| Ultra high performance concrete(UHPC)has the advantages of ultra high strength,high durability and high toughness,however,the cost of UHPC is know to be much higher compared with the average concrete.Iron tail powder(ITP),which is a kind of excessively emissed industrial solid waste,has led to serious environment pollution.Recycled fine aggregate(RFA)is a new type of fine aggregate produced from waste concrete,whose over discharge has occupied a large accumulation of city land.If ITP and RFA could be used to produce UHPC,it may bring not only the reduced cost of UHPC,but also potential environmental benefits.In this study,mechanical properties of UHPC which made use of ITP and RFA as substitutes to quartz powder and river sand,respectively,were experimentally studied.Mechanism of the development of these mechanical properties was revealed on the meso and micro scales,by using both qualitative and quantitative testing techniques.Based on the consideration of engineering application,the optimum mix ratio of UHPC using iron tail powder and recycled fine aggregate was also determined via the orthogonal test design.The findings of this study could be concluded as follows:1.Fluidity of UHPC decreased slightly with the increase of the replacement ratio of iron tail to quartz powder,while the mechanical properties did not change much.Properties of iron tail powder and quartz powder used in this study are similar.It can be seen from the XRD,SEM,and MIP testing resutls that no new hydration products were produced by replacing quartz powder with iron tail powder,meahwhile,the microhardness of the cement paste,the porosity,as well as the pore structure,were similar among UHPC with different ITP replacement ratios.It was also found that the microharness of cement paste on the meso scale is linearly correlated to the mechanical properties of UHPC with ITP,on the macro scale.2.With the increase of the replacement ratio of RfA to natural river sand,the mechanical properties of UHPC decreased.More interfacial transition zones(ITZs)were contained in UHPC with RFA(RFA-UHPC).It was found that the total length of the ITZs contained in RFA-UHPC increased over the RFA replacements,meanwhile the thickness of the diverse types of ITZs were different under different curing conditions.On a basis of correlation analysis,this paper presents a interface parameter which reflects the total area of all the different types of ITZs contained in the tested cross sections(10 mmx10 mm)of RFA-UHPC.A mechanical property development model based on this interface parameter was established.It was revealed that the increase of the total area of ITZs is the mechanism for the decrease of the mechanical properties of RFA-UHPC.3.Mechanical properties of UHPC using ITP or RFA were largely influenced by curing conditions.It was found that under the autoclave curing condition,UHPC with ITP or RFA only,or with both two,presented suprerior mechanical properties,to that under the standard curing condition.The microhardness of ITP-UHPC was proved to be the largest,while the pore structure was much better,and the Ca/Si of hydration product was found the lowest,which was the main reason for the much better mechanical behavior of ITP-UHPC under the autoclave curing condition.For RFA-UHPC,it was found that the microhardness of the newly formed ITZ and ITZ3 was the highnest while the thickness was the smallest under the autoclave curing condition,which explained the better performance of RFA-UHPC on the macro scale under this curing condition.4.Through orthogonal experiment,the optimum mixture ratio was determined as follows:W/C as 0.22,SF/C as 0.35,ITP/C as 0.30,RFA/C as 1,and steel fiber content as 2%.The fluidity of UHPC of this mix ratio was 198 mm.Under autoclave curing,this group of UHPC achieved a compressive strength of 154.41 MPa,a flexural strength of 33.66 MPa,a ratio of fracture to compression of 0.218,at the age of 28 d,which can fully meet the requirements for the mechanical properties of UHPC. |
PDF Full Text Request