Synergistic effect of CaO and red mud on the water resistance of phosphogypsum-based cementitious materials

To enhance the water resistance of phosphogypsum-based cementitious materials and promote solid-waste utilization, red mud (RM) was incorporated to develop phosphogypsum-based composites (RPG). Given the low reactivity of RM due to its inert crystalline phases, calcium oxide (CaO) was introduced as an activator. This study investigates the effects of RM and CaO dosages on the compressive strength, flexural strength, and softening coefficient of RPG. The hydration mechanisms were characterized using XRD, SEM, and MIP. Optimal performance was achieved in the RPG mixture containing 10% RM and 2% CaO, with 28-day compressive strength, flexural strength, and softening coefficient of 39.27 MPa, 9.15 MPa, and 0.85, respectively. These values represent enhancements of 4.3%, 13.8%, and 11.8% compared to the mixture without CaO. This enhancement is attributed to the effective activation of RM by CaO, which promotes the release of Al³⁺ ions. These ions react with Ca(OH)₂ and SO₄²⁻ to form interwoven AFt; the AFt will fill pores and create a dense microstructure. However, a high CaO content of 5% resulted in the lowest softening coefficient (a 4%–7% decrease), as excess CaO led to abundant Ca(OH)₂. This not only introduced inhibitory Ca²⁺ but also promoted a carbonation reaction, compromising performance.