Three-dimensional Analysis of Cement-based Materials by Means of X-ray Tomographic Techniques

The knowledge of the microstructure of cement-based materials plays a fundamental role in predicting their macroscopic mechanical properties and durability. Nowadays, X-ray computed micro-tomography (X-μCT) provides a completely noninvasive tool for investigating, in a three-dimensional way, the inner structure of such materials, with spatial resolution down to the micrometer scale. However, due to the intrinsic complexity of cements, a complete description of all the involved processes requires a multi-disciplinary approach. 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, X-ray diffraction) and computer simulations. During the third year of activity, the research topics studied in the previous year have been further investigated. In particular, the attention has been focused on the accurate characterization of cement-based granular materials, produced following the Mapei HPSS method for the solidification/stabilization (S/S) of contaminated soils and sediments. Secondarily, X-μCT measurements on cement pastes have been carried out using a microfocus system in order to study the effect of water to cement ratio. In addition, further experiments have been performed in order to assess the effectiveness of coupled diffractiontomography (XRD-CT) and synchrotron X-μCT for the three-dimensional mapping of selected phases (C-S-H in this particular case) within the cement paste. 1Characterization of the HPSS cementitious pellets The solidification/stabilization (S/S) techniques allow to reduce the solubility and mobility of solid and liquid pollutants by mixing contaminated materials with a binder (typically Portland cement), in order to obtain a final product with increased physical stability and workability (Glasser, 1994). The Mapei HPSS (High Performance Solidification/Stabilization) system aims at producing a centimeter-sized S/S granular material (pellets) by mixing the contaminated ground with Portland cement, water and organic superplasticizers (SPs). With our experiments, we showed that these latter play a crucial role in the improvement of the performances of the final S/S products. The efficiency of the HPSS process has been evaluated on a soil contaminated mainly by heavy metals, collected in an industrial area close to the city of Venice (Italy) where a mill for the manufacturing of artistic glass operated until 1990. X-μCT experiments were carried out using a microfocus system at the Tomolab laboratory (Trieste, Italy); two sets of pellets, prepared with and without the addition of superplasticizers were analyzed. The air void fraction was extracted from the reconstructed 3D datasets by applying a threshold to the grey values histograms and the total void content was then calculated. Samples prepared without and with SP showed a mean porosity of 5.1% and 2.0% respectively. Moreover, in additivated samples, voids appeared to be distributed over a narrower range of sizes, with a shift of the distribution’s peak towards lower values (Fig. 1). The reduction in porosity is a direct effect of the lower water to solid ratio (w/s) and viscosity achievable when using SPs. This of course has beneficial effects on all the final properties and the durability of the granular materials, as confirmed by the results of several physico-mechanical and leaching tests carried out on the same samples. In addition, in order to understand the nature of the contamination and the mechanisms of stabilization of the contaminants within the cement matrix, Micro-PIXE (Particle-Induced X-ray Emission) pointanalyses and chemical mapping have been carried out. PIXE spectroscopy is a powerful technique for the multi-elemental analysis of materials, allowing the detection of trace elements, even if present at very low concentrations (ppm level), i.e. well below the detection limits of conventional SEM-EDS systems. Micro-PIXE experiments were carried out using a 2.5 MeV Van de Graaff accelerator (AN-2000 accelerator) installed at the Laboratori Nazionali di Legnaro (LNL, Legnaro, Padua, Italy) of the Istituto Nazionale di Fisica Nucleare (INFN). An H ion beam with an energy of 2 MeV was adopted for this study; the beam current value ranged from 0.5 to 1 nA. The system is equipped with an energy dispersive high-purity germanium detector mounted at 45° with respect to the beam. The micro-PIXE spectra were Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010 2 analyzed by means of the GUPIX software (Campbell et al., 2000). In the investigated samples, the main source of contamination was recognized to be related to metal-bearing inorganic compounds (rutile, barite, Mn-dioxide, Fe-Cu-Zn-Sn and Pb alloys and oxides), typically employed in the glass industry as pigments or fluxes, mostly present in the form of μmto mm-sized particles. Traces of Cr and As were also found, finely distributed across the cementitious matrix of the pellets. Figure 1. Upper: cross-sectional reconstructed slices illustrating two S/S grains prepared without (left) and with (right) superplasticizers. A more compact matrix, with a significantly lower air void content can be achieved following the HPSS method. Lower: pore size distribution histograms calculated from the analysis of tomographic data for the two pellets represented above. The relative fraction of the total pore volume was plotted versus the equivalent sphere diameter. Bin width: 20 μm. Due to limitations in the spatial resolution of the technique, pores smaller than 10 μm were not detectable, so they were not included in the calculations. 