Preparation And Properties Of Nano-Silica-Steel Fiber Reinforced MPCM Concrete

The main problems of phase change concrete with structural and functional integration are that phase change material will lead to significant strength loss of concrete and decrease of modulus in different degrees.These problems greatly limit the application of phase change concrete structure.This research relies on the key RESEARCH and development project of Hunan Province"Research and Application of The Key Technology of Passive Adaptation of Green Building Materials and Building Energy Conservation integration"（2018WK2111）.MPCM is mixed into concrete using the encapsulation form of microcapsule encapsulated phase change material（MPCM）,which is more suitable for concrete system.Meanwhile,Nano-Si O₂and steel fiber are used to improve the mechanical properties of phase change concrete and broaden its application field.Firstly,to explore the influence trend and degree of various factors on the properties of composite materials,the orthogonal experiment was designed with nano-Si O₂,MPCM and steel fiber as variables.The preparation part of the specimen needs to be pre-adapted for each experimental group by adjusting the dosage of water-reducing agent,to ensure the workability of the mixture in each experimental group and make the specimens of each group can be formed smoothly.The shrinkage,compressive strength,flexural strength,and elastic modulus of concrete were tested respectively,and the influence of each factor on concrete performance was obtained through orthogonal analysis.Secondly,the thermal properties of concrete are studied through thermal analysis.Finally,the influence of various factors on concrete performance is explained from micro level through micro analysis.The results are as follows:（1）The addition of MPCM will reduce the strength of concrete to a certain extent.The compressive strength of concrete with 3%MPCM is about 5%lower than that of the group with 1%MPCM.The addition of MPCM has no significant effect on the shrinkage of concrete,but its addition can reduce the elastic modulus of concrete.The bending-toughness ratio and bending-toughness index decreased slightly with the addition of MPCM,but the overall range was small.The addition of steel fiber significantly improves the compressive and flexural properties of concrete and plays a positive role in restraining the shrinkage of concrete.The standard compressive strength and initial flexural cracking strength of concrete with 0.8%steel fiber volume content can reach 65.68NPa and6.41MPa,respectively increased by 48%and 37%compared with the control group.Both the bending toughness ratio and bending toughness index increased with the addition of steel fiber.The addition of steel fiber has little effect on the improvement of elastic modulus.The addition of nano-Si O₂can significantly improve the compressive performance and elastic modulus of concrete.Among them,the concrete strength and elastic modulus of nano-Si O₂blended with 3%cementation material reach 64.15MPa and3.31×10⁴MPa,respectively increased by 44%and 7%compared with blank group.But for concrete,shrinkage effect is not obvious.In the aspect of bending resistance,its dosage has positive effect on the bending toughness ratio but has no obvious effect on the bending toughness index.The range analysis shows that the flexural properties of the composites include initial cracking strength,bending toughness ratio(I_5,I₁₀,I₂₀)and bending toughness index K,which are mainly affected by the addition of steel fiber,and the orthogonal range is much larger than the other two factors.The elastic modulus of composites is mainly affected by nano-Si O₂content.（2）DSC was used to characterize the thermal properties of phase change materials and composites.The results showed that although the MPCM material was lost during the preparation process and the DSC curve showed a decrease in the peak value of heat absorption and release,it was still proved that MPCM could be successfully preserved in the composites and retained part of the thermal properties.The endothermic latent heat value and exothermic latent heat value of the composite are 15.9J/g and 6.6 J/g respectively.（3）SEM microscopic analysis shows that cracks around MPCM develop through MPCM,indicating that the addition of MPCM will accelerate the development of cracks,which explains the reason why the addition of MPCM leads to the decline of mechanical properties of concrete from the microscopic level.After the addition of nano-Si O₂,the products with chain structure were found to increase by SEM,which was proved to be C-S-H gel combined with EDS and related literature.In addition,more dense structures can be observed under high concentration of nano-Si O₂,which is the main reason why nano-Si O₂enhances the compressive strength and elastic modulus of concrete.Finally,it is observed by electron microscope that steel fiber can connect concrete and prevent the development of cracks,which effectively improves the crack resistance of concrete.