, calcium sulfoaluminate, is found in all portland cement concretes and is commonly referenced in petrographic reports. Calcium sulfate sources, such as gypsum, are added to portland cement to prevent rapid setting and improve strength development. Sulfate is also present in supplementary cementitious materials and admixtures. Gypsum and other sulfate compounds react with calcium aluminate in the cement to form ettringite within the first few hours after mixing with water. Essentially all of the sulfur in the cement is normally consumed to form ettringite within 24 hours.
The formation of ettringite results in a volume increase in the fresh, plastic concrete. Due to the concrete’s plastic condition, this expansion is harmless and unnoticed. If concrete is exposed to water for long periods of time (many years), the ettringite can slowly dissolve and reform in less confined locations. Upon microscopic examination, harmless white needle-like crystals of ettringite can be observed lining air voids.
Any form of attack or disintegration of concrete by freeze-thaw action, alkali-silica reactivity (ASR), or other means, accelerates the rate at which ettringite leaves its original location in the paste to go into solution and recrystallizes in larger spaces such as voids or cracks. Both water and space must be present for the crystals to form. The space is often provided by cracks that form due to damage caused by frost action, alkali-silica reaction, drying shrinkage, or other mechanisms. Ettringite crystals in air voids and cracks are typically two to four micrometers in cross section and 20 to 30 micrometers long. Under conditions of extreme deterioration, the white ettringite crystals appear to completely fill voids or cracks. However, ettringite, found in its preferred state as large needle-like crystals, should not be interpreted as causing the expansion of deteriorating concrete.
Another term used in petrographic reports is delayed ettringite formation (DEF). This refers to a condition usually associated with heat-treated concrete. Certain concretes of particular chemical makeup which have been exposed to temperatures over about 158 degrees Fahrenheit during curing can undergo expansion and cracking caused by later ettringite formation. This can occur because the high temperature decomposes any initial ettringite formed and holds the sulfate and alumina tightly in the calcium silicate hydrate (C-S-H) gel of the cement paste. The normal formation of ettringite is thus impeded.
In the presence of moisture, sulfate and alumina desorb from the confines of the calcium silicate hydrate to form ettringite in cooled and hardened concrete. After months or years of desorption, ettringite forms in confined locations within the paste. Since the concrete is rigid and if there are insufficient voids to accommodate the ettringite volume increase, expansion and cracks can occur. In addition, some of the initial ettringite formed before heating may be converted to monosulfoaluminate at high temperatures and upon cooling, revert back to ettringite. Because ettringite takes up more space than monosulfoaluminate from which it forms, the transformation is an expansive reaction.
Only extreme cases of delayed ettringite formation result in cracking, and often DEF is associated with other deterioration mechanisms. Air voids can help relieve the stress by providing a location for the delayed ettringite to form. Finally, some petrographers or concrete technologists use the term “secondary ettringite” to refer to both delayed ettringite formation and harmless ettringite found lining voids (often listed under secondary deposits in petrographic reports).
1. PCA, Ettringite Formation and the Performance of Concrete, Portland Cement Association, 2001.
2. Lerch, William, Effect of SO3 Content of Cement on Durability of Concrete, R&D SN0285, Portland Cement Association, 1945.
3. Day, Robert L., The Effect of Secondary Ettringite Formation on the Durability of Concrete: A Literature Analysis, Portland Cement Association, 1992.