Porosity and Ion Diffusivity of Latex-Modified Cement
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POROSITY AND ION DIFFUSIVITY OF LATEX-MODIFIED CEMENT Takashi Nishi*, Osamu Kuriyama*, Masami Matsuda*, Koichi Chino*, and Makoto Kikuchi** * Energy Research Laboratory, Hitachi Ltd., 1168 Moriyama, Hitachi, Ibaraki 316 Japan ** Hitachi Works, Hitachi Ltd., 3-1-1 Saiwaicho, Hitachi, Ibaraki 317 Japan ABSTRACT Latex-modified Portland cement, which would be expected to have low permeability and ion diffusivity, was studied for possible application as a solidification agent for radioactive wastes generated from nuclear power plants. In order to predict the leaching ratio of radionuclides from the cementitious waste forms, the effect of water and latex content in the fresh cement paste on total porosity and ion diffusivity of hardened paste was quantitatively estimated. Total porosity of hardened cement paste decreased with the reduction of water content in the fresh paste and it was also reduced by latex addition. This latter effect could be attributed to the latex emulsion forming a water-proof film and filling the capillary pores. Also Cs ions diffusivity, which is the ratio of the diffusion coefficient in pore water to that in bulk water, showed an exponential correlation with total porosity for both cement materials. An empirical equation, expressing ion diffusivity as a function of total porosity, was derived from the consideration that the water constrictivity in this porous medium could cause an increase of the apparent viscosity of pore water. These results suggested a possibility that the transport behavior of radionuclides through the cementitious matrix could be estimated from the mixing parameters of the original cement pastes. INTRODUCTION Cementitious materials have been developed as solidification agents for radioactive waste or artificial barriers for waste disposal sites. It is well known empirically that the permeability and ion diffusivity through the hardened cement are influenced by its porosity [1] [21, so studies have been carried out on the reduction of porosity using admixtures such as polymer or silica fume [31 [4]. However the relationship between pore structure of hardened cement and the diffusion behavior of aqueous ions in the pore water is so complex that the migration mechanism of radionuclides or the effect of admixtures have not necessarily been clarified. In this paper, we studied latex-modified cement which would be expected to have low permeability and low ion diffusivity. The influence of the water and latex content in fresh cement paste on total porosity of hardened paste was quantitatively estimated. Also diffusivity of cesium ions was derived experimentally as a function of total porosity. The relationship between total porosity and ion diffusivity was discussed using a diffusion model for ions through such a cementitious, porous medium.
Mat. Res. Soc. Symp. Proc. Vol. 176. @1990 Materials Research Society
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EXPERIMENTAL 1. Preparation of specimens Ordinary Portland cement, OPC (containing 64.7% of CaO and 21.5% of Si02) and a latex emulsion of styrene-butadiene rubber (t
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