The results of phase 1 of an interlaboratory test, coordinated by the RILEM TC 267-TRM "Tests for Reactivity of Supplementary Cementitious Materials" showed that the R-3 (rapid, relevant, reliable) test method, by measurement of heat release or bound water, provided the most reliable and relevant determination of the chemical reactivity of supplementary cementitious materials (SCMs), compared to other test methods. The phase 2 work, described in this paper aimed to improve the robustness of the test procedure and to develop precision statements for the consolidated test procedure. The effect of the pre-mixing and mixing conditions, and the impact of the mix design on the test method robustness were assessed and fixed for optimal conditions to carry out the R-3 heat release test. The effect of the drying step was evaluated to define the R-3 bound water test procedure in more detail. Finally, the robustness of the consolidated final test methods was determined by an interlaboratory study to define the precision statements.