Durability Of Recycled Aggregate Seawater Sea-sand Concrete

With the development of island construction in China,a large number of original old buildings have been demolished,and the problem of construction waste pollution is becoming increasingly serious.The use of construction waste as Recycled Aggregate（RA）to reduce the resource consumption of Natural Aggregate（NA）in the production of ordinary concrete has been more and more recognized in the construction industry.If recycled aggregate,sea water and sand can be used to thesis concrete for Marine engineering construction,it can not only realize the resource utilization of construction waste,but also alleviate the shortage of natural resources.However,the preparation of Recycled Aggregate Sea-sand Concrete（RASSC）and its durability mechanism in Marine environment are still not clear,which seriously restricts the application and promotion of RASSC.In view of this,the basic mechanical properties and durability of RASSC were studied in this thesis,and the improvement effect of mineral admixtures on defects of RASSC and potential alkali aggregate reaction was explored.The main research contents and achievements of this thesis are as follows:（1）The orthogonal experimental study of recycled aggregate seawater sand concrete was carried out to explore the influence of material variables on the work and compressive properties of RASSC in combination with macro mechanical properties and microstructure.The results show that:The Interfacial Transition Zone（ITZ）is the key region for controlling RASSC strength,and the rhomboid silica fume particles and fly ash beads filling this Zone are surrounded by Calcium Silicate Hydrate（C-S-H）and Calcium Aluminate（C-A-S-H）can effectively strengthen the weak zone of RASSC and mortar strength.Due to the double improvement effect of silica fume,fly ash micro-aggregate gradation and pozzolanic reaction,the compressive strength of RASSC is significantly affected by silica fume,fly ash and recycled aggregate in turn.Combined with the test results,the optimal ratio of RASSC is proposed.（2）The long-term mechanical properties evolution of RASSC in laboratory simulated seawater environment was studied.The results show that the RASSC axial compressive strength,flexural strength and elastic modulus of mixed ore powder can be divided into performance growth stage（0～180 days）and performance degradation stage（180～360 days）under seawater immersion environment.Under the condition of fly ash and silica fume,hydration and pozzolanic reaction are carried out continuously inside concrete to refine the pores of concrete and enhance the mechanical properties of concrete.With the deepening of chloride erosion on concrete,a channel conducive to the penetration of harmful ions is formed.Chloride ions and sulfate ions react with tricalcium aluminate and tetrachalcium ferroaluminate to generate Friedel’s salt crystals,resulting in the expansion,damage and degradation of concrete.（3）The time-varying law of RASSC expansion in seawater immersion environment for a long time was studied.The results show that:There is no obvious alkali aggregate reaction in RASSC,but its expansion rate will gradually increase with the extension of soaking time,the increase of ambient temperature and the increase of recycled aggregate content.However,the addition of fly ash has a good effect on inhibiting expansion,which is attributed to the pozzolanic reaction of fly ash can consume Ca（OH）₂.C-S-H and C-A-S-H gels can fill the internal pores of concrete,reduce the diffusion rate of water,K⁺and Na⁺,and prevent the occurrence of expansion.