Surface whitening mechanism of ettringite rich decorative mortar

Abstract

This study investigates the surface whitening mechanism of ettringite rich decorative mortars. Surface whitening was triggered through an exaggerated test scenario with a highly porous simplified mortar and a series of wet/dry cycles. Based on correlative light-electron microscopy observation, soak solution and mineralogical composition analyses, a mechanism is proposed: surface whitening is due to large agglomerates (>100 μm) of further hydrates (AFt phase and AFm phase) being formed in the large pores of the mortar surface during the drying process. The degree of surface whitening does not follow a continuous increase over cycle numbers, but rather evolves to a maximum after which more cycles lead to reduced surface whitening. This surface clearing and reduced whitening is explained by the disappearance of the hydrate agglomerates caused by full carbonation of AFt phase and AFm phase, transforming to small crystals of CaCO₃ and secondary Al(OH)₃, releasing the color of the pigments.