Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2008-2009

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THREE-DIMENSIONAL ANALYSIS OF CEMENT-BASED MATERIALS BY MEANS OF X-RAY TOMOGRAPHIC TECHNIQUES

Ph.D. candidate: MATTEO PARISATTO

Tutor: Prof. GILBERTO ARTIOLI Cycle: XXIII

Abstract

Monitoring the microstructural evolution and the chemical reactions taking place in cement-based materials during their setting and hardening is one of the most challenging goals of modern materials science. Due to the intrinsic complexity of such materials, a complete description of all the involved processes requires a multidisciplinary approach. During the last decade, X-ray micro-tomography (X-µCT) has become a widely used non-invasive technique for 3D microstructural investigations. The aim of this research project is to apply the tomographic techniques to the characterization of different cement-based materials and to integrate the results with the information obtained from other established experimental methods (e.g. SEM microscopy and diffraction) and computer simulations. In particular, during this second year of activity, the attention has been focused on two major topics. The first one is a combined study of the hydration of Portland cements by means of synchrotron X-µCT and diffraction-tomography. In the second study, conventional X-µCT, coupled with other experimental methods, has been applied in order to assess the properties and the environmental compatibility of cementitious pellets (HPSS pellets) employed for the stabilization of polluted grounds. Introduction

1- Combined X-µCT and XRD-CT study of the hydration of Portland cements X-µCT is an established technique, successfully applied during the last decade to the cement and concrete research field for the extraction of qualitative and quantitative information (e.g. Bentz et al., 2000, Helfen et al., 2005, Gallucci et al., 2007). It provides a map of the variations of the X-ray attenuation coefficient within a sample in a totally non-invasive manner, allowing 3D images to be reconstructed with a spatial resolution approaching the sub-micrometric scale. However, while voids and cracks inside the cement matrix are easily identified, phases having similar absorption (i.e. similar density and mean atomic number) can hardly be discriminated. Nowadays, synchrotron-based diffraction-tomography (XRD-CT, Bleuet et al., 2008) offers a great tool to overcome these limitations, enabling to map non–destructively the spatial distribution of selected crystalline or amorphous phases in heterogeneous materials, with a resolution depending on the incident beam size. Furthermore, also the fluorescence signal from the sample can be measured simultaneously, in order to generate chemical maps of the investigated area (XRF-CT). 2- Characterization of the HPSS cementitious pellets The study of soil contamination is nowadays a major topic of investigation in environmental sciences; in particular, the requalification and re-utilization of lands contaminated by hazardous wastes represent a complicated issue. Nowadays, the solidification/stabilization (S/S) techniques allow to reduce the solubility and mobility of solid and liquid pollutants by mixing the contaminated materials with a binder (typically Portland cement), in order to obtain a final product with increased physical stability and workability (Glasser, 1994). In conventional S/S processes, a re-use of the stabilized waste is usually not foreseen, whereas in the recently introduced HPSS (High Performance Solidification/Stabilization) system, the contaminated ground is mixed with Portland cement and water in order to obtain a centimeter-sized granulated material (pellets) that can be re-employed in situ as an aggregate into cementitious conglomerates. In the typical formulation, also organic superplasticizers are added; these allow to reduce the water/cement ratio of the mix and therefore the porosity of the solidified pellets, with consequently increased mechanical properties and durability. Laboratory and in situ leaching tests have widely demonstrated that the S/S techniques can substantially reduce the concentration of contaminants into the ground, well below the limits fixed by laws for the residential use.

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2008-2009

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Methods and sample description

The first experiment here described has been carried out at the ID22 beamline of the ESRF in Grenoble (France). Glass capillaries were filled with cement pastes or mortars, prepared using a CEM I 52.5R cement, with a water/cement ratio (w/c) of 0.5. In the formulation of two samples, also organic superplasticizers were added, in order to modify the kinetics of the reactions and the rheology of the pastes. For each of the samples prepared, several X-µCT scans were performed at different times from mixing (from 1 to 72 hours) in order to monitor, almost in real time, the microstructural evolution of the entire sample volume; each scan took approximately 40 minutes. Due to the considerably longer acquisition times (6-7 hours per scan), only a few selected slices of the same samples were investigated with XRD-CT. A simplified scheme of XRD-CT data processing is presented in figure 1.

