Chemical Education Today www.JCE.DivCHED.org • Vol. 83 No. 10 October 2006 • Journal of Chemical Education 1425 Report Concrete by Mary Anne White Figure 1. The addition of water and aggregate to cement gives a concrete slurry that is suitable for pouring. Photo courtesy J. H. Findorff and Son, Inc. Concrete is an important construction material, com- posed of Portland cement and water, combined with sand, gravel, and other relatively inert materials, referred to collec- tively as aggregate. Given cost considerations, no more Port- land cement is used than required to coat the aggregate surfaces and fill the voids (10–15% by volume) (1). Port- land cement production in the U.S. is more than 100 mil- lion metric tons annually (2), indicating annual concrete use of around 1 billion metric tons in the U.S. Surely this famil- iar 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 con- crete are among the least understood materials (3). Cement Chemistry The dry portion of Portland cement is a complex mix- ture of about 63% calcium oxide (CaO), 20% silica (SiO 2 ), 6% alumina (Al 2 O 3 ), 3% iron(III) oxide (Fe 2 O 3 ), and small quantities of other matter (4). Cement is prepared in a two- step process: high-temperature mixing and processing of lime- stone, 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 H 2 O and CO 2 , and causes re- actions between the solids. The main components of cement powder at this stage are tricalcium silicate and dicalcium sili- cate. 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 in- clude 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 co- agulate 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 eas- ily (12). Details of cement chemistry, including a chemist’s guide to the traditional cement notations (for example “C 2 S” corresponds to dicalcium silicate, Ca 2 SiO 4 ) are presented else- where (8). One of the more amazing features is that chemi- cal reactions are still taking place at a significant rate more than 100 days after the addition of water to cement (14). Concrete as a Building Material Concrete is an excellent construction material, with com- pressive strengths ranging from 70 to 350 MPa (15), and it can be modified in many ways for specific applications. Con- crete can be reinforced with steel rods or wire mesh, provid- ing 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 con- ductive 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 re- ducing 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
Transcript
Chemical Education Today
www.JCE.DivCHED.org • Vol. 83 No. 10 October 2006 • Journal of Chemical Education 1425
Report
Concreteby Mary Anne White
Figure 1. The addition of water and aggregate to cement gives aconcrete slurry that is suitable for pouring.
Photo courtesy J. H. Findorff and Son, Inc.
Concrete is an important construction material, com-posed of Portland cement and water, combined with sand,gravel, and other relatively inert materials, referred to collec-tively as aggregate. Given cost considerations, no more Port-land cement is used than required to coat the aggregatesurfaces and fill the voids (10–15% by volume) (1). Port-land cement production in the U.S. is more than 100 mil-lion metric tons annually (2), indicating annual concrete useof around 1 billion metric tons in the U.S. Surely this famil-iar material should make an appearance in our curriculum:quite likely the buildings in which we teach are made, at leastin part, of concrete. Even more surprisingly, cement and con-crete are among the least understood materials (3).
Cement Chemistry
The dry portion of Portland cement is a complex mix-ture of about 63% calcium oxide (CaO), 20% silica (SiO2),6% alumina (Al2O3), 3% iron(III) oxide (Fe2O3), and smallquantities of other matter (4). Cement is prepared in a two-step process: high-temperature mixing and processing of lime-stone, sand, and clay to produce a cement powder, followedby hydration of the cement in preparation for the additionof aggregate to produce concrete (5, 6, 7). The resulting slurrycan be poured or cast (Figure 1).
The heating step releases H2O and CO2, and causes re-actions between the solids. The main components of cementpowder at this stage are tricalcium silicate and dicalcium sili-cate. Addition of alumina can reduce the temperature andtime required for this stage, which is an important economicand 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 inthe solution in the interstices of this heterogeneous system.After an induction period, precipitation of new solids takesplace when their solubility limit is reached. These solids in-clude calcium silicate hydrate and calcium hydroxide. As thehydration processes proceed, anhydrous materials are replacedwith hydrates that take up more than twice the volume (9),leading to a decrease in porosity. However, immediately onaddition of water to cement powder, the slurry tends to co-agulate due to the high ionic strength of the aqueous phase(10). Coagulation of the slurry must be prevented; onemethod 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 contentleads to a more porous product, which can degrade more eas-ily (12). Details of cement chemistry, including a chemist’sguide to the traditional cement notations (for example “C2S”corresponds to dicalcium silicate, Ca2SiO4) are presented else-where (8). One of the more amazing features is that chemi-cal reactions are still taking place at a significant rate morethan 100 days after the addition of water to cement (14).
Concrete as a Building Material
Concrete is an excellent construction material, with com-pressive strengths ranging from 70 to 350 MPa (15), and itcan be modified in many ways for specific applications. Con-crete can be reinforced with steel rods or wire mesh, provid-ing an even stronger material. The surface can be madenon-slip by the addition of aluminum oxide grains (1). Lowerdensity concrete can be made with spongy aggregates, or bythe addition of aluminum powder to the cement, which givesoff hydrogen bubbles from the reaction between the lime andthe aluminum, resulting in 70% reduction in density yet agood compressive strength of 7 MPa (1). An electrically con-ductive concrete, known as Marconite, produced in the UKby Carbon International Ltd., can be used for radio-frequencygrounding of electrical equipment (16). [Students can testhow a modification to concrete affects its properties in thismonth’s JCE Classroom Activity (p 1472 A–B).]
