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APPLICATIONS OF X-RAY TOMOGRAPHIC TECHNIQUES TO THE STUDY OF CEMENT-BASED MATERIALS
No full textThe 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
CT for industrial metrology - Accuracy and Structural Resolution of CT Dimensional Measurements
No full textVerification 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
High energy X-ray tomography of Bronze Age copper ingots
No full textA 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
Practical method for determining the metrological structure resolution of dimensional CT
No full textThis 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
Application of advanced x-ray micro-tomography to the characterization of the 3D microstructure in cement pastes
No full text
Characterization of cementitious granular materials produced from contaminated soils
No full textIn the present paper, an improved solidification/stabilization (S/S) process for the production of cement-based granular
materials from contaminated soils is described. The presented method is based on the use of Portland cement as a
binder and superplasticizers (SPs). The effectiveness of this process on the immobilization of inorganic hazardous
wastes has been tested on a contaminated soil from an industrial site, showing high levels of several heavy metals and metalloids such as copper, zinc, arsenic and lead.
At first, a characterization of the contamination was carried out. The nature and distribution of contaminants have been
investigated in detail by means of SEM-EDS (scanning electron microscopy with energy dispersive spectroscopy) and
micro-PIXE (particle-induced X-ray emission) chemical mapping, both in the original soil and in the cement grains
after the S/S treatment. The coupling of the high imaging capabilities of SEM with the excellent detection limits of
micro-PIXE, allowed the identification of several metal-bearing phases in the investigated samples. The analyses
showed that the main source of pollution is related to the presence of μm- to mm-sized particles of inorganic
compounds, employed as pigments or additives by the glass production plant formerly operating at the studied area.
In a second stage, the physico-chemical properties of the granular S/S materials were evaluated by means of
mechanical and leaching tests. In particular, the attention was focused on the role played by superplasticizers in the S/S
process. For this purpose, the performances of samples produced following the presented method have been compared with those of similar grains prepared without the addition of superplasticizers. Due to the lower demand of mixing
water, the samples produced using superplasticizers showed a general improvement of performances in terms of
decreased porosity, reduced leaching of contaminants and improved mechanical properties. The laboratory tests that
were carried out showed that the granular materials produced with this improved S/S technique may be suitable for an in-situ re-use as filler or concrete aggregate and may be employed in several other large scale applications
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