Aluminum distribution in C-(A)-S-H and calcium aluminate hydrate phases of Portland cement – metakaolin – limestone blends studied by 27Al and 29Si NMR spectroscopy

Calcined clays, combined with limestone, exhibit significant potential as SCMs for achieving high clinker substitution levels. This is partly related to their high aluminum and silicon contents. Binary Portland cement – calcined clay (PC²) blends and ternary blends with limestone (LC³) have been examined using metakaolin (MK) as calcined clay for blends with low and high MK/clinker ratios. The hydration reactions (up to 420 days) and aluminum distribution within the C-(A)-S-H and calcium aluminate hydrate phases have been quantitatively assessed by ²⁷Al and ²⁹Si NMR spectroscopy. The presence of limestone induces higher amounts of ettringite in the LC³ blends, resulting in lower amounts of Al(4) in the C-(A)-S-H. For the high MK/clinker ratio blends, the increase of aluminum incorporated in the C-(A)-S-H phase after prolonged hydration coincides with a leveling off for the amount of AFm phases for the binary and ternary blends at 60 % and 40 % of MK reaction, respectively.