Changes in cement paste during accelerated mortar bar testing for pyrrhotite containing aggregate

The accelerated mortar bar test (AMBT) for pyrrhotite (Fe_1-xS) containing aggregates accelerates expansion using two regimes: Phase I, during which Fe_1-xS oxidation to iron hydroxide is accelerated by bleach and storage at 80 °C; Phase II that promotes thaumasite formation with continued bleach soaking and storage at 4 °C. Two bleach concentrations and one aggregate containing 0.5 wt% Fe_1-xS are tested. SEM-EDX data indicate that the oxidized iron precipitates as iron hydroxide at the place of the Fe_1-xS, leading to significant expansion (0.22%) during Phase I. The released S distributes in the cement paste but leads only to limited amount of additional ettringite and thaumasite and thus only to limited expansion (0.07%). The NaOCl bleach reduced during the test resulting in chloride and thus to Friedel's salt formation and ettringite stabilization at 80 °C. Only during the prolongation of Phase II, the formation of thaumasite and additional expansion was observed.