Calcareous fly ashes are high-potential reactive supplementary cementitious materials, which present an interesting alternative to slowly reacting siliceous fly ashes and to slags, which availability is more and more limited. Themain factor against an accurate qualification and a wide use of calcareous fly ashes in cement is their significant and inherent heterogeneity and variability. Current techniques often fail to characterize the dominant, most reactive, amorphous fraction of the ashes. As a result these fly ashes have not been researched much and many aspects of their reactivity in cement remain ill understood. These include the phase assemblages, microstructure development, strength and performance in aggressive environments. This thesis focuses first on the characterization of the calcareous fly ashes. A new technique based on SEM-EDS full element mapping was developed to identify and characterize the different glassy components in fly ash. This technique was further developed to track the consumption of the individual anhydrous fly ash glasses in hydrating cement paste. A detailed study using synthetic glasses was carried out to investigate the factors influencing glass reaction in cement. Dissolution experiment in alkaline solution and the consumption of the glasses in paste indicated that the key factors of the glass reactivity are the fineness and the degree of depolymerisation of the glass structure. The latter was determined from the chemical composition of glass. The detailed knowledge of the fly ash reaction and the amounts of the elements supplied were linked the phase assemblage, notably the composition of the C-S-H, the amount of AFmphases and the maximumamount of limestone that can react in the given system. Matrix development in composite cements was followed and related to macroscopic aspects of strength and permeability. There are many ways to optimize the replacement of clinker with SCMs. This thesis illustrates an approach that attempts to link the micro- and macro- structural properties not only to a particular fly ash or slag but rather to the amounts and reactivity of their constituent glasses. This way a more fundamental and general understanding is gained, which could forma base for a systematic and deliberate research of the composite cements.