Research On The Properties Of Cement Stabilized Recycled Mixed Aggregates Materials

With the continuous acceleration of urbanization in China,a large number of old buildings have been demolished,resulting in a huge amount of construction and demolition waste(C&DW).These C&DW are nowhere to be stacked and difficult to be effectively utilized.At the same time,infrastructure construction consumes lots of non-renewable natural sand and gravel and the price of sand and gravel are increasing a lot.If a large amount of C&DW can be used to make recycled mixed aggregates(RMA)instead of natural sand and gravel,which can be used in cement-stabilized materials in road base,it will well alleviate the problem of sand and gravel shortage and reduce the cost of construction projects,and also effectively solve recycling problem of huge amount of C&DW.However,the performance of RMA is poor,and the impact on the technical performance of cement-stabilized materials is not clear The lack of systematic research and sufficient theoretical support has greatly limited the application of RMA in cement-stabilized materials.For this reason,based on testing the basic properties of RMA,firstly,the effect of cement content,aggregate grading type and curing age on the mechanical,deformation and durability properties of cement stabilized recycled mixed aggregate(CSRMA)materials were systematically studied.And its impact mechanism was explored through micro-technical means such as XRD/MIP/SEM.Secondly,the dry shrinkage deformation properties of CSRMA materials was optimized and improved by incorporating fly ash to find a suitable fly ash replacement rate and explore its modification mechanism.Finally,lay the test section of CSRMA materials base and test its road performance to compare and analyze with cement stabilized natural aggregates base.Further,compare and analyze the mechanical structure response of these two kinds of base under vehicle load by using ABAQUS finite element method.The main research contents and conclusions are as follows:(1)The basic properties of RMA were tested and compared with natural aggregates.The performance of RMA is worse than natural aggregates,but meets the technical index requirements for second and lower grades highway cement-stabilized materials in the code.Its main components are concrete stones,bricks,porcelain glasses,and wood chips,which account for 61.3%,31.7%,6.6%,and 0.4%,respectively,and belong to continuous graded aggregates.Its mineral composition are quartz,mullite stone,iron oxide,limestone,etc.The microscopic morphology results show that the brick aggregate has many holes,and there are some speckle-like particles on the aggregate surface,which may be activated brick powder or old mortar powder attached to the surface of the aggregates.(2)The effect of cement content on the mechanical,deformation,durability of CSRMA materials were systematically studied,and the effect mechanism was explored.The results showed that cement is the basis for the strength formation of CSRMA materials.With the increase of cement content,the unconfined compressive strength and indirect tensile strength of CSRMA materials were gradually increased,the ultimate compressive strain and ultimate tensile strain were also gradually increased,and the dry shrinkage performance was decreased while the resistance to erosion resistance was improved.The relationship between compressive strength and indirect tensile strength was fit=0.11fc.The compressive rebound modulus and split rebound modulus showed a linear increase tendency with the cement content.Its unconfined compression and indirect tensile failure showed three typical stages:(i)compaction stage(ii)elasticity stage(iii)yielding stage.Microscopic test results showed that cement has both binding and filling effect on CSRMA materials.The CSH gel produced by the hydration of the clinker mineral C2S&C3S in the cement acts as a binder between the aggregates,and can also fill the micro-pores of the CSRMS materials and optimize its pore structure distribution.(3)The effect of three aggregate gradation types of Suspended Dense(SD),Framework Dense(FD)and Suspended-Framework Dense(SFD)on the technical performance of CSRMA materials were analyzed and compared.Compared with the traditional SD and FD type,SFD type of CSRMA materials show more excellent mechanical properties,dry shrinkage performance and erosion durability.Because both coarse aggregates can interlock and overlap to form a skeleton,and there are enough fine aggregates to fill and wrap coarse aggregates in Suspended-Framework Dense gradation type.(4)Dry shrinkage performance of CSRMA materials was modified by mixing fly ash.The research results showed that the incorporation of fly ash could reduce the early strength of CSRMA materials,but it is conducive to its later strength development and significantly improves its drying shrinkage deformation properties.This is because the incorporation of fly ash can reduce the consumption of cement clinker minerals and CSH gel formation in the early stage of CSRMA materials.However,with the development of curing age,the fly ash was constantly hydrated and the cement clinker mineral consumption and CSH gel formation gradually increased in the later period.Fly ash can reduce the porosity and average pore size of CSRMA materials,and significantly optimize the pore structure distribution,reducing the proportion of harmful macropores and increasing the proportion of harmless macropores,thereby improving its dry shrinkage deformation performance.It is recommended that the replacement rate of fly ash is 20%(5)Based on the road performance testing and three-dimensional finite element simulation in the test section,the performance difference between the CSRMA materials base and cement-stabilized natural aggregates base was compared and analyzed.The road performance test results showed that compared with cement-stabilized natural aggregates base,CSRMA materials base had higher water content and lower dry density,but had better compaction and flatness.Its strength and modulus were lower than those of cement-stabilized natural aggregate base,resulting in greater overall road deflection value.The road performance indicators of CSRMA materials base meet the requirements of urban roads for cement-stabilized base.The finite element simulation results showed that the mechanical responses of the two kinds of base road structures under the load of vehicles were similar.The CSRMA materials base will increase the vertical displacement of the pavement,but can reduce the tensile stress of the surface and the bottom of the base.