Although supplementary cementitious materials (SCMs) are now commonly accepted and widely used, consensus has not been reached for methods to test their chemical reactivity. A multitude of test methods exist but often fall short on one or more of the key features of a proper test-that is, width of scope, practicability, reproducibility, and relevance of the result. A rapid, robust, and relevant chemical reactivity test applicable to a wide range of SCMs would therefore not only serve as global benchmark but it would also remove present ambiguities in regards to classification.

In response, a new, so-called "R3" test was conceived at Ecole Polytechnique Federale de Lausanne and is now being further developed and tested in RILEM TC 267 TRM, The test method was initially based on a screening of calcined clays in portlanditealkali-sulfate systems by isothermal calorimetry. Subsequently, the system formulation was more systematically studied and compared to strength development of blended cement mortar bars for a wide range of calcined kaolinitic clays. Remarkably good correlations between strength development and heat release were found. This was confirmed for other Sells and other measurable system properties. In particular; bound water content and chemical shrinkage correlated remarkably well to the isothermal calorimetry results. The origin of this correspondence can be traced back to the hydration reaction. Indeed, it is the solidification of water that directly links heat release; chemical shrinkage; and, obviously, bound water content.

This contribution traces back the origins and the first inroads leading up to the present state of development of the test method and concludes on future perspectives.