Micro- and nano-X-ray computed-tomography: A step forward in the characterization of the pore network of a leached cement paste Nathan Bossa a,b,c, ⁎, Perrine Chaurand a,c , Jérôme Vicente d , Daniel Borschneck a,c , Clément Levard a,c , Olivier Aguerre-Chariol b , Jérôme Rose a,c a Aix-Marseille Université (AMU), CNRS, IRD, CEREGE UM34, BP 80, 13545 Aix-en-Provence, Cedex 4, France b INERIS, Parc Technologique Alata, BP2, 60550 Verneuil-en-Halatte, France c iCEINT, CNRS, Duke Univ. International Consortium for the Environmental Implications of Nanotechnology, Aix-en-Provence, France d Aix-Marseille Université, CNRS, IUSTI UMR 7343, 13013 Marseille, France abstract article info Article history: Received 28 March 2014 Accepted 22 August 2014 Available online 15 October 2014 Keywords: Porosity (B) Pore size distribution (B) Image analysis (B) Nano-CT (B) Micro-CT (B) Pore structure of leached cement pastes (w/c = 0.5) was studied for the first time from micro-scale down to the nano-scale by combining micro- and nano-X-ray computed tomography (micro- & nano-CT). This allowed assessing the 3D heterogeneity of the pore network along the cement profile (from the core to the altered layer) of almost the entire range of cement pore size, i.e. from capillary to gel pores. We successfully quantified an increase of porosity in the altered layer at both resolutions. Porosity is increasing from 1.8 to 6.1% and from 18 to 58% at the micro-(voxel = 1.81 μm) and nano-scale (voxel = 63.5 nm) respectively. The combination of both CT allowed to circumvent weaknesses inherent of both investigation scales. In addition the connectivity and the channel size of the pore network were also evaluated to obtain a complete 3D pore network character- ization at both scales. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction For many years, cement-based materials have been used as structural or filling materials in a great variety of applications, not only as building materials but also as radioactive waste disposal matrices. The durability of these materials is affected as they are altered in contact with water (percolating water such as rain or sea water) and exposed to various cli- matic conditions (e.g. freezing–thawing cycle). This dissolution-induced alteration is considered to be one of the major factors that alter the physico-chemical properties of cement such as strength, permeability, barrier properties and ion diffusion coefficient [1–5]. Cement-based materials are porous with a large pore size distribu- tion ranging from few nanometers (nm) up to tens of micrometers (μm) [6]. Pore structure of hardened cement paste is usually divided into gel pores (from a few nm to 0.2 μm), capillary pores (from 0.2 μm to 10 μm) and air voids (above 10 μm) [7,8]. Gel pores are intrinsic to calcium silicate hydrates (designed as C–S–H in cement notation). Capillary pores are created by chemical shrinkage, i.e., by the reduced of volume occupied by the hydrated phases compared to the volume of the anhydrous phases plus water. Air voids correspond to the air trapped during hydrated cement paste formation [6,9,10]. It is well established that cement leaching and dissolution of cement hydrates (i.e. primary cement phases) result in an increase of the poros- ity and changes in the pore structure. These phenomena result in a porosity gradient from the cement core to the surface altered layer [1]. Cement pore structure plays a fundamental role in the transport pro- cesses of elements (major and trace elements, e.g. metals) within the cement matrix and hence their potential release in the environment. Accurate description of the heterogeneity of the cement pore structure of the altered layers remains a challenging goal, but necessary to help understanding the cement long-term performance. A variety of experimental techniques such as mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and gas ad- sorption techniques have been used to characterize the pore structure of cement-based materials whether freshly hydrated, or altered [5,9, 11–13]. MIP is able to quantify almost all cement pore size range from few nm to 375 μm [6]. All these techniques provide basic information (porosity, pore sizes) but are too constraining to assess precisely pore connectivity and spatial heterogeneity of the porosity. For instance MIP is not able to characterize the porosity heterogeneity along the altered profile and therefore, is usually used to characterize the porosity of unaltered core [5]. The ideal methodology to assess spatial heterogeneity of internal pore structure would reach the following performances: three- dimensional (3D), non-destructive and spatially-resolved investigation (non-bulk dependent), no constraining specific specimen preparation requirement (e.g. drying), direct measurement without interpolation Cement and Concrete Research 67 (2015) 138–147 ⁎ Corresponding author at: CEREGE CNRS, Aix-Marseille University, CNRS, IRD, UM34, UMR 7330, 13545 Aix en Provence, France. E-mail address: bossanathan@gmail.com (N. Bossa). http://dx.doi.org/10.1016/j.cemconres.2014.08.007 0008-8846/© 2014 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp