Solubility of ettringite (Ca-6[Al(OH)(6)](2)(SO4)(3) center dot 26H(2)O) at 5-75 degrees C

The solubility of ettringite (Ca-6[Al(OH)(6)](2)(SO4)(3). 26H(2)O) was meas ured in a series of dissolution and precipitation experiments at 5-75 degre es C and at pH between 10.5 and 13.0 using synthesized material. Equilibriu m was established within 4 to 6 days, with samples collected between 10 and 36 days. The log K-SP for the reaction Ca-6[Al(OH)(6)](2)(SO4)(3). 26H(2)O reversible arrow 6Ca(2+) + 2Al(OH)(4)(-) + 3SO(4)(2-) + 4OH- + 26H(2)O at 25 degrees C calculated for dissolution experiments (-45.0 +/- 0.2) is not significantly different from the log K-SP calculated for precipitation expe riments (-44.8 +/- 0.4) at the 95% confidence level. There is no apparent t rend in log K-SP with pH and the mean log K-SP,K-298 is -44.9 +/- 0.3. The solubility product decreased linearly with the inverse of temperature indic ating a constant enthalpy of reaction from 5 to 75 degrees C. The enthalpy and entropy of reaction Delta H degrees(r) and Delta S degrees(r), were det ermined from the linear regression to be 204.6 +/- 0.6 kJ mol(-1) and 170 /- 38 J mol(-1) K-1. Using our values for log K-SP, Delta H degrees(r), and Delta S degrees(r) and published partial molal quantities for the constitu ent ions, we calculated the free energy of formation Delta G degrees(f,298) , the enthalpy of formation Delta H degrees(F,298), and the entropy of form ation Delta S degrees(f,298) to be -15211 +/- 20, -17550 +/- 16 kJ mol(-1), and 1867 +/- 59 J mol(-1) K-1. Assuming Delta C-P,C-r is zero, the heat ca pacity of ettringite is 590 +/- 140 J mol(-1) K-1. Copyright (C) 1999 Elsev ier Science Ltd.