From Rocks to Skyscrapers: the Life of a Cement Grain

By Linda M. Hills, Senior Materials Scientist • CTL GROUP • www.ctlgroup.com

From sidewalks and basement foundations, to patterned and pigmented architectural concrete and 80-story skyscrapers, concrete is a part of everyday life. In fact, concrete is the world's most used construction material. An understanding about portland cement is important for anyone involved with concrete - from concrete finisher, to architect, contractor, engineer, and owner. Why? Because understanding how a material works makes it easier to work with and to get the final result you need.

Numerous characteristics of the cement, established during its manufacturing process, influence cement hydration, which in turn influence concrete performance. Many books and research papers discussing details of cement and concrete are available, and not all the details can be covered here. This article provides an overview of cement, from manufacturing to its hydration, and its role in concrete.

Cement in the making
Cement is made by first burning ground raw materials at around 1480°C to form cement clinker (Figures 1A-1D). The clinker is ground and combined with gypsum to form cement powder (Figure 1E). More detailed information about cement manufacturing, including an animation of the process, can be obtained at the Portland Cement Association's (PCA) website: www.cement.org.

Manufacturing conditions such as raw feed fineness, burning rate, burning temperature, and cooling rate all influence the microstructure formed within the cement grain. This in turn affects concrete performance.

A. Raw materials containing the right chemical composition (such as limestone, sand, clay, iron ore) are quarried, ground to a fine powder,... B. ...burned in a kiln at 1480°C... C. ...to form cement clinker.
D. This process recombines the chemicals into hydraulic compounds that give cement its setting and strength properties. E. The clinker is ground with gypsum to produce cement.

Four major compounds
are formed during clinkering


F. Tricalcium silicate, C3 S
G. Dicalcium silicate, C2 S
H. Tetracalcium aluminoferrite, C4 AF
I. Tricalcium aluminate, C3 A
   
Concrete's Main Ingredients   Photos A-E (Above):
A. Quarry at a cement plant
B. Inside a kiln
C. Cement clinker balls
D. A cross section of clinker showing the hydraulic compounds, as seen by reflected light microscopy.
E. Cement powder compared to the width of a human hair
7-15% Cement
Paste

Mortar
14-18% H2O
Fine Aggregate Concrete
Coarse Aggregate

Cement hydration process
When water is added to cement, the clinker compounds chemically combine with water (hydrate) to form new compounds, as outlined below. Further information on cement hydration can be found in a publication by Kosmatka, Kerkhooff, and Panarese (2002).

  1. Calcium silicates (C3S and C2S) hydrate to form 1) calcium hydroxide and 2) calcium silicate hydrate (C-S-H). It is worth noting that calcium silicate hydrate is the most important cementing compound in concrete. Setting, hardening, and strength depend primarily on calcium silicate hydrate.
  2. C3A, calcium sulfates, and water form calcium trisulfoaluminate hydrate (ettringite). C3A and ettringite combine with water to form calcium monosulfoaluminate, and other compounds. C3A reacts with water and calcium hydroxide to form tetracalcium aluminate hydrates.
  3. C4AF reacts with water and calcium hydroxide to form calcium aluminoferrite hydrates.

These hydration reactions occur at different rates. Actual images of cement paste microstructure at various hydration stages are shown below in scanning electron photomicrographs of hydrated cement paste as hydration time progresses from "Stage 1" to "Stage 3". Images at 1,000x magnification show overall view and texture differences, while those at 5,000x show hydration products forming with time from the original cement grains.

