Effect of gypsum dosage on the hydration and strength of alite-ye'elimite cement synthesized at 1300 °C

Alite-calcium sulfoaluminate (ACSA) clinker containing about 45 wt% alite and 2 wt% ye'elimite was synthesized at 1300 °C using copper slag and phosphorus slag as raw materials. The effect of different dosages of gypsum on the compressive strength and hydration products at the early ages and long terms of this clinker was investigated. The compressive strength, hydration rates, hydration products evolution and the microstructures development of the pastes were studied. With the gypsum dosage of 3.4 wt% (i.e., the SO₃/Al₂O₃ molar ratio 1.0) in this clinker, the compressive strength was always higher than the synthesized ACSA clinker without gypsum addition and the P·Ⅱ 52.5 at all the curing ages. Especially, the 1 day compressive strength of this case was improved to be 17.7 MPa with AFt largely formed and the alite hydration accelerated. At early ages (1d and 3d), the morphology of hydrates and pore structures can be refined with appropriate dosage of gypsum. At the late ages (more than 28 days), the pore sizes of the transitional pores are slightly increased with the addition of gypsum. However, there is 5.8% of self strength loss after 90 days in this case, which is mainly attributed to the decomposition of AFt. The critical dosage of gypsum can significantly increase the early strength and decrease the usage of clinker, which will make this clinker less carbon dioxide emissions.