Abstract
Sulphate ions induce severe chemical deterioration in a cement matrix due to expansive product (e.g. ettringite and gypsum) formation or sodium sulphate crystallization. Considerably high sodium sulphate concentration can cause U-phase [(CaO)4(Al2O3)0.9(SO3)1.1(Na2O)0.5:16H2O] formation, which leads to increased deterioration of the cement matrix after prolonged exposure to service aqueous environments. However, U-phase stability in cementitious materials is not well understood. This study evaluated U-phase formation in hydrated white Portland cement (WPC) and slag-blended WPC (42 and 70% slag ratio) and observed its degradation in water and sodium sulphate and magnesium sulphate solutions. The U-phase was found to coexist with ettringite in WPC and slag-blended WPC when hydrated with 13% Na2SO4 solution, wherein the hydration reaction and slag addition promoted U-phase formation. The U-phase-containing hydrated cement was immersed in Na2SO4 and MgSO4 solutions for 28 days to investigate phase changes. In addition, the hydrated samples were exposed to water, 1300 mmol/L Na2SO4, and 1300 mmol/L MgSO4 for 1 year to determine the changes in spatial mineralogical distribution. Ettringite formed due to the reactions between sulphate and cement hydrates as well as U-phase destabilisation. This ettringite formation was found to be the primary degradation mechanism in hydrated cement exposed to water and Na2SO4. However, MgSO4 degradation was attributed to gypsum formation. By replacing a proportion of the cement with slag in the slag-blended cement, the material exhibited a very high resistance to external Na2SO4 attack, but it was severely susceptible to damage in the MgSO4 environment.
Published Version
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