Research On Performance Improvement Technology Of Recycled Brick-Concrete Powder

Recycled brick-concrete powder(RBCP)has pozzolanic activity due to the presence of SiO2,Al2O3,CaO and potential hydraulicity due to the presence of unhydrated cement.In recent years,the use of mineral admixtures such as fly ash has increased year by year,and the price has also increased.If RBCP can be used as a supplementary cementitious material,it will not only help the recycling of construction waste,but also ease the tight situation of supplementary cementitious material resources in China.At the same time,the preparation of recycled products can also bring significant social and economic benefits,and is an effective way for sustainable development strategies.However,the physical and chemical properties of the RBCP are not clearly enough.The potential activity has not been effectively activated,and the effect of RBCP on the workability and mechanical properties of cementitious materials has not been supported by sufficient theory,which greatly limits the research and application of RBCP.Therefore,based on the in-depth analysis of the physical and chemical properties of RBCP,the effects of replacement ratio of RBCP on the workability and mechanical properties of cementitious materials were explored.The effect and mechanism of physical,chemical and nano modification methods on RBCP were also analyzed;At the same time,in the presence of fly ash,the superposition effect of modified RBCP and fly ash and its influence on the properties and microstructure of cementitious materials were studied.Finally,a kinetics model of cement hydration under the action of RBCP was established to reveal the effect of RBCP on the hydration process of cementitious materials.The main research contents and conclusions are as follows:1.Chemical composition analyzer,laser particle size analyzer,X-ray diffractometer and scanning electron microscope are used to determine the chemical composition,particle size distribution,phase composition and micro-morphology of RBCP.The results indicated that the RBCP had potential pozzolanic activity due to its chemical composition similar to that of fly ash.At the same time,the RBCP existed as coarse particles,uneven size distribution and irregular particle edges.Activation medicated the particle sizes and distribution,and optimized the shape and surface cracks.Although the addition of modified RBCP will reduce the fluidity of cement paste,but also make its workability more similar to cement paste.2.Mechanical modification,chemical modification and nano-modification are used to activate the activity of RBCP,and the effect of modified RBCP on the workability and mechanical properties of cementitious materials is studied.The hydration process of cementitious materials was quantitatively analyzed by thermal analysis and qualitative analyzed by X-ray diffraction and scanning electron microscopy.The results shown that the addition of modified RBCP can improve the compressive strength and hydration of cement paste.Among them,chemical modification has the best effect on improving compressive strength,followed by physical modification and nano modification.The addition of modified RBCP to the cement paste was helpful to generate the hydration products and made the microstructure denser.3.The effect of modified RBCP on the effect of cementitious materials under the fly ash was evaluated.The cement paste was prepared by changing the replacement ratio of RBCP and fly ash.The optimal compatibility ratio was determined by the compressive strength of cement paste under 28d,and auxiliary analysis was performed by means of scanning electron microscope,X-ray diffraction and thermal analysis.According to the data fitting of the compressive strength of cement paste,and considering the convenience of engineering,it was finally determined that the optimal amount of RBCP is 10%,that is,the compatibility ratio of modified RBCP and fly ash is 1:1.In this case,the chemically modified RBCP also exhibited a good activation effect.The orientation index of CH decreased significantly,and the hydration degree gradually decreased with the increase of the replacement ratio.When the replacement ratio was 10%,the hydration degree decreased slightly.When it reached to 30%,the hydration degree was still in an acceptable range,but when it reached to 50%,the hydration degree decreased sharply.Therefore,it was considered that 30%was a reasonable replacement ratio,and the chemically modified RBCP had the best effect.4.Based on the hydration heat test of RBCP and modified RBCP on cementitious materials,a kinetic model of cement hydration under the RBCP was established and the hydration mechanism was explained.The effect of different modified RBCP on the hydration process of cementitious materials was analyzed by the K-D hydration kinetics model.It was found that the incorporation of RBCP would shorten the crystal nucleation and growth(NG)stage,and the hydration degree at this stage was also decreased.Correspondingly,the earlier the hydration entered the phase boundary reaction(I)stage,the longer the time of this stage.The time for the hydration to enter the diffusion(D)stage was also improved.At the same time,the mechanical modification and chemical modification RBCP had a shorter residence time in the NG phase than original RBCP,and they had been extended in the I phase.This was also consistent with the results of previous compressive strength and micro analysis.