Figure 1: Reconstruction scheme of the XRD-CT direct analysis: for every translation (y) and rotation (ω) of the sample, a 2D diffraction pattern is collected by a FReLoN camera, thus obtaining a set of integrated linear diffraction patterns. Regions of interest corresponding to a particular 2θ range (e.g. a single peak) are selected on the linear diffraction patterns to build up sinograms, representing the variations of the diffracted intensity as a function of y and ω. Sinograms are then used to reconstruct cross-sections mapping the spatial distribution of selected phases of interest.

For what concerns the HPSS study, all the investigated samples come from the "ex-Conterie" industrial site in Murano (Venezia) which represents the first example (currently operating) of an industrial-scale application of the HPSS process for the requalification of a contaminated area. Until 1990, this was the site of a mill for the manufacture of artistic glass which introduced a significant environmental contamination, mainly due to inorganic colourings based on heavy metals and arsenic. The X-µCT experiments were carried out at the Tomolab laboratory (Basovizza, TS, Italy) using a microfocus CT-system. Two different sets of pellets, prepared with and without the addition of a superplasticizer (SP), have been investigated. Moreover, in order to map the distribution of contaminants within the pellets, also SEM micro-analyses and chemical mapping have been carried out. Preliminary results

The 3D volumes of the samples investigated by means of synchrotron X-µCT were reconstructed using the filtered back projection algorithm (Kak & Slaney, 1988). Selected slices taken exactly at the same position from the reconstructed X-µCT datasets and corresponding to different stages of cement hydration

Ny x Nω diffraction patterns

Reconstructed slices

Selective phase sinograms

Ny x Nω CCD images

Azimuthal integrations

Global sinogram

y

ω

Sum pattern

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2008-2009

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were compared for each sample, in order to extract qualitative and quantitative information. Besides, the processing of XRD-CT data, performed with the XRDUA software (De Nolf, 2006), allowed the reconstruction of slices mapping the distribution of selected phases inside the sample. Good results were obtained by following the diffracted signal of the (100) peak of ettringite (Ca6Al 2(SO4)3(OH)12·26H2O), being this phase very fine grained and well distributed through the hydrated cement paste. Major difficulties were encountered when trying to map the distribution of the unreacted clinker phases or the quartz grains included in a mortar sample. Good agreement can be found between two almost corresponding sections obtained from X-µCT and XRD-CT, in spite of the considerable differences in spatial resolution of the two techniques. The results obtained so far confirm that synchrotron micro-tomography is a powerful technique for the investigation of cement-based materials; although the spatial resolution and the contrast between features are still lower than those obtained by SEM-BSE imaging, X-µCT offers great advantages such as the total non-invasiveness and the access to the third dimension. Furthermore, the combination of absorption, diffraction and fluorescence tomographic techniques represents a new multimodal approach to the understanding of the physical and chemical complexity in heterogeneous materials. At present, the main drawback is related to the non-simultaneous measurements of the absorption and diffraction/fluorescence signals due to the different experimental geometries and operational modes. Moreover, as a cement sample is a rapidly evolving system, further improvements in XRD-CT time-resolution (i.e. shorter acquisition times) are needed for an accurate characterization of the hydration reactions, in particular during the earlier stages.

X-µCT images of the Murano pellets highlighted the presence of several high-absorbing features within the samples, with sizes ranging from the µm to the mm scale. SEM-EDS microanalyses revealed that these are mostly iron oxides and hydroxides, pyrite, barite and zircon, usually not classified as contaminants. Low amounts of As and metals like Cr, Cu, Sn, Pb and Zn were recognized only in a very small number of grains. Furthermore, SEM chemical maps didn't allow us to clarify the distribution of dangerous elements within the cement matrix, probably because their concentrations were below the detection limit of the technique. For this reason, a PIXE (Particle-Induced X-ray Emission) experimental session has been scheduled for the next months; we expect that the high sensitivity of this technique will allow a reliable quantitative analysis of the distribution of contaminants, both in the raw material (i.e. the contaminated soil) and the final product (i.e. the pellets). This is fundamental in order to identify the nature of the phases responsible for the contamination and the mechanisms of uptake by the hydration phases of cement. Tomographic data also showed the presence of a significant amount (roughly 2 % by volume) of air voids trapped inside the pellets, apparently not connected at least at the scale of the tomographic resolution. For a more accurate description of the pore network, down to the nm scale, mercury intrusion porosimetry tests were performed on the same samples. The pellets including a superplasticizer in their formulation exhibited a significantly lower total porosity if compared to those prepared without SP. References

