Preparation And Performance Study Of Controlled Low Strength Materials For Temperature Control With Phase Change

With the rapid development of Chinese transportation,it becomes more and more important to establish modern integrated transportation system.At present,road and railway are the main transportation structure between cities.Water transportation has been paid more and more attention in recent years because of its advantages such as low cost,small pollution and large volume of traffic.In order to speed up the construction of land and water transportation and give full play to the advantages of waterway transportation,it is urgent to dredge the river and ensure the waterway capacity.However,a large amount of sediment generated by dredging engineering is usually abandoned and buried,and cannot be effectively used,which not only occupies land and wastes resources,but also seriously affects the stability of the surrounding ecological environment.Controllable low strength materials(CLSM)are a kind of self-leveling,self-compacting and has a certain strength of mobile backfill materials.As the raw materials of CLSM can be widely selected,it provides favorable conditions for the effective engineering application of dredged sediment.Therefore,in order to improve the utilization rate of dredged sediment and solve the problem of heat diffusion in the heating pipeline laying area affecting the surrounding soil and environment,this thesis adopted dredged sediment as the main fine aggregate,and adsorbed paraffin in rice husk ash to produce secondary fine aggregate with phase transition property.By adding cement and water to the fine aggregate,A kind of dredging sediment base CLSM with temperature control characteristics was formed.The effects of water-cement ratio and raw material content on the working and mechanical properties of CLSM were studied through laboratory experiments,so as to determine the optimal mix ratio of CLSM.The interaction mechanism between raw materials of CLSM was analyzed based on microscopic experiments.Meanwhile,a series of studies were carried out on the thermal and durability properties of CLSM under the optimal mix ratio.To explore the ability of CLSM to control warm water and long-term service.The main conclusions are as follows:(1)When the mass ratio of rice husk ash to paraffin is 2:5,the leakage rate of PCM is the lowest.The particle size of the composite PCM mainly ranges from 2mm to 5mm.The initial temperature of phase transformation is 26.96℃,the melting point is 56.03℃,and the latent heat of phase transformation is 116.05J/g.There is no chemical reaction between the organic phase and the matrix in the composite phase change material.(2)The water-cement ratio,cement content,and composite phase change material content all have certain influence on the fluidity and strength of CLSM.With the increase of water cement ratio,the fluidity of CLSM increases,but the strength decreases gradually.With the increase of cement content,the fluidity of CLSM decreased and the strength increased obviously.The fluidity of CLSM increased with the addition of PCM,but the strength of the specimen decreased gradually.(3)The optimal mix of phase change CLSM is 2.4 water cement ratio,20%cement mass content and 20%composite phase change material volume content.According to the laboratory test results,the fluidity of phase change CLSM at the optimal mix ratio is 205mm,and the unconfined compressive strength at 3,7 and 28 days is 0.65MPa,1.05MPa and 1.67MPa,respectively.(4)The thermal properties of CLSM showed that the initial temperature of phase transition was 46.15℃,the melting point was 50.65℃ and the latent heat of phase transition was 4.10J/g.The temperature curve of the material showed that phase change CLSM had good temperature control ability in the range of 25℃～60℃.The mass and volume of CLSM decreased gradually after the phase transition cycle,while the strength increased.(5)The mass and volume of phase change CLSM increased gradually after the dry and wet cycles.The strength of the specimen and the pH of the water-soluble supernatant of the broken test block increased first and then decreased with the increase of the number of cycles.The presence of chlorine salts in the soaking solution is one of the main reasons for the above changes.The physical adsorption and chemical binding of chlorine salts by hydration products result in the improvement of the quality,volume,strength and pH of the test pieces.With the increase of the number of cycles,the degree of water damage was aggravated,resulting in a decrease in the strength and pH of phase change CLSM.Friedel’s salt diffraction peak was added to the XRD pattern of CLSM after wetting and drying cycles,and the surface porosity showed a trend of decreasing first and then increasing with the increase of the number of wetting and drying cycles.