Editors Note: Recent changes in admixture technology are addressing mix-set time. With that in mind, we revisit the basics of mix chemistry in the second of a two-part report. In the first part, the authors discussed the importance of sulfate in cement mixtures and provided five case studies demonstrating examples of common cement sulfate and aluminate characteristics, descriptions of what happens during hydration, and the resulting paste properties that would be expected.

Total SO₃ content is a common QA/QC factor used at the cement plant. While that value provides some information, it does not reveal the entire picture of the available sulfate. The examples here are only a few situations that can affect how much sulfate is needed and what is available.

“So, what is ideal sulfate?” Optimum sulfate can be discussed in terms of both strength development and setting properties. The amount of sulfate in solution needed to achieve the desired early-hydration reactions in a cement paste depends on properties of the clinker and cement (aluminate content and size, alkali aluminate content, particle size distribution) as well as the properties of sulfate (amount, form, particle size) present in the cement. In a concrete mix, chemical and mineral admixtures often play a role in the reactions. The cement sulfate requirement for use in field concrete when admixtures are used may be higher than for ASTM paste tests, often by 0.5 percent -1.0 percent SO₃.

In order to determine the “ideal” sulfate conditions or analyze setting problems, diagnosing the reactions of a particular cement or cement/admixture combination is critical.

“How do I diagnose the reactions in my cement?” Several tests help to determine what is going on in the cement itself, and in the resulting paste:

• Thermal analysis can be used to quantify the amount of gypsum and plaster in the cement. Methods include Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), and Differential Thermal Analysis (DTA).

• Particle-size analysis combined with chemical extractions can determine the fineness of the gypsum particles. Microscopic techniques can also be used to identify large gypsum particles.

• The Conduction Calorimeter measures heat produced by a sample versus time. Cement hydration reactions can be monitored starting immediately upon introduction of water through a period of several days. Figure 1 demonstrates C₃A reactions for cements with and without proper control of aluminate hydration.

• Mini-slump cone tests performed on cement pastes determine early stiffening properties. Paste is prepared in a high shear blender using mixing speeds that closely simulate conditions during concrete mixing. The paste is consolidated into a mini-slump cone, and at two minutes the cone is lifted to allow the paste to slump (Figure 2, page 46). The remaining paste is further mixed, after which 5-, 15-, 30- and 45-min. tests are carried out. The workability, or flow property, of the cement paste is demonstrated in the size of the pat formed after slumping, i.e., the larger the area of pat, the more workable the paste. Pat size at 5-min. and 15-min. tests, which are taken after the paste is remixed, has been shown to correlate better with flash or false set in the field compared to current ASTM methods (larger pat size after remixing indicates false set; smaller size indicates flash set). See Figure 3, page 46.

“How do I optimize my sulfate?”

Establishing the reactions for a particular cement or concrete mix using the above tests will help identify where improvements to optimize sulfate can be made. Plant personnel are best able to determine the appropriate action from there. Here are some examples:

• If the ratio of gypsum to plaster is too low, often the most feasible alternative at the plant is to decrease the mill temperature and thus minimize formation of plaster. Another option is to use water spray to cool the mill and increase mill relative humidity. Or, evaluate clinker grindability to determine if a difficult-to-grind clinker is causing the increased mill retention time. Increasing the ease of clinker grinding should decrease retention time and prevent excess gypsum dehydration. Another alternative may be substitution of some of the gypsum by natural anhydrite.

• Cases of insufficient soluble sulfate may require several improvements. The total sulfate content may need to be increased, and/or the solubility of the sulfate present adjusted. Some plants using anhydrite as part of the gypsum source may need to minimize its addition due to its slow dissolution rate, and increase the amount of more soluble sulfate, such as plaster. More plaster could be created by increasing the finish mill temperature or decreasing volume of water spray.

• In the case of coarse gypsum particles, a new source of softer-grinding gypsum may be needed. If clinker is preground before the finish mill, a method of pregrinding the gypsum should help prevent gypsum particles from being too coarsely ground in the final product. Pregrinding gypsum may be especially useful for plants operating finish mills equipped with high-efficiency separators.

• The cement sulfate may need to be tailored for use in particular concrete jobs, with care not to exceed ASTM C 150 requirements. Sulfate optimized according to ASTM C 563 for one-day strength may not be suited to other field applications such as those involving elevated temperatures or use of chemical or mineral admixtures. For example, if a fly ash with high aluminate is used, the sulfate balance must be taken into account. While it seems the most logical course of action would be to use fly ash with lower aluminate content, that is not a practical solution for the concrete producer. In order to provide a more “compatible” cement for use with these types of additions, the content and form of available sulfate needs to be considered.

CONCLUSION

Specifics of sulfate additions may sometimes be overlooked, but the resulting stiffening properties of the cement usually are not. Although total sulfate is a common measurement at the cement plant lab, it does not provide a complete picture of available sulfate. The balance of sulfate form and amount, with respect to reactive aluminate components in the cement and other materials, is a critical factor. Essential components of this relationship include: sulfate content, sulfate form, sulfate particle size, grain size and amount of clinker aluminate phase, and cement (and other cementitious material) alkali content. In concrete mixes with fly ash, the amount of C₃A, or alkalies, can affect the balance. Aside from rheological properties, conditions for controlling early stiffening are also linked to other performance characteristics, such as strength and durability, as noted.

Flash set and poor strength development can result from insufficient sulfate, whether due to a cement with a low ratio of sulfate to aluminate content, a concrete mix with high C₃A fly ash, or poor distribution of sulfate ions from large gypsum particles. Without sufficient sulfate ions in solution to control aluminate hydration, voluminous hexagonal aluminate hydrates will form, resulting in flash set and poor strength development.

False set can result from too much sulfate in the form of plaster. Since plaster goes into solution more quickly than gypsum, many calcium and sulfate ions are available to control aluminate reactions, therefore less C₃A reacts. The plaster will form crystals of secondary tabular gypsum particles, which interlock and cause false setting. False set is generally less of a problem than flash setting, as it can be overcome by continuing to mix concrete through the stiffening phase for a proper length of time. If a short mix cycle is used in the field and this setting problem occurs, water is often added to the concrete to attempt to improve the workability. However, this “remedy” may reduce concrete strength and durability.

Identifying the sulfate properties is a first step in preventing or resolving stiffening issues. Once the cause of cement behavior is determined, proper manufacturing solutions can be implemented. Solutions involve finish mill temperature or humidity, type and grindability of sulfate added, and even clinker grindability.

REFERENCES

1. ASTM C 563: Standard Test Method for Optimum SO3 in Hydraulic Cement Using 24-h Compressive Strength, American Society for Testing and Materials, West Conshohocken, Pa.

2. ASTM C 150: Standard Specification of Portland Cement, American Society for Testing and Materials

3. Tang, F. J., Optimization of Sulfate Form and Content, Research and Development Bulletin RD 105T, Portland Cement Association, 1992.

Linda Hills, Senior Scientist, and Fulvio Tang, Principal Scientist, both work for Construction Technology Laboratories, Inc., Skokie, Ill. © 2004 IEEE. Reprinted, with permission, from the 2004 IEEE/PCA Cement Industry Conference Technical Conference in Chattanooga, Tenn., April 26-29.