It is well established that a reduced rate of hydration is observed in composite cement materials, but there remains a lack of knowledge regarding the early age reaction kinetics of the separate phases. This may become important when considering early removal of formwork in practice. Furthermore, the reduced Ca(OH)2 content, characteristic of composite cements, presents additional limitations when considering resistance to carbonation induced corrosion. The decrease in portlandite content results in a reduced pH; allowing carbonation to progress more readily. This behaviour is likely to become more pronounced with increasing levels of replacement. Moreover, the expected retardation of the rate of CO2 ingress typically observed in PC systems upon carbonation is a result of densification of the microstructure. This, however, is not concurrent with the behaviour exhibited in composite cements with high replacement levels, in which carbonation may lead to a coarser microstructure and greater porosity. This study investigates the effects of carbonation following short curing periods (72 h) on CEMI and composite cement systems (30% PFA & 30% GGBS). Carbonation behaviour changed compared to ‘idealised’/28 day lab cured samples and accelerated carbonation testing. Carbonation of Ca(OH)2 and C-S-H did not occur simultaneously, with decalcification of C-S-H only beginning once no more Ca(OH)2 was available. 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 appears 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 is dependent on both the level of replacement and the replacement material.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)