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Why do black, colored and gray concrete block surfaces appear lighter in the course of time?

Fading concrete block surfaces –

→ 1 Concrete masonry blocks colored with Bayferrox iron oxide pigments exposed 25 years to outdoor weathering

→ 2 Concrete slabs colored in black shade; a part of the slabs is visibly lightened

Bonding stability of pigments in hardened cement paste

The needle-shaped structures existing in the concrete block are ideal bonding spots for metal oxide pigments such as Bayferrox iron oxide pigments, titanium dioxide, chrome oxide or cobalt spinel and some other inorganic pigments. These dipolar pigment particles are electrostatically bonded into the cement (what does not apply to carbon or organic pigments) and are furthermore mechanically captured in the hardened cement paste matrix. These metal oxide pigments are not able to leave the cement matrix as has been revealed by outdoor...

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