Study On The Performance Of Controllable Low-strength Material (CLSM) Prepared By Excavation Slag Combined With Industrial Solid Waste

There will be a large number of excavation and backfilling works in the process of urban construction.Due to the narrow space available for construction operation and the dead angle between backfilling and structure interface,the backfilling quality of traditional backfilling materials is difficult to guarantee,which will induce engineering diseases.Waste of resources and destruction of ecological environment will be caused in the process of waste slag and backfilling.At the same time,in the process of urban infrastructure renewal,some sections need to re excavate the existing traffic lines.If the backfill materials with re excavatability are selected in the initial construction,the subsequent cost can be reduced.Controllable Low Strength Material（CLSM）has the characteristics of easy pouring,self leveling,self filling,self compacting and fast hardening.It is especially suitable for pipeline backfilling,structural filling,hole backfilling,pipeline cushion and other projects and structures.CLSM has low requirements for raw materials.It can take local materials and make full use of industrial solid waste and construction waste.Using this material to replace the traditional graded sand and stone for backfilling will effectively solve the engineering diseases caused by difficult compaction.From the perspective of engineering demand and green development,this thesis uses excavated residue,industrial solid waste red mud and fly ash to prepare a widely used controllable low strength material.Firstly,the influence of water-solid ratio and ash water ratio on CLSM is studied through indoor test,and the basic water-solid ratio and ash water ratio of CLSM are determined.Secondly,the effects of different proportions of solid waste on the working and mechanical properties of CLSM were studied.Thirdly,the microstructure and mineral composition changes of the mixture after hardening are studied through micro test,and the formation mechanism and main influencing factors of CLSM strength are analyzed.Finally,the stability of CLSM is determined through indoor tests.The main conclusions are as follows:（1）The fluidity of CLSM is mainly affected by the water-solid ratio.The greater the water-solid ratio,the greater the fluidity,and the reasonable range is between 0.4～0.5.The compressive strength of CLSM is mainly affected by the cement water ratio.The greater the cement water ratio,the higher the strength,and the reasonable range is 0.3～0.6.The strength of CLSM increases with the increase of age.When the water-solid ratio is 0.45 and the cement water ratio is 0.5,the strength of CLSM specimens at 3d,7d and 28d is 1.13Mpa,1.78Mpa and 4.58Mpa respectively.For different engineering needs,CLSM can be prepared by adjusting the water-solid ratio and ash water ratio.（2）When solid waste is used to replace some cement,adding red mud will reduce the fluidity of CLSM,while adding fly ash will increase the fluidity of CLSM.When the amount of cement replaced by red mud is 20%～30%,the CLSM strength increases slightly.The addition of fly ash will reduce the CLSM strength,but it can improve the strength growth rate of the specimen.The amount of cement replaced by fly ash should not exceed 20%.When red mud and fly ash are mixed to replace cement,it can produce excitation.When the replacement amount of red mud and fly ash is 30%and 20%,the strength loss of CLSM is the smallest.The age strengths of 3D,7d and 28d are 1.08Mpa,1.49Mpa and 3.77Mpa respectively,which can reach 95%,84%and 82%of the strength of the no solid waste addition group.This mix proportion can reduce the cement consumption by 50%,obtain greater economic benefits,and meet the application requirements of CLSM.（3）there are a lot of Si O₂,Al₂O₃ and Fe₂O₃ in red mud,which react with Ca²⁺to produce hydrated calcium silicate（C-S-H）,hydrated aluminum silicate（C-A-H）and hydrated iron silicate gel（C-F-H）.These hydrated gelled products are the main factors for CLSM to have certain intensity.Si O₂ and Al₂O₃ account for 83%of the total content in fly ash,so the contribution of fly ash to strength depends more on the later pozzolanic reaction.The combined effect of red mud and fly ash makes the internal structure of CLSM more uniform.Because of its strong alkalinity,red mud will decompose and polymerize with the substance in fly ash to form new gel.（4）In terms of water stability,the strength of CLSM increases first and then decreases after immersion.This is because the water enters the interior of the specimen during repeated immersion,which makes the insufficient hydrated cementitious material further hydrated.With the increase of immersion times,the surface of the specimen is damaged by cracks,cracks,waterways and spalling,resulting in the decrease of strength.When the amount of red mud replacing cement is 20%～30,the first immersion will increase the strength of the specimen,and then the compressive strength of the specimen decreases with the increase of immersion times.The substitution of fly ash for cement is not conducive to the water stability of the specimen,and its substitution amount should not exceed 20%.（5）The dry shrinkage of CLSM specimen increased with age and remained unchanged from the 6th day.The reason for shrinkage is that the volume loss is caused by the loss of water.The two reason is that the compression of the specimen results in the shrinkage of the specimen after the gel is generated.The addition of red mud and fly ash will increase the dry shrinkage of the specimen.The CLSM parts with 30%red mud instead of cement and 20%fly ash instead of cement in the mixture were tested for heavy metal leaching after 90 days of curing,and the detection limit of heavy metals met the requirements.