James V. Bothe

The stabilities of calcium arsenates at 23±1°C

Abstract The stabilities of calcium arsenate compounds were established by analysis of suspensions made with varying molar Ca/As ratios. Solution chemistry analyses determined the concentrations of calcium and arsenic and pH. The phases that were shown to form in order of descending pH were Ca 4 (OH) 2 (AsO 4 ) 2 ·4H 2 O, Ca 5 (AsO 4 ) 3 OH (arsenate–apatite), Ca 3 (AsO 4 ) 2 ·3 2 3 H 2 O, Ca 3 (AsO 4 ) 2 ·4 1 4 H 2 O, Ca 5 H 2 (AsO 4 ) 4 ·9H 2 O — ferrarisite, Ca 5 H 2 (AsO 4 ) 4 ·9H 2 O — guerinite and CaHAsO 4 ·H 2 O. The analytical concentrations of calcium and arsenic and pH were used in estimating solubility products. The estimated values were then refined through the comparison of the analytical data with calculated K sp values using the computer program PhreeqC. From the refined solubility products, the free energies of formation of the calcium arsenate hydrates were calculated as follows: Ca 4 (OH) 2 (AsO 4 ) 2 ·4H 2 O (−4941 kJ/mol), Ca 5 (AsO 4 ) 3 OH (−5087 kJ/mol), Ca 3 (AsO 4 ) 2 ·3 2 3 H 2 O (−3945 kJ/mol), Ca 3 (AsO 4 ) 2 ·4 1 4 H 2 O (−4085 kJ/mol), Ca 5 H 2 (AsO 4 ) 4 ·9H 2 O — ferrarisite (−7808 kJ/mol), Ca 5 H 2 (AsO 4 ) 4 ·9H 2 O — guerinite (−7803 kJ/mol), and CaHAsO 4 ·H 2 O (−1533 kJ/mol). Unlike other solubility studies on arsenate immobilization, this study was the first to consider the complete array of calcium arsenate hydrates that can form and to use the associated ions, CaAsO 4 − , CaHAsO 4 0 and CaH 2 AsO 4 + in determining their solubility products.

Phase formation in the system CaO–Al2O3–B2O3–H2O at 23±1°C

Abstract Soluble borates are known to interfere with the hardening of cement. Phase formation in the quaternary system CaO–Al 2 O 3 –B 2 O 3 –H 2 O at 23±1°C was studied and the range of mole ratios permitting the removal of soluble borates from solution has been established. Hydrating the powder composition with molar ratio Ca(OH) 2 :H 3 BO 3 :C 3 A 1 =3:4:1 resulted in the formation of phase pure 6CaO·Al 2 O 3 ·2B 2 O 3 ·39H 2 O after 3 months of equilibration. During the early stages of hydration, though, the hexagonal calcium aluminate hydrate 4CaO·Al 2 O 3 ·1/2B 2 O 3 ·12H 2 O and the calcium borate hydrate CaO·B 2 O 3 ·6H 2 O crystallized but were later consumed by the reaction to form phase pure 6CaO·Al 2 O 3 ·2B 2 O 3 ·39H 2 O. At powder ratios of 3:3:1 and 3:2:1, 6CaO·Al 2 O 3 ·2B 2 O 3 ·39H 2 O and 4CaO·Al 2 O 3 ·1/2B 2 O 3 ·12H 2 O were present together at equilibrium. When powders were mixed in the molar proportion of 1:2/3:1, well crystallized 4CaO·Al 2 O 3 ·1/2B 2 O 3 ·12H 2 O formed directly as a single phase within 5 h. The present study also demonstrates that boron may take on two coordinations, depending on which hydrate forms. Similarities between the hexagonal hydrates 4CaO·Al 2 O 3 ·1/2B 2 O 3 ·12H 2 O and 4CaO·Al 2 O 3 ·1/2CO 2 ·12H 2 O suggest boron to be three-coordinated in the AFm structure, whereas the literature shows boron to be four-coordinated in 6CaO·Al 2 O 3 ·2B 2 O 3 ·39H 2 O, which has the ettringite structure.

The formation of ettringite at elevated temperature

Abstract The formation of ettringite was investigated by isothermal calorimetry over the range of temperature from 50 to 70°C. Ettringite was formed using three sources of alumina: tricalcium aluminate, high alumina cement and calcium aluminate sulfate. In accord with a diffusionally controlled reaction mechanism, the kinetics of ettringite formation exhibit minimal temperature dependence over this range. C 4 A 3 S and HAC evolve comparable amounts of heat in ettringite formation while the heat evolved when the alumina-containing reactant is C3A is significantly higher.