APPLICATIONS OF X-RAY TOMOGRAPHIC TECHNIQUES TO THE STUDY OF CEMENT-BASED MATERIALS

The knowledge of the microstructural properties of cement-based materials plays a fundamental role in predicting their macroscopic behaviour in terms of performance and durability. However, due to the intrinsic microstructural and chemical complexity of such materials, a multi-disciplinary approach is often required. Most classical experimental techniques such as XRD, XRF or mercury porosimetry (MIP) only provide overall information about selected properties (phase and chemical composition, porosity, etc.) but give no indications about their real spatial distribution within the investigated sample. Over the past decades, modern experimental methods for microstructural analysis such as SEM imaging have lead to great advances in our understanding of the complex mechanisms occurring during cement hydration. However, the lack of access to three-dimensional (3D) information represents the main limitation of SEM and other 2D imaging techniques. Furthermore, as sample preparation is often quite invasive, the microstructure of cement may result completely altered. For such reasons, the development of non-destructive techniques for the 3D microstructural investigation of materials has become necessary. Nowadays X-ray computed micro-tomography (X-μCT) provides a totally non-invasive tool to investigate in a three-dimensional way the inner structure of materials, with a spatial resolution reaching the sub-μm level when the most advanced systems are employed. X-μCT allows to reconstruct 3D maps of the variations of the X-ray linear attenuation coefficient (μ) within a sample without perturbing its structure. The aim of this research project is to assess the potential of X-μCT for the microstructural study of several features of interest in cementitious materials. The evolution of the microstructure during setting and hardening, the effects of water-cement ratio (w/c), the role of superplasticizers and the pore space properties are among the major topics that have been investigated. The results obtained from X-μCT at the microscopic scale can then be correlated with the corresponding macroscopic properties observed in real applications. In order to compare the capabilities of the two most common types of X-μCT setups, experiments were carried out using both conventional laboratory instruments and synchrotron-based systems. A synchrotron study of cement evolution during the early hydration stages was successfully performed, focusing the attention on the effect of superplasticizers (chapter 4). The high spatial resolution achievable allowed to follow the evolution of porosity and anhydrous cement fraction as a function of hydration time. In chapter 5, conventional laboratory X-μCT was applied to the study of cement paste samples prepared at different w/c ratios in order to get insights on the microstructural features that determine the variations of strengths in macroscopic samples with varying water contents (chapter 5). In addition, the capabilities of a novel experimental technique (diffraction tomography, XRD-CT) were tested for the first time on cementitious samples (chapter 6). By combining the principles of X-ray micro-diffraction with those of tomographic reconstruction, XRD-CT allows to map the distribution of selected crystalline or amorphous phases within a sample in a totally non invasive manner. In this way, one of the main limitations of X-μCT, related to the poor sensitivity to small absorption variations between different phases can be overcome. Despite the fact that data analysis is not straightforward and requires further developments, the preliminary results presented in this thesis show that crystalline and amorphous phases growing during cement hydration such as ettringite and C-S-H can be successfully mapped without perturbing the system. In the last part of the thesis (chapter 7), a practical application example of X-μCT is reported. The tomographic technique was employed to characterize the pore space properties and the microstructure of cementitious granular materials produced from the solidification and stabilization process (S/S) of soils contaminated by heavy metals. The results of X-μCT analyses were then combined with those obtained using other established experimental methods (e.g. MIP, physico-mechanical and leaching tests) in order to evaluate the performances and environmental compatibility of an innovative method of contaminated grounds remediation.La conoscenza delle proprietà microstrutturali dei materiali cementizi gioca un ruolo fondamentale nel predire il loro comportamento macroscopico in termini di prestazioni e durabilità. Tuttavia, a causa dell’intrinseca complessità microstrutturale e chimica di tali materiali, un approccio multi disciplinare è spesso richiesto. La maggior parte delle tecniche sperimentali classiche come XRD, XRF o la porosimetria a mercurio (MIP) forniscono solamente informazioni complessive riguardo determinate proprietà (composizione mineralogica e chimica, porosità, etc.) ma non danno alcuna indicazione sulla loro reale distribuzione spaziale all’interno del campione studiato. Nel corso degli ultimi decenni, i moderni metodi sperimentali per l’analisi microstrutturale come la microscopia elettronica a scansione (SEM) hanno portato ad importanti avanzamenti delle nostre conoscenze sui complessi meccanismi che avvengono nel corso dell’idratazione del cemento. Tuttavia, l’impossibilità di accedere ad informazioni tridimensionali (3D) rappresenta la principale limitazione della tecnica SEM e degli altri metodi di imaging 2D. Inoltre, poiché la preparazione del campione è spesso piuttosto invasiva, la microstruttura del cemento può risultare completamente alterata. Per tali ragioni, si è reso necessario lo sviluppo di tecniche non distruttive per lo studio microstrutturale in 3D dei materiali. Oggigiorno, la micro-tomografia computerizzata a raggi X (X-μCT) fornisce uno strumento totalmente non invasivo per studiare in modo tridimensionale la struttura interna dei materiali, con una risoluzione spaziale che può raggiungere il livello sub-micrometrico quando vengono utilizzati i sistemi più avanzati. La X-μCT consente di ricostruire mappe in 3D delle variazioni del coefficiente di attenuazione lineare dei raggi X (μ) all’interno di un campione senza perturbarne la struttura. Lo scopo di questo progetto di ricerca è quello di verificare le potenzialità della X-μCT per lo studio microstrutturale di diversi aspetti di interesse nei materiali cementizi. Tra le principali tematiche che sono state affrontate vi sono l’evoluzione della microstruttura durante la presa e l’indurimento, gli effetti del rapporto acqua-cemento, il ruolo degli additivi superfluidificanti e le proprietà dello spazio poroso. I risultati ottenuti dalla X-μCT alla scala microscopica possono essere correlati con le corrispondenti proprietà microscopiche osservate nelle applicazioni reali. Al fine di confrontare le potenzialità delle due principali tipologie di strumenti per X-μCT, sono stati effettuati esperimenti utilizzando sia sistemi convenzionali da laboratorio sia sistemi da sincrotrone. Uno studio al sincrotrone sull’evoluzione del cemento nel corso degli stadi iniziali dell’idratazione è stato portato a termine con successo, ponendo l’attenzione sull’effetto dei superfluidificanti (cap. 4). L’elevata risoluzione spaziale ottenibile ha consentito di seguire l’evoluzione della porosità e della frazione di cemento anidro in funzione del tempo di idratazione. Nel capitolo 5, la X-μCT convenzionale da laboratorio è stata applicata allo studio di campioni di paste di cemento preparati a diverso rapporto acqua-cemento al fine di ottenere indicazioni sui parametri microstrutturali che determinano le variazioni delle resistenze meccaniche in campioni macroscopici al variare del contenuto d’acqua. Inoltre, le potenzialità di una tecnica sperimentale recentemente sviluppata (diffraction tomography, XRD-CT) sono state testate per la prima volta su campioni cementizi (cap. 6). La tecnica della XRD-CT, combinando i principi della micro-diffrazione a raggi X con quelli della ricostruzione tomografica, consente di mappare la distribuzione di determinate fasi cristalline o amorfe all’interno di un campione in una maniera del tutto non invasiva. In questo modo, una delle principali limitazioni della X-μCT legata alla scarsa sensibilità nei confronti di ridotte variazioni di assorbimento tra diverse fasi può essere superata. Nonostante l’analisi dei dati non sia semplice e richieda ulteriori sviluppi, i risultati preliminari presentati in questa tesi mostrano che alcune fasi, sia cristalline sia amorfe, che si sviluppano nel corso dell’idratazione del cemento (come ad esempio l’ettringite o il C-S-H), possono essere mappate con successo senza perturbare il sistema. Nell’ultima parte del lavoro è riportato un esempio pratico di applicazione della X-μCT. La tecnica tomografica è stata utilizzata per caratterizzare la porosità e la microstruttura di materiali cementizi granulari prodotti dal processo di solidificazione e stabilizzazione (S/S) di suoli contaminati da metalli pesanti. I risultati delle analisi di X-μCT sono stati poi combinati con quelli ottenuti usando altri metodi sperimentali classici (ad esempio MIP, test fisico-meccanici e di cessione) al fine di valutare le prestazioni e la compatibilità ambientale di un metodo innovativo di bonifica dei terreni inquinati

