Electromagnetic Functional Aggregate Made By Magnetically Separated Fly Ash And Study On Properties Of Electromagnetic Mortar

The cement-based material represented by concrete is an important civil engineering material and has many advantages.In the past,it was mostly used as structural material.With the advancement of science and technology and the development of society,the requirements for cement-based materials in various engineering applications have been continuously improved.It is no longer satisfied to be used only as structural materials,but it is given certain functions to achieve structural and functional integration.At present,researches on multi-functional cement-based composites mostly incorporated the functional phases into cementitious materials such as cement,and the functional phase only exists in the hardened cement paste,which occupies a very limited volume and was separated by a relatively inertial aggregate.It cannot be evenly distributed throughout the system,which limited the performance of multi-functional cement-based composites.If it can give aggregates with corresponding functions to make it functional,it will greatly improve the performance of cement-based functional materials.The iron oxide content of fly ash produced in southwest China is higher than that of elsewhere,some iron oxide can be easily separated from the fly ash by using a magnetic separator,its unique electrical and magnetic properties can be used to prepared functional aggregates,which will increase the volume occupied by the functional phase in the cement-based material and improve the performance of the cement-based functional material.In this dissertation,starting from the properties of the iron oxide component in magnetically separated fly ash,the preparation and properties of electromagnetic functional aggregates were investigated for the first time,and their electrical and microwave absorption properties is especially concerned.The electromagnetic functional aggregate is used to prepare conductive mortar,thermoelectric mortar and microwave absorbing mortar,and the corresponding performance of the functional mortar is studied,which provides a certain theoretical reference for the optimization of the performance of cement-based functional materials and its practical application.By collecting and analyzing the chemical composition of fly ash from different regions which has exhibited good representatives of the nation,based on the electrical and magnetic properties of magnetite,the conductive aggregate was prepared in reducing atmosphere and the microwave absorbing aggregate was prepared in inert atmosphere.The effect of calcination atmosphere and calcination temperature on the apparent density,water absorption,porosity and electrical resistivity were studied.The optimal calcination conditions of conductive aggregate and microwave absorbing aggregate are a reducing atmosphere and a temperature of 1150°C and an inert atmosphere and a temperature of 1150°C.The conductive phase of conductive aggregate was iron,which formed a continuous electrical path and provided an extremely high conductivity.The microwave absorbing phase of microwave absorbing aggregate was magnetite,its microwave loss mechanisms was magnetic loss,and with the magnetite content increased,the magnetic loss increased,the temperature increased.The particle size of aggregates has a greater influence on the heating rate,the optimal particle size for heating was 2.36 mm.The conductive mortar prepared with conductive aggregate has excellent electrical conductivity and stable heating capability.The effect of age,carbon fiber content and conductive aggregate content on the electrical resistivity was investigated,the results indicated that the electrical resistivity increased rapidly with the increasing age when the conductivity of conductive mortar was low and this trend slowed after 28 days.But when the conductivity of conductive mortar was high,the age has less influence on the electrical resistivity.It also indicated that the optimal aggregate content was 30%~40%,electrical conductivity of that dropped sharply beyond this range,doped with very small amount of carbon fiber could improve the electrical conductivity of conductive mortar.The electrical resistivity of conductive mortar was much lower than that of ordinary mortar at the same carbon fiber content,but the carbon fiber is also indispensable for the conductivity.There is some amorphous substance in the conductive aggregates,which improved the strength of the conductive mortar.The conductive mortar prepared with conductive aggregate has excellent electrical conductivity and stable heating capability,which can reach above 90°C in 60 min at 8V and can maintain stability after heating cycle.The conductive mortar can gain better heating performance under AC power rather than DC power.The Seebeck effect of thermoelectric mortar made by calcined magnetically separated fly ash aggregate and the enhanced Seebeck effect of the thermoelectric mortar doped with metallic oxide powders were investigated.It results that the Seebeck coefficient and the electrical conductivity of thermoelectric mortar were quite high and the Seebeck coefficient was increased significantly by adding metallic oxides which is higher than that of thermoelectric mortar by one order of magnitude,and its electrical conductivity didn’t change a lot.Therefore,the thermoelectric mortar with high power factor was obtained.Some P-type and N-type thermoelectric mortars were selected and arranged in series electrically and in parallel thermally as thermoelectric generator,the open circuit voltage of this thermoelectric device is 1.97 V at 35°C temperature difference.What’s more,a self-heating and self-sensing smart system was designed and prepared with the conductive mortar and the thermoelectric mortar.At last,the microwave absorbing mortar was prepared by functional aggregate,the effect of aggregate content and other microwave absorber on the heating rate and deicing time was investigated.The results indicated that with the aggregate content increased,the surface temperature and microwave absorption capacity were increased,which leads to less deicing time.In addition,the deicing time was mainly affected by the ambient temperature,and the ice layer thickness had little effect on this.Doping with steel fiber can improve its microwave absorption capacity by improve its electrical loss but doping with carbon fiber increased its reflectivity which reduced its deicing capacity.According to its nonuniform surface temperature,graphite and graphene oxide were added to the microwave absorbing mortar to improve its heating uniformity.