Study On The Hydration Kinetics Model Of Cement-slag Composite Cementitious Materials

Development of the reaction kinetic model is an important method to study the hydration of cement-based materials.Firstly,the hydration kinetic model of cement helps to clarify the specific cement hydration mechanisms,which is beneficial to understand and characterize the hydration behavior of cement.Secondly,some cement hydration kinetic model can predict the phase compositions of cement paste,which provides a basis for modeling the mechanical properties or durability of cement-based materials.At present,large amounts of supplementary cementitious materials?SCMs?have been used in the preparation of cement-based materials.However,the research on the hydration kinetics model of composite binders is limited,since the existing models are generally established based on the hydration of plain Portland cement.Hence it is very important to develop a new kinetic model to simultaneously describe the hydration of cement and the pozzolanic reaction of SCMs.In this research,by taking the composite binder containing ground granulated blast-furnace slag as the research object,the hydration kinetic model of composite binder is established.A dissolution-precipitation coupled kinetic model is established to describe the hydration of C₃S.In the new developed model,the ions concentrations are calculated by considering the release of ions from the dissolution process and the consumption of ions from the precipitation of products.Conversely,the ion concentration influences the saturation degree of C₃S and CSH,which definitely influences their dissolution and precipitation rate.The precipitation process of hydration product is modeled according to the similar thought of classical Boundary Nucleation and Growth model.But the growth rate of CSH in the new developed BNG model is modified by a reaction zone hypothesis.The new developed kinetic model of Portland cement ignored the initial dissolution process.The nucleation and growth of hydration products are described by the modified BNG model,and the followed diffusion controlled period is characterized by a modified Jander's equation.The influence of water/cement ratio on the value of kinetic parameters is analyzed and the relation equations are given.The apparent activation energy of various kinetic parameters is determined in this study.The influence of inert mineral admixtures on the hydration of cement is classified as the nucleation effect?additional nucleation site?,dilution effect?increasing the actual water/cement ratio?and accelerated dissolution effect?essentially increasing the nucleation and growth rate?.Based on the hydration kinetic model of Portland cement,these mechanisms of the effects of inert mineral admixtures are characterized by introducing some corresponded kinetic parameters.The value of these parameters in the cases of various content of quartz with different fineness is determined and the value-taking functions of these parameters are given in this study.Finally,the hydration kinetic model of composite cementitious materials is established.In the complete kinetic model of composite binders,the hydration of cement is still represented by the model of“cement&quartz”.The pozzolanic reaction of slag is supposed to be successively controlled by the nucleation and growth of pozzolanic reaction products and the diffusion of free water.These two reaction process are also characterized by the modified BNG model and Jander's equation,respectively.The species and content of pozzolanic reaction products are analyzed according to the chemical composition of slag and the reaction equation is determined.The pozzolanic reaction starting time,the nucleus density,the growth rate and the diffusion rate are determined by simulative fit of the complete model to the experiment data.