Study On Design And Application Of Cementitious Radiative Cooling Coating

The energy of sunlight reaching the earth’s surface is about 1000 w/m²,most of which is located in the ultraviolet,visible and near-infrared regions（0.25-2.5μm）.Sun is the main source of heat for buildings.At the same time,the sky is transparent to the light lying in the"atmospheric window"area（8-13μm）.Radiative cooling coating involves spontaneously cooling a structure by reflecting solar and radiating heat through the atmospheric transparent window into the cold universe.As the most widely used engineering materials at present,the research on the radiative cooling performance of cementitious materials and the preparation of radiative cooling cementitious coatings are of great importance in improving the energy-saving efficiency of buildings and promoting the development of cementitious materials.This paper systematically explores the heat dissipation potential of cementitious materials.Firstly,the optical properties of cement pastes under sunlight with various water-and-cement ratios（w/c）and curing ages were studied.Then the effects of mineral admixtures such as slag,fly ash,silica fume and whiting agents such as Ca CO₃ and Ti O₂ on the optical properties of cementitious materials were compared in detail.Finally,by optimizing the composition and microstructure,a cementitious coating that can achieve both relatively high reflectance（82%）and emissivity（87%）was fabricated.Mechanical properties and durability of the coating were studied,as well as the thermal insulation performance test,which includes numerical simulation and infrared lamp heating experiment.Based on the above experimental research,the following conclusions can be drawn:（1）The abundant Si-O-Si,Si-OH,Ca-OH bonds in cementitious materials ensure the high emissivity in the mid-infrared region.The emissivity of cement pastes and cementitious composite materials in this paper are all higher than 85%.After hydration,the reflectance in the near-infrared region decreases significantly,which is mainly due to the vibration absorption of O-H in the hydration products.（2）The reflectance of white cement paste increases with the increase of water-cement ratio（w/c）and curing age.Curing age and w/c rarely changes the shape of the reflectance curves,but have a greater impact on intensity of the reflectance spectrum.When the slag content is low,the reflectance of cement paste can be improved.Calcium carbonates,whether added as admixture or formed by carbonation,significantly increase the reflectance in the visible region.At the same time,with the increase of Ti O₂ content,the increase of the reflectance of binary cement paste slowed down.（3）When the number of pores ranging from 250 to 700 nm increased,or 0.92μm Ti O₂ was mixed,the reflectance of the matrix was significantly improved.The main reason may be that these micron-scale interfaces provide the space where sunlight in the visible and near-infrared region could be strongly backscatter.（4）Through orthogonal experimental design,optimizing the composition and mixing ratio,a radiative cooling coating made of white cement,calcium carbonate,Ti O₂,quartz sand and HPMC is prepared,which can simultaneously meet the optical properties of reflective thermal insulation coatings and the mechanical requirements of the putty on the external wall of the building.（5）Through numerical calculations,it can be found that the net radiative cooling power of the material surface increases with the increase of the surface temperature,reflectance and emissivity values.Both temperature field simulation and indoor lighting simulation experiments have proved that the cementitious coating RC3-5 has good thermal insulation performance,showing the great potential of using cementitious composite materials in build energy-saving buildings.