Citrate sorption on cement hydrates

The interaction of citrate with hydrated Portland cement and its main hydrates: C-S-H (0.8 ≤ Ca/Si ≤ 1.4), ettringite, monosulfo-, hemi- and monocarboaluminate is investigated. The solubility of citrate-AFm and citrate-AFt was determined. Batch sorption experiments at pH = 13.2 showed that citrate sorbs relatively strongly on AFm phases and ettringite. Citrate sorbed only on the outer surfaces sites of monosulfoaluminate, monocarboaluminate and ettringite resulting in a low sorption capacity of ≈5–8 mmol/kg, while in the case of hemicarboaluminate sorption occurred also in the interlayer resulting in a sorption capacity of ≈1000 mmol/kg. The sorption on C-S-H was found one order of magnitude lower and increased with increasing Ca/Si as its uptake is promoted by the Ca-citrate surface complexes. The sorption coefficients of citrate measured on hydrated Portland cement were with R_d = 10–30 L·kg⁻¹ comparable to the sorption observed on high Ca/Si C-S-H and AFm phases, underlining the strong uptake of citrate in hydrated cements.