The precipitation of calcium hydroxoaluminate hydrate powders from sodium hydroxoaluminate solutions: Precipitate phases and precipitation mechanisms

Calcium hydroxoaluminate hydrates were precipitated from different sodium hydroxoaluminate and hydroxoaluminate-excess hydroxide solutions at ambient temperature (at C_Al = 0.1 to 0.3 M and at XS OH/Al = 0 to above 8). The precipitations were monitored by potentiometric (pH) measurements. Precipitate morphologies were examined by optical microscopy and precipitate compositions were determine by chemical analysis, infra-red spectrophotometry and thermal analysis. Generally at OH/Al ratios of 4 to 4.5 (XS OH/Al = 1 to 1.5), the compound 2 CaO · · Al₂O₃ · 8 H₂O (C₂AH₈) was precipitated with some aluminium hydroxide; then at OH/Al ratios of 5 to above 11 (XS OH/Al = 2 to above 8), the compound 2 CaO · Al₂O₃ · 8 H₂O was precipitated with increasing amounts of the compound 4 CaO · Al₂O₃ · 13 H₂O (C₄AH₁₃).