Optimization Design And Simulation Of The Hydration Process And Micro-structure Formation Of Blended Cement

The production of blended cement can reduce the usage of clinker and recycle a large amount of industrial waste residue.It is one of the important ways to achieve energy conservation and emission reduction in the cement industry and has been promoted by many governments and organizations.However,due to the low hydraulic activity of industrial waste residue,blended cements with industrial waste residue usually present low early age compressive strength,results in that the usage of industrial waste residue in blended cement and the application range of blended cement are limited.In order to increase the usage of the supplementary cementitious materials?SCMs?in cement industry,many attempts have been made to enhancement the activity of SCMs,and the performance of blened cement improved.While the research on the relationship between the fundamental properties of raw materials in cement-based materials and the micro-structure,macroscopic properties of hardened paste remains at the experimental and empirical stage,and the study about the relationship between raw material properties and the packing or connection of particles in the hardened paste,and the relationship between the structure and macroscopic properties of the hardened paste are relatively split,none unified theory is available for the adjusting of the micro-composition and structure of cement-based materials paste,let alone use for improving the hydration efficiency and durability of cement-based materials.Thanks to the development of computer technology,some numerical models were established to simulate the hydration and micro-structure development of cement paste,while most of them are used for simulating the hydration of Portland cement.The basic datas on the difference of hydration activity and interaction of cementitious materials in blended cements are limited,and because the size,gradation and constitutes of particles in existing models is pre-set,so it is not suitable for the study of optimization design of blended cement.The existing models are difficult to deal with the hydration of blended cements.Based on the hydration kinetics and hydration filling ability of the cementitious materials,a hydration numerical model of blended cement was established in this study.The hydration process,the change of the pore solution chemistry and the evolution of micro-structure of the blended cement were simulated with the model.Then the relationship between the contact area of particles and the mechanical properties of cement paste,the relationship between the connectivity of pores and the property of resistance to chloride diffusion of paste were studied.With the blended cement hydration model,an optimization design method of blended cement is proposed based on the relationship between the micro-structure parameters and the mechanical properties,permeability of the paste.The detailed works and conclusions of this paper are given as follows:The hydration filling ability and hydrates characteristics of cementitious materials such as Portland cement,slag and fly ash were studied.The results showed that the amount of calcium hydroxide produced during the hydration of cement used in this experiment was 30.71g/100g;the hydrates density was 2.02 g/cm³,and the volume expansion ratio of cement was 187.17%,the hydration rate of cement particles was 0.0426?m/h at 1 day.The consumption of calcium hydroxide during the hydration of slag was 7.15g/100g,the density of slag hydrates was 1.76g/cm³,the overall volume expansion ratio of slag was 194.93%,the net volume expansion ratio was 203.86%,and the average hydration rate was 9.07×10^-3?m/h at 1 day.The density of the fly ash hydrates was 2.01g/cm ³ when hydrated in water,the overall volume expansion ratio of fly ash was 132.01%,and the net volume expansion rate was 157.45%.The density of the fly ash hydrates was 1.84g/cm³ when hydrated in the simulated cement pore solution,and the volume expansion ratio was 144.25%and 170.21%respectively.The average consumption of calcium hydroxide during the hydration of fly ash was 65.24g/100g.The hydration rate of fly ash in water was 2×10^-3?m/h at 1 day,and that was 6.5×10^-3?m/h in simulated cement pore solution.Based on the hydration filling factor of the cementitious material particles and the interaction factor between the cementitious materials during the hydration process of the blended cement obtained in previous research,a hydration numerical model of the blended cement was established.The hydration and micro-structure formation process of the pure cement,binary and ternary blended cement were simulated.The hydration degree of the cementitious materials in paste,the pH of pore solution and porosity obtained from the model were basically consistent with the test results,and the rationality of the model was verified.A tensile simulation model of cement paste was established.The relationship between the tensile strength of the pure cement paste with different water-cement ratio and hydration age was consistent with that of the test results.The relationship between the micro-structure parameters,such as contact area per unit volume,porosity,gel-space ratio and the tensile strength of the blended cement paste were analyzed.Results showed that among those micro-structure parameters,the best correlation was between the contact area per unit volume of paste and the mechanical properties,followed by porosity and that of gel-space ratio was lowest.A steady-state diffusion model of chloride ions in cement paste was established.The rationality of the model was verified by the test results.The steady-state diffusion coefficients of chloride ions in pure cement,binary and ternary blended cement pastes were simulated.The relationship between the micro-structure parameters such as the connectivity coefficient of the pores and the steady-state diffusion coefficient of chloride ions in paste were revealed.The diffusion coefficient calculated from the min-porosity of triply connected pore was almost equal to the diffusion coefficient obtained from simulation.That was to say,the min-porosity of triply connected pore determined the steady-state diffusion coefficient of chloride ions of the paste.Based on the strength,environmental load and durability,the design principles of blended cement were proposed.The relationship between the structural factor that reflected the physical packing in the paste,the chemically active factor that reflected the hydration capacity of the cementitious particles and the parameters such as porosity of paste,surface area,hydration rate were discussed.The relationship between the micro-structure parameters?contact area per unit volume of paste,min-porosity of triply connected pore?and the initial trait parameters of the paste was analyzed by the hierarchical progression method;and then the distribution range of the optimal structural factor and active factor were obtained when the design goal of blended cement was based on the strength,resource energy consumption and durability.The contribution rate of the type and particle size distribution of cementitious materials to each microstructural parameters was experimentally studied.Results showed that the slag in the fine-grained region contributed the most to the reduction of the min-porosity of triply connected pore and increased the contact area per unit volume of paste,followed by the fine cement and the contribution of ground quartz sand was the smallest;and for the coarse-grained region,the cement has the largest contribution rate,followed by fly ash,the quartz sand contributes the least.When the contact area per unit volume of blended cement paste in age of 3d and 28d were greater than 0.033?m²/?m³ and 0.080?m²/?m³ respectively,the mortar strength meets the requirement of strength class 42.5,and the corresponding structure factor range of the paste was 1.1?m².0?m,the distribution of the activity factor ranged from 0.005 to 0.038.According to the design principles of blended cement,with the help of blended cement hydration model,the optimization design process and method of blended cement was proposed.