Formulation of an alkali-activated grout based on Callovo-Oxfordian argillite for an application in geological radioactive waste disposal C. Dupuy a,b , A. Gharzouni a , I. Sobrados c , N. Tessier-Doyen a , N. Texier-Mandoki b , X. Bourbon b , S. Rossignol a,⇑ a IRCER: Institut de Recherche sur les Céramiques (UMR7315), 12 rue Atlantis, 87068 Limoges Cedex, France b Andra: Agence Nationale pour la Gestion des Déchets Radioactifs, 1-7 rue Jean-Monnet, 92298 Châtenay-Malabry Cedex, France c ICMM-CSIC, Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, C/Sor Juana Inés de la Cruz, 3, 28049 Madrid, Spain highlights  An alkali-activated grout containing COx argillite has been developed.  Use of boric acid or sodium tetraborate allows controlling setting time and pH values.  The decrease of pH value during a thermal treatment at 90 °C has been investigated.  A network reorganization occurs during the thermal curing of a consolidation sample. graphical abstract article info Article history: Received 9 July 2019 Received in revised form 5 September 2019 Accepted 4 October 2019 Keywords: Geopolymer Grout COx argillite Bore pH value Setting time abstract In this study, the formulation of alkali-activated grouts based on Callovo-Oxfordian argillite, with a long setting time (up to 24 h) and a moderate pH value (10–11), is investigated. The first step was to select the reference mixture based on calcined argillite (650 °C) and metakaolin. The selected binder presented an initial setting time of 11 h and a pH value of 12.7. In this mixture, the addition of 2.5 % weight boric acid produced a 24-h setting time. The pH value decreased to 12.4 and 12.3 with the incorporation of boric acid and sodium tetraborate, respectively. Finally, samples cured at 90 °C (the high-level waste disposal cell temperature under operating conditions) are studied. The high temperature leads to a decrease in the pH value down to a minimal value of 11.1. Analysis of the cured samples reveals a reorganization of the geopolymer network during the treatment. The formation of zeolite phases in the network, which can trap alkaline cations, is considered and can explain the pH value decrease. Ó 2019 Elsevier Ltd. All rights reserved. 1. Introduction Cigéo is the geological disposal facility project for radioactive waste managed by the French National Radioactive Waste Management Agency (Andra) for high-level and long-lived intermediate-level radioactive wastes. This disposal is planned to take place 500 m below ground level in a specific geological layer composed of the Callovo-Oxfordian (COx) argillites [1]. This study investigates the valorization of this argillite and presents the formulation of an alkali-activated grout based on COx argillite. Previous works have already studied COx argillite to elaborate on alkali-activated materials. First, Gharzouni et al. focused on the thermal treatment of raw argillite to increase the chemical reactiv- ity of these argillites [2]. This thermal treatment (up to 700 °C) con- tributes to the decomposition of the carbonate species available in https://doi.org/10.1016/j.conbuildmat.2019.117170 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: sylvie.rossignol@unilim.fr (S. Rossignol). Construction and Building Materials 232 (2020) 117170 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat