The formation of calcium sulfoaluminate hydrate compounds Part II

The formation of ettringite (3CaO . Al2O3. 3CaSO(4). 32H(2)O) and monosulfa te (3CaO . Al2O3. CaSO4. 12H(2)O) from tricalcium aluminate (3CaO . Al2O3), gypsum (CaSO4. 2H(2)O), and sodium hydroxide (NaOH) solutions were investi gated by isothermal calorimetry and X-ray diffraction analyses. Tricalcium aluminate/gypsum mixtures with a molar ratio of 1:1 sulfate-to-aluminate we re hydrated at constant temperatures from 30 to 90 degrees C in deionized w ater and in 200 and 500 mM of sodium hydroxide (NaOH) solutions. Hydration in deionized water produced ettringite and monosulfate as the dominant crys talline phases, regardless of temperature. Complex assemblages of phases fo rmed in 200 and 500 mM of sodium hydroxide; these included ettringite, mono sulfate, and U-phase, at all temperatures. Calorimetry and X-ray diffractio n data indicated that U-phase formation becomes the dominant reaction as so dium hydroxide concentrations increase. U-phase is the only AFm phase obser ved after hydration in 1.0 M of NaOH. The data also indicated that the pres ence of sodium cations promotes AFm formation. (C) 2000 Elsevier Science Lt d. All rights reserved.