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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: [•] The chloride diffusion coefficient is measured as a function of the saturation level by impedance spectroscopy. [•] 4 Types of materials made of Portland cement and blended cement are tested in a saturation level ranging from 0.2–1. [•] The evolution of the diffusion coefficient with the saturation level is closely related to the pore size distribution. [Copyright &y& Elsevier]

The consequences of accelerated carbonation at 3% CO 2 were compared with those of natural carbonation (0.04%). Cement pastes (CEM I and CEM V/A) as well as the three major constitutive phases (C-S-H of different C/S ratios, portlandite and ettringite) were used and changes in the mineralogy, microstructure, water retention and cracking were investigated. The main conclusion was that accelerated carbonation at 3% CO 2 was representative of natural carbonation although it promoted the precipitation of metastable calcium carbonate (aragonite and vaterite) in place of calcite. The results also showed that the presence of aragonite and vaterite were characteristic of the carbonation of ettringite and C-S-H respectively. [ABSTRACT FROM AUTHOR]

In unsaturated conditions, the durability of concrete structures is strongly dependent on the evolution of the amount of free water within concrete porosity. Reliable durability assessment of concrete structures in relation to their environment thus requires accurate unsaturated water transport description as well as reliable input data. The effect of carbonation on water transport remains poorly studied and data are lacking. It was then the purpose of this article to acquire all the data needed to describe unsaturated water transport in carbonated cementitious materials (porosity, water retention and unsaturated permeability). Four hardened pastes made with four different binders were carbonated at 3% CO 2 to ensure representativeness with natural carbonation. Beyond the modification of the water retention curve and porosity clogging, significant microcracking due to carbonation shrinkage was observed. The consequence on permeability highlighted a competition between porosity clogging and microcracking that was dependent on the initial mineralogical composition. [ABSTRACT FROM AUTHOR]

Abstract: Relative permeabilities and water vapor diffusion reduction factor coefficients are essential data regarding the modeling of moisture transport and hence the prediction of structure lifetimes. The dependency of these parameters on the humidity history and the chemical evolutions of the material limits their experimental measurement since tests are long to carry out. For cement-based materials, the most usual approach for the calculation of relative permeabilities is the one developed for soils by Van Genuchten, requiring calibration on experimental water retention curves. Moreover, the model involves a pore network connectivity factor, the value of which varies from one author to another. The water vapor diffusion reduction factor is often calculated via Millington's semi-empirical model. This paper presents new methods for calculating transport factors based on physical and statistical considerations associated with a pore network model developed for the prediction of water sorption isotherms. [Copyright &y& Elsevier]

Abstract: The healing of cracks plays an important role with regard to the air- and water-tightness of concrete structures and the durability of cement based materials in general. This paper aims at further characterization and better comprehension of the healing phenomenon induced by the precipitation of calcite in localized cracks. The experimental program consisted in generating a localized crack in a cement paste or concrete specimen and healing the crack created by percolation tests with carbonated water or gas (CO2-air mixture). For tests with liquid, results show that the healing process depends on physical parameters like crack width, pressure gradient and carbonate content in the percolating fluid. For tests with gas, the supply of the crack with Ca2+ ions to form calcite depends on the moisture transport mode on the crack edges, conditioned by the relative humidity of the percolating gas mixture. A simplified model of the leakage rate evolution is proposed. It provides indications concerning the effect of each test parameter on the healing process and allows the experimental results to be reproduced. Application of the model to other tests from the literature shows its limits and gives guidance for future investigations. [Copyright &y& Elsevier]

The influence of atmospheric carbonation on gas diffusion was investigated using four hardened cement pastes (CEM I, CEM III/A, CEM V/A, and a low-alkalinity binder) with common water-to-binder ratio (0.4). The diffusivity of the non‑carbonated and carbonated pastes with respect to helium and nitrogen was measured at different relative humidities. Carbonation decreased the diffusivity of the CEM I paste, whereas that of the other binders significantly increased after carbonation. These results show the competition between porosity clogging and cracking induced by carbonation. The consequences of carbonation are therefore believed to depend on the considered binder. Clogging dominates in ordinary Portland cement (OPC), leading to a decrease in its transport properties after carbonation; cracking dominates in blended cements, leading to a significant increase in its transport properties after carbonation. [ABSTRACT FROM AUTHOR]