Research On The Preparation And The Thermal Conductivity Of Carbon/cement-based Composites

The low thermal conductivity of cement-based materials determines that the heat energy generated by sunlight on the road surface cannot be dissipated and heat accumulation occurs,thus severely shortening the road life and threatening traffic safety.Therefore,there is an urgent need to develop high thermal conductivity cement-based composites that can transfer thermal energy to the road surroundings in a timely manner,and high thermal conductivity cement-based composites can also increase the comfort of road users,which is important for the future development of the transportation sector.Unfortunately,conventional cement-based composites have low thermal conductivity and are unable to meet the existing and future thermal conductivity requirements.In this regard,this paper starts with reducing the porosity of cement-based materials in order to reduce the internal thermal resistance of cement.Afterwards,high thermal conductivity cement-based composites were prepared by adding high thermal conductivity fillers such as asphalt-based carbon fiber,flake graphite and expanded graphite to the low porosity cement to achieve a significant improvement in the thermal performance of cement pavements.At the same time,the effect laws of reducing porosity and high thermal conductivity fillers on thermal conductivity were compared and analyzed by testing and characterizing thermal conductivity,porosity,mechanical properties,effect of heat transfer and microscopic morphology of high thermal conductivity cementbased composites.In addition,to study the durability of the above cement-based composites,all cement-based composites were subjected to freeze-thaw cycles.The main research contents and results are as follows:Pouring method was used to form cement and pitch-based carbon fiber cementbased composites.To address the problem of low thermal conductivity of the cement structure,the effect of porosity on the thermal conductivity of the cement was investigated,followed by the effect of pitch-based carbon fiber on the thermal conductivity with minimum porosity.The results showed that the lower the watercement ratio,the lower the porosity of the cement,the higher the corresponding thermal conductivity,the higher the flexural strength and the faster the heating of the composite surface,the higher the amount of pitch-based carbon fiber,the higher the thermal conductivity and the faster the heating of the composite surface.After 40 freeze-thaw cycles,the thermal conductivity,flexural strength and heated surface temperature after heating decreased less for cement with a water-cement ratio of 0.30,but the thermal conductivity,flexural strength and surface temperature after heating decreased more for pitch-based carbon fiber cement-based composites.The thermal conductivity reached0.384 W/(m·K)at a water-cement ratio of 0.30 and 0.418 W/(m·K)at a pitch-based carbon fiber doping of 1.0wt%.The reduction in the water-cement ratio reduced the amount of water in the cement that was not involved in the hydration reaction and therefore the pores left by the evaporation of this water were effectively reduced,thus reducing the thermal resistance due to the pores.In addition,the pitch-based carbon fibers built thermally conductive channels within the cement,thus further improving the thermal conductivity of the cement-based composites.To address the problem of limited improvement in thermal conductivity of pitchbased carbon fibers,a dry pressing method was used to form scaled graphite cementbased composites to study the effect of flake graphite on the thermal conductivity of cement-based composites.The results showed that the larger the flake graphite content,the lower the porosity,the higher the thermal conductivity,the higher the flexural strength,the faster the heating of the composite surface.After 40 freeze-thaw cycles,the thermal conductivity of the cement specimens containing flake graphite decreased more,the flexural strength decreased less and the surface temperature decreased more after heating.The thermal conductivity reached 4.255 W/(m·K)when the flake graphite was mixed at 20 wt%.Flake graphite had high strength and high thermal conductivity properties and could build efficient thermal conductivity channels in cement,but its volume change was different from that of cement due to low coefficient of thermal expansion thus creating pores and making mechanical properties degraded.In order to improve the thermal conductivity more significantly,the expanded graphite cement-based composites were formed by dry pressing method to study the effect of expanded graphite on the thermal conductivity of cement-based composites.The results showed that the higher the content of expanded graphite,the higher the porosity and flexural strength,the higher the corresponding thermal conductivity and the faster the heating of the composite surface.After 40 freeze-thaw cycles,the thermal conductivity and flexural strength of the expanded graphite cement-based composite decreased less,but the surface temperature dropped more after heating.The thermal conductivity reached 5.877 W/(m·K)when the expanded graphite was dosed at 20 wt%.This was because expanded graphite had both high thermal conductivity and high specific surface area,making it easy to form inter-lap thermal conductivity networks in the cement matrix,which ultimately was a significant increase in the thermal conductivity of the cement-based composites.