2X-μCT analyses of cement pastes prepared at different w/c It is well known that water to cement ratio (w/c) plays a fundamental role in the determination of the final mechanical properties of cement and concretes. A reduction in the amount of mixing water causes a higher packing of the clinker particles and a lower porosity, with consequent improved mechanical strength. However, one of the main limitations related to a lower w/c is the loss of workability of the fresh cement paste or concrete. For this reason, superplasticizers (SPs) are often added in many formulations. In fact, modern SPs consist of water soluble polyelectrolytes which dramatically reduce the viscosity of fresh cement mixtures and allow to cast flowable, easy to place concretes characterized by a Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010 3 low W/C (≤ 0.4). In the experiment described here, microfocus X-μCT was employed in order to characterize, from a microstructural point of view, the effect of w/c on cement pastes. Three set of samples were prepared by mixing a CEM I 52.5 R ordinary Portland cement with distilled water at w/c ratios of 0.35, 0.5 and 0.65. The fresh pastes were injected into cylindrical glass tubes having a diameter of ~3 mm and analyzed by means of the X-μCT system in use at the Tomolab facility (Trieste, Italy). For each sample, a tomographic scan was acquired at 24 hours, 7 days and 28 days from preparation, in order to follow the evolution of the microstructure with hydration time. The effect of w/c may be clearly recognized by simply comparing the absorption of each sample in the 2D radiographs (i.e. the raw data used for 3D reconstruction). Of course, with the increase of water content, an appreciable reduction of the absorption of X-rays has been recorded. For each sample, the evolution with time was studied by comparing corresponding reconstructed slices and calculating grey values (GV) histograms. Even though the spatial resolution of microfocus X-μCT cannot reach the sub-μm levels of synchrotron-based systems, some changes in the microstructure of cement with hydration can be appreciated. In particular, a sharpening of the GV histogram can be noticed, due to the growth of the hydration products (intermediate GV) at the expenses of unreacted clinker phases (brighter GV) and pores (darker GV). 3Combined X-μCT and XRD-CT study of the hydration of Portland cements Due to the intrinsic microstructural complexity of cement-based materials, a multi-disciplinary approach is necessary for a complete understanding of the reactions taking place during hydration. Nowadays, the information achievable with X-μCT in complex materials can be enhanced if combined with a spatially resolved diffraction signal (diffraction-tomography, XRD-CT, Bleuet et al., 2008). In a recent experiment performed at the ESRF (Artioli et al. 2010) we successfully showed that XRD-CT can provide valuable 3D information even in extremely complex multiphase systems as cements. Despite the fact that data analysis is not straightforward and requires further developments, our results showed that crystalline phases growing during the cement hydration, such as ettringite, can be successfully mapped without perturbing the system. With this new research project, we aim at showing that it is also possible to map the distribution of pseudo-amorphous phases, such as C-S-H, by carefully selecting for image reconstruction specific portions of low-angle XRD patterns, corresponding to the pseudo-tobermorite peaks. XRD-CT experiments have been carried out at the ID-22 beamline of the ESRF in Grenoble (France) on cement paste samples, aged from a few hours to 28 days, contained in glass capillaries with a diameter of ~400 μm. In addition, the samples have been investigated by means of X-μCT, using the setup available at the same beamline. Tomographic scans were acquired at very high spatial resolution (0.35 μm), resulting in an excellent image quality of the final reconstructed slices which also showed a very good contrast between different hydration phases (ettringite, C-S-H, portlandite). With respect to our previous synchrotron experimental session, this can be considered a significant step forward for what concerns tomographic image quality. On the other hand, XRD-CT data need further accurate processing in order to assess the effectiveness of the technique in mapping C-S-H. Another complicated issue is related to the reconstruction of the diffracted signal originated by larger crystals (clinker phases, portlandite) whose behaviour cannot be assimilated to that of a powder. References Artioli, G., Cerulli, T., Cruciani, G., Dalconi, M.C., Ferrari, G., Parisatto, M., Rack, A. & Tucoulou, R. (2010) X-ray diffraction microtomography (XRD-CT), a novel tool for non-invasive mapping of phase development in cement materials. Analytical and Bioanalytical Chemistry, 397 (6), 2131-2136. 