Deciphering mineralogical changes and carbonation development during hydration and ageing of a consolidated ternary blended cement paste

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 mm in width. Carbonation was not observed for this time scale, but was found to affect the first 100 mm of cement located around macropores in a sample cured for six months. Regarding this carbonation, the only mineral detected was calcite.

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“…Since the combinations will occur on time-scales that are not accessible to experimentation, we must develop modeling approaches that can help to predict how barriers will evolve in time and space (Claret et al 2018b). Reactive transport modeling is probably one of the most efficient techniques used to account for and quantify the complexity of coupling processes over a long period of time.…”

“…A comprehensive description of these equations can be found in Steefel et al (2015b). Recently, the significance of the parameters in Equations 1and 2, their relationship in case of mutual dependence (e.g., diffusion and saturation permeability and saturation) and the way they can be approximated using different empirical or based on theory laws have been reviewed (Claret et al 2018b). Therefore, it will not be detailed again here.…”

“…Equation 1also highlights the fact that a porosity has to be defined for the considered materials. However, some of them (e.g., steel and other metals, glass) are not porous materials, and, unfortunately, reactive transport models are, by definition and according to Equation 1, not suited to include non-porous media (Bildstein et al 2007;Claret et al 2018b). To unravel this problem, the volume clearance that exists at the interface between the canister or the glass and the surrounding material, also known as the "technological gap", is often included in the numerical cells that represent these materials.…”

“…As the hydration reaction proceeds due to the cement-water interaction, the anhydrous phases, calcium silicates (C3S or C2S) and calcium aluminates (C3A and C4AF) are converted into hydrates such as portlandite, C-S-H (calcium silicate hydrate), ettringite, monosulfate or monocarbonate. This decreases the bulk porosity, since the molar volume of the hydrates is much larger than that of the anhydrous phases (Van Damme et al 2013;Gaboreau et al 2017;Claret et al 2018b). Concrete has a long history that began in the pre-Roman age, and its formulation, which was greatly improved at the beginning of nineteenth century with the invention of Portland cement, has recently become more and more sophisticated.…”

“…Many databases have also been used: EQ3/6 (Wolery 1983), CEMDATA2007 (Lothenbach and Wieland 2006;Lothenbach and Winnefeld 2006), Nagra/PSI database (Hummel et al 2002), THERMOCHIMIE (Blanc et al 2015a,b), THERMODDEM (Bartier et al 2013). The geometries (1D or 2D Cartesian, 1D radial, 1D or 2D axisymmetric and 2D with cylindrical coordinates), the mesh size (usually in the cm range), the transport parameters, the way mineral dissolution or precipitation was accounted for (either at local equilibrium and/or in kinetics) have been recently reviewed (Claret et al 2018b). Otherwise, in the case of the mass balance calculations reported earlier in this chapter, all reported studies are consistent with very limited spatial extension, in terms of mineral precipitation and dissolution, of cement perturbation in clay materials and vice versa.…”