JCE Classroom Activity: #83
Instructor Information
Chemistry of Cement Patricia Mason Delphi Community High School, 501 Armory Road, Delphi, IN 46923;
[email protected] In this Activity, students use a commercial cement mix to produce concrete. They investigate how the addition of various substances, called admixtures, affects properties such as setting time, hardness, and plasticity. Portland cement was patented in England in 1824 by Joseph Aspden and named Portland because when it hardened it reminded him of stone from the Isle of Portland (1). Prior to Aspden’s patent, the compositions of mixtures used in cement were kept secret. Portland cement was not introduced into the U.S. until 1871. The chemistry of making cement involves two distinct steps: (i) setting, the process of changing the plasticity (ability to be shaped or molded) of the cement to stiffness, and (ii) hardening, the development of strength. If cement sets too quickly, workers experience difficulty in getting the material in place. Adding gypsum, CaSO4, to Portland cement prolongs the plastic period. The most important compounds present in cement are: 3CaO·Al2O3, tricalcium aluminate; 3CaO·SiO3, tricalcium silicate; 2CaO·SiO3, dicalcium silicate; and CaO, calcium oxide. The 2CaO·SiO3 reacts slowly with water to yield Ca(OH)2 and H2SiO3. This reaction not only helps in holding the material together, but also makes the concrete less pervious to water. The hardening process is due in part to the hydration of the compounds present and is probably influenced by the crystallization of these hydrates. Essentially, cement is a highly polymerized cross-linked matrix (1). Setting and hardening both require the presence of water, but not of air. Compounds formed as cement hardens into concrete are insoluble in water, which is why cement can harden underwater.
Integrating the Activity into Your Curriculum This Activity complements the American Chemical Society’s theme for National Chemistry Week 2006, “Your Home— It’s All Built on Chemistry”. It introduces students to mixtures and solutions as well as to experimental design in the context of a familiar consumer product. Students prepare mixtures and change key variables such as concentrations, curing temperatures, and addition of admixtures. photo by P. Mason
perforated
About the Activity
Although concrete can be made from scratch using sand, gravel, and cement, the use of a commercially prepared mix such as Sakrete allows for ease in preparation. Bags of mix are at hardware and home improvement stores. One such mixture contains 12% Portland cement, 34% sand, 48% crushed stone, and 6% water (see Student Activity ref 3). Students should take care not to inhale the dust from the concrete mix. An empty 1-lb. cottage cheese tub works well as a scoop and a mold. Larger 2–3-lb. plastic margarine tubs serve as mixing containers. Paint stirrers or craft sticks are sturdy enough for mixing. After the addition of water and thorough mixing, the material should have the consistency of thick mud. If the mixture is too dry, students may carefully add more water. Students can vary the procedure in several ways; suggestions are in the Student Activity. JCE articles describe the effects of some admixtures (2). There are several ways to test the product. If students have access to a hydraulic press in the school’s auto shop, use it to break the product samples. The greater the force required, the stronger the concrete. Less quantitative tests are to hit the sample with a hammer or drop a heavy object such as a plumb bob on it from different heights, using a long plastic tube (such as for fluorescent lights) as a guide. In the Activity, students drop the concrete sample from different heights, since this does not require any additional materials. In all of these tests, students should wear eye protection and take care not to damage the surface under the sample. Layers of newspaper can offer protection. The sample may be wrapped in a plastic bag or thin cloth before being dropped to reduce the possibility of scattering the chunks.
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
fold here and tear out
Background
Answers to Questions 1. 2. 3. 4.
Answers will vary, depending on the variable chosen. Answers will vary. See “About the Activity” for possible answers. Concrete is a heterogeneous mixture because its individual parts can be distinguished by the naked eye. Radon gas is radioactive and naturally breaks down into other harmful substances that also release radiation, which can cause health problems. 5. Fly-ash is a waste product that needs proper disposal. Using it in concrete solves the solid waste disposal problem.
References, Additional Related Activities, and Demonstrations 1. MacLaren, Douglas C.; White, Mary Anne. Cement: Its Chemistry and Properties. J. Chem. Educ. 2003, 80, 623–635. 2. Daugherty, Kenneth E.; Robertson, Les D. Practical Problems in the Cement Industry Solved by Modern Research Techniques. J. Chem. Educ. 1972, 49, 522–525. White, Mary Anne. Cement. J. Chem. Educ. 2006, 83, 1425. JCE Classroom Activities are edited by Erica K. Jacobsen and Julie Cunningham
www.JCE.DivCHED.org •
Vol. 83 No. 10 October 2006 •
Journal of Chemical Education
1472A
JCE Classroom Activity: #83
Student Activity
Chemistry of Cement In 2000, a steel and concrete pedestrian bridge collapsed at a motor speedway (see WWW ref 1). Nearly 100 people were injured, and testimony at a subsequent court case brought chemistry to the courtroom (see WWW ref 2). The prosecutor claimed evidence would show that a product added to the concrete contributed to the bridge’s collapse. Concrete is a hardened material that forms when a mixture of cement, sand, crushed stone, and water is poured into molds and cured, or hardened. Curing can be affected by the proportion of water, crushed stone, or cement used, or by other additives. Cement is made from limestone Puddling removes (CaCO3) and clay that are heated in a kiln until pea-sized masses called clinker form. Clinker air bubbles and is ground into powder, which is called cement; generally, the finer the powder, the stronger moves pieces of the cement. In this Activity, you will use a commercial cement mix to make concrete and stone away from investigate how the addition of various substances affects the concrete’s properties. the surface (step 5).
