Abstract
To understand the main properties of cement, a ubiquitous material, a sound description of its chemistry and mineralogy, including its reactivity in aggressive environments and its mechanical properties, is vital. In particular, the porosity distribution and associated sample carbonation, both of which affect cement's properties and durability, should be quantified accurately, and their kinetics and mechanisms of formation known both in detail and in situ. However, traditional methods of cement mineralogy analysis (e.g. chemical mapping) involve sample preparation (e.g. slicing) that can be destructive and/or expose cement to the atmosphere, leading to preparation artefacts (e.g. dehydration). In addition, the kinetics of mineralogical development during hydration, and associated porosity development, cannot be examined. To circumvent these issues, X-ray diffraction computed tomography (XRD-CT) has been used. This allowed the mineralogy of ternary blended cement composed of clinker, fly ash and blast furnace slag to be deciphered. Consistent with previous results obtained for both powdered samples and dilute systems, it was possible, using a consolidated cement paste (with a water-to-solid ratio akin to that used in civil engineering), to determine that the mineralogy consists of alite (only detected in the in situ hydration experiment), calcite, calcium silicate hydrates (C-S-H), ettringite, mullite, portlandite, and an amorphous fraction of unreacted slag and fly ash. Mineralogical evolution during the first hydration steps indicated fast ferrite reactivity. Insights were also gained into how the cement porosity evolves over time and into associated spatially and time-resolved carbonation mechanisms. It was observed that macroporosity developed in less than 30 h of hydration, with pore sizes reaching about 100-150 µm in width. Carbonation was not observed for this time scale, but was found to affect the first 100 µm of cement located around macropores in a sample cured for six months. Regarding this carbonation, the only mineral detected was calcite.
Highlights
Taking advantage of our recent developments in C-SH characterization that allow us to discriminate between this phase and the amorphous components of cement (Grangeon, Claret, Lerouge et al, 2013; Grangeon, Claret, Linard & Chiaberge, 2013; Grangeon et al, 2016, 2017), we present results obtained by synchrotron X-ray diffraction computed tomography (XRD-CT) of a cement paste formulation consisting of blended Portland cement, fly ash and blast furnace slag (Chen et al, 2012), as is expected to be used for nuclear waste disposal applications (Bildstein & Claret, 2015)
The composite cement used in this study was Rombas’s CEM V/A (Calcia), a blended cement with enhanced durability obtained by mixing about 50 wt% clinker with 25 wt% blast furnace slag (BFS) and 25 wt% fly ash (FA, mainly silica fume)
The observed mineralogy is consistent with that described in the literature for a dilute system
Summary
One of the main improvements made to its formulation occurred at the beginning of nineteenth century, with the invention of Portland cement. Since that time, it has not changed significantly (Camoes & Ferreira, 2010), the formulations have become more sophisticated [e.g. lowalkaline concrete (Lothenbach et al, 2012) and alkaliactivated slag binders]. Porosity changes and mineralogical reactions can occur after the initial hydration stage because of the reactivity of cement with its environment (e.g. well casing integrity during geological CO2 sequestration; Jun et al, 2017)
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