High-temperature Properties Of Cement-based Materials Incorporated With Graphene Oxide

Thermal damage and cracking of cement-based materials in a high temperature environment or fire will seriously affect the mechanical properties such as strength and stiffness,and even lead to the loss of their bearing capacity,which has great safety risks.It is necessary to find solutions that can effectively enhance the high-temperature properties of cement-based materials.With the development of nanotechnology,more attention has been paid to adding nanomaterials into cement-based materials to improve their high-temperature properties.Graphene oxide(GO)is tremendously attractive,and has become a reinforcing material with great application prospects for improving the properties of cement-based materials.Nowadays,it has realized the structural analysis and mechanism exploration of materials from the mesomicroscopic level through the advanced technical means.While,in addition to the high cost,these methods cannot meet the in-depth research at the nanoscale,whose data that experiments cannot obtain.Therefore,molecular simulation has become a powerful tool to explore the relationship between macroscopic properties and microstructure of materials,and can deeply explore the interaction between GO and hydration products under high temperatures.In conclusion,the effect of GO on the high temperature properties of cement-based materials and its mechanism were studied in the paper.Two key requirements need to be meet at the same time for GO-reinforced cement-based materials: one is the dispersion of GO in cement paste,and the other is the interaction between GO and hydration products.GO has a large number of oxygen-containing functional groups on the surface to increase the electrostatic repulsion between the layers,weaken the van der Waals force,and enhance the interaction with other media,which provides the possibility of ensuring good dispersion of GO in the matrix and strong bonding between GO and hydration products.Firstly,the real dispersion of GO in the cement paste is explored.Then the effect of GO on the mechanical properties of cement paste and mortar under high temperatures and the mechanism are analyzed.Finally,molecular dynamics is used to study the interaction between GO and hydration products.By combining experimental with simulation,multi-scale research can help to systematacially reveal the mechanism of GO on the high-temperature properties of cement-based materials.This provides theoretical guidance and experimental data support for the improvement of high-temperature properties of cement-based materials as well as the promotion and application of GO.The main works are as follows:(1)GO generates irreversible aggregates in the cement hydration environment.Ensuring the uniform and stable dispersion of GO in the cement-based material can make full use of the specific surface area of the nanomaterial and make it fully play a role in the cement-based material.GO was modified by two kinds of water reducing agents PC1 and PC2(PC-GO),then assisted ultrasonic dispersion for 20 min to analyze the dispersion of GO in cement-based materials.The results showed that the optimal mass ratios of GO to PC1,PC2 were 1:3.5 and1:4,which could realize the well dispersion in simulated pore solution.The UV-vis was used to characterize the effects of PC1 and PC2 on the dispersion and stability in simulated pore solution,and adsorption of GO on the solid particles’ surface in cement-based materials.The dispersion and stability of GO modified by PC2 are better than that of PC1 in simulated pore solution,and the combination between PC2 and GO are better.In real cement hydration environment,GO is not only well dispersed in pore solution,but also adsorbed on the solid particles’ surface.The probability of each form is the same.Combined with macro tests of micro slump and mechanical strength,the effectiveness of PC2 on GO dispersion is verified.(2)The effect of GO on the mechanical properties of cement paste at high temperature(105 ℃,200 ℃,300 ℃,450 ℃,600 ℃,and 750 ℃)was investigated.The effect of 5%CMK instead of the cement and the incorporation of GO on the high-temperature properties of cement paste was conducted.The effect of GO on the mortar and its interfacial transition zone after high temperature was analyzed.It is concluded that well-dispersed GO can improve the compressive strength of the cement paste,delay the strength loss of the cement paste after high temperatures,and improve the crack resistance and toughness of the cement paste at high temperatures.The compressive strength of CPKG4 with 5% CMK replacing part of cement is higher than that of CPG4,increased by nearly 30%.The incorporation of CMK and GO has a synergistic effect on the compressive strength of cement paste after high temperatures.GO can enhance the high-temperature properties of cement mortar.(3)According to TGA,FTIR,Raman,XRD,MIP,SEM,DIL and other characterization methods,it is found that the addition of GO slightly improves the hydration degree of cement.More high-density C-S-H with better thermal stability is formed,which be broken requires higher temperatures,improving the polymerization degree of C-S-H and inhibiting the disordered of the local C-S-H gels structure.It can also refine the size of CH crystal,decrease the thermal expansion coefficient of cement paste and act as a “bridge” between the matrix,thus improving the pore structure of cement matrix,decreasing the porosity,compacting the microstructure at high temperatures.These are beneficial to the improvement of mechanical properties of cement paste.The volcanic activity of CMK could consume the CH and form more strength-friendly C-S-H and C-A-S-H.The cross-linking of C-A-S-H makes hydration products have higher density and better thermal stability.The ITZ of mortar is significantly improved.(4)Based on the Reax FF molecular dynamics simulation,the structural,dynamic properties and the mechanical properties under uniaxial tensile tests of the GO/C-S-H and GO/C-A-S-H were studied at the temperture of 300 K,600 K,750 K,900 K,and 1050 K.GO carbon-based materials was found to be relatively stable under high temperatures.The bonding chance of atoms in the interface regions between GO and C-S-H or C-A-S-H was increased due to the presence of epoxide and hydroxyl functional groups on the GO surface.The stability of the whole structure directly depends on the formation of hydrogen and covalent bonds(Ca-O)between interfaces.The maximum stress,failure strain of tensile fracture,and the Young’s modulus of GO/C-S-H and GO/C-A-S-H decreased with temperature increased.However,GO/C-A-S-H showed higher Young’s modulus and better ductility.Due to the presence of Al atoms,there is a stable connection(Al-Oc/Al-Os)between the silica aluminate structure and the functional groups of GO,which improves the thermal stability of the structure.