Try This
photo by P. Mason
You will need: commercial cement mixture (it contains sand, stone, and cement in the correct proportions), 2 or 3 plastic containers such as 2–3-lb. margarine tubs, 2 or 3 smaller Be Safe! Take care not to breathe plastic containers such as 1-lb. cottage cheese tubs, marker, sturdy wooden sticks such as the cement dust. During the hardcraft sticks or paint stirrers, graduated cylinder or measuring cup, water, and your choice of ness test, wear eye protection and additives, such as calcium chloride, sugar, table salt, Epsom salts, nails, and polystyrene pea- protect the surface on which you drop the sample. Be careful not nuts. To change the curing temperature, you will need access to a refrigerator or freezer. __1. Use a marker to label one 1-lb. cottage cheese tub “control”, another “variation”. These to drop the sample on your feet. will be your molds. __2. Into one of the tubs from step 1, measure about 500 cm3 of a commercial cement mixture (nearly to the top of the 1-lb. tub). Take care not to breathe the dust. Place the mixture in a larger plastic container such as a 2–3-lb. margarine tub, which will serve as your mixing bowl. __3. Add 110 mL (~1/2 cup) of tap water (measured with a graduated cylinder or measuring cup) to the mixture. __4. Stir the water into the cement mixture using a sturdy wooden stick such as a craft stick or painter stirrer. Mix until no dry mix remains and the mixture has the consistency of thick mud. Add an additional 10 mL (~2 tsp) of water if the mixture still looks dry. Pour the wet mixture into the tub labeled “control”. __5. Carefully move the wooden stick up and down in the mixture in the mold, which will remove any air and leave no pieces of stone on the surface. This process is called puddling. __6. Repeat steps 2–5 using the tub labeled “variation”, but change one of the variables. Some suggestions are: adjust the volume of water, cure under water, cure in refrigerator or freezer, include an additive in step 4 (salt, sugar, Epsom salts, calcium chloride, nails), replace several rocks with polystyrene pieces. Predict how the variation will affect the cured sample. __7. The cement is ready to cure or harden. Allow the two molds to sit undisturbed for a minimum of 48 hours. __8. After the curing time, remove the hardened sample from the mold and note the properties (smoothness, cracking, visible gravel, air pockets, dryness, etc.) of the outside surface of your concrete. __9. Test each cured sample for hardness. Drop the sample from a given height onto a hard surface. Begin by dropping it from 15 cm (~6 in.); if it withstands the impact, increase the dropping height. CAUTION: Wear eye protection. Protect the surface on which you drop the sample (use several layers of newspaper on the floor or perform this test outdoors). Wrap the concrete sample in a plastic bag or thin cloth before dropping to minimize any mess. After the sample cracks, note the properties of the inside surfaces as in step 8.
Questions 1. 2. 3. 4.
Compare the cured “control” and “variation” samples. How did the variation you used affect the cured sample? What is another reproducible, measurable way you could test a cured sample for hardness? What kind of mixture is concrete, heterogeneous or homogeneous? Explain your answer. Aggregate materials other than gravel have been used in concrete. Some of them turned out to be sources of radon gas. What problems would this cause for people who lived in houses with this type of concrete for their basements? 5. Fly-ash, a waste product of coal-fired power plants, is sometimes used as an additive in cement. Why would using this product in cement be beneficial to society?
Information from the World Wide Web (accessed Aug 2006) 1. Cause of Overpass Collapse outside NC Speedway Sought. http://archives.cnn.com/2000/US/05/21/walkway.collapse.06 2. Testimony Begins in LMS Bridge Collapse Case. http://www.wsoctv.com/news/1952003/detail.html 3. The Chemistry of Cement. http://www.uvi.edu/Physics/SCI3xxWeb/Structure/ChemOfCement.html This Classroom Activity may be reproduced for use in the subscriber’s classroom.
1472B
Journal of Chemical Education
• Vol. 83 No. 10 October 2006 •
www.JCE.DivCHED.org
