Experimental Study On Mechanical-thermal Properties Of Light-weight Aggregate Concrete Mixed With Microcapsule Phase Change Materials And Theoretical Model Construction

Building energy consumption in China ranks first in energy consumption,among which building energy consumption of residential buildings accounts for 37% of the country’s total building energy consumption.The development of energy-saving building materials has become the mainstream trend of the current and future development of the building industry.Development of lightweight concrete wall mixed with phase change materials can not only reduce the self-weight of the building structure,but also improve the performance of thermal insulation and heat storage,which is of great significance to reduce building energy consumption.In order to get a better understanding of the basic properties of lightweight aggregate concrete mixing with microcapsule phase change materials(MPCM-LWAC),the optimization of the ratio,mechanical-thermal characteristics,pore structure characteristics and theoretical calculation model of MPCM-LWAC are studied by experimental and theoretical research,which lays a foundation for the application and popularization of MPCM-LWAC.The main research contents of this paper are as follows:(1)The mixture ratio of MPCM-LWAC with maximum energy storage is obtained.Firstly,the basic properties of major raw materials such as shale ceramsite,shale ceramsite,ordinary Portland cement and phase change microcapsule were investigated.Then,the influence of watercement ratio on collapse degree,dry density and compressive strength of lightweight aggregate concrete(LWAC)was studied.The quantitative relationship between water-cement ratio and compressive strength and dry density was obtained.Based on this quantitative relationship,the water-cement ratio of MPCM-LWAC to be prepared is determined.Finally,the adding methods of MPCM to LWAC,the best mixing process and mixture slump were determined by the experiment,which reveals the change rules of dry density,compressive strength and porosity of MPCM-LWAC with the content of MPCM.According to the above research,the upper limit of MPCM content of MPCM-LWAC with mechanical properties that meet the requirements and maximum energy storage is determined.(2)The influences of water cement ratio,curing age,phase state,MPCM content and phase change cycle number on the tensile strength,compressive strength and elastic modulus of MPCM-LWAC were revealed.The quantitative relationships between the MPCM content and the thermal conductivity,specific heat capacity and thermal diffusion of MPCM-LWAC were determined.The change process of enthalpy and specific heat capacity of MPCM-LWAC in phase change temperature region is studied during heating and cooling.The influences of number of phase transition cycles on enthalpy and specific heat capacity of MPCM-LWAC are investigated.(3)The pore structure characteristics of MPCM-LWAC are visualized and quantitatively studied.CT tests were carried out to obtain visual CT images reflecting the internal pore structure of MPCM-LWAC.Porosity,flattening,sphericity and roundness were extracted by coupling with CT images and digital image processing technology.By studying the distribution rules and statistical information characteristics of these four parameters,the influence of MPCM content on the pore structure characteristics of MPCM-LWAC was investigated.(4)A theoretical calculation model of thermal conductivity of MPCM-LWAC based on“two-step method” was established by considering porosity and pore shape characteristics.The existing theoretical calculation model of thermal conductivity of MPCM-LWAC simplifies the MPCM-LWAC as a two-phase composite composed of "phase change microcapsule and concrete matrix".However,this simplified model ignores the influence of pore structure on thermal conductivity of MPCM-LWAC.Based on this,the theoretical calculation of thermal conductivity of “pore-phase change microcapsule-concrete matrix” composite material considering pore characteristics is established in this paper.In addition,the shape characteristic parameters which can express the shape factor in the theoretical calculation model of thermal conductivity are determined.The effectiveness of the new theoretical calculation model is verified by comparing the theoretical and measured values of thermal conductivity.Finally,the theoretical calculation model of specific heat capacity of MPCM-LWAC is given by using proportion mixing method,and the theoretical calculation model of enthalpy of MPCM-LWAC is derived based on the binary mixing hypothesis and liquid phase ratio.(5)A thermodynamic parameter testing method considering both the phase change characteristics and structural characteristics of MPCM-LWAC was proposed.An integrated thermophysical parameter testing equipment was developed for the proposed method.The specific heat capacity and thermal conductivity of MPCM-LWAC in different temperature ranges are tested by using this new equipment.Meanwhile,the attenuation heat of LWAC with different MPCM contents were measured by the equipment.Based on the above research results,the change law of thermodynamic parameters of MPCM-LWAC considering both phase change characteristics and structural characteristics was revealed.(6)The influence of different temperature conditions on thermal response characteristics of MPCM-LWAC was revealed.Indoor model tests were carried out to study the influence of different outdoor ambient temperature,duration of outdoor ambient temperature and fluctuations of outdoor ambient temperature on the temperature and heat flux of MPCM-LWAC under heating and cooling conditions.The energy dissipation process of MPCM-LWAC in different outdoor temperature environment was firstly investigated,then the optimal use environment of MPCMLWAC was determined.An outdoor model test was carried out under real climate conditions(summer in Xi’an)to study the effect of MPCM-LWAC on indoor temperature in summer.