A decrease in pH occurs because the added [Ca.sup.2+] in the gypsum reacts with soluble alkalinity (HC[O.sub.3.sup.-], C[O.sub.3.sup.2-], Al[(OH).sub.4.sup.-], and O[H.sup.-]) to form precipitates of calcite, tricalcium aluminate, and hydrocalumite
(Grafe and Klauber 2011).
is a solid closer to hydrotalcite-type materials based in brucite-type corrugated layers with an ordered array of hepta and hexa-coordinated [Ca.sup.2+] and [M.sup.3+] ions, and a fixed [Ca.sup.2+]/[M.sup.3+] ratio of 2:1 .
Dermatas, "Evaluation of ettringite and hydrocalumite
formation for heavy metal immobilization: literature review and experimental study," Journal of Hazardous Materials, vol.
Adsorbents such as sulphuric acid-treated peanut shell, hydrocalumite
, ettringite, AlP[O.sub.4], biopolymeric materials, aluminum-based water treatment chemicals, hardened cement paste, cement minerals, aluminum oxides, iron oxyhydroxides, iron-coated sand, and zero valent iron [16-23] have also been tested for Se removal.
The major difference in the mineral composition between the hydrated samples after 7 days is the disappearance of CaO and the portlandite Ca[(OH).sub.2] phase, as well as the formation of a secondary Ca-Al hydrate phase hydrocalumite
and a hydrous Ca-Al sulfate phase ettringite.
and Leroux, F.: 2004, In situ polymerization of interleaved monomers: A comparative study between hydrotalcite and hydrocalumite
host structures, J.
The sulfate-dominant rhombohedral analog of hydrocalumite
Heterogeneous basic catalysts commonly used in the transesterification reaction are Ca-Al hydrocalumite
, MgO , CaO [14-16], BaO , Li-CaO , CaO-ZnO , CaO/SBA-15 , MgO-CaO , CaO/[Al.sub.2][O.sub.3] , CaO-[La.sub.2][O.sub.3]  calcium/chitosan spheres , and natural calcium .
The major difference in mineral composition between the hydrated samples after 7 days and fresh ash is the disappearance of oldhamite (CaS) and the formation of the secondary Ca-Al hydrate phase hydrocalumite
and the Ca-Al sulphate-hydrate phase ettringite.
Zhao et al., "Efficient and controllable phosphate removal on hydrocalumite
by multi-step treatment based on pH-dependent precipitation," Chemical Engineering Journal, vol.
, also known as Friedel's salt  or AFm phase in cement science , belongs to the family of layered double hydroxides (LDHs) .
In the PF ash, the transformation proceeds with the precipitation of hydrocalumite
([Ca.sub.2]Al[(OH).sub.7] x 3[H.sub.2]O), which suggests that the ettringite formation is limited by the availability of dissolved sulphate, while the excess of aluminium is precipitated as Ca-aluminate(hydrate)-type phases.