Properties And Hydration Mechanism Of Cement-based Materials Based On The Characteristics Of Waste Clay Brick

Waste clay brick is one of the main components of construction waste currently,and it is more difficult to recycle than waste concrete due to low strength and high water absorption.Therefore,exploring the new recycling methods based on the characteristics of clay brick is the core problem of efficient resource utilization.The solution of this problem is of great significance to improve the utilization of construction waste and reduce the carbon emission.In this study,On the basis of properties of clay brick aggregate,the recycled concrete was prepared with clay brick as coarse and fine aggregates,and a strength prediction model of recycled clay brick concrete based on mix parameters was established.The physical and chemical properties of clay brick powder(CBP)and the enhancement mechanism of pozzolanic activity by mechanical grinding were studied systematically.The hydration mechanism of ternary composite cementitious material(CCM)containing PC,CBP and slag was revealed by studying the early-age hydration kinetics and long-term hydration performances.Main research contents and results of this study are as follows:(1)The physical and micro structural properties of clay brick aggregate were studied,and a model for predicting strength of recycled concrete was established according to the influence law of mix parameters(water-binder ratio,aggregate-binder ratio,volume rate of sand and admixture replacement rate)on the compressive strength.The results indicate that the aluminosilicate phase in clay brick aggregate is loose and flake-like,its hardness is lower than the quartz phase.Those phases are separated by a large number of micro-scale pores,and the average porosity is 47.3%.The microstructural characteristics above lead to the low density,low strength and high water absorption in clay brick aggregate.The compressive strength development of recycled concrete will be limited by the low-strength clay brick aggregate,after reaching above 35MPa,the effect of improving strength by adjusting the mixing parameters is limited.The average absolute error of strength predication model established by BP neural network technology is 1.64MPa,and the average absolute value of relative error is 5.78%,indicating that this model can accurately reflect the nonlinear relationship between the compressive strength of recycled concrete and mix parameters.(2)The chemical components,phase compositions and particle characteristics of CBP were analyzed by means of multi-technical combination,and the pozzolanic activity of CBP and the effect of mechanical grinding on activity improvement were systematically studied.The results indicate that the volume fraction of quartz and aluminosilicate phases are almost the same.The quartz phase has a dense microstructure and the crystalline degree is much higher than aluminonsilicate phase.Mechanical grinding can not only greatly increase the specific surface area of CBP,make the particle shape tend to be spherical,but also destroy the original crystal structure,break and recombine chemical bonds,which can reduce the chemical binding energy of Si-O and Al-O.However,excessive grinding will also cause agglomeration and reduce the grinding efficiency.The activity index of CBP is about 0.71～0.77,and replacing 30%cement by CBP cause great loss to the early mechanical properties of mortar,but in the later age of hydration,the pozzolanic reaction of CBP will continue to consume CH,and the finer the CBP,the higher the consumption of CH.This reaction process can be observed by SEM+BSE,and the reaction product C-(A)-S-H gel grows in pores,optimizing the microstructure,so the mechanical properties of mortar mixed with CBP can increase continuously in the later stage.(3)The early-age hydration mechanism was studied by analyzing the hydration characteristics and kinetics parameters of CCM with different admixture replacement rate,CBP to slag ratio and temperature.The results indicate that increasing admixture replacement rate accelerates cement hydration and leads to the advance of exothermic peaks.However,the reduced C3S content in blended cement reduces the peak intensity of the second exothermic peak and reaction rates of NG-I-D(nucleation and crystal growth-phase boundary reaction-diffusion)processes.Slag promotes the nucleation and growth of C-S-H gel slightly more than CBP before the third exothermic peak.Then,the slag reaction in the deceleration period will greatly enhance the third exothermic peak and release more hydration heat.The increase of temperature will obviously accelerate the hydration heat release,resulting in a shortening of the induction period,and the second and third exothermic peaks are greatly advanced and gradually merged.High temperature provides nucleation driving force to accelerate the cement dissolution and ions migration,thus,the reaction rates of NG-I-D processes are increased as the temperature rises.(4)The macro-and micro-mechanical properties of CCM under long-term hydration,the types of reaction products of each component,the morphological characteristics and the evolution of microstructure were studied.The synergistic reaction mechanism of CBP and slag in the long-term hydration of CCM was discussed.The results indicate that the depolymerization of glass phase in slag consumes OH-with strong polarity,but the slag reaction can proceed spontaneously once starting,therefore,the demand for CH is greatly reduced.The rapid reaction of slag results in the retardation of ion diffusion,the inter C-A-S-H gel will be generated in situ.In the later stage of hydration,CBP can consume CH for continuous hydration,and the Si and Al generated by the depolymerization of CBP under the action OH-diffuse outward into the pore solution and combine with Ca to form a honeycomb C-A-S-H gel,which optimizes the pore structure.The synergistic effect of CBP and slag enables CCM to have a high degree of hydration in the early stage and continue to hydrate in the later stage.The elastic modulus values of the quartz and aluminosilicate phases in CBP are 113.12±14.58GPa and 69.46±11.15GPa respectively,which are much higher than the hydration products of CCM,while the hydration products of slag and PC show no significant difference in micromechanical properties,indicating that there is no obvious correlation between the micromechanical properties and the contents of Mg and Al in the hydration products.