Feasibility Study of Waste Gypsum as a Full Replacement for Fine Aggregates of Controlled Low-Strength Material

: The waste gypsum used in this study was a by-product of petroleum coke burning by petrochemical companies which has been treated with hydration. The waste gypsum has been stored in the atmosphere for over ten years and can be considered an inert filler. Its main chemical components were calcium sulfate dihydrate (47.90%), calcium hydroxide (21.64%) and calcium carbonate (14.80%). In this study, Portland cement and fly ash were used as cementitious materials, and waste gypsum of the particle size from 9.53 mm to 0.149 mm was selected as the fine aggregate to produce a controlled low-strength material (CLSM) and to verify the suitability of reusing waste gypsum. The water to binder ratio of 0.65 was used for the specimen. The test results showed that the CLSM specimen with a high amount of waste gypsum had air-hardening properties. The placement of the specimen in water caused abnormalities, such as cracking and disintegration of the specimens. The compressive strength of atmospherically maintained specimens increased with age, with 4.71 MPa and 6.08 MPa at 28 and 56 days, respectively. No significant changes in weight or volume were measured after the specimens had been left for 56 days and then immersed in seawater and water for 28 days. As specimens were immersed in seawater for up to 100 days, needle-shaped ettringite and C-S-H colloids filled the interface between the pores and the colloids. In accordance with the concept of eco-engineering, special consideration should be given to avoid long-term contact with water and to ensure the safety and durability of waste gypsum reuse through the design of multiple protective layers.