| As an important component of concrete,fine aggregate determines the performance of concrete directly.Due to the growing shortage of natural sand resources and the strict implementation of environmental protection policies,it is imperative to replace natural sand with manufactured sand.Manufactured sand can improve the strength of concrete effectively.However,a large amount of stone powder and harmful clay components are often mixed in the process of mining,transportation and storage of manufactured sand,and excessive fine powder content reduce the working performance of concrete.At present,the rationality of the existing evaluation methods for key technical indicators of fine aggregates is still unclear,and its test methods have the disadvantages of cumbersome operation,long time-consuming and poor precision.The rapid evaluation and detection methods for fine powder content in fine aggregates are different.Research on the effects of types of stone powder and clay on the performance of concrete is scarce.This paper firstly determines the key technical indicators of fine aggregates,then optimizes the traditional methylene blue method,and a method for rapid and accurate determination of composition and content of hazardous clay in laboratory and engineering fine aggregate was established.Moreover,the effects of stone powder content and clay composition on the performance of concrete in manufactured sand were systematically studied by means of rheological tests of cement-based materials,early shrinkage test of concrete,mechanical properties test,chloride ion permeability resistance test and single-sided salt freezing test.The main conclusions are as follows:(1)According to the correlation analysis and grey correlation calculation results,the key technical indicators affecting the workability of cement-based materials were determined to be methylene blue value and stone powder content,among which methylene blue value was mainly controlled by the component of expansive clay in the manufactured sand.In addition,the problem of limiting the large-scale promotion and use of manufactured sand is mainly that the content of stone powder and methylene blue value exceed the standard requirements.Therefore,in concrete engineering,the key technical indicators for controlling the quality of manufacturedsand are the content of stone powder and the existing expansive clay.(2)The optimized methylene blue method is easy to operate,and the test results are accurate and reproducible.It only takes 10 min to test a sample,realizing rapid laboratory and field detection of harmful clay composition in fine aggregate;Optimized methylene blue method can distinguish between expanded clay and non-expanded clay,and there is a linear correlation between the MBV of the manufactured sand and the clay content.(3)When the stone powder content in the manufactured sand ?10% and does not contain the expanded clay,the stone powder has no harmful effect on the concrete performance and can even improve the mechanical properties of the concrete.Therefore,it is recommended that the industry standard should appropriately increase the limit value for the stone powder content below 0.075 mm.If the manufactured sand contains expanded clay,the workability of the cement-based material will be reduced,and the self-shrinkage of the concrete will increase.When the clay content in the manufactured sand ?3%,the mechanical properties of the concrete can be improved,and the clay content in the manufactured sand can be relaxed in the projects with low requirements on the workability of concrete(4)The stone powder in the manufactured sand can promote the durability of concrete,mainly by filling the pores in the concrete structure,so as to increase the density of the concrete structure.Expansive clay exists in concrete,which will absorb moisture and then produce expansion effect.After losing water,it will shrink in volume,resulting in excessive micro-cracks and voids in concrete,thus leading to reduced durability of concrete. |
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