Progressive Changes in the Structure of Hardened C ₃ S Cement Pastes due to Carbonation

Abstract

The structures of partially carbonated hardened C 3 S cement pastes have been investigated by a combination of 29 Si magic angle spinning nuclear magnetic resonance spectroscopy and analytical transmission electron microscopy, supported by X‐ray diffraction and thermogravimetric analysis. Progressive changes in structure are reported for thin slices for a paste carbonated in pure CO 2 for times from 1 to 16 h, and the results are compared with those for a paste carbonated for 2 months in air. C‐S‐H gel of reduced Ca:Si ratio and increased silicate polymerization was formed during the early stages of carbonation. The morphology of the original C‐S‐H was, in the main, retained. A cross‐linked silica‐rich gel formed at later times in paste carbonated in CO 2 but not up to the time of 2 months in air. Calcium carbonate took the form of microcrystals of vaterite and calcite which formed dense masses between gel fibrils and around partially reacted CH crystals, possibly accounting for the observed slowing in the rate of reaction of CH with time.