Abstract: The solidification effect of contaminated soil degrades under wet-dry (W-D) cycles and acid rain. Acidic dry-wet cycle tests for Cr-contaminated soil solidified by alkali-activated granulated blast furnace slag (GGBS) are carried out. Toxic leaching test and accelerated leaching test are performed to study the leaching characteristic and mechanism. Scanning electron microscopy and energy spectrum analysis are used to investigate the microscopic mechanism. The long-term stability is evaluated through the apparent diffusion coefficient. The results show that a few W-D cycles at pH=7 will cause additional hydraulic reaction of GGBS and thus reduce the leaching concentration of total Cr and Cr(VI). Along with W-D cycles more AFt is generated. The expansion of AFt results in micro-fracture and thus more Cr leaching. In acidic W-D cycles, AFt dissolves first, releasing Cr immobilized by ion exchange. With the increasing acidity, C-S-H gels dissolve and more gypsum is generated, resulting in more micro-fractures. Consequently, the encapsulation effect weakens, resulting in more Cr leaching. However, the C-A-S-H gels remain stable. The slopes of the logarithmic curves of cumulative leached fraction versus time range from 0.373 to 0.675. The errors of fitting by a pure-diffusion analytical solution are mainly below 0.5%, indicating that diffusion is the dominant leaching mechanism. However, after 18 W-D cycles at pH=3, the effect of dissolution increases and the diffusion-dominated criteria are not satisfied. The mobility of Cr under neutral, weak acidic, and strong acidic W-D cycles is low, moderate, and high, respectively. It is necessary to take measures to reduce acid rain infiltration and W-D cycles when utilizing solidified soil. This research provides a reference for evaluating the long-term stability of solidified contaminated soil.

Key words: Cr-contaminated soil, solidification/stabilization, long-term stability, acidic dry-wet cycle, accelerated leaching test