Ultramarine blue pigment: A non-conventional pozzolan

Ultramarine blue pigment (UB), is used like an inorganic pigment due to its stability with external agents. However, when UB is combined with Portland cement, it loses its coloring stability. In this paper, the interaction between UB and white Portland cement are evaluated by mineralogical methods, compressive strength and durability (sulfate attack and capillary suction). Results show that UB has a non-conventional pozzolanic behavior due to that it reacts with the portlandite, generated from hydration of cement, forming additional phases like C–S–H and mainly ettringite. These phases densify the matrix and improve compressive strength and durability of the blended mortars. All this happens despite of crystalline structure and chemical composition of UB, which are very different to the properties of typical pozzolans. Calcium hydroxide is fixed by UB with an ionic interchange, through of the Hauyne phase (Al6Ca2Na6O32S2Si6), and this produces the destruction of the chemical structure of UB.