Development of Low Alkaline Cement Considering Pozzolanic Reaction for Support System in HLW Repository Construction
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Development of Low Alkaline Cement Considering Pozzolanic Reaction for Support System in HLW Repository Construction Masashi NAKAYAMA1, Keishiro IRIYA2, Atsushi FUJISHIMA3, Morihiro MIHARA3, Koichiro HATANAKA1, Yuji KURIHARA2 and Mikazu YUI3 1 Horonobe Underground Research Center JAEA, Hokushin 432-2, Horonobe, Hokkaido 0983224, Japan 2 Civil Engineering Division, Obayashi Co., Tokyo, 204-8558 Japan 3 Geological Isolation Research and Development Department, JAEA, Tokai 319-1194 Japan ABSTRACT Cementitious materials have been studied for their applicability as support and lining in the construction of shafts and tunnels of the underground facilities for HLW disposal system. However, in the case of Ordinary Portland Cement (OPC), the buffer materials and the host rock may be altered due to the high alkalinity of groundwater contacting the OPC in the long-term. Low alkaline cements with high content of pozzolanic material were developed in order to reduce such hyper alkaline alteration. This paper shows that the pH of the leaching solution of this cement is about 11, and that it can be applied for full scale lining and for shotcrete. INTRODUCTION In 2001 in Horonobe, Hokkaido, Japan, JNC started the Horonobe Underground Research Laboratory (URL) project to understand the geological environment in the deep underground of sedimentary soft rock and to develop and apply disposal technologies. In the Horonobe URL project, the shafts of the underground facilities will extend to around 500 m from the ground surface with drifts for experiments being constructed at several elevations. Repositories of HLW and TRU waste, which contain long live radionuclides, will be constructed in a deep geological environment. The concept of the repository is to isolate the waste from the biosphere by a multi barrier system in which an engineered barrier is combined with the natural barrier of the host rock. Bentonite is the most effective candidate as the engineered barrier, because it self-seals by swelling and has superior colloid filtration function [1]. However, bentonite and the host rock may be altered by a hyper alkaline solution with a pH of 13, which is leached from cementitious materials as the tunnel support [2]. Since a leaching solution from OPC shows high alkalinity by dissolution of portlandite (Ca(OH)2), decrement of portlandite production in hardened cement is required in order to reduce the pH of the leaching water of the cementitious materials. JNC has been developing cement with a lower leaching solution pH from cement hydrate. In this paper, the type of cement is defined as low alkaline cement and the type of concrete using low alkaline cement is defined as low alkaline concrete. The leaching solution of cement hydrate is the solution that is equilibrated with cement hydrate, and it shows the same condition as the solution leached from concrete to groundwater in HLW repositories [3,4]. The low alkaline cement consists of OPC and pozzolanic materials such as fly ash (FA) and silica fume (SF) with the content of pozzolanic
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