The present paper introduces a new rapid, relevant and reliable (R-3) test to predict the pozzolanic activity of calcined clays with kaolinite contents ranging from 0 to 95%. The test is based on the correlation between the chemical reactivity of calcined clays in a simplified system and the compressive strength of blends in standard mortar. The simplified system consists of calcined clay portlandite and limestone pastes with sulfate and alkali levels adjusted to reproduce the reaction environment of hydrating blended cements. The pastes were hydrated for 6 days at 20 degrees C or for 1 day at 40 degrees C. The chemical reactivity of the calcined clay can be obtained first by measurement of the heat release during reaction using isothermal calorimetry and second by bound water determination in a heating step between 110 degrees C and 400 degrees C. Very good correlations were found between the mortar compressive strength and both measures of chemical reactivity. (C) 2016 Elsevier Ltd. All rights reserved.