Greening effect in slag cement materials Domitille Le Cornec a, c, * , Qirong Wang b , Laurence Galoisy a , Guillaume Renaudin b , Laurent Izoret c , Georges Calas a a Institut de Mineralogie, de Physique des Materiaux et de Cosmochimie, UPMC, Sorbonne Universite, UMR CNRS 7590, MNHN, IRD, 4 Place Jussieu, F-75005 Paris, France b Universite Clermont Auvergne, CNRS, SIGMA Clermont, ICCF, F-63000 ClermonteFerrand, France c Association Technique de l’Industrie des Liants Hydrauliques (ATILH), 7 place de la Defense, 92974 Paris-La-Defense Cedex, France article info Article history: Received 25 November 2016 Received in revised form 21 June 2017 Accepted 28 August 2017 Available online 31 August 2017 Keywords: Ground granulated blast-furnace slag (GGBS) Cement Concrete Coloration Polysulfides Diffuse reflectance spectroscopy abstract This article presents the first spectroscopic data describing the processes responsible for the temporary blue-green coloration that forms during the hydration of various materials containing Ground Granu- lated Blast-furnace Slag (GGBS) under anoxic conditions. UV-visible-near infrared Diffuse Reflectance (DR) spectra demonstrate a striking similarity of the coloring center forming during the curing of a broad range of GGBS-bearing materials (pure GGBS with different compositions, mix Portland cement/GGBS (30/70), concrete and mortar). All spectra are similar to those of polysulfide complexes contained in the interlayer spacing of a synthetic green-colored hydrated calcium aluminate phase (AFm). This “greening effect” demonstrates a progressive oxidation of sulfide-based compounds initially contained in these materials during curing of GGBS bearing materials. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction In order to decrease the environmental impact of concrete within an environmentally sustainable design and reduce CO 2 emissions due to its production, Ground Granulated Blast-furnace Slag (GGBS), a glassy by-product of pig iron and steel manufacturing, is commonly used to produce blast-furnace slag cements and GGBS-concrete mixtures [1]. However, concrete based on CEM III cements, or on equivalent binders of GGBS in combi- nation with Portland cement, tends to develop, under certain cir- cumstances, a temporary blue-green color immediately after casting [2]. This coloration, sometimes referred to as a “greening effect”, has been noticed since early uses of GGBS in the cement industry, at the end of the 19th century. Since that time, it has raised some degree of concern due to a lack of knowledge of its origin. However, there are neither indications of inferior engi- neering properties or adverse consequences nor evidence of im- pacts on occupational health or environmental consequences during demolding or aging of the concrete. In particular, it does not affect mechanical strength or other concrete properties but it im- pacts the visual aspect with mediatized consequences. Such coloration is temporary: the color vanishes at the surface of these materials after a few days or weeks of exposure to air and adopts the typical light grey color of slag cement concrete, even though the inside may remain green for years. When old GGBS-based concrete is being demolished, it often shows internal evidence of greening when it is fractured or drilled. A similar coloration has been recently observed in solidified cementitious waste forms contain- ing GGBS, which showed a deep blue fading coloration that has raised concerns about its influence on other physical properties of this solidified waste over long times [3]. In the absence of experimental or analytical data, the origin of the blue-green coloration of GGBS-based materials has only been speculated: formation of new phases during GGBS hydration, such as FeS or MnS [4], or charge transfer processes in various phases or complexes, such as green rusts [5], hydrated ferrous sulfates [2,5] or ferro/ferricyanide complexes and Prussian-blue type pigments [6]. The presence of S 2 in hydrated calcium aluminates with substituted Fe(III) has also been speculated as being responsible for the coloration during GGBS hydration [7]. As the sulfide anion S 2 does not absorb in the visible region, it cannot be a coloring agent * Corresponding author. E-mail address: domitille.le-cornec@espci.org (D. Le Cornec). Contents lists available at ScienceDirect Cement and Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp http://dx.doi.org/10.1016/j.cemconcomp.2017.08.017 0958-9465/© 2017 Elsevier Ltd. All rights reserved. Cement and Concrete Composites 84 (2017) 93e98