Design And Performance Improvement Of Thermal Storage Cement-based Material Incorporated With SiO₂ Nano/Micro-encapsulated Pcm

The promotion of passive ultra-low-energy green buildings is a crucial strategy for the green,ecological,and sustainable development of China's construction industry.The thermal storage cement-based material(TSCM)is one of the most potential functional materials for the passive ultra-low-energy buildings.Phase change material(PCM),as the core component of TSCM,has given cement-based materials outstanding functions such as recycling clean energy and regulating the building micro-environment.Therefore,TSCM research has become an international frontier in the field of functional cement-based material.Numerous literatures have evaluated the mechanism and performance of TSCM based on microencapsulated phase-change materials with organic shells,and found that the incoordination between the organic capsules and the cement matrix led to various defects such as delayed hydration of TSCM,loose structure,and strength degradation.Inorganic SiO₂ microencapsulated PCM(SiO₂-MPCM)has potential in overcoming the defects caused by organic capsules.However,the usage of SiO₂-MPCM in TSCM has never been implemented and studied.Hence,this paper focuses on the design and property enhancement of TSCM incorporated with SiO₂nano-/micro-encapsulated PCM based on the National Natural Science Foundation of China(51572207).The effects of nano-SiO₂-MPCM(Nano-MPCM)and micro-SiO₂-MPCM(Micro-MPCM)on TSCM hydration,microstructure evolution,performance,etc.were intensively researched to establish an effective method for the improvement of TSCM with SiO₂-MPCM.The main contents and results are as follows:1.Nano-MPCM and Micro-MPCM were prepared by interfacial polymerization method with n-octadecane as the core and SiO₂ as the shell,and their composition,morphology,and thermal properties were evaluated.The effects of the amount of emulsifier,p H,and core-shell ratio on the assembly dynamics of SiO₂-MPCM shell were determined,and the mechanism of preparation parameters in the morphology and thermodynamic properties of SiO₂-MPCM was explored.Two multiple regression models and response surfaces related on the morphology and enthalpy of Micro-MPCM were established to obtain the optimal preparation parameters and clarify the interaction of the three factors.The performance of micro-/nano-SiO₂-MPCM was systematically evaluated.The above studies provide support for the application of SiO₂-MPCM in thermal storage cement-based materials.2.The effects of SiO₂-MPCM on the rheology and hydration reaction kinetics of TSCM slurry have been studied systematically,and the mechanism of the hydration reaction of TSCM was clarified.Through the study of fluidity and rheological behavior over time,the effect of SiO₂-MPCM on the rheological parameters was simulated and calculated.The correspondence between the changes in the yield stress and plastic viscosity of TSCM slurry and the evolution of the internal microstructure was proved.The mechanisms of SiO₂-MPCM heterogeneous nucleation,reduction of activation energy of hydration reaction and acceleration of cement hydration were clarified using hydration heat,non-contact resistivity and chemical shrinkage.Nano-MPCM TSCM gave higher hydration heat release rate,acceleration index and chemical shrinkage than Micro-MPCM paste.However,the phase transition absorbed the hydration heat and masked the hydration acceleration effect of Micro-MPCM.The resistivity results revealed that Nano-MPCM and Micro-MPCM dominated the pore solution and microstructure development of TSCM respectively by accelerating ion dissolution and increasing pore bypass induced by the filling effect.3.The performance of TSCM based on micro/nano SiO₂-MPCM was studied through a series of experiments such as strength,deformation effects,static/dynamic heat transfer,and temperature control in a mini-room.Based on the structure and distribution characteristics of hydration products of TSCM,the effects of SiO₂-MPCM on TSCM microstructures were clarified,and the inherent causes of TSCM performance were revealed.SiO₂-MPCM increased the degree of hydration of the cement matrix and C-S-H polymerization,and significantly reduces the pore volume and pore size of TSCM,thereby promoting the development of TSCM mechanical properties.The highest strength was obtained at a replacement of 10%and 5%for Nano-MPCM and Micro-MPCM,respectively;however,smaller pore size and higher hydration products leaded to enhanced capillary pressure and dry shrinkage for TSCM.Nevertheless,Micro-MPCM only increased the early dry shrinkage,but decreased the dry shrinkage below that of the blank after 7 d.SiO₂-MPCM was embedded in the pores or hydration product of TSCM with compact interface combination,which promoted the effective transmission of heat and enabled TSCM to have a high resistance to thermal deformation.15%of Nano-MPCM and Micro-MPCM reduced the thermal deformation of TSCM by about 75%and 50%,respectively.TSCM exhibited significant temperature control and time lag effects,which can maintain a relatively stable indoor thermal environment and reduce temperature fluctuations in the mini-room.15%of Nano-MPCM reduced the peak temperature of the inner wall and indoor center of the mini-room by 3.9°C and 2.2°C;and 15%of Micro-MPCM reduced those temperatures by 5.5°C and 2.9°C.4.Aiming at the synergistic improvement of the workability,mechanical and thermal properties of TSCM,a rigid framework microspheres made of porous?-Al₂O₃ ceramic(APNC)was prepared to load PCM with different phase changing temperatures and used as functional fine aggregates to broaden the temperature control range of TSCM.Further,carbon nanotubes(CNTs)were also employed to build internal bridges to enhance heat transfer,forming a general multi-stage phase transition,heat storage,and effective heat transfer TSCM(HTSCM).In the range of-5?40°C,the HTSCM test room showed an obvious multi-stage temperature control range.The peak temperature of the HTSCM wall and the indoor center was decreased by 9°C and 6.3°C,respectively.Under the season simulation test,HTSCM exhibited good peak-cutting and peak-shifting effects,which facilitates various application environments and multi-stage heat storage.The research results of this paper lay a theoretical foundation for the design and evaluation of high performance TSCM,provide key technical support for the application of cement-based thermal storage materials in passive ultra-low-energy green buildings and structural durability,and are of great significance to promote the innovative development of passive ultra-low energy green buildings.