Blended binder systems in concrete have been in use for many years. However, over the recent discussions on the sustainability of cement and concrete the topic became of paramount importance for the construction industry. In Sweden blended binder systems in concrete were applied fairly recently and supplementary cementitious materials (SCM) were not used largely before. In Sweden over the last 30 years engineering structures were built, with exceptions, with a low alkali, sulfate resistant Portland cement due to domestic requirements. Therefore, in context to blended binder systems durability is a major point being discussed. Most interest lays in the resistance towards frost, carbonation and chloride penetration and the experience with blended systems was limited. It is therefore the goal of this study to investigate blended binder systems over a longer hydration period of up to two years in context to durability. The results presented here will focus on the porosity and hydrate phase development within the first 90 days of hydration. In this study in total 9 different binder blends were studied. The blends included different replacement levels with granulated blast furnace slag from a Swedish producer, fly ash from a Danish power plant and commercially available metakaolin. The matrix included also one set of factory-blended cements (with interground fly ash and slag). The pore structure was investigated by mercury intrusion porosimetry (MIP), phase development by powder x-ray diffraction (XRD). The results are based on the evaluation of binder paste samples. Compared to a reference series consisting of a CEM I the slag containing series showed a continuous decrease of the total pore volume with increasing slag content over 90 days of hydration. The phase development showed a constant increase in monosulfate and hydrotalcite. The amount of these phases increased with increasing slag content. Notable was consumption of C4AF, which increased with the amount of slag and a decrease of C2S reaction. Compared to the reference Portland cement paste the fly ash containing pastes showed increased total pore volumes when the fly ash was just added. For the cement paste with interground fly ash the porosity development was the same as for the reference paste. The phase development for these series showed up to 28 days of hydration formation of ettringite, hemicarbonate and monocarbonate. After 91 days monosulfate was dominating and the amount of hemicarbonate was decreasing. The paste series with 15 % metakaolin showed a strong decrease in total pore volume compared to the reference series within 2 days. Afterwards the pore volume almost leveled to a constant value. The hydration phases included in the beginning large amounts of monosulfate, ettringite and hemicarbonate but changed after 91 days in favor to ettringite and monocarbonate. The metakaolin blend showed the lowest amounts of portlandite in all series, indicating a strong pozzolanic reactivity. The different hydrate phases, in particular AFm phases can help to explain a different behavior towards, e.g. chloride binding. The pore structure development has a large impact not only on strength development but also for ion and moisture transport as well as for carbonation in all series.