Rb. Perkins et Cd. Palmer, Solubility of chromate hydrocalumite (3CaO center dot Al2O3 center dot CaCrO4 center dot nH(2)O) 5-75 degrees C, CEM CONCR R, 31(7), 2001, pp. 983-992
A secondary precipitate was consistently observed in a series of dissolutio
n and precipitation experiments conducted on synthesized Ca-6[Al(OH)(6)](2)
(CrO4)(3). 26H(2)O. X-ray diffraction (XRD) patterns, scanning electron mic
roscopy, thermogravimetry, and solid digest analyses demonstrate that d-spa
cings, crystal morphology, water content, and Ca/Cr/Al ratios for the secon
dary solid are consistent with chromate hydrocalumite (3CaO . Al2O3. CaCrO4
. 15H(2)O). Steady-state aqueous ion concentrations indicate that the exper
imental aqueous solutions were in apparent equilibrium with both the Cr(VI)
analog of ettringite and chromate hydrocalumite. Over the pH range 10.9 to
12.2, the log of the solubility product (log K-SP) for the reaction
3CaO . Al2O3. CaCrO4. 15H(2)O=4Ca(2+) + 2Al(OH)(4)(-) + CrO42- + 4OH(-) + 9
H(2)O
at 25 degreesC is -30.38 0.28. The temperature dependence of the log K-SP o
btained from six additional temperatures from 5 degreesC to 75 degreesC is
logK(SP) = (-2042.1/T) -23.480.
Delta Gr(r)(o) and DeltaH(r)(o) for the reaction are 173.1 +/- 3.7 and 39.1
+/- 3.2 0 mol(-1) and DeltaS(r)(o) is -450 +/- 10 J mol(-1) K-1. Using the
se values and published partial molal quantities for constituent ions, we c
alculate DeltaG(f,298)degrees= -9905 +/- 15.7 kJ mol(-1), DeltaH(f)(o) = -1
1303 +/- 8.3 kJ mol(-1), and DeltaS degrees = 1439 +/- 89 J mol(-1) K-1 for
3CaO . Al2O3. CaCrO4. 15H(2)O. (C) 2001 Elsevier Science Ltd. All rights r
eserved.