Modification Of Recycled Coarse Aggregate And Its Effect On Concrete Properties

With the rapid development of China's infrastructure in the past decades,the consumption of natural aggregate and the demand for commercial concrete have increased rapidly.However,the shortage of natural resources,the rise in aggregate prices,the stack of solid waste and other problems were increasingly prominent.Due to the demand of economic development,a large amount of construction waste was generated in the construction demolition and road reconstruction projects,while the conventional treatment methods are landfill and stacking.The environmental pollution and land waste will restrict the implementation of China's sustainable development strategy.Large volume proportion of aggregate is required for concrete as the foundation of building engineering,and it is limited to the shortage of natural aggregate.Therefore,the rational use of waste recycled aggregate will undoubtedly be an effective means to reduce the consumption of natural resources and increase the utilization of construction waste.However,the untreated recycled aggregate has high porosity and low hardness,and the interface transition zone of the recycled concrete is complex,which seriously affects the working performance,mechanical strength and durability of the concrete.Based on this,in order to obtain the stable performance of recycled concrete,the modification of recycled aggregate has become the focus of attention in recent years.In this paper,on the basis of exploring the basic properties of recycled coarse aggregates such as water absorption,crushing index,sphericity and pore size distribution,the ternary paste of silica fume,fly ash and slag with different water-cement ratios was used to coat coarse aggregates;The effects of water-cement ratio and different activities coated paste on the thickness of aggregate coating layer and the micro-hardness were studied.Furthermore,three kinds of water-cement ratios were increased on the basis of paste modification to study the effect of lower viscosity paste and the immersion of water glass on water absorption,apparent density and microstructure of recycled coarse aggregate.Finally,the workability,compressive strength and microstructure of interfacial transition zone of recycled concrete manufactured by coated and soaking modified aggregate was researched.The results show that:(1)The main reason of the recycled coarse aggregate with many angularities,high water absorption and high crushing index is that its surface adhered with a large amount of old mortar.The coating method can effectively coat the surface of recycled aggregate,and the paste with high viscosity has a better coating effect on the aggregate.(2)The workability of recycled aggregate concrete generally improved with the decrease of the coated thickness of the aggregate,in which the fly ash in the coated paste significantly improved the workability of recycled concrete,while the use of silica fume on improvement effect was not obvious.In addition,the lubrication provided by the slag paste coated aggregate was between the two at the front.(3)Too high or too low water-cement ratio of the coated paste was not conducive to the development of concrete strength.Based on the pozzolanic effect of the material,when the water-cement ratio of the fly ash and slag paste were 0.8 and 1.0,respectively,the mechanical performance of recycled concrete was best improved;and with the use of fly ash and slag,the compression strength at 28 days were improved 3.3% and 4.0%,and the micro-hardness of the interface transition zone has an increased range of 11.8% and 28.1%,respectively.After silica fume was used as the paste material,because the water-cement ratio of silica fume was too high,many unfavorable pores were formed,and the compressive strength of concrete and micro-hardness of interface transition zone were lost.(4)In the soaking method,the active paste mainly reduced the porosity and water absorption rate of the recycled coarse aggregate by going deep into the internal cracks and pores in the recycled aggregate.The modification effect of liquid water glass and silica fume with higher specific surface area were better,and the use of fly ash and slag paste to improve the aggregate was general.(5)A paste with a high water-cement ratio has a lower viscosity,which was more likely to penetrate into the pores and cracks in the recycled aggregate.At the same viscosity,the more active components penetrated into the aggregate,the more sufficient pozzolanic reaction or solidification will occur,and the better the modified effect of the recycled aggregate.The most suitable paste viscosity of silica fume,fly ash,slag and water glass were about 30 mPa·s.(6)After the soaking treatment of active materials(silica fume,fly ash and slag)and water glass,the water absorption of recycled aggregates were reduced from 6.59% to 5.32%,5.67%,6.30% and 4.64%,respectively,and the compressive strength of the concrete manufactured by soaked aggregates correspondingly increased from 37.55 MPa to 45.64 MPa,41.58 MPa,43.93 MPa and 51.34 MPa.The active material can produce pozzolanic reaction while filling the pores and cracks.After soaking the recycled aggregate with silica fume paste and water glass,the pore structure of the concrete interface has improved,and the micro-hardness of interface can be increased by 36.56% and 39.56%,respectively.After the use of fly ash paste and slag paste,there were still some pores in the concrete interface,and the micro-hardness in this area can also be correspondingly increased by 12.35% and 0.50%,respectively,and the improvement effect was relatively weak.