Evolution Process Of Interfacial Transition Zone And Influence On Performance In Recycling Concrete

Steel slag, coal gangue, limestone and some adjustive raw materials were used to preparewaste-based artificial aggregates and artificial aggregates whose mineral compositions aresimilar to that of Portland cement clinker. The two kinds of artificial aggregates substitute fornatural aggregates to produce recycling concrete. These two kinds of concrete once abandonedcan completely regenerate raw cement by means of adding a bit adjustive raw materials,without the process to separate the aggregates and cement paste, thus the reuse of concrete canbe truly realized.Recycling concrete is prepared with artificial aggregates. The composition, structure andevolution of the Interfacial transition zone （ITZ） of recycling concrete are different with thatformed by nature aggregates and cement. The changes of ITZ will affect the concrete’smechanical properties.Waste-based artificial aggregates and artificial aggregates whose mineral compositions aresimilar to that of Portland cement clinker were used to prepare recycling concrete respectively.The microscopic morphology, Ca/Si ratios, CH orientation index, micro-hardness andmechanical property of interfacial transition zone （ITZ） of two kinds of concrete were analyzedby SEM-EDS, XRD, Vickers and compression testing machine. The microstructurecharacteristics, evolution process and the influence rule of that on the mechanical properties ofconcrete were studied in this paper. The main research contents and the obtained results andconclusions are as follows:1. Study on ITZ microstructure of recycling concreteThe main mineral compositions of the artificial aggregates are C₂S, C₃S, C₃A and C₄AF,whose compositions are similar to cement clinker. Comparing with normal concrete, thereexist a small amount of CH crystals of the ITZ of the concrete prepared by artificial aggregateswhose mineral compositions are similar to that of Portland cement clinker, most of the spaceare filled with dense hydration products. The Ca/Si of ITZ of recycling concrete is less than thatof ordinary concrete. The width of the recycling concrete’s ITZ is about45μm, which is lessthan50μm of ordinary concrete’s ITZ. The CH orientation index of1.70in the recyclingconcrete’s ITZ is lower than that of2.34in traditional concrete’s ITZ, and the micro-hardness of recycling concrete is higher than that in traditional concrete on the whole.Waste artificial aggregates with different mix proportion of raw materials have a greateffect on recycling concrete’s ITZ. The main mineral compositions of the artificial aggregatesprepared with40%steel slag,40%pulverized blast furnace slag and20%coal gangue are C₂S,CS, CA₂, CF₂and Ca₃Al₁₀O₁₈. The artificial aggregates have large water absorption, and thereexist some pores on the surface. As a result, the ITZ of this kind of concrete prepared withartificial aggregates has small Ca/Si and good microstructure. Compared with traditionalconcrete, The ITZ of recycling concrete prepared with artificial aggregates （40%steel slag,40%pulverized blast furnace slag and20%coal gangue） has fewer pore and densermicrostructure. The ITZ’s width is about40μm, which is less than that of50μm of ordinaryconcrete. The CH orientation index is1.64, which is less than that of2.34of ordinary concrete,and the micro hardness is higher than that of ordinary concrete on the whole.2. Influence of W/C on the ITZ microstructure of recycling concreteSimilar to ordinary concrete, the water-cement ratio has a great influence on the ITZ ofrecycling concrete, and the W/C increase will bring about the decrease of the ITZ’smicro-performance. The higher water-cement ratio, the heavier micro bleeding, this will lead tocrack width increase between artificial aggregates and cement. As a result, the cohesiveness ofartificial aggregates and cement decreases, the pore size and pore quantity, Ca/Si, and theinterface width of recycling concrete’s ITZ increase, and the micro hardness decreaseobviously.3. The evolution process of ITZ microstructure of recycling concreteAt the early ages of1-14days, there exist much pore in the ITZ of recycling concrete, theCa/Si is relatively large, and the interface width is about60-80μm. At the age of28days, withthe hydration degree deepening, the developments of ITZ’s structure turn better. Most of thespace is filled with dense hydration products, the Ca/Si decrease largely, and the interface widthis about40-50μm. At the ages of56days and120days, there exist little changes of Ca/Si, theITZ’s structure turns stable and the interface width is about40μm. CH orientation index wasgradually increasing with curing age from1day to28days, which decrease with the curing agefrom28days to120days due to secondary hydration.4. Influence of ITZ’s microstructure on mechanical properties of recycling concrete The ITZ’s structure of recycling concrete prepared with similar clinker compositionartificial aggregates and waste-based artificial aggregates have some improvement comparingwith that of ordinary concrete. The compressive strength also has some certain improvement,28days’ compressive strength of recycling concrete prepared with similar clinker compositionartificial aggregates increase7.4%, and that prepared with waste-based artificial aggregatesincrease16.4%.Water-cement ratio is one of the most important factors that affecting the compressivestrength of concrete. With water-cement ratio increasing, the ITZ of recycling concrete has abig Ca/Si, high CH orientation and large interface width, but the micro hardness decrease. Themicro-performance degradation eventually leads to the decline of the compressive strength ofthe recycling concrete.