Microstructure and Phase Assemblage of Low-Clinker Cements during Early Stages of Carbonation

Though it is well established that a reduced rate of hydration is observed in composite cement materials, both the PC and the SCM will hydrate simultaneously, there is still a significant lack of knowledge regarding the early age kinetics of the reactions taking place. This becomes of great importance when considering the early removal of formwork in practice, where the reliance is on models established for PC systems. A comprehensive understanding of the complex relationship between slower composite hydration, drying of the sample surface and phase carbonation kinetics is imperative. Current models derived from idealistic, i.e. fully hydrated and non-carbonated, materials, are ineffective for durability predictions. In addition to this, the existing models do not consider the effects that insufficient curing and phase carbonation have on the phase assemblage composition, development of the microstructure and the subsequent consequences for the transport properties. This study investigates the effects of carbonation following short curing periods (72 hours) on CEMI and composite cement systems (30% PFA, 30% & 60% GGBS) at a low (0.40) and high (0.57) w/b ratio. Modifications in carbonation behaviour were observed compared to idealised/28 day cured samples and natural carbonation studies in the literature. Although the carbonation of the hydrate phases was observed to occur simultaneously, it was only once CH had been completely consumed, or was no longer accessible, that carbonation of the other hydrate phases (C-S-H, AFt and AFm) was permitted to occur more extensively. Decalcification and dealumination of the C-S-H phase occurred following exposure to ambient [CO2], and CaCO3 microcrystals were observed in the outer product (Op) regions only. A reduction in the Ca/Si ratio of the Ip C-S-H appeared to be a result of migration of the Ca ions, driven by a concentration gradient. Furthermore, the rate and extent of carbonation and the nature of the carbonate species formed was dependent on the level of replacement, the replacement material and the degree of hydration. The formation of vaterite appeared to be related to the carbonation of C-S-H, particularly low Ca C-S-H, and silica gel was observed to form in the CEMI and blended materials following carbonation at ambient CO2 concentrations for 60 days. AFt was determined to be more resilient to carbonation compared to the AFm phases, remaining in heavily carbonated CEMI pastes in which portlandite was absent. The carbonation of the AFm phases was observed to occur in two stages; first, phase transformation from SO4 (monosulfoaluminate) to CO3 (hemi- and monocarboaluminate) bearing species was observed to occur, ultimately decomposing by decalcification. The availability of CH, however, prevented complete decomposition. Low w/b ratios were unable to mitigate the effects of short curing periods for systems with high levels of substitution but improved the carbonation resistance of cement systems with moderate levels of substitution (30%). The availability of CH was determined to be the rate determining factor for carbonation, but its consumption is primarily controlled by the ability of CO2 to diffuse through the microstructure.