Comparative Study On The Effect Of Different Nano-materials On The Properties Of Cement-based Materials

Since the 21st century,the rapid development of urbanization and civil engineering industry have caused cement output to reach new highs.The traditional cement mechanics and durability are insufficient,and the auto-shrinkage and cracking affect the appearance and lead to higher maintenance frequency and cost.Its modification has gradually become the focus of attention of the construction industry and many scholars.Cement modification is mainly through the incorporation of various modified materials,including the application of mature macro-level such as steel fibers,glass fibers and asbestos fibers,as well as micro-level new nano-materials such as nano-Ca CO₃,nano-graphene and plant nano-fibers.At present,the existing research are mainly focuses on nano-particle materials and carbon-based nanomaterials.There are few detailed studies on the influence of NCC on various properties of cement-based materials.To this end,based on the research of previous achievements,while using NCC to make cement-based composite materials,nano-SiO₂and CNTs were introduced.Through experimental research and theoretical analysis,the difference of the effect of three nano-materials on cement-based materials in mechanical properties,water absorption,thermal conductivity and the resistance of dry-wet and freeze-thaw cycles was systematically compared.At the same time,with the help of SEM scanning and XRD phase analysis,the reasons and mechanism of performance improvement are explained from the micro level.Finally,the strength-benefit ratio is introduced to comprehensively compare and analyze the economics of the strength improvement of the three nanomaterials.The main research contents and results are as follows:（1）The effect of three kinds of nanomaterials on the performance of cement-based materials and their comparative study.By testing the mechanical properties of the three nanomaterials at 7d and 28d,as well as the water absorption,thermal conductivity and durability at 28d,it was found that the incorporation of the three nanomaterials increased the peak strength of the composite material,and the strength varies with the dosage,which increase first and then decrease;only the lowest value of water absorption of the nano-SiO₂group is lower than that of the blank control group（N0）,and the lowest value of the CNTs and NCC groups is greater than that of the N0;The thermal conductivity of nano-SiO₂and NCC increases first and then decreases with the dosage of nanomaterials,while the thermal conductivity of the CNTs group always increases;the quality and strength loss rates of the three nano-cement-based composite materials first decrease and then increase with the dosage under dry-wet and freeze-thaw cycles,provin gthat the durability is improved,about 10 times wet-dry cycles and 20 times freeze-thaw cycles can promote the quality growth of cement-based materials,and about 20 dry-wet and freeze-thaw cycles are helpful to improve the strength of composite materials.（2）The microstructure of three kinds of nanomaterials and the exploration of the principle of improving the performance of cement-based materials.The experiment found that the three nanomaterials have different ways to improve the working performance of cement-based materials.Scanning electron microscopy results show that:nano-SiO₂mainly increases the material density by filling and acting as a nucleus to continuously hydrate the cement on its surface;CNTs and NCC mainly enhance the various work performance of material through the effects of bridging,lap joint and pull-out enhancement.However,too much content will cause the internal compactness of the test piece to decrease,which will lead to a decrease in working performance.The results of XRD phase analysis shows that the incorporation of the three nanomaterials reduces the peak and content of C₃S,increased the peak value of the hydration product AFt and decreased the orientation index of CH crystals,which promotes the shift of the hydration balance to the right and increases the chaotic growth ratio of CH crystals.（3）With the aid of the strength-benefit ratio,the economic efficiency of the three nanomaterials on the improvement of mechanical properties was compared,and it was found that the economic benefits of the three nanomaterials on the improvement of different mechanical properties were different.Among them,the compressive strength-benifit ratios of nano-SiO₂is the lowest at 7d and 28d,which are respectively 5.518 yuan/m³and 10.784 yuan/m³;and the flexural strength-benifit ratio of NCC is lowest,which are 5.800 yuan/m³and 4.673 yuan/m³at 7d and 28d,respectively.At the same time,the strength-benefit ratio of nano-SiO₂and CNTs at 7d is lower,while the strength-benefit ratio of NCC at 28d is lower,indicating that nano-SiO₂has a better economic efficiency for improving the early compressive strength,and NCC is more economical in increasing the later flexural strength.