Research On The Effect And Mechanism Of Microfines In Manufactured Sand On Concrete Properties

Microfines refer to particles finer than 75?m in manufactured sand(MS),and the quantity as well as quality of microfines can significantly affect concrete properties.Focusing on the problems that characteristic characterization and harmfulness evaluation on microfines in MS remained to be improved,influence rules of microfines on concrete property and limits on microfines content in specification were disputable,resource waste and environment pollution caused by removing microfines,etc,this dissertation had systematically investigated characteristics of microfines in MS,effects of microfines characteristic and synergistic effects of microfines quality and quantity on fresh and hardened concrete properties.Besides,the influence and mechanism of microfines on concrete properties had been revealed,and the evaluation method on harmfulness of microfines in MS had also been developed,which can provide theoretical foundation and technical support to relaxation of microfines content limit and high quality application of manufactured sand concrete.(1)Various microfines were collected from MS samples by sieving,washing and grinding.The physical and chemical properties of microfines were tested and characterized,the characteristic differences among microfines prepared by various methods were compared,and the correlations between characterizing indexes were analyzed.The results show that microfines in MS differ prominently in fineness,mineral composition,water demand,absorption,activity,and characteristics of microfines are affected remarkably by preparation methods.The characteristic differences of various microfines essentially lie in variations of activity and water demand,which are found to be highly related with fineness and MBV(MB value),respectively.(2)Microfines of different finenesses and MBV-s were incorporated to study the influence of microfines characteristic on concrete property.The results indicate that the increase of microfines fineness improves the compatibility between cement and water reducer,fresh concrete workability,and compressive strength and chloride-ion penetration resistance of hardened concrete to some extent,yet it has little influence on drying shrinkage and frost resistance.The increase in MBV of microfines decreases the compatibility between cement and water reducer,increases the amount of water and water reducer needed to achieve constant slump,decreases frost resistance and increases drying shrinkage.Moreover,it also decreases compressive strength and chloride-ion penetration resistance of C30 and C60 concrete with constant W/C when MBV exceeds 10 and 7.The influence of variation of microfines MBV on property of C60 concrete is greater than that of microfines MBV on property of C30 concrete.Negative effects of microfines with high MBV are mainly caused by increase of water demand under constant workability.(3)The influence of microfines quality and quantity on concrete property had been studied by means of incorporating microfines of different MBV-s into concrete with various contents.It is found that the influence of microfines on concrete property is not only related to content,but also highly correlated with MBV.The influence rule of microfines on concrete property is determined by synergistic effect of microfines content and MBV.The higher the MBV and content of microfines is,the greater the decrease of workability,frost resistance and the increase of drying shrinkage.When microfines MBV exceeds 7 and microfines content exceeds 7%,the decrease of compressive strength and chloride-ion penetration resistance of C60 concrete is so obvious that they start to deteriorate.However,compressive strength and chloride-ion penetration resistance of C30 concrete haven't been adversely affected with variations of MBV-s and contents of microfines.(4)Through testing MBV,percentage of passing 2.36 mm,microfines content of MS samples and MBV and content of microfines,the relationships among modified MBV(defined as the multiple of microfines MBV and content),MS MBV and microfines MBV were identified.Moreover,the correlations between modified MBV and properties of mortar and concrete were analyzed,the dual-control system of restrictting clay content in MS using microfines MBV and modified MBV was established and suggestion on limit of microfines content in MS was proposed according to classification.It is found that modified MBV is proportional to microfines MBV and positive linear relationship can be developed between microfines MBV and MS MBV.High correlation can be established between microfines MBV and properties of mortar and concrete,which confirms the index of modified MBV can be applied in assessing harmfulness of microfines in MS.It is suggested that modified MBV of MS used in ordinary C30 and C60 concrete should be no more than 140 and 100,and modified MBV of MS used in concrete required frost and permeability resistance be no more than 100 and 70.(5)The mechanisms of various microfines,including stone dusts,clays and clay-contaminated stone dust,on concrete property were analyzed using XRD,DTG,hydration heat,SEM and MIP.The results reveal that the effect of microfines on cement hydration is not only related with mineral characteristic of microfines,but also in connection with water to cement ratio(W/C)and hydration age.Microfines can significantly accelerate hydration in initial stage,the acceleration effects of bentonite and clay-contamionated stone dust of high MBV are found to be more prominent than the others,but no significant effect is observed after acceleration stage.The addition of stone dust and clay-contaminated stone dust of low MBV in cement can promote early stage hydration,yet bentonite and clay-contaminated stone dust with high MBV will prevent late stage hydration at low W/C.The incorporation of clay-contaminated stone dust in C30 concrete can improve interfacial transition zone(ITZ)and pore structure;however,it will weaken ITZ and deteriorate pore structure of C60 concrete when MBV of clay-contaminated stone dust exceeds 10(namely modified MB value exceeds 100).The deterioration effect of microfines with high MBV can be recognized as physical physical mechanism of increasing W/C and chemical mechanism of preventing hydration and deteriorating ITZ and pore structure.