Long-term immobilisation of Cd2+ at the Tulul Al Hammam natural analogue site, central Jordan

•Hazardous Cd occurs in weathered combustion-metamorphic marbles entrapped in Cd-rich portlandite.•Natural (Ca1−xCdx)[OH]2 is isostructural with the Ca[OH]2 and β-Cd[OH]2 phases.•The (Ca1−xCdx)[OH]2 composition is inherited from the precursor (Ca1−xCdx)O phase.•Reliable long-term fixation of Cd in (Ca1−xCdx)[OH]2 solid solution is buffered by C-S-Hs.

The Tulul Al Hammam area in central Jordan is a natural analogue site where to study long-term Cd2+ retention during the weathering of Cd-enriched combustion-metamorphic marbles with clinker-like mineralogy. Natural Cd-rich portlandite (Ca0.67−0.45Cd0.33−0.55)[OH]2 was discovered and characterised by single-crystal XRD, SEM/EDS, and microprobe analyses, and Raman spectroscopy. The Ca[OH]2 – Cd[OH]2 solid solution ((Ca1−xCdx)[OH]2) is isostructural with Ca[OH]2 and β-Cd[OH]2 crystals in the CdI2-type of structure. Cd-rich portlandite is a common secondary phase in slightly altered massive marbles, where it replaces and rims single-crystals of the high-temperature precursor (Ca1−xCdx)O. The Cd:Ca molar ratios of (Ca1−xCdx)[OH]2 are inherited from the precursor phase (Ca1−xCdx)O. Reliable long-term immobilisation of Cd in (Ca1−xCdx)[OH]2 is assumed for pore water pH values from 10.5 up to 12 buffered by C-S-Hs formed mainly during spurrite hydration. The Tulul Al Hammam site may have some bearing on long-term Cd2+ retention in the concrete matrix.