High energy X-ray tomography of Bronze Age copper ingots

A few samples from the Bronze Age settlement of Santa Caterina Tredossi, Cremona, Italy generically identified during the excavation as metallurgical slags have been investigated by high energy X-ray computed tomography (XCT) in order to explore the possibility of using XCT as a non-invasive diagnostic tool for metal-related materials, and eventually of calibrating the absorption signal toward the actual composition of the object. The experiments were performed using the high energy X-ray source available at the Museum Research Laboratory of the Getty Conservation Institute, Los Angeles (GCI) and the experimental setup developed through by a collaboration between the GCI and the University of Bologna. By changing the distance between the sample, the GOS screen, and camera the setup makes it possible to optimize the resolution of the measured images even for large objects. The objects turned out to be ingots of pure copper with a very thick alteration layer. The experiments showed that through XCT it is possible to clearly identify the core of pristine copper metal and its shape underneath the thick layers of cuprite and secondary copper minerals, mainly malachite and brochantite. The grey-scale segmentation of the layers based on the absorption contrast was successively confirmed by sacrificing a few samples and by direct chemical check of the layer compositions with electron probe microanalysis, in order to carefully calibrate the absorption contrast vs the copper content of the material. The interlayer surfaces can be used to image in 3D the shape of the object and the depth of alteration

