Formation and stability of U-phase in cementitious materials under sulphate attack

Author(s): Yogarajah Elakneswaran, Li Chuang, Tomohiro Kajio, Eiji Owaki, Yuka Morinaga, Toyo-haru Nawa
Paper category: Proceedings
Book title: Proceedings of the International Conference on Sustainable Materials Systems and Structures (SMSS2019) Durability, Monitoring and Repair of Structures
Editor(s): Ana Baričević, Marija Jelčić Rukavina, Domagoj Damjanović, Maurizio Guadagnini
ISBN: 978-2-35158-217-6
e-ISBN: 978-2-35158-218-3
Publisher: RILEM Publications SARL
Publication year: 2019
Pages: 39-46
Total Pages: 813
Language: English

Abstract: Cementitious materials have been considered as main component for low- and intermediate-level radioactive waste disposal facilities. The liquid wastes are partially evaporated and solidified with cementitious materials, which leads high concentration of salts such as sodium sulphate (higher than 25 wt %) in the solidified waste. A critical concern of highly concentrated sodium sulphate nuclear waste with co-hydrating of cementitious materials is the degradation of the materials by chemical interaction of sulphate with cement hydrates and crystallisation of sodium sulphate as well as the deterioration due to external sulphate attack. A very high concentration of sodium sulphate solution could induce to form U-phase [(CaO)4(Al2O3)0.9(SO3)1.1(Na2O)0.5:16H2O] and this may cause deterioration to the solidified material. However, the stability of formed U-phase in cementitious materials expose to service aqueous environments has not well understood. In this study, U-phase formation in hydrating white Portland cement (WPC) and slag-blended cement (42 % of cement replacement) and its degradation in water and sodium and magnesium sulphate solutions were examined. The experimental results show that U-phase coexists with ettringite in hydrating both WPC and its slag-blended case in 15% of Na2SO4. Furthermore, the hydration reaction and replacement of slag increase the U-phase formation. The WPC and slag-blended WPC were hydrated in 13% of Na2SO4 solution for 28 days before exposure to water and 1300 mmol/L of Na2SO4 and MgSO4 for one month. Solid phases in the specimens from the exposure surface to its core were quantified by XRD/Rietveld analysis. The type of exposure solution and the replacement of slag influence the dissolution of U-phase and consequent formation of ettringite. The diffusion of ions from pore solution to exposure solution destabilises the U-phase in cementitious materials and the degradation.

Online publication :2019
Publication type : full_text
Public price (Euros) : 00