Effects of polycarboxylates with different adsorption groups on the rheological properties of cement paste

The adsorption of polycarboxylate (PCE) superplasticizer is the key step in the dispersing of cement particles, which is closely associated with problems in practice (e.g. compatibility). The adsorption process could be finely tuned by the modification of adsorption groups. However, the fundamental effect of different adsorption groups on the rheological properties of cement pastes was still not so clear. In this paper, a series of PCEs with different types (carboxyl group, sulfonate group and phosphate group) and proportions (1/3 and 2/3 substitution ratio) of adsorption groups have been synthesized and characterized via gel permeation chromatography (GPC), Fourier-transform infrared (FTIR) spectroscopy and H-1 nuclear magnetic resonance (H-1 NMR). The effects of PCEs with different adsorption groups on the solution conformation and adsorption conformation, adsorption amount, dispersion and rheological properties have been systematically explored to reveal the mechanism. The results show that for PCEs with sulfonate groups, both the lowest adsorption amount and poorest affinity toward negatively-charged surfaces lead to poorest dispersion performance. For PCEs with phosphate groups, the excellent dispersion performance and relative low yield stress is attributed to the strong electrostatic effect and Ca2+ complexity at high dosage. Differences between the PCEs containing phosphate groups relied on the difference of adsorption affinity and conformation. Meanwhile, the dispersing performance at low PCE dosage is quite different with expectation owing to the "bridging" effect. In summary, the above findings will be useful for the development of high-performance superplasticizers.