Chemical Education Today
Report
Concrete by Mary Anne White Photo courtesy J. H. Findorff and Son, Inc.
Concrete is an important construction material, composed of Portland cement and water, combined with sand, gravel, and other relatively inert materials, referred to collectively as aggregate. Given cost considerations, no more Portland cement is used than required to coat the aggregate surfaces and fill the voids (10–15% by volume) (1). Portland cement production in the U.S. is more than 100 million metric tons annually (2), indicating annual concrete use of around 1 billion metric tons in the U.S. Surely this familiar material should make an appearance in our curriculum: quite likely the buildings in which we teach are made, at least in part, of concrete. Even more surprisingly, cement and concrete are among the least understood materials (3). Cement Chemistry The dry portion of Portland cement is a complex mixture of about 63% calcium oxide (CaO), 20% silica (SiO2), 6% alumina (Al2O3), 3% iron(III) oxide (Fe2O3), and small quantities of other matter (4). Cement is prepared in a twostep process: high-temperature mixing and processing of limestone, sand, and clay to produce a cement powder, followed by hydration of the cement in preparation for the addition of aggregate to produce concrete (5, 6, 7). The resulting slurry can be poured or cast (Figure 1). The heating step releases H2O and CO2, and causes reactions between the solids. The main components of cement powder at this stage are tricalcium silicate and dicalcium silicate. Addition of alumina can reduce the temperature and time required for this stage, which is an important economic and energetic consideration for the production of cement (8). When cement is mixed with 30–40% water by volume, complex chemistry, not fully understood, ensues. In brief, dissolution produces calcium, silicate, and aluminate ions in the solution in the interstices of this heterogeneous system. After an induction period, precipitation of new solids takes place when their solubility limit is reached. These solids include calcium silicate hydrate and calcium hydroxide. As the hydration processes proceed, anhydrous materials are replaced with hydrates that take up more than twice the volume (9), leading to a decrease in porosity. However, immediately on addition of water to cement powder, the slurry tends to coagulate due to the high ionic strength of the aqueous phase (10). Coagulation of the slurry must be prevented; one method of prevention is the addition of dispersants (11, 12), to allow workability with the minimum addition of water (12, 13). The latter is important because higher water content leads to a more porous product, which can degrade more easily (12). Details of cement chemistry, including a chemist’s guide to the traditional cement notations (for example “C2S” corresponds to dicalcium silicate, Ca2SiO4) are presented elsewhere (8). One of the more amazing features is that chemical reactions are still taking place at a significant rate more than 100 days after the addition of water to cement (14). www.JCE.DivCHED.org
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Figure 1. The addition of water and aggregate to cement gives a concrete slurry that is suitable for pouring.
Concrete as a Building Material Concrete is an excellent construction material, with compressive strengths ranging from 70 to 350 MPa (15), and it can be modified in many ways for specific applications. Concrete can be reinforced with steel rods or wire mesh, providing an even stronger material. The surface can be made non-slip by the addition of aluminum oxide grains (1). Lower density concrete can be made with spongy aggregates, or by the addition of aluminum powder to the cement, which gives off hydrogen bubbles from the reaction between the lime and the aluminum, resulting in 70% reduction in density yet a good compressive strength of 7 MPa (1). An electrically conductive concrete, known as Marconite, produced in the UK by Carbon International Ltd., can be used for radio-frequency grounding of electrical equipment (16). [Students can test how a modification to concrete affects its properties in this month’s JCE Classroom Activity (p 1472 A–B).] The Environmental Council of Concrete Organizations has indicated a number of ways in which concrete can be considered to be an environmentally friendly material (17). The ingredients of concrete are earth materials that are in abundant supply, and they are extracted locally thereby reducing the energy required for shipping. The aggregate can include a wide variety of materials that could otherwise be waste, such as slag from blast furnaces, recycled polystyrenes, fly ash and even old concrete (Figures 2 and 3). Concrete requires less energy for production than many materials: its primary energy requirement is in the range of 1–10 MJ kg᎑1, comparable to clay bricks and tiles, and much less than glass (12–25 MJ kg᎑1), steel (20–100 MJ kg᎑1), plastics (50– 100 MJ kg ᎑1), and aluminum (200–250 MJ kg᎑1) (18). Whether precast or poured in place, concrete can be cast in a wide variety of shapes and sizes, with little waste. In use, concrete provides buildings with higher energy efficiency due
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Chemical Education Today
Report photo: Portland Cement Associaton
photo: Portland Cement Associaton
Figure 2: Old concrete can be broken up and recycled to provide aggregate for use in new concrete. Reproduced with permission of the Portland Cement Association.
