Molecular Dynamics Simulation Of Thermal Conductivity Of Calcium Silicate Hydrate Gel

Nowadays,concrete has become an indispensable part of the development of modern civilization.The global per capita annual consumption of concrete is about 1 m³,making it the most used artificial stone.Its thermal conductivity has attracted much attention in the construction and use process.With the development of test technology,the performance characterization and improvement of concrete gradually extend to micro and nano scale,which can reveal the mechanism of action and multi-scale difference of performance at different scales.Calcium silicate hydrate（C-S-H）is the main cement hydration product used to characterize the micro and nano scale properties of concrete.There have been systematic studies on mechanical properties,but few studies on its thermal conductivity.In this paper,the thermal conductivity of layered ordered C-S-H gel molecular structure will be systematically studied based on molecular dynamics simulation method.The effects of calcium-silicon ratio and water content on the thermal conductivity of C-S-H gel will be studied by using equilibrium and non-equilibrium molecular dynamics respectively,and the difference of action mechanism and research methods will be analyzed.Firstly,11?tobermorite was randomly deleted from Si O₂ groups to construct C-S-H monocell models with 1.3,1.4,1.5,1.6 and 1.7 ratios of calcium to silicon.The C-S-H monocell models with five ratios of calcium to silicon were respectively subjected to GCMC water adsorption.The C-S-H monocell models of H₂O/Si=0,H₂O/Si=1.0 and H₂O/Si=1.65 were obtained by controlling the number of adsorbed water molecules.The equilibrium molecular dynamics method and non-equilibrium molecular dynamics method were used to calculate the C-S-H thermal conductivity with different Ca/Si ratio and different water content,respectively,and the simulation method was proved to be feasible.It was found that the thermal conductivity of C-S-H gel with different Ca/Si ratios of 1.3,1.4,1.5,1.6 and 1.7 in x,y and z directions was calculated by non-equilibrium molecular dynamics.The thermal conductivity and mechanical properties of C-S-H gel showed strong anisotropy,>>.The thermal conductivity in the y direction decreases with the increase of the Ca/Si ratio,while the thermal conductivity in the x and z directions has no obvious rule.The average thermal conductivity of C-S-H with water content of 0 is about 0.92W/m K,which is close to 0.96 W/m K of 11?tobermorite.The average thermal conductivity of C-S-H with water content of 1.0 tends to decrease with increasing Ca/Si ratio.The influence of water content on the thermal conductivity of C-S-H gel was investigated.It was found that the thermal conductivity of C-S-H gel increased with the increase of water content calculated by non-equilibrium molecular dynamics method.When the thermal conductivity of C-S-H gel with different Ca/Si ratios of 1.3,1.5 and 1.7 in x,y and z directions was calculated by equilibrium molecular dynamics,the order of>>was obtained,The average thermal conductivity of C-S-H is close to that of C-S-H.The influence of Ca/Si ratio and water content on thermal conductivity is discussed by comparing the fluctuation region of running thermal conductivity.With the increase of Ca/Si ratio,the thermal conductivity decreases,which is most obvious in y direction.By comparing the results of equilibrium and non-equilibrium molecular dynamics,it is found that the value of equilibrium calculation is generally larger.The difference may be due to the divergence and loss of some heat flow in the process of heat flow transfer in non-equilibrium calculation of thermal conductivity,resulting in a small overall calculation result.Because the calculation of thermal conductivity by equilibrium molecular dynamics method still needs to be solved,the simulation system is anisotropic material,need to be modified in specific conditions.By studying the atomic and phonon transport information of calcium silicate hydrate--phonon state density,it is found that the phonon state density curves of C-S-H with water content of 0 are basically the same,while the phonon state density curves of C-S-H with water content of 1 fluctuate greatly,but the trend is the same.The addition and increase of water molecules change the vibration mode of C-S-H phonons.The water content is from 0 to 1.0,and the water molecules adsorb into the C-S-H layer,increasing the heat transfer channel,and enhancing the matching degree between silicon chains,so as to strengthen the heat transfer of C-S-H and improve the thermal conductivity of C-S-H.When the water content is from 1.0 to1.65,the peak phonon state density of C-S-H increases significantly,and the peak phonon state density of Ca atom,Si atom and O atom also increases significantly.Therefore,the thermal conductivity of C-S-H increases with the increase of water content.