Three-dimensional distribution of primary melt inclusions in garnets by X-ray microtomography

X-ray computed microtomography (X-μCT) is applied here to investigate in a non-invasive way the three-dimensional (3D) spatial distribution of primary melt and fluid inclusions in garnets from the metapelitic enclaves of El Hoyazo and from the migmatites of Sierra Alpujata, Spain. Attention is focused on a particular case of inhomogeneous distribution of inclusions, characterized by inclusion-rich cores and almost inclusion-free rims (i.e., zonal arrangement), that has been previously investigated in detail only by means of 2D conventional methods. Different experimental X-μCT configurations, both synchrotron radiation-and X-ray tube-based, are employed to explore the limits of the technique. The internal features of the samples are successfully imaged, with spatial resolution down to a few micrometers. By means of dedicated image processing protocols, the lighter melt and fluid inclusions can be separated from the heavier host garnet and from other non-relevant features (e.g., other mineral phases or large voids). This allows evaluating the volumetric density of inclusions within spherical shells as a function of the radial distance from the center of the host garnets. The 3D spatial distribution of heavy mineral inclusions is investigated as well and compared with that of melt inclusions. Data analysis reveals the occurrence of a clear peak of melt and fluid inclusions density, ranging approximately from â to of the radial distance from the center of the distribution and a gradual decrease from the peak outward. Heavy mineral inclusions appear to be almost absent in the central portion of the garnets and more randomly arranged, showing no correlation with the distribution of melt and fluid inclusions. To reduce the effect of geometric artifacts arising from the non-spherical shape of the distribution, the inclusion density was calculated also along narrow prisms with different orientations, obtaining plots of pseudo-linear distributions. The results show that the core-rim transition is characterized by a rapid (but not step-like) decrease in inclusion density, occurring in a continuous mode. X-ray tomographic data, combined with electron microprobe chemical profiles of selected elements, suggest that despite the inhomogeneous distribution of inclusions, the investigated garnets have grown in one single progressive episode in the presence of anatectic melt. The continuous drop of inclusion density suggests a similar decline in (radial) garnet growth, which is a natural consequence in the case of a constant reaction rate. Our results confirm the advantages of high-resolution X-μCT compared to conventional destructive 2D observations for the analysis of the spatial distribution of micrometer-scale inclusions in minerals, owing to its non-invasive 3D capabilities. The same approach can be extended to the study of different microstructural features in samples from a wide variety of geological settings

Practical method for determining the metrological structure resolution of dimensional CT

This work deals with a practical approach for determining the metrological structure resolution in X-ray Computed Tomography (CT) for dimensional measurements in manufacturing. Advantages over other applicable approaches are discussed. The experimental results obtained from the implementation of the method using a micro-CT system are compared with the geometrical unsharpness of CT reconstructions

CT for industrial metrology - Accuracy and Structural Resolution of CT Dimensional Measurements

Verification of dimensional measurement accuracy and other metrological characteristics of X-ray Computed Tomography (CT) systems is necessary both for establishing traceability of CT dimensional measurements and for achieving comparability of CT to other dimensional measuring techniques used in industrial metrology. This paper summarizes the state of the art in accuracy evaluation of CT dimensional measurements, discussing methods for metrological performances verification and traceability establishment. The work is based on experimental results obtained both from (i) the first international interlaboratory comparison on CT dimensional metrology and from (ii) additional CT measurements performed at University of Padova specifically for a more in depth examination of specific metrological characteristics. Particular attention is given to the evaluation of a specific metrological characteristic that too often is neglected when testing CT systems: the structural resolution for dimensional measurements. After discussing possible methods for determining the structural resolution, a new method is proposed, based on a novel calibrated reference standard that has been developed expressly for facilitating the evaluation of structural resolution. Preliminary results of an experimental investigation are discussed and conclusions are reported

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

Tourmaline nodules occurring in the Capo Bianco (Elba Island, Italy) aplitic rocks are here investigated by X\u2011ray 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\u2011ray 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