Figure 3: Supplementary materials that can be added to aggregate for concrete: from left to right, fly ash (Class C), metakaolin (calcined clay), silica fume, fly ash (Class F), slag, and calcined shale. Reproduced with permission of the Portland Cement Association.
to the large thermal mass, acting as a sensible heat storage material (19). Furthermore, phase-change materials can be added to concrete for enhanced heat storage from, for example, solar energy (20).
matics, and aldehydes, leading to self-cleaning building walls (26), as used in the building shown in the photograph on the right in Figure 4. Literature Cited
Active Areas of Research Research to improve concrete is ongoing and important, especially considering the amount of concrete in our homes, office buildings, highways, and bridges. One serious problem is cement corrosion, which can be accelerated in steelreinforced concrete, and by the action of water. The corrosion can be physically based, especially from the freeze–thaw cycle of water in the porous material, or chemical in nature, such as from the action of acid converting calcium hydroxide to calcium carbonate (8). Cement corrosion costs North Americans nearly a billion dollars a year (21). One area of active research is advancing understanding of the corrosion processes (22, 23). Another is the use of steel fibres to produce concrete with a compressive strength of 800 MPa, close to that of steel (23). Polymer fibers also have been used to strengthen concrete without corrosion (24). [Recent research on the effects of using fly ash in concrete is reported on p 1420 of this issue of JCE .] Other research aims to improve the cement process. For example, belite cement—which has more belite (-dicalcium silicate) and less alite (tricalcium silicate) than Portland cement—requires less limestone as a raw material and is processed at lower temperatures with less CO2 production (25). Finally, we note that concrete’s properties are still being improved in novel ways. A particularly interesting innovation is the addition of photocatalysts such as the anatase form of TiO2, to reduce pollutants including NOx, ammonia, aro-
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1. Brady, G. S.; Clauser, H. R.; Vaccari, J. A. Materials Handbook, 14th ed; McGraw-Hill: New York, 1997. 2. Cement and Concrete Basics Web Page of the Portland Cement Association Web Site. http://www.cement.org/basics/cement industry.asp (accessed Aug 2006). 3. Coveney, P. V.; Davey, R. J.; Griffin; J. L. W.; Whiting, A. Chem. Commun. 1998, 1467. 4. West, A. R. Solid State Chemistry and its Applications; Wiley: Chichester, 1984. 5. Blezard, R. G. The History of Calcerous Cements. In Lea’s Chemistry of Cement and Concrete, 4th ed., Hewlett, P. C., Ed.; Arnold: London, 1998. 6. Hewlett, P. C.; Hunter, G.; Jones, R. Chemistry in Britain 1999, 35 (1), 40. 7. Daugherty, K. E. ; Robertson, L. D. J. Chem. Educ. 1972, 49, 522. 8. MacLaren, D. C.; White, M. A. J. Chem. Educ. 2003, 80, 623. 9. Powers, T. C. J. Am. Ceram. Soc. 1958, 41, 1. 10. Nachbaur, L.; Mutin, J. C.; Choplin, L.; Nonat, A. Cem. Concr. Res. 2001, 31, 183. 11. Lootens, D.; Lécolier, E.; Hébraud, P.; Van Damme, H. Oil Gas Technol. 2004, 59, 31. 12. Flatt, R. J.; Martys, N.; Bergström, L. MRS Bull. 2004, 29 (5), 314. 13. Pellenq, R. J.-M.; Van Damme, H. MRS Bull. 2004, 29 (5), 319. 14. Odler, I. Hydration, Setting and Hardening of Portland Ce-
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Chemical Education Today
Figure 4: Dives in Misericordia (at left), a church designed by Richard Meier and constructed in Rome in 2003 with TiO2-containing selfcleaning concrete. (Image © www.erco.com/Frieder Blickle; used with permission.) At the right is a schematic view of the action of TiO2 acting as a photocatalyst for self-cleaning.
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ment. In Lea’s Chemistry of Cement and Concrete, 4th ed., Hewlett, P. C., Ed.; Arnold: London, 1998. Shackelford, J. F. Introduction to Materials Science for Engineers, 2nd ed.; Macmillan: New York, 1988. Carbon International Ltd. Conductivity Products Web Site. http://www.carboninternational.co.uk/carbon/conductivity/ indexconductivity.html (accessed Aug 2006) Why Concrete? Page of the Environmental Council of Concrete Organizations. http://www.ecco.org/why.htm (accessed Aug 2006). Gupta, T. N. MRS Bull. 2000, 25 (4), 60. White, M. A. Heat Storage Systems. In 2002 McGraw-Hill Yearbook of Science and Technology; McGraw-Hill: New York, 2001; pp 151–153. Hawes, D. W.; Banu, D.; Feldman, D. Solar Energy Materials and Solar Cells 1992, 27 (2), 103. Luma, C. Coping with Corrosion and Fouling: Protect Concrete from Corrosion. In Plant Operation and Maintenance— Part 2: Best Practices and Procedures; Access Intelligence: New York, 1998; Chapter 8, 149–150. Bournazel, J. P.; Moranville, M. Cem. Concr. Res. 1997, 27, 1543. Vernet, C. P. MRS Bull. 2004, 29 (5), 324. Trottier, J.-F.; Mahoney, M. Concrete International 2001, 23 (6), 23. Ishida, E. H.; Isu, N. MRS Bull. 2001, 26 (11), 895. Cassar, L. MRS Bull. 2004, 29 (5), 328.
Mary Anne White is University Research Professor of Chemistry and Physics and Director of the Institute for Research in Materials at Dalhousie University, Halifax, Nova Scotia B3H 4J3, Canada;
[email protected] www.JCE.DivCHED.org
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