Mechanical Properties And Microscopic Characterization Of Polypropylene Fiber And Road Waste Aggregate-Modified Cement Mortar

In recent years,the path of green sustainable development has become a major issue of common concern to the world,and environmental protection and economic benefits have been fully valued.The resource reuse of construction waste is highly in line with the needs of green development and has received widespread attention.The recycled aggregate prepared from construction waste is highly in line with China’s "green development" and "carbon-free city" concepts,and has good application prospects.As one of the branches of cement-based materials,cement mortar is an indispensable part of the construction process.The contradiction between the demand for sand and gravel of cement mortar and the shortage of natural sand and gravel is becoming increasingly prominent,so it is of great practical significance to explore the practicality of different recycled aggregates.However,the preparation process of recycled aggregate has gone through crushing,screening and other processes,and the surface damage is serious,the crushing value is high,and the water absorption rate is large,which brings great challenges to its use in practical engineering.As a fiber material with good tensile performance and high economic benefits,polypropylene fiber can be used in cement-based materials to increase strength,ductility,etc.,so polypropylene fiber is used as a fiber material to make up for the defects of recycled aggregates.A large amount of waste generated during the road demolition process can be crushed and screened into recycled aggregates.In this paper,the recycled aggregate was substituted for river sand in the traditional cement mortar,and on this basis,polypropylene fiber was mixed with recycled aggregate mortar specimens(referred to as "RAM")and fiber recycled aggregate mortar samples(referred to as "RAFM")with M5 as the design strength.Among them,the replacement rate of recycled aggregate was 0,20%,30%,40%,50%,60%,80% and 100%,respectively.The length of the fiber is set to 3mm,6mm,9mm,12 mm,and the dosage is set to0.3%,0.6%,0.9%.Through particle screening test,compressive test and flexural test,the feasibility of applying recycled aggregates for road demolition in cement mortar and the modification effect of fiber were explored,and the optimal ratio of RAM and RAFM was obtained,and the mechanical prediction model was obtained by fitting the surface.After the macroscopic mechanical test,SEM microscopic photography was performed on the specimen,and the interface transition zone inside the specimen was studied,and the microstructure inside the specimen was characterized.The main research results of this paper are as follows:(1)The analysis of the particle screening test results shows that the addition of recycled aggregate can improve the grading interval of the granules.When RSA < 80%,the fine aggregate is medium sand,and the grading interval is 2 zones,which belongs to the I.grade sand.When the recycled aggregate replaces 40% of the river sand,the fractal dimension value of the fine aggregate is the smallest,and the density between the aggregate and the river sand particles is the densest.(2)The compressive and flexural test results of recycled aggregate mortar show that with the increase of the replacement rate of recycled aggregate,the compressive and flexural strength show a trend of first increasing and then decreasing,and the compressive strength of recycled aggregate mortar reaches the maximum when the RSA is 40%.The brittle index and fracture energy calculation of recycled aggregate mortar show that the specimen is the least brittle when the replacement rate of recycled aggregate is 40%,and the specimen has the strongest ability to resist fracture and deformation.(3)On the basis of the replacement rate of recycled aggregate of 40%,polypropylene fiber was incorporated and subsequent mechanical tests were carried out,and the results showed that different lengths and amounts of fibers had a great influence on the mechanical properties of recycled aggregate mortar,and the sample strength value was more sensitive to the change of fiber length.The compressive strength of RAFM specimens increased first and then decreased with the increase of fiber content,and increased with the increase of fiber length.The flexural strength showed an increasing trend with the increase of fiber length and content.Comprehensive strength,brittleness index,breaking energy,the optimal length and content of fiber modified recycled aggregate mortar are 12 mm and 0.6%.(4)The RSAF compressive strength prediction binary quadratic polynomial model and the flexural ternary quadratic polynomial model defined by fiber length and fiber content at different ages were established.The strength prediction model has high fitting accuracy,which can predict the intensity of RSAF without experiments,which provides reference for practical engineering applications.(5)SEM microscopic test shows that the interface transition zone in cement mortar generally develops around the aggregate with a thickness of about 5μm.After adding recycled aggregate,cement hydration products are bonded to river sand and recycled aggregate to form an overall agglomerate.After adding fibers,a small amount of fibers play a bridging role in the recycled aggregate mortar,and the specimen is deformed when it is damaged by partial tensile force;Excessive fibers will be entangled and knotted inside the sample,which inhibits the tensile ability of the fiber,resulting in a decrease in the strength of the sample.