Virtual materials for the prediction of concrete mechanical properties

Physical properties such as compressive strength and elastic moduli are of the utmost importance for the structural stability and design of cement-based materials. These properties are strictly related to the microstructure of the binder paste, which in turn varies in time, as a function of the hydration kinetics. Therefore, the development of the elastic properties and mechanical strength can in principle be controlled by affecting the microstructure and hydration kinetics. This can be achieved through an appropriate mix-design, which encompasses a careful selection of phase proportions, grain-size distribution, amount of water and aggregates, and use of additives. Changing such variables by a trial-and-error process can be extremely time consuming and has a significant impact in terms of resources employed. Moreover, a fully quantitative approach to the study of the cement microstructure and hydration kinetics requires significant efforts in terms of experimental testing, often encompassing analytical techniques such as X-ray diffraction, scanning electron microscopy and isothermal calorimetry, among others. In this contribution, an alternative quantitative characterization of the cement paste in time is illustrated, based on the numerical modeling of cement-based systems. Virtual cement pastes and mortars are generated using the software VCCTL (http://www.nist.gov/el/building_materials/inorganic/vcctl.cfm), using as input parameters the clinker phase composition, the water/cement ratio, and the size and shape distribution of the particles. The elastic moduli and compressive strength of such virtual samples is then computed from the developed microstructure by a finite element method. Extensive calibration and testing has been performed against experimental data, and the good agreement between the calculated and measured elastic and mechanical properties shows that VCCTL can be used as a truly predictive tool. Although experimental testing remains a fundamental aspect of concrete science, the coupling of experiments with computational methods provides a viable tool towards a knowledge-based mix design, with a potential reduction of costs and environmental impact

Virtual materials for the prediction of concrete mechanical properties

Physical properties such as compressive strength and elastic moduli are of the utmost importance for the structural stability and design of cement-based materials. These properties are strictly related to the microstructure of the binder paste, which in turn varies in time, as a function of the hydration kinetics. Therefore, the development of the elastic properties and mechanical strength can in principle be controlled by affecting the microstructure and hydration kinetics. This can be achieved through an appropriate mix-design, which encompasses a careful selection of phase proportions, grain-size distribution, amount of water and aggregates, and use of additives. Changing such variables by a trial-and-error process can be extremely time consuming and has a significant impact in terms of resources employed. Moreover, a fully quantitative approach to the study of the cement microstructure and hydration kinetics requires significant efforts in terms of experimental testing, often encompassing analytical techniques such as X-ray diffraction, scanning electron microscopy and isothermal calorimetry, among others. In this contribution, an alternative quantitative characterization of the cement paste in time is illustrated, based on the numerical modeling of cement-based systems. Virtual cement pastes and mortars are generated using the software VCCTL (http://www.nist.gov/el/building_materials/inorganic/vcctl.cfm), using as input parameters the clinker phase composition, the water/cement ratio, and the size and shape distribution of the particles. The elastic moduli and compressive strength of such virtual samples is then computed from the developed microstructure by a finite element method. Extensive calibration and testing has been performed against experimental data, and the good agreement between the calculated and measured elastic and mechanical properties shows that VCCTL can be used as a truly predictive tool. Although experimental testing remains a fundamental aspect of concrete science, the coupling of experiments with computational methods provides a viable tool towards a knowledge-based mix design, with a potential reduction of costs and environmental impact

In situ analysis of garnet inclusion in diamond using single-crystal X-ray diffraction and X-ray micro-tomography

none9A single crystal of garnet enclosed in a diamond from the Jericho kimberlite (Slave Craton, Canada) has been investigated using X-ray diffraction and X-ray micro-tomography. The novel experimental approach allowed us to determine the crystal structure of the garnet. The unit-cell edge a and fractional atomic coordinates of oxygen were used to determine the composition via an updated Margules model for garnets. The composition is Pyr(0.41(5))Alm(0.36(7))Gro(0.22(1))Uva(0.01(1)), which is indistinguishable from the eclogitic garnets found in other Jericho diamonds. We also demonstrated that residual pressures on the inclusion of up to 1 GPa do not affect significantly the determination of the garnet composition by structure refinement.noneFABRIZIO NESTOLA;M. MERLI;PAOLO NIMIS;M. PARISATTO;M. KOPYLOVA;A. DE;M. LONGO;L. ZIBERNA;M. MANGHNANINestola, Fabrizio; M., Merli; Nimis, Paolo; Parisatto, Matteo; M., Kopylova; A., De; Longo, Micaela; Ziberna, Luca; M., Manghnan

From HPC to HPSS: The use of superplasticizers for the improvement of S/S technology

This paper presents an S/S treatment which is based on the principles of High Performance Concrete (HPC). By using superplasticizers and hydrophobic additives, the proposed process allows the transformation of contaminated soils and sediments into low water to cement ratio (W/C) granular materials. These grains are characterized by lower porosity, lower leaching rate of inorganic contaminants and lower water absorption as well as higher mechanical properties and improved durability compared to solidified products obtained without additives. Moreover, the mixing water may be reduced more than 25%. The results of X-ray microtomography and mercury intrusion porosimetry measurements carried out on samples prepared with and without additives are illustrated here. Volatile and/or semi-volatile organic pollutants, if present, can be removed from the granular material by a steam distillation step, at relatively low temperature (max 250\ub0C) and under vacuum (P < 0.1 bar) in excellent yield. The resulting aggregates are suitable for the re-utilization as backfilling, concrete aggregates or may be employed for covering landfills and in other civil engineering projects

The pigments of the frigidarium in the Sarno Baths, Pompeii: Identification, stratigraphy and weathering

In the present research is used a multi-analytical approach to study the wall paintings from the Sarno Baths, located in the southern part of Pompeii. In particular the investigation is focused on the frescos of the frigidarium, though a few samples from other rooms were also analysed. Twenty wallpaintings fragments were analysed by laser scanning confocal microscopy (LSCM), optical microscopy (OM), micro-Raman, scanning electron microscope with energy dispersive spectrometry (SEM-EDS), portable X-ray fluorescence spectrometry (p-XRF) and X-ray powder diffraction (XRPD). The XRPD data were used for the mineralogical semi-quantitative phase analyses (SQPA) and the estimation of the hematite crystals size. The obtained data allow the identification of the pigments and the techniques used, and address new question such as whether talc and aragonite were used routinely in the formulation of pigments. The chemical composition of red and yellow pigments is also discussed and compared with data available from the recent literature. The wall paintings are badly preserved and weathering products occur on the pictorial surface. Eight samples of efflorescence salts and patinas were analysed by XRPD: all the samples are composed mainly by alkali sulphates. The systematic difference between the salts present on the northern and the western walls is likely related to the materials inserted during the 19th century restauration

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