Production of cementitious materials causes the emission of CO2 gas, which has detrimental impact on the environment augmenting the global warming process. Using by-products such as slags is a possible strategy to limit the environmental impact of cementitious materials. Consequently, there is an increasing use of supplementary cementitious materials (SCMs), either pre-blended with ground clinker or added during fabrication of concrete. However, it is well known that these SCMs generally react slower than cement clinker so the levels of substitution are limited. However, substitution by SCMs should not compromise the development of mechanical properties especially at early ages. In order to better understand the factors affecting the degree of reaction of SCMs it is essential to have an accurate method to evaluate the actual rate of reaction of these materials independent from the degree of reaction of the clinker component. To this end, the contribution of slag in blended cements can be monitored and characterised as a function of time. In this thesis, methods of characterisation of anhydrous materials were initially improved and provided the starting point for the study of hydrated systems. Secondly, the effect of slag on clinker phases was identified. It was found that slag does not significantly affect the overall hydration of aluminate phase. Although, the slag favoured the hydration of the ferrite phases and significantly retarded the hydration of belite and, consequently, the degree of reaction of cement. It was also observed that the slag modified the composition of hydrates. Analyses of hydrated cements with and without slag have shown two major effects: a) no significant decrease in calcium hydroxide content (normalized to cement content) in blended systems, b) higher substitution of Al for Si and lower C/S ratio in outer C-S-H in blended systems. To measure the reactivity of slag in blended pastes at later ages, five methods were studied. As selective dissolution and differential scanning calorimetry have shown to be unreliable, even if SEM-IA-mapping is time consuming, it appeared to be the only accurate method to quantify the degree of reaction of slag. The computation of difference in cumulative calorimetry and chemical shrinkage curves of slag and its comparison to inert filler allowed the reaction of the slag to be isolated. Calibration of these techniques using the SEM-IA-mapping results proved to be a promising method to understand and quantify the reactivity of slag. Using the overall degree of reaction, it was established that increasing reaction in the slag corresponds to an increasing strength in blended mortars. Comparing the strength with calculated total porosity, it was concluded that the contribution of the slag seemed to be more than just filling the microstructural space by producing hydration products. Slag was observed to also enhance the strength by its interaction with other phases. The study will focus on differentiating the two effects to elicit the influence of slag on development of strength.