A new approach for assessing the potential alkali expansivity of slowly reactive siliceous aggregates in concrete

The phase assemblage formation resulting from hydration of a tetracalcium aluminoferrite (ferrite) powder (4CaO-Al203.Fe2O3) was investigated by isothermal calorimetty, X ray diffraction analyses and scanning electron microscopy (SEM). Hydration of the ferrite powder was carried out isothermally at 50°C in I.OM magnesium sulfate solution for 30 h. Monosubstituted calcium aluminosulfate hydrate (monosulfate) (3CaO.Al2O3. CaSO4.12H2O) was the dominant crystalline phase. Other crystalline phases present included ettringite (3CaO-Al2O3-3CaS04.32H2O), magnesium aluminate hydroxide hydrate (Mg4Al2(OH)]4.3H2O) and calcium aluminate carbonate hydrate (4CaO.Al2O3.1/2CO2.I2H2O). The amorphous solids produced include a calcium/ironrich gel and a magnesium/aluminium/sulfate-rich phase. The calcium/iron-rich gel is the only iron-rich phase observed in the hydrated phase assemblage. SEM indicates magnesium aids in the movement of aluminate from the ferrite phase during hydration, the reason for increased ferrite reactivity. Iron in the ferrite phase exhibits little mobility, since most of the iron is found in residual ferrite grains. SEM also indicates monosulfate can be expansive; a five-fold increase in the lengths of the ferrite grains is observed.