Effects of Soaking Time and Na2SO4 Concentration on Engineering Properties of Recycled Concrete Aggregate Treated by Using Cement—Fly Ash Slurry

The exploitation of natural resources provides raw materials for construction such as river sand, crushed stone, and so on. They are non-renewable resources and overexploitation causes them to gradually deplete. Besides, breaking up buildings for various purposes creates a large amount of construction and demolition waste (CDW), which occupies most of the landfill sites. As a result, the replacement of crushed stone in concrete with recycled concrete aggregate (RCA) from CDW is necessary. However, the engineering properties of concrete using RCA are worse than those of concrete using crushed stone. The aim of this study was to investigate the effects of soaking time and Na2SO4 concentration on engineering properties of RCA treated by using cement—fly ash slurry to evaluate the effectiveness of this technique for improving the properties of RCA. In this study, the RCA had several types including untreated RCA (0RA), RCA treated in a slurry of cement and fly ash for 48 h (TRA), RCA treated in a slurry of cement, fly ash, and 2% Na2SO4 for 24 h (2TRA24N) or 48 h (2TRA48N), and RCA treated in a corresponding slurry with 4% Na2SO4 for 24 h (4TRA24N) or 48 h (4TRA48N). The results showed 4TRA24N had crushing value and water absorption which were thought to be optimal as compared to 0RA and other treated RCAs. The higher the soaking time and Na2SO4 concentration, the lower the crushing values and water absorption of treated RCA. In summary, soaking time of 24 h and a slurry of cement, fly ash, and 4% Na2SO4 were the optimum solution for improving the engineering properties of RCA.