Alkali silica reaction (ASR) is a long-term reaction between certain aggregates containing amorphous silicate phases and the alkalis from the cement paste. These silicates react with the alkalis present in the pore solution of the cement paste and form an expansive gel in the presence of water, resulting in the macroscopic expansion and cracking of concrete. Supplementary cementitious materials (SCM), replacing a part of the Portland cement (PC) in blended pastes, are known to reduce or even stop expansion due to ASR. Studies indicate that the main reason for this is the decrease in alkalinity of the pore solution of the cement paste, which in turn is attributed to the change in composition of the C-S-H, the main cement hydrate. However, knowledge on the effect of SCMs on ASR control is incomplete, especially the role of aluminium. The first part of this work focuses on the effect of aluminium and silicon incorporation in C-S-H, provided by SCMs, on the composition of the pore solution of blended pastes. It was found that, contrary to the common idea, the incorporation of aluminium in C-S-H does not increase its alkali fixation capacity, suggesting that the greater effectiveness of SCMs containing alumina is due to other reasons. In a second part, it is proposed that the additional aluminium acts directly on the reactive phases of the aggregates. Marine chemistry and geology theories about the dissolution mechanisms of amorphous silicates were applied to cementitious systems. Aluminium species, provided by certain SCMs and present in the pore solution, are incorporated in the silica surface and limit the dissolution of amorphous silica of the aggregates, limiting ASR. The effect of aluminium was shown through a study of reactive aggregates in simulated pore solutions. The mechanism was explained through a more fundamental study with pure amorphous silica plates put in simulated pore solutions. Finally, the impact of various alkali cations on ASR was studied to better understand the reactions inducing gel formation.