Structural changes in C–S–H gel during dissolution: Small-angle neutron scattering and Si-NMR characterization Ana Trapote-Barreira a, ⁎, Lionel Porcar b,c , Jordi Cama a , Josep M. Soler a , Andrew J. Allen b a Institute of Environmental Assessment and Water Research (IDAEA), Barcelona 08034, Catalonia, Spain b National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899, USA c Large Scale Structure Group, Institut Laue Langevin, Grenoble, France abstract article info Article history: Received 28 August 2014 Accepted 11 February 2015 Available online 7 March 2015 Keywords: Calcium-Silicate-Hydrate (B) Small-Angle X-Ray Scattering (B) Cement (D) Modeling (E) Dissolution Flow-through experiments were conducted to study the calcium–silicate–hydrate (C–S–H) gel dissolution kinetics. During C–S–H gel dissolution the initial aqueous Ca/Si ratio decreases to reach the stoichiometric value of the Ca/Si ratio of a tobermorite-like phase (Ca/Si = 0.83). As the Ca/Si ratio decreases, the solid C– S–H dissolution rate increases from (4.5 × 10 -14 to 6.7 × 10 -12 ) mol m -2 s -1 . The changes in the microstructure of the dissolving C–S–H gel were characterized by small-angle neutron scattering (SANS) and 29 Si magic-angle-spinning nuclear magnetic resonance ( 29 Si-MAS NMR). The SANS data were fitted using a fractal model. The SANS specific surface area tends to increase with time and the obtained fit parameters reflect the changes in the nanostructure of the dissolving solid C–S–H within the gel. The 29 Si MAS NMR analyses show that with dissolution the solid C–S–H structure tends to a more ordered tobermorite structure, in agreement with the Ca/Si ratio evolution. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Portland cement concrete is used worldwide to build all types of constructions with different purposes. Houses, factories, bridges, storage facilities, etc. are examples of cement-based structures. In particular, concrete is the predominate material in engineered barriers in low-level nuclear waste disposal facilities [1]. A combination of diffusion-transport effects and chemical reactions promotes the alteration of the microstructure of the material when subject to a flow of water: dissolution of cement constituents such as portlandite (calcium hydroxide, denoted CH in cement notation) and calcium silicate hydrate (C–S–H). The C–S–H gel, which constitutes at least 60% of the fully hydrated cement paste by volume, is the main strength-giving phase, also responsible for the durability and radionuclide barrier properties of cement owing to the features of its microstructure (porous structure and alkaline solution inside the pores that limit the solubility of radionuclides [2,3]). In this context, there is a need to study any alteration of the microstructure of the cement, together with any associated changes in the C–S–H gel micro- structure caused by the presence of water. In recent years, considerable research on cement degradation has been conducted to understand the relevant mechanisms governing this complex process. The complexity and demanding nature of this research has required several different methodologies and techniques to be applied. A common methodology, used due to its simplicity, is based on laboratory leaching experiments (i.e., closed systems) in which decal- cification of the solid C–S–H microstructure within the gel (after dissolu- tion of CH) and consequent changes in the cement mechanical properties are studied [4–9]. Among the techniques used to study the cement and concrete structures, 29 Si magic-angle-spinning nuclear magnetic reso- nance ( 29 Si MAS-NMR) and small-angle neutron scattering (SANS) are particularly useful for investigating porous structures like C–S–H gel, given that the amorphous nature of C–S–H renders diffraction ineffective. In addition, specimens can be studied in their natural saturated state, thus avoiding complications associated with drying the C–S–H gel [8]. SANS data are effective in probing features in the 10 Å to 1000 Å (1 nm to 100 nm) size range (short-ranged crystalline order) that defines critical aspects of the solid C–S–H structure within the gel [7], providing quanti- tative information about microstructural features (e.g., particle size, shape, surface area and fractal properties). SANS covers the range of scattering q values from 0.002 Å -1 to 0.2 Å -1 , where q = (4π/λ)sin(θ), λ is the neutron wavelength and 2θ is the scattering angle. SANS data permit determination of the fractal exponent and fractal morphology of the C–S–H gel over a large scale range, and this can be quantified through application of a fractal microstructure model [8,10–14]. Cement and Concrete Research 72 (2015) 76–89 ⁎ Corresponding author at: Jordi Girona 18-26, Barcelona 08034, Catalonia, Spain. Tel.: +34 934006100. E-mail address: anatrapotebarreira@gmail.com (A. Trapote-Barreira). http://dx.doi.org/10.1016/j.cemconres.2015.02.009 0008-8846/© 2015 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp