TRACK CEMENT CHEMISTRY TO HELP PREDICT PERFORMANCE

Ignoring the mill certificate that comes with your cement means losing out onvaluable information. Often, producers overlook this useful document because they are not familiar with the terminology. Understanding mill certificate information does not require an advanced degree in chemistry - just some basic knowledge of the composition of cement and terms used to describe it.

Portland cement is manufactured using naturally occurring raw materials combined in a controlled manner, processed in a kiln into clinker, and then ground to the appropriate size in grinding mills. Raw materials used in the manufacture of Portland cement may include limestone, marl, iron ore, clay, shale, sand, and in some cases, industrial by-products. Because a large range of raw materials is used in the manufacture of Portland cement, the resulting product has a complex chemistry. In fact, a chemical analysis of portland cement identifies 12 oxides and determines their weight percentages. These values may be reported on the mill certificate. However, cement chemists have developed a system of quantifying four principle components in portland cement based on the oxides present. The calculations are based on a system developed by R.H. Bogue. Therefore, the calculated composition is often referred to as the "Bogue Compounds."

Of the four Bogue compounds, tricalcium silicate (C₃S) in cement chemist notation) is typically the most abundant. In Type I, II and III portland cement,C₃S will be more than 50% of the total compounds present. C₃S hydrates and hardens quickly, and consequently, influences concrete's setting time and early age strength significantly. Most strength gained from C₃S occurs in the first four weeks.

Early strength development of C₃S is accompanied by generation of heat. Therefore, when control of heat of hydration is important, such as in Type IV cement, the amount of C₃S will be reduced.

Dicalcium silicate (C₂S), the second most abundant compound in Portland cement, develops strength more slowly than C₃S. Most of its strength is contributed after four weeks. It also produces less heat upon hydration than C3S. These factors make higher percentages of C₂S important in cements used to produce concrete when early age strength is not as important as controlling heat generation. In typical portland cement, C₂S will be between 20% and 50% of the total compounds.

Tricalcium aluminate (C₃A) plays a major role in the characteristics of fresh concrete. It starts the hydration process quickly upon exposure to water and generates much heat during the first few days. It also contributes to the very early age strength of one to three days. But it may somewhat reduce advanced age strength. High early-strength cement, Type III, will have higher concentrations of C₃A, often between 10% and 15%.

Cements with low percentages of a cement often will require changes in the amount of gypsum added and may also affect chemical admixture dosage levels. To a large degree, gypsum is added to cement to slow the rate of hydration of C₃A and to prevent too rapid of setting.

The final Bogue compound, tetracalcium aluminoferrite (C₄AF) is primarily a result of materials used in the cement manufacturing process to lower the temperatures required in the kilns. C₄AF hydrates rapidly, but contributes little actual strength. Perhaps its most significant effect on concrete is its influence on color. Higher concentrations of C₄AF will result in darker color concretes. In white cement the percentage of C₄AF is kept low, often about 1% to 2%.

In addition to the Bogue compounds, two other values reported on a mill certificate are important to consider. The total alkalies (sometimes referred to as total equivalent alkalies) will help determine if the cement may be used with aggregates that are potentially reactive with alkalies. Cement with alkali levels below 0.6% are considered low-alkali cements and may be suitable for use with some reactive aggregates. Sulfate level (S0₃) can be important in controlling initial and final set time of concrete and performance with chemical admixtures.

Making a habit of reviewing cement mill certification reports can be an informative and important tool in your overall quality control program. Likely, by tracking the changes in cement chemistry you will be able to predict their influence on the performance of your product. Remember, the cement mill certification can and should be used to do more than just show that the cement meets relevant specifications.

Ron Burg is principal engineer for Construction Technology Laboratories Inc., Skokie, III. Ha may be reached at 708-965-7500 fax: 708-965-6541.