Experimental Study On Aggregate Optimization Of Slag-metakaolin Base Polymer Recycled Concrete

The performance and behaviour of composite materials depends on the performance of their main components,and in ordinary concrete,aggregates account for 60%to 80%of the total volume of the material,so they play a vital role in the concrete system.With the economic and social development of China and the increase in infrastructure construction,the demand for sand and gravel is increasing steeply,and the long period of mining has made natural sand and gravel resources increasingly scarce,people turn their attention to the study of recycled aggregates in order to reduce the mining of sand and gravel resources and achieve sustainable development.In the production process of recycled aggregates,the poor quality and unstable performance of the recycled aggregates are caused by the poor strength grade of the virgin concrete,the unknown source,the presence of impurities and the lack of fixed gradation,which seriously restrict the improvement of the quality of recycled concrete and its engineering applications.In view of the above problems,this paper conducts an in-depth experimental study on recycled aggregates.The factors affecting the performance of the aggregates were selected and optimisation schemes were designed to compare the effect of aggregate optimisation,to investigate the effect of each factor on the performance of the aggregates and concrete,and to obtain the optimum ratio of aggregates for each performance of concrete.The main research contents and conclusions of this paper are as follows:(1)By reviewing the literature,the factors affecting the properties of the aggregates were identified and used as test parameters.The physical indices of recycled and natural aggregates were tested in comparison.The test results showed that the properties of recycled aggregates were inferior to those of natural aggregates in a number of aspects,making it necessary to optimise the design of recycled aggregates.(2)An orthogonal test was designed to optimise the coarse aggregate ratios and to test the aggregate packing density,the flowability of the fresh concrete and the mechanical properties of the hardened concrete.The results show that the size of the coarse aggregate is a significant factor affecting the performance of the aggregate and the concrete,and that a set of optimum coarse aggregate ratios can be obtained by analysing the test results.In addition,the recycled aggregates within the concrete were crushed and damaged during the compression of the geopolymer recycled concrete,indicating that poor quality recycled aggregates may have become a weak link in the strength of the concrete.(3)A single factor sand rate test was designed to investigate the optimum value of volumetric sand rate.The test results show that changes in the volumetric sand rate have approximately the same effect on aggregate packing density,the flowability of the fresh concrete and the mechanical properties of the hardened concrete.The results show that there is an optimum sand rate that allows the aggregate and concrete properties to reach their peak.(4)Based on the experimental data,a multiple regression analysis was conducted to establish the prediction formula for the compressive strength of cube of recycled concrete based on slag-paraffin base aggregates,and the predicted values were compared with the actual measured values to verify the accuracy of the formula.The results show that the formula can predict the compressive strength of the recycled concrete based on the test parameters well,thus providing a theoretical reference for its application to engineering practice.