Effect of MgO and superfine slag modification on the carbonation resistance of phosphogypsum-based cementitious materials: Based on hydration enhancement and phase evolution regulation

This study investigates the role of superfine slag and reactive MgO (0%, 0.33%, 0.66% and 1.0%) on macro-, micro- and nano-scale performances in phosphogypsum-based excess-sulphate slag cement (PESSC). The specimens are cured for 150 d and further accelerated carbonated at 70% RH and 20% CO2 concentration for 28 d and 60 d. The mechanical properties, phase assemblage and morphology, and silicon chain evolution of gels are further characterised with the assistance of strength tests, X-ray diffraction, Fourier transform infrared spectroscopy, thermal analysis, scanning electron microscope, and 29Si nuclear magnetic resonance. The results indicate that additional MgO and superfine slag favour the generation of ettringite and improve the hydration degree of PESSC before carbonation. CO2 curing decreases the strength of PESSC as the direct decalcification and decomposition of gels, ettringite and unreacted slag. As a buffer, the existence of MgO can react with extensive CO2, and delay the formation of calcite with great crystallinity. More vaterite and aragonite with greater solubility are detected, which inhibit the decalcification of the gel, as well as the aluminium uptake in C-A-S-H gel. The samples incorporating MgO display serious carbonation of ettringite, while more gels with low Ca/(Si+Al) and high main chain length (MCL) are formed in neat PESSC. Supplied MgO within 1.0% and superfine powder are considered potential methods to reduce the carbonation degree effectively and improve the durability of this eco-friendly cementitious material to meet the requirements of application.