Synthesis of ettringite from portlandite suspensions at various Ca/Al ratios

Synthesis of ettringite, Ca 6 Al 2 (OH) 12 (SO 4 ) 3 ·26H 2 O has been attempted in suspensions of portlandite, Ca(OH) 2 under CO 2 -free condition, agitating by a magnetic stirrer for constant 3 h at room temperature. Al-sulfate hydrate, Al 2 (SO 4 ) 3 ·(14–18)H 2 O solution (0.01 M) was used for Al 2 O 3 and SO 3 sources and appropriate amount of portlandite was mixed with the solution together with sucrose for boosting portlandite dissolution. The Ca/Al atomic ratio in suspensions was varied from 1.0 to 5.0 at intervals of 0.5. Run products were collected by filtration and washing and air-dried and evaluated by XRD, XRF, and RF-SEM. No ettringite formed at Ca/Al = 1.0, but ettringite began to form at Ca/Al = 1.5 and over. Measurement of pH of supernatants indicated pH 9.86 as a critical point of ettringite formation, and this mineral was always identified in a pH region above this point. Minute crystallites of ettringite were obtained up to 2 μm. The Yield of ettringite was suspended around 95 wt% as dry-base even in stoichiometric mixing due to the presence of portlandite remaining undissolved as relicts. As a consequence, no marked difference of the yield was found among Ca/Al 2.0 to 3.5 runs. The remaining portlandite was apt to decompose into crystalline calcite during the air-dry process. However, formation of amorphous calcite was estimated in some runs during this decomposition process.