Deterioration mechanism of supersulfated cement paste exposed to sulfate attack and combined acid-sulfate attack

Supersulfated cement is mainly composed of granulated blast furnace slag, gypsum and a small amount of alkaline activator. The objective of this work is to study the deterioration mechanism of supersulfated cement paste containing phosphogypsum (SSC) exposed to sodium sulfate and hydrochloric acid-sodium sulfate solutions, which was compared to that of ordinary Portland cement (OPC) paste. The mechanical properties of SSC were characterized by compressive strength and its retention rate. Microscopic techniques such as XRD, FTIR, TG-DTG, MIP and SEM were employed to analyze hydration products changes and microstructural alterations in SSC paste, unveiling the deterioration mechanism. Soaked in Na2SO4 solution, OPC generated erosion products ettringite and gypsum, which led to expansion and cracking of the cement paste. In contrast, the hydration products of SSC remained largely unchanged, demonstrating high resistance to sulfate attack. Soaked in HClNa2SO4 composite solution, H+ and SO42- reacted with Ca(OH)2 in OPC leading to gypsum formation and subsequent deterioration of the cement paste. The reaction of SSC with H+ and SO42- in the environment resulted in ettringite decomposition, C-S-H decalcification and gypsum formation, which damaged the structure of SSC paste. Nevertheless, compared to OPC, SSC exhibited the higher ability to maintain mechanical performance under aggressive environment.