[en] The dissolution behavior of montmorillonite under three different alkaline conditions, which is considered low-level radioactive waste disposal facility environment, was investigated using a flow-through dissolution experiments. For the measurement of dissolution rate under low-temperature condition, dissolution experiment (I) was carried out using NaOH-NaCl mixed solution in the pH range from 9 to 13.5 at 15degC. In this experiment, dissolution rate of montmorillonite was slightly increased with increase pH of reactive solution. A pH dependence of dissolution rate at 15degC was smaller than that of previous report at the temperature range from 30 to 70degC. This result showed that the pH dependence of dissolution rate of montmorillonite is depended upon reactive temperature as Sato et al. (2004) have previous suggested. For the understanding of dissolution behavior in the solution of Ca(OH)₂ that is major component of cement materials, dissolution experiments (II) was carried out using Ca(OH)₂-CaCl₂ mixed solution in the pH range from 9 to 12 at 50degC. The dissolution rate of montmorillonite in Ca(OH)₂-CaCl₂ mixed solution was increased with increase pH of reactive solution. The calculated dissolution rates from dissolved Si and dissolved Al were shown similar values, which is refracted congruent montmorillonite dissolution. The pH dependence of dissolution rate in Ca(OH)₂-CaCl₂ mixed solution was similar to previous that of dissolution rate in NaOH-NaCl mixed solution and KOH-KCl mixed solution. This result implies that the pH dependence of dissolution rate would be independent with the kind of alkaline solutions. In order to estimate the dissolution behavior of montmorillonite in the leaching solution of real cement materials, dissolution experiments (III) was carried out using fly ash mixed low heat Portland cement (LPC-FA) leaching solution at 50degC. The LPC-FA leaching solutions were prepared by equilibrating LPC-FA (<250 μm) with distilled water at solid:liquid ratio of 1:100 (solution 100) and 1:1000 (solution 1000). The dissolution rates of montmorillonite in each solution were slower than that in dissolution experiment (II). This result implies that the dissolution rate in leaching solution of LPC-FA was affected from saturation index of reactive solution because of leaching solution of LPC-FA precedently contained dissolved Si and Al. (author)