| As a way to deal with construction waste,the utilization of recycled fine powder can refine the recycling way of construction waste and realize the reuse value of construction waste.Cement will consume a lot of energy in the production process.Using recycled fine powder as a mineral admixture to replace part of cement can save energy,reduce greenhouse gas emissions,and realize the recycling of construction waste.The alkali-activated cementitious material is considered to be the most likely material to replace cement due to its early strength,good durability,and green low carbon.A new type of green cementitious material was prepared by combining recycled fine powder with alkali-activated cementitious material.Exploring a new way of using recycled fine powder is of great significance for disposing solid waste and saving raw materials.In this paper,waste concrete and waste red brick were crushed,grinding and screened to produce recycled concrete and red brick fine powder.The two kinds of fine powder were mixed in five ratios(3:1,2:1,1:1,1:2,1:3)to obtain composite recycled powder.The particle sizes of the three kinds of fine powder were all ≤0.15 mm.Three kinds of recycled fine powder were used in cement system,alkali-activated slag system and alkali-activated slag+fly ash system with 0 %,10 %,20 %,30 % and 40 %respectively,to explore the influence law of the amount of recycled fine powder on the flow and mechanical properties of cement system and alkali-activited system,and study the activity effect of recycled fine powder.The effects of recycled fine powder on the rheological properties and microscopic properties of cement paste and alkali-activated paste were studied by rotational rheometer and scanning electron microscope.The heat release process of hydration was analyzed by microcalorimeter to study the effect of recycled powder on the hydration process and alkali-activated hydration process of cement.Finally,the optimum content of recycled fine powder and the hydration mechanism of recycled fine powder in alkali activation system are obtained.The main results are as follows :(1)The flow performance of cement mortar was reduced with the increase of the content of three kinds of recycled fine powder,and the decrease range was in the order of composite recycled powder > recycled red brick powder > recycled concrete powder.The incorporation of recycled concrete powder increases the fluidity of alkali-activated mortar,while the incorporation of recycled red brick powder reduces the fluidity of mortar.The incorporation of composite recycled powder also reduces the fluidity of the alkali-activated system mortar,and the higher the proportion of recycled red brick powder in the composite powder,the greater the fluidity loss.(2)The incorporation of three kinds of recycled powder will reduce the mechanical properties of cement mortar.According to the method of JG / T573-2020,the activity indexes of recycled concrete powder and recycled red brick powder are 60.3 % and72.3 %,respectively,when the content of recycled powder is 30%.The activity indexes can meet the requirements of grade II(activity index ≥ 60 %)and grade I(activity index ≥ 70 %).The suitable content of recycled concrete powder in alkali-activated system is 10 %-30 %,and the mechanical properties reach the best at 10 %.The appropriate content of recycled red brick powder in alkali-activated system is 10 %-40 %,within which the compressive strength loss is not more than 10 %.In alkali-activated slag + fly ash,the suitable content is not more than 30 %.The incorporation of composite powder has little effect on the mechanical properties of the alkali-activated system,and the strength reaches the maximum when the ratio of RCP to RBP is 1 : 1 and 1 : 3.(3)The cement paste and alkali-activated paste mixed with recycled powder still conform to the rheological characteristics of Bingham fluid.The yield stress and plastic viscosity of cement paste decrease with the increase of recycled concrete powder content.With the increase of the content of recycled concrete powder,the plastic viscosity of alkali-activated slag slurry decreases,and the yield stress increases first and then decreases.The increase of recycled concrete powder content makes the yield stress and plastic viscosity of alkali-activated slag + fly ash slurry show an upward trend.The increase of recycled red brick powder content makes the yield stress and plastic viscosity of cement paste and alkali-activated slag paste decrease,and the yield stress and plastic viscosity of alkali-activated slag + fly ash paste increase first and then decrease.The incorporation of composite powder makes the yield stress and plastic viscosity increase with time.(4)After adding recycled powder,the hydration products are mainly C-S-H and C-A-S-H gel,and there are some N-A-S-H gels in the alkali activation system.The active small particles in the recycled powder can fully participate in the reaction,and the inactive large particles are encapsulated by the gel material to form a dense structure.The increase of the content of recycled concrete powder and recycled red brick powder will make the microstructure of hydration products rough and uneven.When the proportion of recycled red brick powder in the composite powder is more,its microstructure is denser.(5)The increase of recycled powder content will reduce the hydration heat and hydration heat release rate of alkali-activated system.The recycled concrete powder delays the peak time of the second exothermic peak and prolongs the duration.The recycled red brick powder has little effect on the peak time of the second reaction exothermic peak,but the duration is prolonged.The addition of composite opowder reduces the total heat release of cement hydration,and the more the proportion of recycled concrete powder,the lower the total heat release of hydration.It can promote the hydration reaction of cement,advance the time of the second reaction exothermic peak,and the more the proportion of recycled red brick powder,the earlier the reaction peak reaches the peak.The addition of composite powder reduces the total amount of hydration heat and the rate of hydration heat release of the alkali-activated system,and the more the proportion of recycled red brick powder,the higher the total amount of hydration heat release. |
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