Experimental Study On Mechanics And Durability Of Geopolymer Recycled Concrete

With the development of urbanization in China,a large number of concrete buildings（structures）that do not meet the use requirements have been demolished or transformed,resulting in a large amount of construction waste,especially waste concrete.The treatment of these construction waste not only costs a lot of money,but also faces severe environmental challenges.The traditional utilization of waste concrete has the disadvantages of high energy consumption and low utilization rate.Therefore,the research and development of green recycling technology is of great significance to solve the problem of waste concrete.In this paper,the geopolymer is prepared by activating and modifying the waste concrete powder with fly ash and blast furnace slag,and then the geopolymer is mixed with recycled aggregate to prepare recycled concrete.The workability,mechanical properties and durability of geopolymer recycled concrete are studied through experiments,so as to realize the complete recycling of waste concrete.The main research contents and achievements of this paper are as follows:（1）Taking waste concrete powder as the basic powder and fly ash and blast furnace slag as the external powder（single or compound）,the effect of the amount of external powder on the physical and mechanical properties of geopolymer at room temperature was studied through laboratory experiments.The density of geopolymer increases,the setting time decreases and the fluidity decreases when the blast furnace slag is mixed alone.The addition of fly ash can effectively improve the density,fluidity and setting time of polymer.The synergistic effect of blast furnace slag and fly ash can be brought into full play and the later strength of geopolymer can be increased.（2）The phase composition,micro morphology and pore distribution of geopolymer were characterized by X-ray diffraction,scanning electron microscope energy spectrum analysis and mercury injection test.Blast furnace slag can promote the dissolution of waste concrete powder and fly ash particles and accelerate the formation of amorphous aluminosilicate,while fly ash has little effect on the alkali excitation reaction of waste concrete powder at room temperature.The addition of blast furnace slag can make the microstructure of geopolymer more compact,reduce the porosity of geopolymer system and increase the proportion of geopolymer gel pores.（3）Geopolymer recycled concrete was prepared by mixing geopolymer cementitious material and recycled aggregate.The effects of mass ratio of solid powder to aggregate,water consumption and content of recycled fine aggregate on the workability of geopolymer recycled concrete were studied.The results show that the slump and setting time of geopolymer recycled concrete increase gradually with the increase of mass ratio of solid powder to aggregate and water consumption;With the increase of recycled fine aggregate content,the slump and setting time of geopolymer recycled concrete decrease.（4）In order to obtain geopolymer recycled concrete with excellent mechanical properties,the effects of the mass ratio of solid powder to aggregate,water consumption,the content of recycled fine aggregate and the grade of recycled coarse aggregate on the mechanical properties of geopolymer recycled concrete were studied through laboratory tests.The results show that the compressive strength of geopolymer recycled concrete increases with the increase of the mass ratio of solid powder to aggregate,decreases with the increase of water consumption,and first increases and then decreases with the increase of the content of recycled fine aggregate.Under the condition of the same mass ratio of solid powder to aggregate,the strength of geopolymer recycled concrete with different gradation is different,which is related to the difference of geopolymer slurry wrapping aggregate,the amount of water absorbed by aggregate and the overall compactness of concrete.（5）The sulfate resistance of geopolymer recycled concrete is studied through Na₂SO₄erosion test and Na₂SO₄ erosion dry wet cycle coupling test,and the change mechanism of durability of geopolymer recycled concrete is analyzed through micro test.With the increase of Na₂SO₄ concentration,the quality and strength loss of geopolymer recycled concrete accelerate,and the variation range of dynamic elastic modulus increases.The coupling of sulfate and dry wet cycle accelerates the process of polymer recycled concrete eroded by sulfate.The addition of Nano-Si O₂ can obtain more dense geopolymer recycled concrete,so as to slow down the erosion process of Na₂SO₄ solution.The strength reduction of geopolymer recycled concrete is mainly due to the erosion of Na₂SO₄,which leads to the transformation of its internal microstructure from the original dense state to the loose state.The pore structure of geopolymer recycled concrete is mainly harmless pores.The addition of nano Si O₂ can reduce the porosity of harmful pores and harmful pores of geopolymer recycled concrete,and the coupling of sulfate and dry wet cycle will increase the porosity of harmful pores.（6）The carbonation resistance of geopolymer recycled concrete is studied through carbonation test and carbonation freeze-thaw cycle coupling test,and the change mechanism of durability of geopolymer recycled concrete is analyzed through micro test.The carbonation depth of geopolymer recycled concrete increases with the increase of carbonation age.During the carbonization age,the compressive strength and dynamic elastic modulus of geopolymer recycled concrete increase all the time,while the coupling of carbonization freeze-thaw cycle increases the compressive strength of geopolymer recycled concrete first and then decreases.In the process of carbonation,CO₂ will react with the alkali solution in the pores to reduce the amount of crystallization on the concrete surface.The freeze-thaw cycle will lead to the deterioration of the structural integrity of geopolymer recycled concrete,thus speeding up the carbonation process.Nano Si O₂ plays a filling role in the carbonization process of geopolymer recycled concrete,which can effectively reduce the porosity of less harmful pores and harmful pores,while the freeze-thaw cycle will increase the porosity of less harmful pores of geopolymer recycled concrete.