Predicting Cement-Admixture Incompatibilities with Cement Paste Rheology

The complex interaction between cement and the chemical and mineral admixtures in concrete mixture sometimes causes unpredictable concrete performance in the field, generally defined as concrete incompatibilities. Cement paste rheology measurements-rather than traditional workability tests-can effectively detect these incompatibilities in concrete before the concrete is placed to avoid setting-time, workability, and curing-related issues, which sometimes lead to early-age cracking, especially in severe weather conditions. The objective of the present study is to examine the applicability of the Superpave (R) dynamic shear rheometer to measure cement paste rheology and to identify incompatibilities between cement and the admixtures based on cement paste rheological behavior. Extensive laboratory investigation showed that dynamic shear rheometer in modified form could measure cement paste rheology with permissible repeatability and sensitivity and has great potential in identifying the incompatibilities between cement and the admixtures. Heat of hydration data from an isothermal calorimeter test and the setting time results of the mixtures studied have strongly supported the rheology results. A procedure to formulate the rheology-based acceptance criteria has been developed on the basis of the available test results. Further refinement of these acceptance criteria based on detailed work covering a wide range of incompatibilities and validation through a round-robin testing program is warranted. This research will ultimately help concrete producers and district laboratories detect problematic combinations of concrete ingredients during the mixture design process.