17:15 〜 18:45
[AGE29-P05] Establishment of mutual technology for ground improvement and insolubilization of heavy metals by mixing Vietnamese coal ash and mud
キーワード：石炭灰、泥土、重金属、セメンテーション
While most of coal ash and mud are effectively utilized in developed countries such as Japan, in developing countries such as Vietnam, coal ash and mud are landfilled instead of being effectively utilized. In the future, the expansion of thermal power plants and the shortage of remaining capacity of treatment plants are expected to increase the need for more effective utilization of coal ash and mud. It is known that coal ash contains silicate, which causes a pozzolanic reaction and improves the strength and durability of concrete. In this study, the cementation effect of coal ash is expected to be achieved by mixing coal ash from Vietnam with muddy soil containing heavy metals, and the objective is to establish a mutual technology for soil improvement by increasing strength and insolubilization of heavy metals.
The methods used to evaluate soil improvement and insolubilization are (1) compaction test, (2) cone index test, (3) uniaxial compression test, and (4) leaching test. In (1), compaction curves were obtained for coal ash mixing rates of 0, 5, and 10%. In (2), tests were conducted under the optimum water content ratio obtained from the compaction tests. In (3), the stress-strain curves were obtained for the combinations of 0, 5, and 10 wt% coal ash mixture and 30, 40, and 50% moisture content, respectively, and for the coal ash mixture at 0, 1, 7, 14, and 28 days of curing, respectively. In (4), pH, arsenic and lead concentrations were measured.
In (1), samples with a mixing ratio of 0wt% did not give optimum moisture content and maximum dry density. At a mixing ratio of 5 wt%, the optimum moisture content was about 40% and the maximum dry density was about 1.0-1.3 g/cm³. At a mixing ratio of 10 wt%, the optimum moisture content was about 50% and the maximum dry density ranged from 1.05 to 1.15 g/cm³. In (2), tests were conducted according to the optimum moisture content obtained from the compaction tests, with mixing rates of 0wt%, 40% and 50%, respectively. The results showed that the mixing ratio was at least 800 kN/m2 in all cases. In case (3), the compressive strength of the specimens with a moisture content of 30 and 40% was lower than that of the specimens with a mixing ratio of 0wt%, regardless of the coal ash mixing ratio and the curing period. At a moisture content of 50%, the maximum compressive strength at a mixing ratio of 0wt% was about 100 kN/m2, while the specimens at a mixing ratio of 5wt% (curing periods of 1 and 14 days) and 10wt% (14 and 28 days) developed a strength of 160 to 190 kN/m2. In case (4), the pH increased on average by about 0.4 and 0.2, respectively, after 28 days of curing of the samples with mixing rates of 5 and 10 wt% compared to that with 0 wt%. The arsenic concentration of the sample with 0wt% mixing ratio was about 0.017mg/L, which exceeded the arsenic release standard of 0.01mg/L specified in the Japanese Industrial Safety and Health Law No.18, while the arsenic concentration of the sample with coal ash was below the release standard in all cases.
The compaction, cone index, and uniaxial compression tests indicated that the strength of the specimens mixed and cured with coal ash was reduced. From the leaching tests, it was found that heavy metals such as arsenic and lead can be insolubilized and cementation occurs. From the above, it was found that it is possible to establish a mutual technology of soil improvement and insolubilization of heavy metals by mixing Vietnamese coal ash and muddy soil. In this study, the mixing of coal ash and mud with 30, 40, and 50% moisture content was applied, and the increase of strength by coal ash mixing was confirmed only in the case of 50% moisture content. From the above, it is considered that the technology can make the best use of the interaction effects by considering the high-water content condition, increasing the amount of coal ash, and the method of adding slurry, etc. in the future.
The methods used to evaluate soil improvement and insolubilization are (1) compaction test, (2) cone index test, (3) uniaxial compression test, and (4) leaching test. In (1), compaction curves were obtained for coal ash mixing rates of 0, 5, and 10%. In (2), tests were conducted under the optimum water content ratio obtained from the compaction tests. In (3), the stress-strain curves were obtained for the combinations of 0, 5, and 10 wt% coal ash mixture and 30, 40, and 50% moisture content, respectively, and for the coal ash mixture at 0, 1, 7, 14, and 28 days of curing, respectively. In (4), pH, arsenic and lead concentrations were measured.
In (1), samples with a mixing ratio of 0wt% did not give optimum moisture content and maximum dry density. At a mixing ratio of 5 wt%, the optimum moisture content was about 40% and the maximum dry density was about 1.0-1.3 g/cm³. At a mixing ratio of 10 wt%, the optimum moisture content was about 50% and the maximum dry density ranged from 1.05 to 1.15 g/cm³. In (2), tests were conducted according to the optimum moisture content obtained from the compaction tests, with mixing rates of 0wt%, 40% and 50%, respectively. The results showed that the mixing ratio was at least 800 kN/m2 in all cases. In case (3), the compressive strength of the specimens with a moisture content of 30 and 40% was lower than that of the specimens with a mixing ratio of 0wt%, regardless of the coal ash mixing ratio and the curing period. At a moisture content of 50%, the maximum compressive strength at a mixing ratio of 0wt% was about 100 kN/m2, while the specimens at a mixing ratio of 5wt% (curing periods of 1 and 14 days) and 10wt% (14 and 28 days) developed a strength of 160 to 190 kN/m2. In case (4), the pH increased on average by about 0.4 and 0.2, respectively, after 28 days of curing of the samples with mixing rates of 5 and 10 wt% compared to that with 0 wt%. The arsenic concentration of the sample with 0wt% mixing ratio was about 0.017mg/L, which exceeded the arsenic release standard of 0.01mg/L specified in the Japanese Industrial Safety and Health Law No.18, while the arsenic concentration of the sample with coal ash was below the release standard in all cases.
The compaction, cone index, and uniaxial compression tests indicated that the strength of the specimens mixed and cured with coal ash was reduced. From the leaching tests, it was found that heavy metals such as arsenic and lead can be insolubilized and cementation occurs. From the above, it was found that it is possible to establish a mutual technology of soil improvement and insolubilization of heavy metals by mixing Vietnamese coal ash and muddy soil. In this study, the mixing of coal ash and mud with 30, 40, and 50% moisture content was applied, and the increase of strength by coal ash mixing was confirmed only in the case of 50% moisture content. From the above, it is considered that the technology can make the best use of the interaction effects by considering the high-water content condition, increasing the amount of coal ash, and the method of adding slurry, etc. in the future.