Solid–liquid distribution of selected concrete admixtures in hardened cement pastes

The distribution between hardened cement paste and cement pore water of selected concrete admixtures (BZMs), i.e., sulfonated naphthalene–formaldehyde condensate (NS), lignosulfonate (LS) and a gluconate-containing plasticiser used at the Paul Scherrer Institute for waste conditioning, was measured. Sorption data were fitted to a single-site Langmuir isotherm with affinity constants K = (19 ± 4) dm3 g−1 for NS, K = (2.1 ± 0.6) dm3 g−1 for LS and sorption capacities q = (81 ± 16) g kg−1 for NS, q = (43 ± 8) g kg−1 for LS. In the case of gluconate, a two-site Langmuir sorption model was necessary to fit the data satisfactorily. Sorption parameters for gluconate were K1 = (2 ± 1) × 106 dm3 mol−1 and q1 = (0.04 ± 0.02) mol kg−1 for the stronger binding site and K2 = (2.6 ± 1.1) × 103 dm3 mol−1 and q2 = (0.7 ± 0.3) mol kg−1 for the weaker binding site. Desorption of these BZMs from cement pastes and pore water in cement specimens prepared in the presence of the BZMs were then used to test the model. It was found that only minor parts of NS and LS could be mobilised as long as the cement composition was intact, whereas the sorption of gluconate was found to be reversible. The Langmuir model makes valuable predictions in the qualitative sense in that the pore water concentration of the BZMs is reduced by several orders of magnitude as compared to the initial concentrations. In view of the necessity for conservative predictions used in the safety analysis for disposal of radioactive waste, however, the predictions are unsatisfactory in that the measured pore water concentrations of NS and LS were considerably larger than the predicted values. This conclusion does not apply for gluconate, because its concentration in cement pore water was below the detection limit of 50 nM.