In recent years, the behaviour of supplementary cementitious materials (SCMs), used as cement replacements in concrete for civil engineering solutions, continuously increased in its complexity. Among other things, this is due to the employment of upcoming innovative and/or ecological concrete additives as a replacement of cement. To this extend, the fundamental understanding of the theoretical principles that drive these additives to react with cement have not been fully understood, leading a knowledge gap in the correct and optimum usage of it in a cementitious system. Normative restrictions permit their use only to a limited extent or allow them in composite cements or in cementitious blends, respectively. Very often, a clear explanation of a correct usage by means of stoichiometric reaction mechanism or its associated impact on the microstructural development is not provided. Therefore, aim of this study is to emphasis the impact the reactivity of SCMs have on the chemo-mechanical properties concrete mixtures. An introductory investigation on the initial reactivity of SCMs reveals their potential on the one hand, and addresses a link to the scarcity of various reactive raw materials on the other. In this dissertation, possible alternative approaches to the application of various amounts of SCMs, and their interplay in cementitious systems is provided. Resulting strategies for achieving characteristic concrete parameters more efficiently or testing resource-saving alternatives on their suitability are addressed. An evaluation of the international literature on reaction-kinetic as well as on analytical instruments, tools for the systemization, prediction models for the quantification of reactive parameters, are discussed. The reason for this in-depth study on the chemical processes associated with cement hydration is its enormous importance for understanding the common theories in this field, as explained by the following. An extended experimental programme was developed with the aim to find an methodical solution for the behaviour of SCMs in a cementitious system. Therefore, a series of experiments are described in detail, by its chemical nature and mix design. The selection of tested SCMs allowed exploiting the comparative performance of a broad range of reactive materials. Starting with inert and/or slightly reactive fillers and pozzolans to SCMs commonly described in literature as latent hydraulic. A combination of thermal analysis and micro-structural methods on different amounts of SCMs in cementitious systems is applied to different stages of the reaction process. This enabled a thorough validation of a self-established research hypothesis, which is to characterise SCMs by means of their reactivity, to predict their performance at different stages of reaction, their material properties, and their micro-structural development by means of the mineralogical composition of the initial base materials. The dissertation ends with a possible field application as well as a discussion of the result in the context of a wider use, while open research questions are discussed.