Early carbonation curing effects on the microstructure of high initial strength Portland cement pastes

The microstructure of high early strength and sulfate-resistant Portland cement carbonated pastes at two different durations of carbonation (1 h and 24 h) was analysed under the best conditions for carbon dioxide capture as indicated in previous studies by the authors. The aim was to determine, by means of X-ray diffraction analysis, nuclear magnetic resonance spectroscopy and scanning electron microscopy, how different carbonation times affect the microstructure of the pastes and to explain their respective related mechanical and porosity properties. The microstructure of the reference paste was homogeneous. The 1 h carbonated paste showed a more porous region near its external surface, surrounding a much higher inner non-porous homogeneous phase, while the 24 h carbonated specimen showed a single and more porous phase. The longer the carbonation time, the higher was the microstructural porosity of the carbonated regions and the higher the condensed silicate structure of the matrix, as characterised by morphology modifications. The resultant mechanical and physical properties of the pastes were affected by these chemical changes.