The proportion of anhydrous cement in materials is essential data in relation to reactivity potential, particularly in by-products that may be reused it in concrete techniques. This paper presents a method based on Scanning Electron Microscopy (SEM) image analysis and a specific program in Matlab that was developed to statistically determine the proportion of anhydrous cement. Industrial by-products containing anhydrous cement were tested by this method and a percentage of around 15-20% was found in the analyzed by-products. The reliability of these results is noteworthy in comparison with those given by classical image analysis software. The reactive characteristic and the hydration kinetics of the analyzed by-products were also verified using isothermal calorimetry and a mechanical study of standard mortars in which the cement had been partially replaced by these by-products. (C) 2016 Elsevier Ltd. All rights reserved.