Effect Of Recycled Fine Powder/Aggregate Modification On Cement-based Materials And Its Application

Recycling construction and demolition waste into recycled concrete powder(RFP)and aggregate(RA)for reuse in the construction industry saves the shortage of natural materials.It solves the current construction waste disposal problem,which has the prospect of green sustainability.Currently,there is some research on the application of RFP in concrete,but most of it is focused on the macroscopic properties of RFP on concrete.Relatively little work has been done on the microscopic aspects of RFP on cementitious materials,which is mainly focused on activating the potential activity of RFP.However,the depth of research is still shallow.Due to the natural quality defects of RA,which are not conducive to the application of RA in concrete materials,many scholars have conducted research on the modification of RA.However,it also faces the problems of a complicated process,high cost,and even secondary damage to RA.These problems limit the application of construction waste recycled materials in engineering practice.This study aims to conduct corresponding research and modification tests for the shortcomings and characteristics of the two materials,RFP and RA,respectively by RA.This paper investigated the effect of different replacement levels of recycled micronized powder on the performance of composite cement and the mechanism of action in terms of physical properties,mechanical properties,and hydration products.We also explored the modification test of RFP by chemical reagents.Moreover,the slurry reinforcement of recycled aggregate was carried out using recycled micronized cement paste.On this basis,the synergistic effect of carbonation and recycled micronized cement paste on the properties of recycled aggregate and the reinforcement mechanism were investigated.Finally,the recycled aggregate pervious concrete was designed with a reasonable ratio to achieve a specific permeability coefficient and mechanical properties.The effect of the reinforced recycled aggregate on its performance was further investigated.The main conclusions are as follows.(1)The right amount of recycled micronized powder(not more than 30%)will shorten the setting time of the cement and improve the flowability of the cement paste.The more the compound cement is mixed with recycled micronized powder,the less the total accumulated exotherm during the hydration process.Moreover,the induction period will be further lengthened with the gradually increasing content of the recycled micronized powder.The Krstulovic-Dabic model can simulate the hydration kinetics of composite cement well when the amount of recycled micronized powder does not exceed 70%.(2)The low-field NMR technique confirmed that the recycled micronized powder could fill the pores of large and medium pores in cement mortar.The results from nondestructive layered scans revealed that the mortar pore defects of cement containing recycled micronized powder were concentrated in the middle and showed a gradient distribution pattern of gradually decreasing pores from inside to outside.(3)Further,the microscopic pore structure in the mortar of composite cement containing recycled micronized powder was visualized and non-homogeneously characterized by the dynamic distribution law of pore water molecules,which confirmed the better filling effect of recycled micronized powder on the mortar of composite cement.(4)The activation effect of H on the mechanical properties of RFP cement aggregate is not obvious,and the amount of CH dosing over 2.5%will adversely affect the strength of the aggregate.The activation effect of CaSO4 on the strength of RFP cement aggregate is more obvious than that of CH.CaSO4 can promote the formation of C-S-H gel and calcium alumina crystals,which is beneficial to early strength development.The XRD patterns of the specimens when the two were combined showed that the peak intensity of the SiO2 peak decreased while the peak intensity of the silica-calcite was higher.(5)After the treatment of recycled micronized cement paste,the apparent density and crush value of the recycled aggregates were improved to some extent,but the water absorption rate increased significantly.The microscopic analysis shows that the recycled aggregate after the regenerated micronized cement paste synergistic carbonation treatment produces hydrated calcium silicate(C-S-H)and CaCO3 to fill the pores,microcracks,and interfacial transition zone(ITZ)of the recycled aggregate and also reinforces the loose particles on the surface of the recycled aggregate.It was shown that the pore reduction of the recycled aggregate was 25.1%,and the CaCO3 content increased from 11.1%(slurry wrap only)to 20.75%(synergistic effect).(6)Recycled aggregate pervious concrete,in the process of compression,is cement paste bond failure damage,while the tensile splitting process contains cement paste bond failure and recycled aggregate failure damage of two kinds.The peak strain of recycled aggregate pervious concrete is greater than that of ordinary concrete.The modified recycled aggregate makes the peak stress and peak strain of recycled aggregate concrete further increase in the process of compressive damage.In contrast,the plastic deformation in the tensile splitting process decreases,and the brittle damage increases.