Modification of phosphogypsum using circulating fluidized bed fly ash and carbide slag for use as cement retarder

Cement retarder preparation is one of the main avenues for the resource utilization of phosphogypsum, and appropriate modification technology is the key for ensuring that the phosphogypsum is adjusted for achieving the desired setting time. This study uses carbide slag, circulating fluidized bed (CFB) fly ash, and other solid wastes to comprehensively modify phosphogypsum required for preparing the cement retarder; in addition, it discusses the influence of material ratio and aging time on the modification effect of phosphogypsum. The effects of modified phosphogypsum, undisturbed phosphogypsum, and natural gypsum on the properties of Portland cement were comparatively studied. The modification mechanism of carbide slag and CFB fly ash on phosphogypsum and the mechanism of influence of the modified phosphogypsum on the hydration characteristics of Portland cement were revealed by XRD, SEM, and hydration heat analyses. The results show that adding 6% carbide slag and 4% CFB fly ash can effectively reduce the content of soluble phosphorus and soluble fluorine in phosphogypsum, increase the pH value, enhance the strength of modified phosphogypsum, and enable its use as a cement retarder after aging for 7 d. Compared with the undisturbed phosphogypsum, the setting time of Portland cement made from modified phosphogypsum is significantly shorter; the early-strength is higher, and the compatibility with water reducing agent is better. The microscopic test results showed that the CFB fly ash has good self-hardening properties. The CFB fly ash also showed better hydration hardening performance and curing effect on soluble phosphorus and soluble fluorine under the synergistic excitation of alkali-sulfur by carbide slag and phosphogypsum, which is beneficial for the modification of phosphogypsum and improves the retardation effect of Portland cement.