Bentz, D.P., Quenard, D.A., Kunzel, H.M., Baruchel, J., Peyrin, F., Martys, N.S. & Garboczi, E.J. (2000) - Microstructure and transport properties of porous building materials. II: Three-dimensional X-ray tomographic studies. Materials and Structures, 33(3), 147-153. Bleuet, P., Welcomme, E., Dooryhée, E., Susini, J., Hodeau, J.-L. & Walter, P. (2008) - Probing the structure of heterogeneous diluted materials by diffraction tomography. Nature Materials, 7, 468-472. De Nolf, W. (2006) - XRDUA, 2D powder-XRD analysis. http://xrdua.ua.ac.be Gallucci, E., Scrivener, K., Groso, A., Stampanoni, M. & Margaritondo, G. (2007) - 3D experimental investigation of the microstructure of cement pastes using synchrotron X-ray microtomography (µCT). Cement and Concrete Research, 37, 360-368.

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2008-2009

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Glasser, F.P. (1994) - Environmental uses of cement in waste conditioning. Engineering Foundation Conference Proceedings, Advances in Cement and Concrete, M.D. Grutzeck and S.L. Sarkar, eds., American Society of Civil Engineers, New York, pp. 499-507. Helfen, L., Dehn, F., Mikulìk, P. & Baumbach, T. (2005) - Three-dimensional imaging of cement microstructure evolution during hydration. Advances in Cement Research, 17(3), 103-111. Kak, A.C. & Slaney, M. (1988) - Principles of computerized tomographic Imaging. IEEE Press, New York. SUMMARY OF LAST YEAR’S ACTIVITIES

Courses at the Dipartimento di Geoscienze, Università degli Studi di Padova: GLASSER, F.P.: "The materials science of Portland cements"

OMENETTO, P.: "Mediterranean tectonics and metallogenesis"

TEZA, G.: "Introduction to Matlab"

GULICK, L.H: "Advanced English course"

RASSU, A., VINANTE, C., PRATICELLI, N.: "Introduction to Linux"

BULLARD, J., STUTZMAN, P.: "Advanced course on cement characterization and modeling"

Schools, workshops and congresses: 10th SILS School on Synchrotron Radiation: Fundamentals, Methods and Applications. Duino (TS), Italy, 7-18 September.

2009.

Posters: Parisatto, M., Dalconi, M.C., Artioli, G., Cruciani, G., Tucoulou, R., Rack, A., Ferrari, G. & Cerulli, T. - Study of the hydration of Portland cements by means of combined X-ray micro-tomography and XRD-tomography. Presented at the 10th SILS School on Synchrotron Radiation: Fundamentals, Methods and Applications, Duino (TS), Italy. Artioli, G., Dalconi, M.C., Parisatto, M., Tucoulou, R., Cerulli, T. & Cruciani, G. - Ettringite phase distribution in hydrating cement paste by means of diffraction computed micro-tomography. Presented by M.C. Dalconi at the Nanocem - Marie Curie RTN Final Conference, Villars-sur-Ollon, Switzerland. Publications: Nestola, F., Smyth, J.R., Parisatto, M., Secco, L., Princivalle, F., Bruno, M., Prencipe, M. & Dal Negro, A. (2009) - The effect of non-stoichiometry at high pressure on spinel structure: implications for the Earth's mantle mineralogy. Geochimica et Cosmochimica Acta, 73(2), 489-492. Teaching activities: Teaching assistance (25 hours): "Mineralogia" (Prof. L. Secco), Corso di Laurea di primo livello in Scienze Geologiche, A.A. 2008/2009, II semester. Teaching assistance (25 hours): "Mineralogia" (Prof.ssa G. Salviulo), Corso di Laurea di primo livello in Scienze Naturali, A.A. 2009/2010, I semester (in progress).

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2008-2009

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Experimental activities: SEM analyses on cement-based materials at the Dipartimento di Geoscienze, Università degli Studi di Padova and at the Mapei Analytical Laboratory, Milano. Preparation and oven firing (Dipartimento di Scienze della Terra, Università di Modena) of C3S samples for NRRA (Nuclear Resonant Reaction Analysis) experiments. Experimental session (6th-10th May 2009) at the Dynamitron Tandem Accelerator, Ruhr-Universität Bochum (Germany): NRRA experiments on hydrated C3S samples. Series of experiments at the Tomolab facility, Sincrotrone Elettra, Basovizza (Trieste): X-ray microtomography experiments on HPSS cementitious pellets. Mercury intrusion porosimetry tests performed at the Dipartimento di Agronomia Ambientale e Produzioni Vegetali, Università degli Studi di Padova.