Microstructure and phase assemblage of low-clinker cements during the early stages of carbonation

Blending Portland cement with fly ash or ground granulated blast furnace slag effectively reduces its carbon footprint. However, these materials hydrate more slowly than Portland cement. Therefore, their early-stage behaviour is of interest. Fly ash-and slag-containing cement pastes were cured for 72 h and 28 days before exposure to ambient air (ca. 400 ppm). Changes in carbonation behaviour at 0%, 30% and 60% replacement and w/b ratios (0.40 & 0.57) were studied. All Ca-bearing hydrate phases carbonated, but portlandite effectively buffered carbonation of other phases. Complete portlandite consumption permitted decalcification of the other phases, with substantial removal of Ca and Al from C-S-H and increased silicate polymerisation. When portlandite was present, monosulfoaluminate transformed to hemi-and monocarboaluminate, but complete decalcification only occurred when portlandite was no longer available. Reduced w/b ratios improved carbonation resistance for materials with moderate substitution but couldn't mitigate the effects of high replacement levels.