Eco-friendly solid waste-based cementitious material containing a large amount of phosphogypsum: performance optimization, micro-mechanisms, and environmental properties

Abstract

Reusing phosphogypsum (PG) waste to manufacture eco-cementitious materials is sustainable. However, PG faces the challenge of high stacking volume and low utilization rate, and the low-content PG in eco-cementitious materials currently limits its large-scale potential application. This study provides a solution to use high-content PG to fabricate eco-friendly solid waste-based cementitious material (PBC), associated with ground granulated blast furnace slag (GGBS), fly ash (FA), and hydrated lime. Respond surface methodology (RSM) was undertaken to optimize and discuss the influence of independent design factors of GGBS, FA, and hydrated lime content on the unconfined compressive strength (UCS). Besides, the micro-mechanisms and environmental properties of PBC were also investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and the leachate test. The results indicated that the optimal raw material contents were (by mass): 50.57% PG, 28% GGBS, 20% FA, and 1.43% hydrated lime. The UCS of the optimal PBC mixture is 23.57 MPa at 28 days, which was confirmed via experimental verification (24.65 MPa). Hence, the RSM is an effective method for optimizing PBC's UCS. The micro-mechanisms analysis shows that the ettringite is the primary hydration product within the reaction system of PBC, which is attributed to the substantial amount of PG. The leachate test results indicate that the fluoride ion concentration of the leaching solution of PBC remains below the PG, and the pH value falls within the range of 6–9. The leaching test results meet the Chinese environmental standard of GB 8978-1996. The cost and CO2 emission of the optimal PBC mixture could achieve a reduction of 71.25% and 99.98%, respectively, compared to Ordinary Portland cement (OPC). The findings of this study could serve as a theoretical foundation for the practical implementation of large-scale and efficient utilization of PG waste. The developed eco-friendly PBC has excellent potential to partially substitute conventional OPC binders and reduce engineering project costs.