Study On The Characteristic Index Of Residual Mortar Content Of Recycled Fine Aggregates From Waste Concrete

As China’s urbanization continues to accelerate,the demand for construction sand and gravel has increased,leading to a series of problems faced by the construction industry,such as the lack of natural resources.At the same time,the demolition of a large number of old buildings in the process of urban development has produced a large amount of waste concrete,the disposal of which is still mainly in landfills and piles,which occupies a large amount of land and seriously affects the living environment of human beings.At present,domestic and foreign research on recycled aggregate mostly focuses on recycled coarse aggregate,resulting in the utilization rate of recycled fine aggregate is much lower than that of recycled coarse aggregate,so there is an urgent need to study the nature and characteristics of recycled fine aggregate,so that a large number of waste concrete can be more efficiently disposed and utilized,in line with the requirements of national sustainable development.With the funding and support of the National Key R&D Program(SQ2020YFC190071),this paper firstly investigates the quantitative characterization method of the residual mortar content of recycled fine aggregate and the relationship between the residual mortar content and the particle size of recycled fine aggregate,secondly explores the relationship between the residual mortar content of different particle sizes of recycled fine aggregate and its basic properties,and finally conducts a full recycled fine aggregate mortar and concrete The relationship between the residual mortar content of recycled fine aggregate and the mechanical properties of fully recycled fine aggregate mortar and concrete was investigated in the test,and the main research contents and conclusions of this paper are as follows.(1)The quantitative characterization method of the residual mortar content of recycled fine aggregate was studied,and the experimental results of the residual mortar content of silica-calcium recycled fine aggregate were verified by using the single-component verification test of recycled fine aggregate and X-CT imaging analysis.The results showed that the residual mortar content of silica recycled fine aggregate can be quantified by immersion dissolution method using hydrochloric acid solution,and the residual mortar content of silica-calcium recycled fine aggregate can be quantified by grinding dissolution method using salicylic acid-methanol solution.Meanwhile,the applicability of the salicylic acid-methanol solution grinding and dissolution method for determining the residual mortar content of silica-calcium recycled fine aggregates was verified by single-component validation tests and X-ray CT imaging analysis of silica-calcium recycled fine aggregates.(2)Seven recycled fine aggregates were sieved into three different grain sections of0.6～1.18 mm,1.18～2.36 mm and 2.36～4.75 mm,and the relationship between the residual mortar content of recycled fine aggregates and their grain sections was analyzed.The results showed that the residual mortar content was negatively correlated with the particle size of the recycled fine aggregate,and the reduction of the residual mortar content in the 0.6～1.18 mm to 1.18～2.36 mm particle size of the recycled fine aggregate was greater than that in the 1.18～2.36 mm to 2.36～2.36 mm particle size.～4.75 mm particle size,and the residual mortar content may not be directly related to the strength grade of the original waste concrete.(3)The relationship between the residual mortar content of recycled fine aggregates and their basic properties was investigated.The basic properties of recycled fine aggregates were determined according to the current recycled aggregate specifications,and the relationship between the residual mortar content of recycled fine aggregates and their basic properties under three different grain sections was investigated in combination with the residual mortar content of recycled fine aggregates.The results showed that the water absorption and crushing index of recycled fine aggregate were positively correlated with the residual mortar content,and the apparent density was negatively correlated with the residual mortar content.According to the correlation between the residual mortar content and the basic properties of the three different granular aggregates,the corresponding fitted equations were proposed to predict the basic properties of the recycled aggregates with known residual mortar content.(4)The relationship between the mechanical properties and the residual mortar content of fully recycled fine aggregate mortar and concrete at different ages was investigated using recycled fine aggregate with different residual mortar contents.The results showed that the residual mortar content was negatively correlated with the mechanical properties of both fully recycled fine aggregate mortar and concrete,and the contribution of the decreasing residual mortar content to the strength improvement of mortar and concrete was gradually reduced with the increase of age.Based on the correlation between the mechanical properties of fully recycled fine aggregate mortar and concrete and the residual mortar content,the corresponding fitted equations are proposed to predict the mechanical properties of fully recycled fine aggregate mortar and concrete with known residual mortar content.(5)In the fitting relationship between the residual mortar content of recycled fine aggregate and its basic properties as well as with the mechanical properties of fully recycled fine aggregate mortar and concrete,95% confidence interval and 95% prediction interval are introduced to increase the degree of inclusion of the overall data and prediction data for the discrete and heterogeneous nature of recycled fine aggregate,which provides a basis and reference for improving the utilization rate of recycled fine aggregate in the building materials industry in the future.