The need to limit the environmental impact of cementitious materials and to dispose of by-products such as fly ash and slag leads to the increasing use of supplementary cementitious materials (SCMs).In order to better understand the factors affecting the rate of reaction of SCMs, it is essential to have a good method to evaluate the degree of reaction of fly ash and slag in composite cement pastes.In this paper, backscattered electron image analysis (BSE-IA) was developed to measure the degree of reaction of fly ash and slag in composite cement pastes.Grey level along with visual information such as particle size, shape and texture were used for identifying the phases.Where necessary, the elemental composition was determined via spot analysis or mapping using EDS (Energy dispersive X-ray spectroscopy) to confirm the phase present.Although this method is time consuming, it appears to be the only accurate method to quantify the degree of reaction of fly ash and slag in composite cement pastes.The reaction of fly ash is little during the first 3 days confirming that at least 95% of the fly ash particles are visible by IA.The fly ash continues to react at a rather steady rate during the first 28 days, at longer hydration times the reaction rate of fly ash slow down, resulting in a total reaction of the fly ash of about 34% at 360 days.The slag continues to react at a rather steady rate during the first 28 days, at longer hydration times the reaction rate of slag slow down, resulting in a total reaction of the slag of about 64% at 360 days.