| The production of steel slag in China account for more than half of the total steel slag amount all over the world. However, the utilization rate is only about 22% which is far behind other developed countries. The poor volume stability and grindabilty of steel slag limit its largescale application. Additionally, if the steel slag is not properly handled, the toxic substances such as heavy metals can be environmentally hazardous. Carbonation is an effective way to make full use of steel slag by artificially storing carbon dioxide, which absorbing large amount of carbon oxide as well as mitigating greenhouse effects.Vast studies show that utilization of steel slag as aggregate in concrete can enhance the mechanical properties, the durability of concrete, however, will be seriously affected by the exceeded free lime and magnesium content in steel slag. Besides, the angular and recessed hole-structures of steel slag are more likely to form a “self-locking” and barrier effects to reduce the fluidity of fresh concrete and increase cementitious materials. By optimizing the carbonation process and using coarsely grounded steel slag to prepare artificial steel slag aggregate, which not only protect the environment but also eliminate the disadvantage of steel slag in structure and chemical nature. The affinity of carbonated steel slag aggregate and cement paste will also further enhance the performance of concrete.In this study, the five factors and five levels orthogonal experiment was carried out to find out the optimum process parameters of carbonated steel slag aggregate. The aggregate shape and distribution were amended by Fuller distribution as well as packing distribution. The volume stability of aggregate and concrete were evaluated by tasting the free calcium oxide and magnesium oxide and the stability of samples in steaming and boiling environment. The electric flux and rapid chloride migration tests, fast freezing and drawing test etc. were used to study the durability of carbonated steel slag aggregate concrete. The thermal gravimetric analysis, Xray diffraction analysis, scanning electron microscopy, EDX analysis, pore structure analys is, thermal analysis etc. were used to research hydration of aggregate and concrete microstruc t ure and micro properties.Based on the experimental data, the main conclusions are as follows: Basic properties of carbonated steel slag aggregateThe orthogonal experiment results showed that the factors on influencing the compressive strength of carbonated steel slag aggregate were water percentage, CO 2 pressure, carbonation temperature, molding pressure and carbonation time sorted by influence in descending order. Moreover, when the water percentage was 12 wt.%, molding pressure was 9 MPa, carbonation temperature was 60 oC, CO2 pressure was 0.1 MPa and carbonation time was 8 hours, the carbonated steel slag aggregate obtained the highest compressive strength. After carbonation treatment, the f-CaO content was decreased from 7 wt.% to less than 1 wt.%; the harmful pore volume decreased by 24.4%, less harmful pore volume increased by 67.9%; the carbonation products were main spindle calcium carbonated particles which were calcite by XRD. In optimal process conditions, the carbonated steel slag aggregates were prepared with a crushing value of 1830% and an apparent density of 2650 kg/m3, which meet the relevant standards. Although the water absorption(8.91%) of carbonated steel slag aggregate was exceeding limit, the absorption rate is slow that the effective water absorption time was upon 12 hours. Thus, the performance of fresh concrete was not significantly reduced while the bleeding and segregation phenomena was mitigated. The effects of aggregate on the basic properties and volume stability of concrete.Comparing to natural aggregate concrete, the utilization of carbonated steel slag aggregate improved compressive strength of the concrete specimens on 7 days, 28 days and 60 days by 15% to 20%; though the fluidity of fresh concrete was decreased, the bleeding and segregatio n phenomena was mitigated; the surfaces of specimens with carbonated steel slag aggregate were more compact and were with less macroscopic defects than other aggregate. Although carbonated steel slag aggregate contained high MgO content(about 4 wt.%), speculative ly, magnesium content might be a solid solution to most other silicate minerals which were unlik e ly to cause the adverse volume stability. Carbonated steel slag aggregate was not cracked or chalked after boiling and steaming tests; in aggregate akali-activity experiment, expansion of the samples were less than 0.1%, so the volume stability of carbonated steel slag aggregate and concrete met the relevant standards. Compared to then untreated crushed steel slag aggregate, the aggregate severe spalled and cause the unstability of concrete. The effects of aggregate on chloride penetration and resistance to freeze-thaw circles of concrete.Based on the results of electric flux and rapid chloride migration tests, resistance to chloride ion penetration of concrete with carbonated steel slag aggregate was enhanced by about 2-fold compared with ones of natural aggregate. In fresh water, the concrete samples with carbonated steel slag aggregate can endure about 300 times freeze-thaw cycles while the ones with natural aggregate only endure less than 140 cycles; in 3 wt.% salt solution, the concrete samples with carbonated steel slag aggregate can endure about 110 times freeze-thaw cycles while the ones with natural aggregate only endure less than 5 cycles, thus the resistance to salty freeze-thaw damage of concrete with carbonated steel slag aggregate had been further enhanced. By studying the average thickness and hardness of the interfacial transition zone(ITZ) of each aggregate, the thickness of carbonated steel slag aggregate and limestone aggregate were 50 ?m and 35?m respectively while Vickers hardness were about 43 and 28 respectively. Based on the surface morphology of ITZ, gaps with a width of about 2 to 5?m were observed between natural aggregate and cement paste while the areas around carbonated steel slag aggregate were compact and seamless. Therefore, it can be suspected that the voids and gaps around aggregate might be the main factors to affect the performance on freeze-thaw resistance of concrete. The effects of aggregate on self-healing properties of concrete.Based on the results of hydration heats, the hydration heats of 28 days old cement paste and carbonated steel slag were 50J/g and 90J/g respectively, the exothermic rate of aggregate exceeded cement paste at 2nd hydrated day. Based on stress-strain curves of concrete specimens, carbonated steel slag aggregate made obvious contribution to healing effects of concrete that the specimens with initial compressive strength of 67 MPa had gone through 4 times breakhealing cycles to decrease to less than 38 MPa. Before the first three cycles, the specimens were hold upon 60 MPa. The ultrasonic inspection data and Fourier transform analysis showed that the tested sound speed of cracked carbonated aggregate concrete specimens which were cured for 90 days had decreased from 30?s to 29.6?s while amplitude increased from 0.252 V to 0.648 V. The dominate frequency was hold upon 50 kHz. By contrast, normal natural aggregate and crushed steel slag aggregate had no significant self-healing properties. Based on microscopy picture and XRD, the healing mechanism of carbonated steel slag aggregate was by synergistic effects of carbonation and hydration, the final healing products were mainly calcium carbonate, calcium hydroxide, aluminate hydration, calcium and iron aluminide and lots of amorphous silica gel. |
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