Freeze-thaw characteristics and microscopic mechanism of coarse coal gasification slag improved loess filler
In the loess area of northwest China,the subgrade engineering is facing the problem of freeze-thaw diseases owing to the freezing-thawing cycles. A commonly prevention method is using the improved loess as subgrade filling,and now the approach of poor subgrade soil improved by the industrial waste has become a new trend of ground treatment. Considering a huge stock of coal gasification slag and their low utilization rate,and coal gasification slag needs to be immediately treated safely and reasonably. Thus,we proposed a new approach of using the loess improved by the coal gasification slag as subgrade filling in seasonal frozen ground regions,and our previous studies have shown that a certain amount of coal gasification coarse slag improved loess as sub-grade filler is suitable for mechanical properties. In this study,the freeze-thaw characteristics and microscopic mechanism of coal gasification coarse slag modified loess under freeze-thaw cycle conditions were explored through laboratory freeze-thaw cycle tests,electron microscope scanning and CT scanning tests,and the two groups of pure loess and single lime modified loess were tested as comparison groups. The experimental results show that:(1) After the first and second freezing-thawing cycles,the frost-heave ratios of the group of mixing 15％ coarse coal gasification slag were 0.02％ and 0.29％,and the thaw-settlement coefficients were 0 and 0.05％. The frost-heave rates of the groups of pure loess and with 4％ lime were 0.43％ and 0.63％,0.38％ and 0.42％,respectively. The thaw-settlement coefficients were 0.26％ and 0.22％,and 0 and 0.13％,respective-ly. However,after three times of freeze-thawing cycles,the frost-heaving ratio and the thaw-settlement coeffi-cient of the experimental group with only 15％ coarse coal gasification slag began to increase,and the average frost-heaving rate and the thaw-settlement coefficient during five freezing-thawing cycles were 0.38％ and 0.17％,respectively. It is close to or even greater than the values of the experimental groups of pure loess and mixing 4％ lime,that is,the average frost-heave rate and thaw-settlement coefficient in the first two freezing-thawing cycles decreased by 70.8％ and 89.6％,respectively,compared with the pure loess group,indicating that the group of mixing 15％ coarse coal gasification slag can significantly restrain the frost heave and thaw set-tlement of loess at the initial stage of the freezing-thawing cycle action,but with the increase of freezing-thawing cycles numbers,the effect of inhibiting freezing-thawing on loess weakened gradually. (2) The temperature gra-dient at each position of the sample with 15％ coarse coal gasification slag is the smallest,followed by that of the sample with coal gasification coarse slag and lime,and the thermal conductivities of the two groups of samples were the smallest under positive and negative temperature conditions,indicating that the two groups can signifi-cantly reduce the temperature fluctuation at each position of the sample,and will reduce the thermal conductivity of the sample. (3) After five freeze-thaw cycles,the water content of the sample with 15％ coarse coal gasifica-tion slag is the lowest at each height,indicating that the addition of coal gasification coarse slag is conducive to controlling the distribution of water content of the soil sample after the freezing-thawing cycle. (4) The freeze-thaw characteristics of loess can be significantly improved by adding an appropriate amount of coal gasification slag at the initial stage of the freeze-thaw cycle action,and the porosity of the sample before and after the freez-ing-thawing cycle can be reduced by 41.5％ and 47.8％,respectively,compared with that of pure loess,mak-ing the soil much denser. In general,the addition of coal gasification slag in loess can improve the service ability of loess subgrade fill in cold regions when the number of freeze-thaw cycles is less than 3,but its freeze-thaw characteristics under more freeze-thaw cycles need to be further studied. This paper puts forward some new thoughts on the stability of loess roadbed engineering and the utilization of coal gasification slag resources in sea-sonal frozen ground regions.
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