On-site Recycling Of Waste Concrete And Bio-modification Of Recycled Aggregates

With the vigorous promotion of modernization and industrialization,the construction industry consumes a lot of natural resources and produces a lot of construction waste every year.The research on the comprehensive utilization of construction and demolition（C&D）waste is extremely urgent recently.After the construction waste demolished on site is broken and screened,the local materials can be used in temporary engineering construction,which can not only save the engineering cost,but also realize the recycling of C&D waste.On the other hand,low-quality recycled aggregate（RA）carries attached mortar,which has high water absorption and low strength compared with natural aggregate.The performance of recycled concrete prepared from it is also worse than that of ordinary concrete,so it is necessary to strengthen the low-quality RA.In recent years,microbial induced carbonate precipitation（MICP）has gradually come to the fore in the field of building materials,and has been selected for enhanced treatment of RAs because of its many advantages.In order to improve the recycling efficiency of C&D waste and carry out the source reduction,the technology of on-site recycling and utilization of waste concrete was explored.Gradation design and mix design were investigated to make the partial or complete replacement of natural aggregate by RA.Finally,recycled aggregate concrete was prepared,which satisfied the requirement of pavement cushion.Its slump was 120 mm and the compressive strength reached 43.3 MPa.The freezing and thawing durability grade was F250.The urea decomposition precipitation system is commonly used.The effects of calcium source,urea concentration and treatment duration were studied.The MICP treatment process was optimized,considering mass increase,effective precipitation ratio and water absorption change rate.The mechanism of MICP under this system was discussed considering phase composition and micro-morphology of precipitates,pore structure of RA and mechanical properties of interfacial transition zone（ITZ）.It was found that when calcium chloride was used,the precipitation efficiency was the highest while the urea concentration is 0.5 mol/L,which reached 68.8%in 2 hours.When treatment duration was 14 days,the secondary precipitaiton method based on vacuum dipping had the best strengthening effect.The water absorption of treated RA reduced by 26.57%.The effective precipitation ratio was 99.62%.The elastic modulus of ITZ increased by 16.9%and the hardness increased by 40.7%and an obvious calcium carbonate band was seen at the ITZ.The precipitates formed a shell structure on RA surface,which blocked the water transport and reduced the water absorption of RA.The precipitation process under this system was divided into two stages:the first stage was the hydrolysis of urea and the formation of calcium carbonate.The second stage was that the precipitates accumulated on RA surface and were compacted by solution pressure,forming a dense calcium carbonate shell.Ammonia was produced during the process of urea decomposition precipitation system,which was harmful to environment.In order to explore other environmental possibilities of MICP to enhance RAs,another CO₂ capture precipitation system was investigated.The effects of calcium source,carbonation pressure and carbonation duration were studied.Carbonation curing process was optimized considering mass increase of RA,effective precipitation ratio and water absorption change rate of RA.The mechanism of MICP under this system was discussed considering phase composition and micro-morphology of precipitates,pore structure of RA and mechanical properties of ITZ.It was found that the carbonation efficiency of calcium hydroxide was the highest and the carbonation output was increased by pressurized carbonation.Under positive pressure,the mass increase of RA did not rise significantly with the increase of pressure and 0.15 MPa was selected as pressure value.The strengthening effect of soaking carbonation method was better than that of wetting carbonation method.After 28 days,the mass increase of RA reached 4.87%,the effective precipitation ratio reached 98.36%and the water absorption of RA decreased by 13.31%.In this system,the continuity of the precipitation matrix was low and the surface defects of RA was not be covered effectively.The calcium carbonate band at ITZ was discontinuous and the elastic modulus and hardness of ITZ did not change significantly.Carbonation treatment had no significant effect on the surface properties of RA.The main mechanism of this system to strengthen RA was to form carbonized products through MICP carbon fixation,filling the pores of RA from the outside to the inside and improving the pore size distribution,so as to reduce the water absorption of RA.The precipitation process under this system was completely harmless to the environment.This system can not only capture and seal CO₂,but also improve the recycling efficiency of waste concrete,which is worthy of further study.