The role of age on carbon sequestration and strength development in blended cement mixes

While early-age carbonation of concrete has gained interest as an avenue for carbon sequestration, the influence of curing age on carbon uptake for blended cements is an open subject. This has significant practical utility given the popularity of supplementary cementitious materials (SCM) as alternate binders. Pastes and mortar samples were prepared by replacing 20% by wt. cement with four SCMs of distinct composition (fly-ash, silica fume, calcined clay, ground-granulated blast furnace slag) and quartz. The effect of sample age on its carbon storing ability was examined by carbon curing at fresh state (0-24 h' age) and hardened state (24-48 h' age). Carbon curing was done in a chamber using 99% purity CO2 at 1.5 bars for 24 h. Thermo-gravimetric analysis showed that hardened-state curing stores up to 1.5 times more carbon than fresh-state curing. CH carbonation favors higher carbon storage than AFt/C-S-H phases. Hardened state curing reduces the bound water noticeably, but no major changes are observed in fresh state cured samples. Mercury intrusion porosimetry (MIP) revealed microstructure densification in hardened state cured samples (up to 18% reduction in pore volume) due to carbon curing, but no definite pattern is observed in fresh state curing. While no change in early-age strength is observed in hardened state curing, evident improvement is observed for fresh-state cured samples (7-37%). The findings of this study indicate that irrespective of the SCM, carbon curing at hardened state is more effective in storing carbon without degrading the later age strength of the mix.