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

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 beta-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. (C) 2016 Elsevier Ltd. All rights reserved.