The Science of Cement and Concrete

1 Introduction

1.1 Is concrete a scientifically interesting material?

Few would argue that concrete is an important and useful material: after all, concrete is the backbone of the world’s infrastructure, used in vast amounts to make roads, buildings, bridges, and other structures. But sometimes familiarity breeds contempt. Concrete is generally considered a “boring” material, about which everything important must be known by now. Not true! Concrete may be boring to some at the macroscopic scale (although we don’t think so), but at the microscopic scale it is both interesting and complex. And surprisingly enough, due to its complexity there are still things that scientists do not know about the chemistry and microscopic structure, and there is still significant potential for improving concrete to make better and longer-lasting structures. So the answer to the question in the title of this section is yes!

Figure 1-1 illustrates the process by which cement powder hardens after it is mixed with water. These images come from a highly realistic two-dimensional digital model of cement hydration developed at the National Institute of Standards and Technology. Image (a) shows the unreacted cement particles, which are several microns in size. Each cement particle is made up of a few different minerals, which are color coded. Image (b) shows the cement paste after 30% reaction with water. The yellow rims around the cement particles are a new solid phase called calcium-silicate hydrate (C-S-H) gel. The C-S-H gel grows out into the spaces between the particles, binding them together and giving the cement its strength. As shown in image (c), after continued hydration the formation of C-S-H gel decreases the amount of water-filled pore space (black in the images), which makes the cement relatively impermeable to water and dissolved ions.

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Figure 1-: A realistic model of cement reacting with water and becoming hard. a) Digitized image of a real cement powder just after it is combined with water. The various cement minerals are color coded and the water is black. b) Cement paste that is 30% reacted and has set (about 1 day after mixing). c). Cement paste that is 70% reacted and has become much stronger and less porous (about 1 month after mixing). (Images courtesy of NIST).

The configuration of solid phases and pore space illustrated in the images of Fig. 1-1 is called the microstructure, a word that simply means the structure at the scale of microns (there are 1000 microns in a millimeter). Microstructural features are generally too small to be seen without a microscope, but on the other hand they are much larger than the atoms or molecules that make up the fundamental chemical structure. In general, both the chemical structure and the microstructure of a material control its properties. An important difference between the two is that while the chemical structure is relatively fixed, the microstructure depends strongly on how the material is made and can thus be controlled. Thus the microstructure provides a link between processing (how a material is made) and properties (how a material behaves). These relationships are the basis of the growing field of materials science (see Fig. 1-2).

Figure 1-: The paradigm of materials science. The microstructure of a material depends on the way that it is processed, and the properties depend on the

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a) Unreacted cement

b) 30% reacted

c) 70% reacted

microstructure. If these relationships are understood, than material properties can be controlled, predicted, and improved.

Why is the microstructural link in the middle of Fig. 1-2 needed? After all, one could simply process a material in different ways and then measure the resulting effects on the properties of interest. This is called empirical research. While empirical research can be used to find an effective way to make a material, it does not provide any insight into why the properties change. The advantage of the materials science approach shown in Fig. 1-2 is that it provides a deeper understanding of the material that can be used to predict the properties and to design new and improved materials. Of course, the ability to use the materials science approach is limited by the ability to measure and observe the microstructure. The relatively recent emergence of the field of materials science over the last 50 years has paralleled the development of important characterization techniques such as electron microscopy and X-ray diffraction.

1.2 The purpose of this monograph

Concrete is the world’s most widely used man-made material, and there is a huge database of empirical knowledge on how to mix, place, and cure concrete for specific applications. This knowledge has been extensively codified into standards that ensure that concrete has the required strength, durability, and other properties needed to fit a specific application. This means that the professional who works with concrete does not need to understand in detail (or even be aware of) the fundamental chemical processes that give concrete its useful properties. However, possessing some knowledge of the chemical processes that underlie the workability of concrete, the time to set, the ultimate strength, the tendency to shrink on drying, the tendency to deteriorate under adverse environmental conditions and other phenomena can be a great advantage. A professional with a good working knowledge of the materials science of cement can be more confident, creative, and effective in their use of concrete.

Modern concrete has been around for more than 150 years, and there are many examples of structures lasting for more than one hundred years with little signs of deterioration. However, there are many other cases when the durability of concrete is much less than it should be. Concrete structures can be built to last for a hundred years or much more under almost any conditions if the right mix design and curing conditions are specified and followed. However, all too often the mix design was not the best one for the conditions, or the concrete that was actually used differed from what was specified. This is then a failure not of concrete science, or of the code of standards, but of procedure. We believe that one reason that there is so much poor concrete out there is that there is a general lack of awareness of the materials science of concrete, which can be defined as the relationships between the way concrete is formulated and cured and the resulting microstructure, and between the microstructure and the properties such as durability. Human nature dictates that people are more likely to follow the rules if they understand the reasons behind them, so one of the motivations behind this monograph is that increasing the understanding of the science of concrete will improve the quality of concrete.

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1.3 What is (and is not) covered

The focus will be very much on calcium silicate cement, more commonly known as Portland cement, which is, as a class, by far the most widely used type of cement. Blended cements, which are formed by replacing some of the Portland cement with mineral admixtures such as silica fume, slag, and fly ash, are also discussed in detail. This is justified on the basis that blended cements are used in a large and growing fraction of world’s concrete. It also happens that the chemical reactions and reaction products associated with mineral admixtures are quite interesting and can be readily discussed in conjunction with those of Portland cement itself.

