers and  Silos for raw  Kiln  Clinker  Cement Quarry (raw materials) Mills for raw material store mill material blending. If you happen to be a geologist, the quarry is probably the most interesting part of a cement works (although the kiln might be if you view the clinkering process as igneous rocks in the making).. Figure 4.1 Limestone quarry. (Picture courtesy Figure 4.2 Limestone blocks being Castle Cement.) removed for crushing after blasting. (Picture courtesy Castle Cement.). Typical rock types used in cement production are:.

(47) Portland cement manufacturing - the main components of a cement plant. . Limestone (supplies the bulk of the lime). . Clay, marl or shale (supplies the bulk of the silica, alumina and ferric oxide). . Other supplementary materials such as sand, fly ash or ironstone to achieve the desired bulk composition. Quarry management is a complex process; most quarries will have ‘good material’ from which cement can easily be made, but they will also have lessgood material, possibly of harder material, or of less convenient composition. If the ’good stuff’ is all used up first, it may be difficult to make cement out of what is left. Detailed forward planning is needed to make the best use of all the materials available. Raw materials are extracted from the quarry, then crushed and milled as necessary to provide a fine material for blending in the required proportions. They may also need to be dried. The raw materials may be milled together or separately, depending on how hard they are and whether additional mills are available. The fineness of the material is usually expressed in terms of the percentage retained on a 90µm sieve, typically between 5% and 15%. The blended raw material, the raw meal, is stored in a silo before being fed to the kiln. The silo provides a stock of raw meal, enabling production to be maintained for a day or two in the event of failure of equipment or in the supply of materials.. 4.2. The cement kiln. Rotary kilns were first developed in the 1890s, and became widespread in the early part of the 20th century. They were a great improvement on the earlier shaft kilns, giving continuous production and a more uniform product in larger quantities.. www.whd.co.uk. 33. www.understanding-cement.com.

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