基于准弹性中子散射谱分析水化硅酸钙(C-S-H)中受限水的动态

Analysis of the dynamics of water confined in hydrated calcium silica(C-S-H) based on the quasi-elastic neutron scattering spectra

研究水化硅酸钙(C-S-H)中受限水动态的一种重要手段是准弹性中子散射(quasi-elastic neutron scattering,QENS)实验.C-S-H样品的QENS谱数据可通过跳跃扩散和转动扩散模型进行分析拟合,进而导出C-S-H样品微纳孔中水动态的相关物理参数:不动水指数C、转动扩散系数Dr、均方位移<u^2 >、自扩散系数Dt及平均停留时间τ_0.本文对水与水泥质量比为30%的C-S-H样品,测量温度为230—280 K的QENS谱进行了分析,得到的拟合参数可定量描述C-S-H样品内不同尺度的微纳孔中受限水随温度变化的动态过程.转动扩散系数D_r随散射矢量Q的变化可知,Q值较大时,水分子的转动对QENS谱影响较大.均方位移<u^2>随Q值的不同而变化,其拟合值可区分C-S-H样品中的不动水、强受限水和受限水.在Q较小时,D_t和τ_0的拟合值随温度而变化,并分别在230和240 K突然增大,由此揭示温度为230—240 K时,C-S-H样品中受限水分子的动态特性发生了转变.

Cement is a widely used construction material in the world.Calcium-silicate-hydrate（C-S-H）is the main component of aged cement（concrete）.The quality and durability of concrete are strongly affected by the dynamics of water confined in it.Dynamics of the confined water can be studied experimentally by the quasi-elastic neutron scattering（QENS）.In this paper,the jump-diffusion and rotation-diffusion model（JRM）is used to fit the QENS spectra of C-S-H paste samples at different measured temperatures for the whole scattering vector Q-range from 0.3 to 1.7 A^-1）.Five important parameters are extracted to describe the dynamics of water confined in C-S-H samples：the index of immobile water C,the rotational diffusion constant Dr,the mean square displacement（MSD）u^2,the self-diffusion coefficient Dt,and the average residence time between jumps τ0.Here,all the C-S-H samples,each with a 30% weight ratio of water to cement,are measured at temperatures ranging from 230 to 280 K.The fitted parameters can quantitatively describe the dynamics of water confined on different scales of C-S-H gel pores.The results show that the QENS spectra are fitted very well not only for small Q（Q≤1 A^-1））but also for large Q（Q 〉1 A^-1））.The obtained rotational diffusion constant is Q dependent.Thus the contribution of the water molecule rotation to a QENS spectrum increases with the value of Q increasing.The mean square displacement（MSD）u^2 decreases with the increase of Q,which can be used to distinguish the confined water,ultra-confined water,and bound water contained in C-S-H samples.When Q is small,the fitted Dt and τ0 vary with the measured temperature.Dt has a maximum value at 230 K and τ0 has a peak at240 K.These indicate that the dynamics of the confined water shows different behaviors at 230 K and 240 K.There are crossover or critical phenomena for water confined in C-S-H sample at low temperature.