The carbonation of hardened cement pastes

The microstructure of hardened pastes of C₃S and a C₃S/silica fume blend have been examined by transmission electron microscopy before and after partial carbonation in pure CO₂ at a relative humidity of 72·6%. Fibrillar outer product C-S-H and the outer regions of inner product C-S-H gel carbonate without change of morphology to silica gel. Calcium carbonate is formed mainly in outer product regions as microcrystalline vaterite or calcite. Residual CH embedded in vaterite microcrystals is observed in the carbonated C₃S pastes. It is proposed that outer product C-S-H and CH surfaces carbonate relatively rapidly, compared with inner regions of CH or the inner regions of inner product C-S-H, leading to a substantial level of carbonation of thin slices (0·4 mm) of paste within one day, with little further carbonation in the next few days. The implications for the theory of the kinetics of cement paste carbonation are discussed.