Phase assemblages in hydrated portland cement, calcined clay and limestone blends from solid-state 27Al and 29Si MAS NMR, XRD, and thermodynamic modeling

Abstract

The use of calcined clays as SCM’s provides a valuable contribution to the reduction in CO2 emission associated with cement production since they can be produced at significantly lower temperatures and do not involve a calcination reaction. Moreover, it is well known that binary blends of Portland cement and small amounts of limestone powder can increase the strength of the resulting concrete. In this study we have investigated the substitution of cement by metakaolin and limestone as well as blends of cement, metakaolin, silica fume and limestone, in all cases with a 35 wt% replacement level of Portland cement. The phase assemblages of the hydrated blends are characterized by XRD, 27Al and 29Si MAS NMR spectroscopy, and thermodynamic modelling. The X-ray diffractograms show that larger amounts of the AFm phases are formed for the samples containing both limestone and calcined clay whereas 29Si MAS NMR studies of the hydrated pastes provide information about the uptake of Al in the C–S–H phase and structural details about the resulting C–A–S–H phases. 27Al MAS NMR is used to follow the formation of the hydrated aluminate species, in particular the formation of strätlingite. The results from XRD and NMR after prolonged hydration are compared with the phase assemblages predicted from thermodynamic modelling. Overall, good agreements are observed between the experiments and the modeled predictions.