It is probably also useful to mention what will not be covered in much detail. As the title suggests, the focus is on the science, and thus the practical engineering aspects of concrete will be covered only sparingly and in general terms. Thus there will be little discussion of specific mix designs and mix proportioning, of reinforced concrete, or of the design of concrete structures, except as needed to motivate the scientific topics. These important areas are all covered well in books [e.g. 1,2,3] and other publications such as standards [e.g. 4]. For those who want even more detail than this monograph provides about the chemistry of cement manufacture, the details of the hydration process, and other types of cement, we can confidently recommend the standard and excellent book by Taylor [5].

1.4 Basic definitions and terminology

Concrete is composite material, meaning that it is made up of more than one type of material at the macroscopic scale. While concrete can contain a wide variety of different materials, the simplest view divides concrete into just two components: the filler and the binder. This brings us to the most common question people have about cement and concrete, which is… What is the difference?

Cement is the binder component of concrete, the glue that holds the filler together to create a uniform, strong material. The filler in concrete consists primarily of aggregate particles. These can be made out of lots of different materials, but the vast majority of aggregate is just sand, gravel, and rocks, which are cheap, plentiful, and easily obtained from nature. Cement, on the other hand, is a man-made material that requires high-temperature processing, grinding, and other costly and energy-intensive steps (these are discussed in Chapter 3). Pure cement paste would not only be much more expensive to use than concrete, but would not perform nearly as well. This is primarily because cement paste has a tendency to shrink and crack when it dries, and the aggregate helps prevent this. So unless you mix it up yourself, or hang

1 . S. Mindess, J.F. Young, and D. Darwin, Concrete, Prentice Hall, 2nd Ed., 1996.

2 . P.K. Mehta and P.J.M. Monteiro, Concrete: Structure, Properties, and Materials, Prentice-Hall, Englewood Cliffs, NJ, 2nd Ed., 1993.

3 . S.H. Kosmatka, B. Kerkhoff, and W.C. Panarese, Design and Control of Concrete Mixtures, Portland Cement Association, Skokie, IL. 14th Ed., 2002.

4 . Annual Book of ASTM Standards, Volume 4, published by the American Society for Testing and Materials.

5 . H.F.W. Taylor, Cement Chemistry, Thomas Telford, London, 2nd Ed., 1997.

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around a cement research lab, you will rarely see pure cement paste, just concrete. Figure 1- shows the basic ingredients of concrete.

Figure 1-: The basic ingredients of concrete. From left to right: fine aggregate (sand), coarse aggregate, water, and cement. (Image courtesy of the Portland Cement Association). MK5-019

As is true of any complex topic, those who work with cement and concrete have developed some specific terminology. Many of these terms and concepts will be introduced and defined as they are first needed, but it will probably be helpful to go over some of them now. Here are some of the most basic terms:

Cement: This word is used colloquially to mean several very different things: the dry unreacted powder that comes in a sack, the sticky fluid stuff formed just after water is added, and the rocklike substance that forms later on. As noted above, people also tend to use it to refer to concrete. Obviously this won’t work for people who want to have technical discussions. For our purposes, the word cement used by itself refers to the dry unreacted powder.

Cement paste: Cement (see above) that has been mixed with water. Usually the term implies that it has already become hard (see Fresh).

Concrete: A mixture of sand, gravel, and rocks held together by cement paste. The world’s most widely-used man-made material.

Aggregate: The inert filler material that makes up the bulk of concrete. Usually sand, gravel, and rocks. Fibers and reinforcing bars are not considered aggregate.

Mortar: A mixture of cement paste and sand used in thin layers to hold together bricks or stones. Technically, mortar is just a specific type of concrete with a small maximum aggregate size.

Fresh: Refers to cement paste or concrete that has been recently mixed and is still fluid. This is what those big trucks with the rotating container on the back are full of. (These are often called “cement mixers” but now you know why they should be called “concrete mixers”).

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Hardened: Refers to cement paste or concrete that has gained enough strength to bear some load.

Set: The transition from fresh cement paste to hardened cement paste. The terms “initial set” and “final set” refer to specific times when the paste becomes no longer workable and completely rigid, respectively. “Setting” is the process by which this occurs.

Curing/Hardening: Essentially interchangeable terms that mean the process of continued strength gain after the cement paste has set due to chemical reactions between cement and water.

Young: Refers to cement paste or concrete that has recently set and is now actively hardening. What constitutes “young” in terms of time is variable; the term implies that the paste has undergone a only fraction of its full reaction and is thus weak and vulnerable to damage. This could be anywhere from a few hours to weeks depending on the mix design and the temperature.

Mature: Refers to cement paste or concrete that has reached close to its full strength and is reacting very slowly, if at all. An age of 28 days is a very rough rule of thumb for reaching maturity.

Hydration: The chemical reactions between cement and water. Hydration is what causes cement paste to first set and then harden.

Hydration products: The new solid phases that are formed by hydration.

Heat of hydration: Like most spontaneous chemical reactions, the hydration reactions between cement and water are exothermic, meaning that they release heat. Large volumes of concrete can warm up considerably during the first few days after mixing when hydration is rapid. This is generally a bad thing, for reasons that will be discussed.

Placing: The process of transferring fresh concrete from the mixer to the formwork that defines its final location and shape

Segregation: An undesirable process of the aggregate particles becoming unevenly distributed within the fresh cement paste while the concrete is being placed or consolidated.

Bleeding: An undesirable process of mix water separating from the fresh cement paste or concrete while it is being placed or consolidated.

References

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