Mechanism of aluminium incorporation into C-S-H from ab initio calculations

Blended cements have great potential to reduce the CO2 footprint due to cement production. C(alcium)-S(ilicate)-H(ydrate) in these novel materials is known to incorporate a considerable amount of Al. We have for the first time applied large-scale first principles calculations to address the mechanism of Al incorporation into low C/S ratio C-S-H. In agreement with state-of-the-art NMR information, our calculations show that Al substitutes Si in bridging tetrahedra only, and that substitutions in pairing tetrahedra are strongly disfavoured in a wide range of conditions. In broad terms, the energy penalty for having an Al atom in a pairing position is of about 20 kcal mol(-1). Al in bridging tetrahedra is therefore the thermodynamically favoured state, rather than merely a kinetically trapped one in a solid-liquid equilibrium known experimentally to take a very long time to reach. A systematic investigation of Al-Al and defect-Al correlations shows that having two Al atoms as next-neighbours is particularly unfavourable, which gives clues on the limit of Al incorporation into C-S-H. All in all, the current work supports the model and methodology employed to pursue further studies in such materials (e.g., higher C/S ratio systems), in the context of what is still the open question of the structure of C-S-H.