Supplementary cementitious materials (SCM) are known to reduce or even stop expansion due to alkali silica reaction (ASR) in concretes with reactive aggregates. 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. In this paper we study the effect of aluminium and silicon incorporation in C-S-H on the composition of the pore solution in cement pastes containing SCMs. Different blended pastes of silica fume and metakaolin were cast, in order to obtain the same Si/Ca ratio of the C-S-H but with different aluminium contents. EDS micro analysis was made to determine the C-S-H compositions. In parallel pore solutions were extracted and analysed. It is found that the incorporation of aluminium does not increase the alkali fixation of the C-S-H found in real cementitious materials, suggesting that the greater effectiveness of SCMs containing alumina is due to other reasons. (C) 2012 Elsevier Ltd. All rights reserved.