Study on the inorganic synthesis from recycled cement and solid waste gypsum system: Application in grouting materials

Grouting reinforcement technology plays an important role in the field of disaster prevention and control of deep underground engineering. Due to the insufficient workability and mechanical property of recycled cement (RC) under high water to solid ratio, there is still a blank about recycled cement used in grouting materials. In order to propose a new high-performance, low-cost environmental grouting materials and expand the application field of RC, this study examined the effect of phosphogypsum and flue gas desulfurization gypsum on the performance and microstructure of RC thermal activated with different temperatures, high purity natural gypsum and blank sample were designed as control group. Test results indicated that three kinds of gypsum all prolonged the initial setting times, and improved the fluidity and mechanical properties of RC grouts. Especially, 10% phosphogypsum added to the 750 °C thermal activated RC, optimal proportion of RC to gypsum system, can obtain a higher compressive strength (6.33 MPa for 3d and 11.26 MPa for 28d) than P.O.42.5 cement with 1.0 water to solid ratio, prolong the initial setting times from 5 min up to 27 min, which makes the RC capable of using as grouting materials. The microscopic analysis results showed that the increase in the compressive strength was attributed to gypsums can promote the generation process of C-S-H and ettringite in RC, the impurity of phosphogypsum also participate in the hydration process and generated crystalline substances such as calcium phosphate silicate.