Stage 1
Up to 10 minutes
Stage 1: 1,000x magnification Stage 1: 5,000x magnification
Stage 1: up to 10 minutes. Aluminates react with calcium sulfate and water. Gel and short ettringite needles form on surface of cement grains.
Stage 2:
4 to 20 hours
Stage 2: 1,000x magnification Stage 2: 5,000x magnification
Stage 2: 4 to 20 hours. C3S reacts. Calcium hydroxide, calcium silicate hydrate, and longer needles of ettringite form.
Stage 3:
After 1-2 days
Stage 3: 1,000x magnification Stage 3: 5,000x magnification
Stage 3: after 1-2 days. Ettringite reacts with remaining aluminate to form monosulfate plates. C2S will hydrate; fibrous calcium silicate hydrate will continue forming around grains.

Cement characteristics and concrete properties
Cement reactivity and concrete properties are complex. Numerous characteristics of the cement influence the hydration reactions, which influence concrete performance; a few are discussed below.

Cement characteristics that affect concrete properties include chemical composition (bulk composition, alkalies, sulfates, minor components, loss on ignition, insoluble residue), physical properties (microstructure, fineness, particle size distribution), and performance characteristics (setting, compressive strength, heat of hydration, soundness). Properties of gypsum added during grinding can also influence concrete properties.

Examples of how some of these characteristics relate to concrete properties include:

  • Aluminate/sulfate balance is important to early cement reactions. Early reactions are due to the aluminate hydration, which is controlled by calcium sulfate (usually in the form of gypsum) added to the cement. Stiffening properties in the first 15 minutes will depend on properties of aluminate (content and size) and of the calcium sulfate (content, form, and particle size). If the balance is not correct, and the aluminate is not controlled, or if too much sulfate is present, false or flash set could occur. See Hills and Tang (2004) for more information.
  • C3S reacts quicker than C2S during hydration. Therefore, the calcium silicate hydrate formed from the C3S is important to the early strength. Of the compounds formed, calcium silicate hydrate is the most important cementing component in concrete. Setting, hardening, and strength depend primarily on calcium silicate hydrate.
  • Cement particle size distribution is important, as smaller cement grains hydrate quicker. This early reactivity provides an increase in early strength and heat of hydration, and decreases workability and bleed water.

A publication by Johansen, Taylor, and Tennis (2005) provides additional details on these relationships, including summary tables that compare the changes in characteristics of cement with predicted or observed concrete behavior. An excerpt from these tables is provided in the Table 1.


Table 1. Concrete properties affected by changes in cement characteristics (partial listing).
Adapted from Johansen, Taylor, and Tennis (2005).

Conclusion
Details of cement manufacture and resulting characteristics are important to the cement hydration reactions, and ultimately the concrete performance.

References

  • Hills, L. M. and Tang, F. J., "Manufacturing Solutions for Concrete Performance", Conference Record of IEEE-IAS/PCA 2004 Industry Technical Conference, May 2004.
  • Johansen, V. C.; Taylor, P. C.; and Tennis, P. D., Effects of Cement Characteristics on Concrete Properties, EB 226, Portland Cement Association, Skokie, Illinois, 2005, 48 pages.
  • Kosmatka, S. G.; Kerkhoff, B.; and Panarese, W.C., Design and Control of Concrete Microstructures, EB 001, Portland Cement Association, Skokie, Illinois, 2002.

About the Author:
Linda Hills has been employed with CTL Group for 19 years. In her position as Senior Materials Scientist, Linda uses the scanning electron microscope in forensic evaluation of various materials such as concrete, paint, floor tile, and steel. She also uses the microscope to troubleshoot and improve cement production and performance. Linda provides instruction of educational courses and on-site microscopy training for cement plants.

Linda received the First Place Paper Presentation Award at IEEE-IAS/PCA Cement Industry Conference in both 2002 and 2004. She is a Chairman of the International Cement Microscopy Association.

CTL GROUP
5400 OLD ORCHARD ROAD
SKOKIE, ILLINOIS 60077-1030
Phone: 847-972-3136 • Fax: 847-965-6541 email: LHills@CTLGroup.comwww.CTLGroup.com


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© 2005 L&M Construction Chemicals, Inc. | ConcreteNews Summer 2005.

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