Macro and Micro Observations on Mortar Alteration Mechanism Under the Various Solution Conditions
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MACRO AND MICRO OBSERVATIONS ON MORTAR ALTERATION MECHANISM UNDER THE VARIOUS SOLUTION CONDITIONS F
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A. FUJIWARA, S..1SHIRO and K. KATSUYAMA
T. TAKEMURA*,
K. SAKOGAICHI*,
S. YOKOMOTO
*Radioactive Waste Management Center, 2-8-10 Toranomon, Minato-ku,Tokyo, Japan **Mitsui Mining & Smelting Co.,2-1-1 Muromachi Nionbashi, Chuo-ku, Tokyo, Japan ***National Institute for Resources and Environment, Tukuba-shi. Ibaraki, Japan
ABSTRACT Accelerated aging tests have been conducted to evaluate the long-term durability of cementitious material against aggressive ions. In tests, cementitious specimens were immersed in the solutions containing concentrated aggressive ions at high temperature and it promoted diffusion of the ions in the specimen. This method would be suitable for the evaluation on the aging as the alteration of the specimen would be expected to resemble the natural behavior. This paper presents a classification of alteration mechanism in the immersion tests using MgCl 2 and Na2 SO4 solution. This classification was done by relating the changes of compressive strength to microscopic and mineralogical changes. INTRODUCTION Cementitious material is an important candidate for engineered barriers in
radioactive
waste
repositories,
especially,
for
low
and
intermediate
radioactive wastes. The required properties as engineered barriers are mechanical strength, low permeability, low diffusivity, high sorption capacity and property of controlling high pH [1]. Accelerated aging tests have been conducted to predict the long-term performance of these properties. In tests, cementitious specimens were immersed in the solutiyn containing concentrated aggressive ions, such as Mg +, Cl and SO4 at raised temperature[2],[3]. Under these conditions, acceleration of the alteration process could be obtained by high diffusion rate of the ions and the high reaction rate of the harmful products. This method, however, would be suitable for the evaluation on the aging as the alteration of the specimens would be expected to resemble the natural behavior. For example, a phase change of hydrated cement by curing at high temperature[2] should be considered since it might affect the similarity of the alteration mechanism. In this study, the changes of compressive strength of the mortar specimens under the various solution conditions were related to microscopic and mineralogical changes. Using these relations, a classification of the similar alteration mechanism was presented. EXPERIMENT Ordinary Portland cement, Toyoura standard sand and water were mixed at the ratio of 1 : 2 : 0.65 and casted into a size of 4cm x 4cm x 16cm. After curing for 28 days, each specimen was shaped into a prism of 4cm x 4cm x 12cm by cutting 2cm of the both ends. Prior to the immersion, the shaped specimens were coated all but one face using epoxy resin to restrict the ions to uniMat. Res. Soc. Symp. Proc. Vol. 353 ©1995 Materials Research Society
900
directional migration. In the immersion tests, three mortar specimens were placed in a vesse
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