Excess-sulphate phosphogypsum slag cement blended with magnesium ion: Part Ⅱ-the long-term microstructure characterisation and phase evolution

Magnesium ions, as the most destructive ions in seawater, significantly affect the durability of cement-based structure. Excess-sulphate phosphogypsum slag cement (ESPSC) with great resistance to chemical attack, exhibits a potential application in marine engineering. As a continuation of previous research, the long-term performance development and microstructure evolution of ESPSC mixed with water including various magnesium ion concentrations (0–0.5 mol/L), are investigated in this study. The results indicate that incorporating magnesium ions within 0.2 mol/L has optimised the later compressive strength, as well as the pore structure. Furthermore, the components and nanostructure of C-(A)-S-H gels change due to the charge balance and cation compensation caused by magnesium ions. The phase evolution of AlⅣ to AlⅥ and an increase in hydration degree have been detected, where the aluminium phase tends to exist stable as ettringite. The M-S-H gel production is detected in samples incorporating 0.5 mol/L magnesium ions at early stage, which is detrimental to the gelatinous structure and strength development of ESPSC. From the perspectives of technological feasibility and environmental requirements, ESPSC production is an effective method for CO2 emission reduction and phosphogypsum utilisation with scalable potential. Introducing magnesium ions through seawater is also regarded as a modification to enhance the mechanical performance of ESPSC.