Study On Properties Of Alkali Activated Recycled Micro Powder Foamed Concrete

With the acceleration of urbanization construction,a large amount of construction waste has been generated in the demolition and renovation of existing buildings.During the crushing process of waste concrete in construction waste,a large number of construction waste micro powders with uneven fineness and complex components are produced.After further grinding to a fineness of less than 0.16mm,they become recycled micro powders.Due to its low activity and large porosity,the recycled micro powders has not been effectively utilized.Therefore,exploring the large amount and high value-added utilization of recycled micro powder in the building materials industry is of great significance to social and economic development.In this paper,the effects of water cement ratio,Na2O content and mineral admixture on the mechanical properties,dry apparent density,water absorption,dry shrinkage,pore structure and thermal conductivity of the alkali activated recycled micro powder foamed concrete（AARMPFC）were systematically studied by using recycled micro powder,slag powder,fly ash and alkali activator.The influence of mortar and alkali activated recycled micro powder foamed concrete on mechanical properties is expounded,and the formation and evolution of the microstructure of alkali activated foamed concrete are clarified,so as to provide theoretical basis and scientific method for its preparation and application.The main results are as follows:（1）With the increase of water cement ratio,the compressive strength and dry apparent density of foamed concrete decrease gradually and water absorption increases.With the increase of Na2O content,the compressive strength and dry apparent density decreased first and then increased,and the water absorption was on the contrary.When the Na2O content is higher than 8%,the alkali solution prevents the foamed from being evenly distributed,and the foamed stability is poor.With the increase of fly ash content,the compressive strength of foamed concrete decreases,water absorption gradually increases,dry shrinkage decreases,porosity increases,and thermal conductivity decreases.（2）When the curing temperature is 20℃,40℃ and 60℃,increasing the curing temperature can improve the activity of silicon aluminum phase in recycled micro powder,intensify the "depolymerization polycondensation" reaction,significantly speed up the early hydration reaction and improve the early compressive strength of the specimen.When the curing temperature is 80℃ and the curing time exceeds 12h,it will have an adverse impact on the strength.The optimum curing temperature is 60℃ and the curing time is 6h.Under the optimum curing system,the compressive strength of standard mortar specimens at 28 days of age can reach 61.64MPa;The compressive strength of A06 grade alkali activated foamed concrete is increased by more than 50%.（3）Alkali activated recycled micro powder foamed concrete is consistent with the hydration products of cementitious materials,mainly C（N）-A-S-H gel and quartz,calcite and dolomite brought in the raw powder.The curing temperature is too high,which leads to the excessive temperature difference between the inside and outside of the paste specimen,and a large number of micro cracks appear in the microstructure,affecting the later strength development.Fly ash has the function of optimizing the pore size and reducing the pore shape factor,and improving the thermal insulation performance of alkali activated foamed concrete.There is a nonlinear relationship between thermal conductivity and porosity,which can be used as k=1.36e（-φ/27.56）-0.045 fitting equation shows that the correlation coefficient R2 is 0.9012,which can accurately predict the thermal conductivity of foamed concrete with different porosity.（4）The best ratio of alkali activated recycled micro powder foamed concrete is recycled micro powder 60%,slag powder 20%,fly ash 20%,water cement ratio 0.45,excitant modulus 1.4,Na2O content 6%,and the foamed concrete prepared has the best comprehensive performance.The A08 grade alkali activated recycled micro powder foamed concrete has a 28 day compressive strength of 4.13MPa,a water absorption of 42.1%,and a thermal conductivity of 0.1036W/（m·K）.