Experimental Study On The Activity Of Recycled Fine Powder Composite Cementitious Material

With the rapid development of urbanization and renewal in China,it has caused two major problems: the sharp increase in cement demand and the large-scale generation and disposal of waste cement-based materials.Therefore,the recycling of waste cement-based materials has become a research hotspot for researchers.In this paper,waste mortar is used as raw material to prepare the reference recycled fine powder that meets the physical properties of fly ash,and the physical properties of recycled fine powder are optimized by adding grinding aid and heating.The physical properties of single and composite recycled fine powder cementitious materials,such as fineness,fluidity,setting time and strength ratio of cementitious sand,were studied through single mixing and compounding experiments of various kinds of recycled fine powder.The recycled fine powder with different activity index grades was screened out.The influence of particle group characteristics of recycled fine powder on its rheological properties was studied by laser particle size test and rheological test.The early hydration activity mechanism of the recycled fine powders of various activity index grades was further analyzed by hydration heat test.The experimental results show that the specific surface area,mortar fluidity and final setting time of the regenerated fine powder can be reduced to some extent by adding grinding aids,while the setting time of the regenerated fine powder can be increased by heating treatment in a certain range.The activity index of single recycled fine powder retracts in the later stage,but when the recycled fine powder is mixed with fly ash and slag powder in binary or ternary way,there is no retraction phenomenon of activity index in 28 d,and the composite effect of slag powder is better than that of fly ash,and the activity index is obviously increased.Since the single recycled fine powder contains more inactive sand powder,the coarse particles account for a larger amount,and the powder is generally coarser,but the characteristic particle diameter is less than 40?m,which indicates that it still has potential activity.The addition of grinding aid can reduce the fineness of recycled fine pewders to a certain extent and increase the activity of the 28 d mortar.The heat treatment can reduce the overall fineness of the powder,but the effect on the activity is not obvious.The highest activity index level of recycled fine powder binary composite cementitious materials can reach H95,while the activity index level of ternary composite cementitious materials is between H75 and H85.The recycled fine powder and its composite cementitious material belong to the Bingham fluid model and have obvious shear thinning characteristics.The shear stress of binary composite cementitious material slurry increases with the increase of shear rate,while the plastic viscosity decreases with the increase of shear rate.When the powder particle size is similar,the yield stress and plastic viscosity of the binary compound sample slurry are consistent with the increase of the uniformity coefficient,and the yield stress of the ternary compound sample slurry is consistent.The stress decreases with the increase of the uniformity coefficient of the powder compound,and the plastic viscosity increases first and then decreases.The hydration reaction of the recycled fine powder composite cementitious material contains two exothermic peaks,which occur within about 0~0.1h in the early stage of induction,and the hydration exothermic peak in the accelerated period appears in about 10h~13h.In the early stage,especially in 1d,the heat release was large,and the 7d compressive strength and the hydration heat release basically satisfied the positive correlation.Using recycled fine powder to replace cement in engineering not only reduces the consumption of natural resources and environmental pollution in cement production to a certain extent,but also realizes the recycling of waste mortar.In this paper,the technology of preparing recycled fine powder from waste mortar is explored,and the basic properties of recycled fine powder composite cementitious material are systematically analyzed,which has reference value for the production and popularization of recycled fine powder.