The Environmental Council of Concrete Organizationshas indicated a number of ways in which concrete can beconsidered to be an environmentally friendly material (17).The ingredients of concrete are earth materials that are inabundant supply, and they are extracted locally thereby re-ducing the energy required for shipping. The aggregate caninclude a wide variety of materials that could otherwise bewaste, such as slag from blast furnaces, recycled polystyrenes,fly ash and even old concrete (Figures 2 and 3). Concreterequires less energy for production than many materials: itsprimary 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 ina wide variety of shapes and sizes, with little waste. In use,concrete provides buildings with higher energy efficiency due
1426 Journal of Chemical Education • Vol. 83 No. 10 October 2006 • www.JCE.DivCHED.org
Report
Figure 2: Old concrete can be broken up and recycled to provideaggregate for use in new concrete. Reproduced with permission ofthe Portland Cement Association.
Figure 3: Supplementary materials that can be added to aggregatefor concrete: from left to right, fly ash (Class C), metakaolin (cal-cined clay), silica fume, fly ash (Class F), slag, and calcined shale.Reproduced with permission of the Portland Cement Association.
photo: Portland Cem
ent Associaton
photo: Portland Cem
ent Associaton
to the large thermal mass, acting as a sensible heat storagematerial (19). Furthermore, phase-change materials can beadded to concrete for enhanced heat storage from, for ex-ample, solar energy (20).
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 prob-lem is cement corrosion, which can be accelerated in steel-reinforced concrete, and by the action of water. The corrosioncan be physically based, especially from the freeze–thaw cycleof water in the porous material, or chemical in nature, suchas from the action of acid converting calcium hydroxide tocalcium carbonate (8). Cement corrosion costs North Ameri-cans nearly a billion dollars a year (21). One area of activeresearch is advancing understanding of the corrosion processes(22, 23). Another is the use of steel fibres to produce con-crete with a compressive strength of 800 MPa, close to thatof steel (23). Polymer fibers also have been used to strengthenconcrete without corrosion (24). [Recent research on the ef-fects of using fly ash in concrete is reported on p 1420 ofthis issue of JCE .]
Other research aims to improve the cement process. Forexample, belite cement—which has more belite (�-dicalciumsilicate) and less alite (tricalcium silicate) than Portland ce-ment—requires less limestone as a raw material and is pro-cessed at lower temperatures with less CO2 production (25).
Finally, we note that concrete’s properties are still beingimproved in novel ways. A particularly interesting innova-tion is the addition of photocatalysts such as the anatase formof TiO2, to reduce pollutants including NOx, ammonia, aro-
matics, and aldehydes, leading to self-cleaning building walls(26), as used in the building shown in the photograph onthe right in Figure 4.
Literature Cited
1. Brady, G. S.; Clauser, H. R.; Vaccari, J. A. Materials Hand-book, 14th ed; McGraw-Hill: New York, 1997.
2. Cement and Concrete Basics Web Page of the Portland Ce-ment Association Web Site. http://www.cement.org/basics/cementindustry.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’sChemistry of Cement and Concrete, 4th ed., Hewlett, P. C., Ed.;Arnold: London, 1998.
6. Hewlett, P. C.; Hunter, G.; Jones, R. Chemistry in Britain1999, 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. OilGas 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-
ment. In Lea’s Chemistry of Cement and Concrete, 4th ed.,Hewlett, P. C., Ed.; Arnold: London, 1998.
15. Shackelford, J. F. Introduction to Materials Science for Engi-neers, 2nd ed.; Macmillan: New York, 1988.
16. Carbon International Ltd. Conductivity Products Web Site.http://www.carboninternational.co.uk/carbon/conductivity/indexconductivity.html (accessed Aug 2006)
17. Why Concrete? Page of the Environmental Council of Con-crete Organizations. http://www.ecco.org/why.htm (accessed Aug2006).
18. Gupta, T. N. MRS Bull. 2000, 25 (4), 60.19. White, M. A. Heat Storage Systems. In 2002 McGraw-Hill
Yearbook of Science and Technology; McGraw-Hill: New York,2001; pp 151–153.
20. Hawes, D. W.; Banu, D.; Feldman, D. Solar Energy Materialsand Solar Cells 1992, 27 (2), 103.
21. Luma, C. Coping with Corrosion and Fouling: Protect Con-crete from Corrosion. In Plant Operation and Maintenance—Part 2: Best Practices and Procedures; Access Intelligence: NewYork, 1998; Chapter 8, 149–150.
22. Bournazel, J. P.; Moranville, M. Cem. Concr. Res. 1997, 27,1543.
23. Vernet, C. P. MRS Bull. 2004, 29 (5), 324.24. Trottier, J.-F.; Mahoney, M. Concrete International 2001, 23
(6), 23.25. Ishida, E. H.; Isu, N. MRS Bull. 2001, 26 (11), 895.26. Cassar, L. MRS Bull. 2004, 29 (5), 328.
Mary Anne White is University Research Professor of Chem-istry and Physics and Director of the Institute for Research inMaterials at Dalhousie University, Halifax, Nova Scotia B3H4J3, Canada; [email protected]
