The Accelerated Carbonation Behavior And Environmental Evaluation Of Recycled Aggregates Improved By Flue Gas

Reaching the maximum of carbon dioxide emissions by 2030 is an important goal for energy conservation and emission reduction in China.Construction industry consumed 40%of the total energy and generated 36%of the total greenhouse gas emissions in the world,which was a key area of energy conservation and emission reduction.With the rapid growth in economy,the building industry is booming.The increasing concrete production not only leads to the over-exploitation of natural aggregates（NAs）and also results in many environmental problems such as greenhouse gas emissions.At the same time,a large number of construction and demolition（C&D）wastes are dumped to landfills,which leads to a lot of land to be occupied.The utilization of C&D wastes to prepare recycled aggregates（RAs）is an effective method to solve the source of aggregates and the disposal of C&D wastes.However,the quality of RAs is lower than that of NAs,resulting in a low application rate of recycled concrete.Therefore,improving the quality of recycled aggregates is critical for increasing the application of recycled concrete,and is a research hotspot in recent years.Removing and strengthening the attached cement mortar are two main methods to improve the performance of RAs at present.However,some methods need high energy,and the operation processes are complex,or it may cause environmental pollution and the damage of RAs.The accelerated carbonation technology has been verified an effective method to improve the quality of RAs and store carbon dioxide permanently.High-concentration carbon dioxide is often selected as carbon source to carbonate RAs in published papers.However,the amount of carbon dioxide absorbed by RAs is relatively small,so the utilization of high concentration carbon dioxide to treat RAs leads to large amount of carbon emission again.In addition,the cost of separation and purification for carbon dioxide is relatively high,thus,it is still difficult to apply this technology in industry.The efficiency and mechanism of RAs carbonated by flue gas,which was sourced from cement plant and contained 20%carbon dioxide,were studied,and the environmental evaluation of flue gas improving RAs was also conducted in this paper.The main conclusion are as follows:（1）The mechanisms and influencing factors of the accelerated carbonation to treat RAs were summarized.The quality of carbonated RAs and the performance of concrete with carbonated RAs were summarized.（1）The accelerated carbonation of RAs was based on reactions between carbon dioxide and calcium hydroxides,calcium silicate hydrates,ettringites and unreacted cement clinkers to produce calcium carbonate crystals precipitating in the cracks and pores of RAs,which contributed to improve the quality of RAs.（2）The carbonation processes were affected by the carbonation conditions,such as relative humidity,gas pressure,carbon dioxide concentration,carbonation time and temperature,and the properties of RAs for example particle size,raw material strength,calcium content and water content.（3）After accelerated carbonation treatment,the quality of RAs obviously increased.The mechanical properties and durability performance of the concrete incorporating carbonated RAs were improved significantly.（2）The carbonation conditions of cyclic pressurization using flue gas were optimized.Mechanisms of RAs carbonated by flue gas under the cyclic pressurization were clarified.And the performance of concrete incorporating RAs carbonated cyclically by flue gas was evaluated.（1）The rate of carbonation reaction was relatively fast in the early stage,and then gradually slowed down.After 24 hours of carbonation,the carbonation extent was basically stable.（2）The rate of carbonation reaction increased with the increase of the concentration of carbon dioxide,however,the carbon extent did not increase significantly when the carbon dioxide concentration was higher than 20%.（3）When the gas pressure was lower than 0.1 MPa,increasing pressure led to the obvious increase of carbonation extent,while the increase of gas pressure had little impact on the change of carbon extent when the gas pressure was higher than 0.1 MPa.（4）Multi-cycle pressurization treatment was more effective for low concentration（about 20%）than high concentration carbon dioxide to enhance the carbonation degree,and 6 times was proposed as optimal cycle times.（5）There were a lot of calcium hydroxides and other hydration products in the RAs before carbonation.However,these compounds were converted into calcium carbonates filling in cracks or pores after carbonation treatment leading to the reduction of porosity,which benefited to improve the quality of RAs.（6）Compared with untreated recycled aggregate concrete,concrete incorporating RAs treated by flue gas exhibited greater slump,lower dry shrinkage,and higher compressive strength and flexural tensile strength.（7）The carbonation extent and physical properties of RAs treated by the flue gas and 20%CO₂ mixed gas were similar,and were equivalent to the performance of RAs treated by pure carbon dioxide.The material structures and composition of RAs treated with flue gas and 20%CO₂ mixed gas were also similar.（3）The process flow of presureless flow gas to accelerate RAs using flue gas was proposed.And the process and mechanisms of RAs treated by flue gas at the presureless flow through were revealed.（1）High concentration carbon dioxide reached the maximum of carbon extent earlier than low concentration carbon dioxide.The carbonation extent basically kept stable when RAs were carbonated by flue gas for 7 day（2）The carbon extent increased with the increase of the concentration before the concentration of carbon dioxide lower than 20%.The carbon extent of RAs treated by flue gas and 20%mixed gas were basically similar,and were equivalent to that carbonated by pure carbon dioxide.（3）The quality of RAs increased with the increase of gas flow rate（from 0.5-7 L/min）,and 5 L/min was optimal flow rate for 7 days of carbonation duration.（4）After carbonation treatment,the amount of calcium hydroxide in RAs reduced,while the amount of calcium carbonate increased based on microstructure analysis.（5）Compared with untreated recycled concrete,the slump of concrete with carbonated RAs increased,while the dry shrinkage reduced,the compressive strength and flexural tensile strength could increase by 20%,which was equivalent to that of concrete with RAs treated by pure carbon dioxide.（4）The environmental impact law of concrete with RAs treated by flue gas was evaluated.The key stages of environmental impacts caused by the production of concrete with RAs treated by flue gas were identified.（1）The utilization of RAs treated by flue gas could reduce the consumption of resources and energy for concrete production,and reduced the greenhouse gas emission,acidification,photochemical smog,water eutrophication and environmental costs caused by concrete production.The reduction value increased with the increase of the replacement rate of carbonated RAs.（2）The environmental impact of carbonation treatment using flue gas was lower than that of ordinary treatment and heat treatment.The former could significantly reduce the amount of additional cement,and the energy consumption and pollution emissions of the carbonation process were also relatively low.（3）Cement production was the primary factor of environmental impact caused by concrete production,followed by transportation.（4）When using recycled aggregates treated by flue gas to prepare concrete,it was recommended that the distance from the removal point to the aggregate carbonization treatment station should be less than 44 km.