Effects Of Recycled Concrete Aggregate Carbonated Treaments On Permeability And ITZs Of Recycled Aggregate Concrete

Waste concrete after crushing screening can be used in the production of recycled concrete aggregates(RCA).Compared with natural aggregate,the recycled aggregate has high crushing index and high porosity,small density,stronger water absorption,weak bonding ability.The characteristics of recycled aggregate lead to poor low mechanical strength and durability of recycled concrete.Many researchers have proposed different methods to improve the performance of recycled aggregate.Removing and strengthening the adhered mortar are the two common methods for improving the properties of RCA.Removing adhered mortar including mechanical grinding,heat grinding and pre-soaking in acid.Strengthening adhered mortar including polymer emulsion,pozzolan slurry,calcium carbonate biodeposition and carbonation.In these methods,the acid treatment may bring durability problems,and mechanical grinding requires human and financial resources.Carbon dioxide curing is one of the most economical and environmentally friendly method to improving the properties of RCA.Therefore,this paper mainly focuses on the properties of recycled aggregate after carbonated.Meanwhile,the effect of carbonated recycled aggregate on the permeability and interface transition zone of recycled aggregate concrete will be discussed.First,we broken recycled aggregate for carbonation,and then the physical properties of the aggregate were tested.The results showed that the water absorption and crushed value of recycled aggregates are decreased,the apparent density increases after carbonated processing.The compressive strength of recycled concrete shows that the carbonated treatment of recycled aggregate increases the compressive strength of recycled aggregate concrete.In addition,the effect of carbonization treatment of recycled aggregate on the permeability of recycled aggregate concrete was evaluated by water absorption,Autoclam gas permeation,Autoclam water permeation,carbonization and rapid chloride ion transport experiments.The results show that the carbonated recycled aggregate can effectively reduce the water absorption of recycled aggregate concrete and improve the anti-gas permeation and water permeation resistance of recycled aggregate concrete.At the same time,carbonated recycled aggregate improves the carbonation resistance of recycled aggregate concrete and the resistance to chloride ion permeability is also improved.The research on the old and new interface transition zones(ITZs)of fresh concrete interface is mainly based on microhardness and backscattered image analysis.The results of microhardness show that the carbonated recycled aggregate increases the microhardness of the old interface transition zone of the recycled aggregate concrete.Carbonation treatment has no effect on the thickness of the old interface transition zone,but the thickness of the new interface transition zone is from 40-70 mm to 30-50 mm after carbonization.The backscattered image also shows that the carbonated recycled aggregate makes the pores of the interface transition zone of recycled aggregate concrete decrease and the structure is more dense.The results of phase analysis show that the porosity and the content of calcium hydroxide in the interface transition zone of recycled aggregate concrete are reduced.In addition,in this paper,the cement paste was used to simulate the cement powder produced in the production of recycled aggregate.The characteristics of carbonated cement powder were studied,and the influence of cementing on cement hydration was discussed.The results show that the silica powder and calcium carbonate are formed after the carbonization of the cement powder,and the main crystal form of calcium carbonate is calcite.The silica gel extracted from cement paste has the greatest influence on the cement hydration heat,followed by the carbonated cement powder.The silica gel extracted from cement paste increased the hydration rate of the rapid reaction period,and delayed the occurrence of the acceleration period.The addition of carbonated cement powder also accelerates the hydration rate of cement during the rapid reaction period,but the hydration rate is still lower than the silica gel.The addition of uncarbonated powder reduces the strength of the cement paste,and the higher the content,the lower the strength.The strength of the cement paste is increased by the addition of the carbonized cement powder in the range of 0 to 10%.When the dosage is more than 10%,the strength of the cement paste decreases with the increase of the content.