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

Synthesis of ettringite, Ca6Al2(OH)12(SO4)3·26H2O has been attempted in suspensions of portlandite, Ca(OH)2 under CO2-free condition, agitating by a magnetic stirrer for constant 3 h at room temperature. Al-sulfate hydrate, Al2(SO4)3·(14–18)H2O solution (0.01 M) was used for Al2O3 and SO3 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.