Freeze-thaw properties of silty sand modified by paraffin-based phase change materials and cinder
Objective In view of the current situation the lining structure of farmland water transmission channel is frequently damaged due to freeze-thaw cycles in areas with seasonal frozen soil.To solve the leakage problem of paraffin-based phase change materials(PCM)and meet the recycling demand of cinder in local thermal power plants,paraffin-based PCM and cinder are added into the channel soil as additives and the primary focus is on studying the mechanism of improving the freeze-thaw performance of soil.Additionally,the impact of the latent heat effect of paraffin-based PCM on temperature field within seasonally frozen soil is also analyzed.Methods The silty sand collected from a channel in Ningxia was used as the research object.Paraffin-based PCM and cinder were selected as the improver to prepare two distinct types of improved soil:paraffin-based PCM improved soil and paraffin-based PCM-cinder improved soil.Through the volume change rate test and unconfined compressive strength test under the freeze-thaw cycle,the volume change rate and unconfined compressive strength of silty sand and the improved soil were analyzed.Besides,an analysis of the microstructure and latent heat changes of the improved soil before and after the freeze-thaw cycle was also conducted.This comprehensive approach allowed for a thorough investigation into the combined effects of paraffin-based PCM and cinder on the freeze-thaw properties of the improved soil.Results and Discussion The freeze-thaw deformation of silty sand can be inhibited by PCM.However,the volume change of par-affin-based PCM improved soil lacks stability with the increase of the freeze-thaw cycles.When the mass fraction of paraffin-based PCM is 8％,the volume change rate of the two improved soils is the smallest.Cinder can reduce the leakage of paraffin-based PCM,and the volume change rate of paraffin-based PCM decreases with the increase of the content of paraffin-based PCM.In contrast the adsorption effect of cinder on the paraffin-based PCM gradually decreases with the increase of freeze-thaw cycles.The unconfined compressive strength of all soils decreases with the increase of freeze-thaw cycles,of which the decline rate is first fast and then slow.The unconfined compressive strength of PCM-cinder improved soil is greater than that of PCM improved soil under the same cycle times.After 9 freeze-thaw cycles,the unconfined compressive strength of PCM modified soil with a PCM mass fraction of 10％is the lowest.The effect of temperature fluctuations on soil can be alleviated by the absorption or release of latent heat of the two improved soil,and the freezing or melting transitions within the soil can be delayed.The forma-tion of the ice lens is delayed by the incorporation of cinder and PCM,and the particles of the improved soil are connected by the cementing material formed by PCM.This results in a dense and stable soil structure,significantly inhibiting the degree of expan-sion and contraction.Conclusion The integration of paraffin-based PCM and cinder into silty sand effectively and stably manages freeze-thaw deforma-tions in the soil.Hence,the freeze-thaw performance of soil is improved,contributing to a reduction in freezing damage to chan-nel lining structure in seasonal frozen soil area.Simultaneously,the utilization rate of cinder is increased as well.
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