minerals Article Reaction and Alteration of Mudstone with Ordinary Portland Cement and Low Alkali Cement Pore Fluids Keith Bateman * , Yuki Amano, Mitsuru Kubota, Yuji Ohuchi and Yukio Tachi   Citation: Bateman, K.; Amano, Y.; Kubota, M.; Ohuchi, Y.; Tachi,Y. Reaction and Alteration of Mudstone with Ordinary Portland Cement and Low Alkali Cement Pore Fluids. Minerals 2021, 11, 588. https:// doi.org/10.3390/min11060588 Academic Editors: Ana María Fernández, Stephan Kaufhold, Markus Olin, Lian-Ge Zheng, Paul Wersin and James Wilson Received: 13 April 2021 Accepted: 26 May 2021 Published: 31 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Radionuclide Migration Research Group, Japan Atomic Energy Agency, Ibaraki 319-1194, Japan; amano.yuki@jaea.go.jp (Y.A.); kubota.mitsuru@jaea.go.jp (M.K.); ohuchi.yuji@jaea.go.jp (Y.O.); tachi.yukio@jaea.go.jp (Y.T.) * Correspondence: bateman.keith@jaea.go.jp Abstract: The construction of a repository for the geological disposal of radioactive waste will utilize cement-based materials. Following closure, resaturation will result in the development of a highly alkaline porewater. The alkaline fluid will migrate and react with host rock, producing a chemically disturbed zone (CDZ) around the repository. To understand how these conditions may evolve, a series of batch and flow experiments were conducted with Horonobe mudstone and fluids representative of the alkaline leachates expected from a cementitious repository. Both ordinary Portland cement (OPC) and low alkali cement (LAC) leachates were examined. The impact of the LAC leachates was more limited than the OPC leachates, with experiments using the LAC leachate showing the least reaction and lowest long-term pH of the different leachate types. The reaction was dominated by primary mineral dissolution, and in the case of OPC leachates, precipitation of secondary calcium-silicate-hydrate (C-S-H) phases. Flow experiments revealed that precipitation of the secondary phases was restricted to close to the initial contact zone of the fluids and mudstone. The experimental results demonstrate that a combination of both batch and flow-through experiments can provide the insights required for the understanding of the key geochemical interactions and the impact of transport. Keywords: radioactive waste; cement-clay interaction; OPC; LAC; alkaline leachate 1. Introduction The construction of a repository for geological disposal of radioactive waste will by necessity include the use of cementitious materials in a multiplicity of ways, such as fillers, liners, plugs, and seals [1–4]. Ordinary Portland cement (OPC)-based materials will be extensively used in the construction, and following closure, groundwater will saturate the repository and the use of OPC will result in the development of highly alkaline porewater (pH > 12.5), [5,6]. The alkaline fluid will migrate and react with the host rock to form a chemically disturbed zone (CDZ) around the repository [2]. A series of chemical gradients will develop over time and distance from the repository, disturbing the pH, redox, and fluid chemistry of the migrating fluid. It is important, particularly in the case of a radioactive waste repository, to understand the evolution of the CDZ in both time and space and subsequent impacts on the behavior and transport of any radionuclides in the CDZ. The extent of the CDZ, beyond the host rock-cement interface, will depend upon several factors with both physical and chemical properties, i.e., host rock mineralogy, poros- ity/permeability, fracture density, groundwater composition, flow rates, and chemical buffering capacity. Previous studies have shown that in the host rock, silicate minerals dissolve in the highly alkaline pore fluid [2,7,8], followed by the precipitation of secondary minerals (calcium-silicate–hydrate, calcium-aluminum-silicate-hydrate (C-S-H, C-A-S-H), calcite) and aluminosilicates (zeolites, feldspars, feldspathoids) [9]. Other authors [10–12] have reported the interactions between cement pore fluids and argillaceous rocks, which demonstrated the alteration of argillaceous rocks by the high pH fluids and the buffering of Minerals 2021, 11, 588. https://doi.org/10.3390/min11060588 https://www.mdpi.com/journal/minerals