Sulfate optimization of cements using isothermal calorimetry
S. Bishnoi, Indian Institute of Technology Delhi, Delhi, India,
P. Sandberg, Calmetrix Inc, Arlington, USA
Calcium sulfate is added to cement during grinding to control the aluminate hydration. An optimized sulfate content maximizes the strength development especially at early age, and in many cases also improves the workability in concrete when the cement is used with Supplementary Cementitious Materials (SCM) and chemical admixtures. In 1946, Lerch published a famous paper showing the critical importance of sulfate optimization of cement. His results were later used to develop the sulfate limits used in ASTM and other cement standards. While Lerch demonstrated that sulfate optimization can be carried out using several different methods, including compressive strength testing, isothermal calorimetry, or shrinkage testing, compressive strength testing was the only readily available method at the time and thus became the method referenced in most cement standards. The recent development of isothermal calorimetry, however, enables rapid, low cost and very effective sulfate optimization of cements compared to traditional methods based on compressive strength, thereby largely eliminating the need to send samples to an air-conditioned central laboratory. Unlike with traditional compressive strength testing, when using isothermal calorimetry there is no need for a standard temperature controlled laboratory, since the calorimeter itself precisely controls the curing temperature of samples. For example, testing the effect of admixtures, which are known to impact sulfate balance, would be rather impractical using standard cement mortar compressive strength testing, but becomes a very easy task with isothermal calorimetry. The same is true when testing for the effect of supplementary materials, different sulfate sources, or the effect of curing temperature to optimize the cement for winter and summer conditions in different markets. This contribution describes the basic methodology for sulfate optimization using isothermal calorimetry.
Summary CEMENT INTERNATIONAL 04/2016 pp 70-77
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