Luca Valentini

Towards three-dimensional quantitative reconstruction of cement microstructure by X-ray diffraction microtomography

Quantitative characterization of the microstructure of cement-based materials is of fundamental importance for assessing the performance and durability of the final products. However, accessing the three-dimensional microstructural information of hydrating cement pastes without introducing any perturbation is not trivial. Recently, a novel non-invasive method based on X-ray diffraction computed microtomography (XRD-CT) has been applied to cement-based materials, with the aim of describing the three-dimensional spatial distribution of selected phases during the hydration of the cement paste. This paper illustrates a method based on XRD-CT, combined with Rietveld-based quantitative phase analysis and image processing, which provides quantitative information relative to the distribution of the various phases present in the studied samples. In particular, it is shown how this method allows the estimation of the local volume fraction of the phase ettringite within a hydrating cement paste, and construction of a three-dimensional distribution map. Application of this method to the various constituents of a cementitious material, or, more generally, of a composite polycrystalline material, may provide a non-invasive tool for three-dimensional microstructural quantitative characterization.

RieCalc: quantitative phase analysis of hydrating cement pastes

The MATLAB program RieCalc, developed for the quantitative phase analysis of hydrating cement pastes, is presented. Rietveld refined data obtained from in situ X-ray diffraction measurements are used as input and loaded by means of a simple graphical user interface. Time-resolved phase fractions are rescaled by a mass balance method, which calculates the amounts of amorphous phases (calcium silicate hydrate and capillary water) present in the system. Furthermore, RieCalc calculates theoretical curves that simulate those obtained from isothermal calorimetry measurements.

3D imaging of complex materials: the case of cement

Abstract Absorption-based X-ray micro-tomography (X-μCT) provides fundamental in-situ information on the 3D microstructure of complex multiphase materials such as cements. However, since the phases present in a hydrating cement paste may be characterized by similar values of the attenuation coefficient, leading to low absorption contrast between different crystalline or amorphous phases, micro-structural interpretation can be equivocal. 3D phase mapping by X-ray diffraction micro-tomography proved to be a successful technique for investigating the spatial distribution of the products in the paste during the hydration process, in a totally non-invasive mode and with enhanced phase selectivity compared to absorption tomography. Phase-selective maps, in the case of crystalline phases, can be extracted from single Bragg peaks or from the Rietveld-refined scale factor. However, even poorly crystalline and/or amorphous phases present in the cement paste, such as calcium silicate hydrates, can be successfully mapped by the use of selected portions of the measured powder data containing the relevant scattering of the phase. The reconstructed maps can be directly modeled by multifractal analysis and compared with computer-generated distributions.

Imaging of nano-seeded nucleation in cement pastes by X-ray diffraction tomography

Abstract The 3D phase distribution of cement pastes evolves during hydration and controls the rheology and mechanical properties of the paste. Synchrotron powder-diffraction micro-tomographic imaging is here employed to assess the cement phase spatial distribution in a totally non-invasive way. This technique can be used to produce distribution maps of the phases present in the hydrating cement paste. The method is applied to an ordinary Portland cement, hydrated in pure water or in the presence of nucleation seeds. The quantitative description of the phase spatial distribution by radial distribution functions allows the discrimination of different nucleation mechanisms.

Non-invasive assessment of the formation of tourmaline nodules by X-ray microtomography and computer modeling

Abstract Tourmaline nodules occurring in the Capo Bianco (Elba Island, Italy) aplitic rocks are here investigated by X-ray microtomography 3D imaging. This non-invasive technique provides 3D images of the tourmaline nodules, revealing an irregular morphology consisting of branches that extend radially from the cores. The nodules present scale-invariant features that can be described by a box-counting fractal dimension. The value of the fractal dimension is proportional to the size of the nodules and tends asymptotically to a value of 2.5, in agreement with the results obtained from the simulation of virtual nodules, by means of a diffusion-limited aggregation model based on a Monte Carlo Metropolis algorithm, in which the growth probability at the tips of the nodule is an inverse function of the diffusion coefficient. The results support the hypothesis that tourmaline formed by a disequilibrium magmatic process, in which diffusion represents the rate-limiting step, inducing the formation of nodules with irregular shapes. This study shows the potential of X-ray microtomography, in combination with numerical modeling, as a probe for accessing the 3D microstructural information of complex mineral morphologies with a non-invasive approach. The combination of numerical and experimental, non-invasive, 3D techniques represents a fundamental step forward in bridging the gap between the observation of microstructures and the interpretation of the associated processes.