1 Department of Civil Engineering, Tsinghua University, Beijing 100084; 2 China Construction First Group Construction and Development Co., Ltd., Beijing 100102
Abstract: Copper slag (CS) is the by-product of the smelting and refining process of copper. It has been estimated that the production of 1 t blister copper generates 2.2 t slag, and the global copper slag emissions reached 51.7 million tonnes in 2017. As the most common treatment for CS, the landfill not only occupies a great quantity of soils, but also lead to severe ecological environment pollution all around due to the heavy metals in CS. CS, especially the water-quenched one, contains large vitreous fraction, which can be used as the concrete admixture instead of fly ash, slag and other mineral admixtures to relieve the shortage of admixture in some areas. This review summarizes the influence of CS powder on the workability, mechanical properties and durability of concrete, analyzes the environmental safety of copper slag as admixture, and points out several issues that should be paid attention to in the use of copper slag as admixture. It provides a reference for large scale application of copper slag powder in concrete industry. Recent researches revealed that there is high vitreous fraction in water-quenched CS with favorable activity. The vitreous content in cooling CS is low, nevertheless, it presents acceptable activity. Owing to the high heavy metal composition and low activity, the impact of CS on the workability, mechanical properties and durability of concrete is much more complicated than that of common admixtures like slag and fly ash, which hinders the application of copper slag powder as admixture in concrete. Although there are different opinions about the effect of CS, it is generally believed that the utilization of CS powder as admixtures can reduce heat release, water consumption and improve workability, while increase the bleeding rate. Thanks to the low activity and high heavy metal content, the CS powder shows notable retarding effect, yet the extension of setting time caused by using large amount of CS powder can be controlled in most cases. The insufficient early strength of concrete resultes from CS powder can be alleviated by adding activators, improving fineness, and reducing W/B ratio. The negative effect of the CS powder on the strength can gradually ease with age, and even it is beneficial to concrete strength after a certain period of time. Besides, CS powder can also improve the elastic modulus of concrete and reduce shrinkage, and has no obvious adverse effect on the fracture resistance of concrete. However, the role of CS powder on the brittleness of concrete is still controversial. CS powder is helpful to reduce harmful pores like capillary pores and further reduce the water absorption rate and amount, which result in a improved durability of concrete. In the hydration reaction of CS powder, owing to the increment of C-S-H gel formation, the interfacial transition zone is optimized, and the anti-carbonation, sulfate resistance and chloride ion penetration resistance of concrete are improved.
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