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. Campbell, J.L., Hopman, T.L., Maxwell, J.A. & Nejedly, Z. (2000) The Guelph PIXE software package III: alternative proton database, Nuclear Instruments and Methods in Physics Research Section B, 170(1-2), 193-204. 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. Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010 4 SUMMARY OF THIRD YEAR’S ACTIVITIES Courses at the Dipartimento di Geoscienze, Università degli Studi di Padova: NONAT, A.: "Physical chemistry of hydration and setting of cement", 7-9 June 2010. Schools, workshops and congresses: Mapei symposium on cement hydration. Mapei S.p.A., Milan, Italy, 16-17 December 2009. International solidification/stabilization technology forum. Sydney, NS, Canada, 14-18 June 2010. Posters: Parisatto, M., Artioli, G., Ferrari, G. & Gerarduzzi, M. Characterization of optimized S/S granular materials: a multidisciplinary approach. Presented at the International solidification/stabilization technology forum. Sydney, NS, Canada. Publications: Artioli, G., Cerulli, T., Cruciani, G., Dalconi, M.C., Ferrari, G., Parisatto, M., Rack, A. & Tucoulou, R. (2010) X-ray diffraction microtomography (XRD-CT), a novel tool for non-invasive mapping of phase development in cement materials. Analytical and Bioanalytical Chemistry, 397 (6), 2131-2136. Ferrari, G., Artioli, G. & Parisatto, M. (2010) From HPC to HPSS: the use of superplasticizers for the improvement of S/S technology. In: Lake, C.B. & Hills, C.D. (Eds.), Proceedings of the International Solidification/Stabilization Technology Forum, Sydney (NS, Canada), June 14-18 2010, 193-203. Parisatto, M., Artioli, G., Ferrari, G., Gerarduzzi, M. & Mazzoli, C. (2011) – Characterization of artificial aggregates produced from contaminated soils. Submitted for the proceedings of the XIII International Congress on the Chemistry of Cement, Madrid (Spain), July 3-8 2011. Teaching activities: Teaching assistance (25 hours): "Mineralogia" (Prof.ssa G. Salviulo), Corso di Laurea di primo livello in Scienze Naturali, A.A. 2009/2010, I semester. Teaching assistance (25 hours): "Mineralogia" (Prof. L. Secco), Corso di Laurea di primo livello in Scienze Geologiche, A.A. 2009/2010, II semester. Experimental activities: Micro-PIXE experiments on Solidification/Stabilization granular materials at the AN-2000 accelerator, INFN Laboratori nazionali di Legnaro (PD) (December 2009 February 2010). SEM analyses on cement-based materials at the Dipartimento di Geoscienze, Università degli Studi di Padova and at the Mapei Analytical Laboratory, Milano (February-March 2010). Experimental session (25-29 June 2010) at the ID22 beamline, E.S.R.F., Grenoble (France): X-ray micro-tomography and diffraction-tomography experiments on cementitious samples. Series of experiments at the Tomolab facility, Sincrotrone Elettra, Basovizza (Trieste): X-ray microtomography experiments on cement paste samples (May-June 2010) and bronze-age artefacts (October 2010). Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010 5 SUMMARY OF SECOND 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: 10 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 10 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. 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 (6-10 May 2009) at the Dynamitron Tandem Accelerator, Ruhr-Universität Bochum (Germany): NRRA experiments on hydrated C3S samples. Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010 6 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. SUMMARY OF FIRST YEAR’S ACTIVITIES Courses: NESTOLA, F.: “Metodologie analitiche”, Dipartimento di Geoscienze, Università degli Studi di Padova. CÁMARA ARTIGAS F.: "Risoluzione strutturale di sostanze inorganiche a struttura cristallina ignota", Dipartimento di Geoscienze, Università degli Studi di Padova. WALTON, G.: "Inglese scientifico", Dipartimento di Geoscienze, Università degli Studi di Padova. FIORETTI, A., DI TORO, G. & ARTIOLI, G.: "Comunicazione scientifica", Dipartimento di Geoscienze, Università degli Studi di Padova. PESARIN, F. & SALMASO, L.: "Introduzione ai metodi statistici", Dipartimento di Geoscienze, Università degli Studi di Padova. Schools, workshops and congresses: Mineralogical International School, HP-HT Mineral Physics: Implications for Geosciences. Bressanone (BZ), Italy, 11-15 February 2008. 19th ACBM/NIST Computer Modeling Workshop. National Institute of Standards and Technology, Gaithersburg, MD, U.S.A., 16-18 June 2008. Publications: Nestola, F., Smyth, J.R., Parisatto, M., Secco, L., Princivalle, F., Bruno, M., Prencipe, M. & Dal Negro, A. (2008) The effect of non-stoichiometry at high pressure on spinel structure: implications for the Earth's mantle mineralogy. Geochimica et Cosmochimica Acta, in press. Experimental activities: SEM analysis on clinker grains and cement samples at the Dipartimento di Geoscienze, Università degli Studi di Padova. Series of experiments (June-August) at the Tomolab laboratory and the SYRMEP beamline, Sincrotrone Elettra, Basovizza (Trieste): X-ray microtomography experiments on mortar samples. Experimental session (2-8 October 2008) at the ID22 beamline, E.S.R.F., Grenoble (France): X-ray microtomography and diffraction-tomography experiments on cement and mortar samples.