What are the mechanisms of functional monomers' effect on air entrainment of polycarboxylate superplasticizers in cement paste and mortar?

This study focuses on developing three kinds of polycarboxylate superplasticizers (PCEs), i.e., ether-type polycarboxylate superplasticizer (T-PCE), fluorine-type polycarboxylate superplasticizer (F-PCE), and phosphate-type polycarboxylate superplasticizer (P-PCE) synthesized respectively with three functional monomers, i.e., isopentenyl polyoxyethylene ether (TPEG3000), hexafluorobutyl methacrylate (HFBMA), and 2-hydroxyethyl methacrylate phosphate (HEMAP) for improved dispersion and low air entrainment in cementitious systems. The molecular structures of PCEs were characterized by Fourier Transform Infrared Spectroscopy (FTIR), Gel Permeation Chromatography (GPC), and Proton Nuclear Magnetic Resonance (1H NMR). Further, the influence and mechanism of the functional monomers on the dispersion and air entrainment of PCEs in cement paste and mortar were investigated. Finally, the mechanisms of the functional monomers' effect on the air entrainment of PCEs in the cement paste and mortar were discussed. The results showed that T-PCE, F-PCE, and P-PCE were successfully synthesized, and P-PCE had the optimal effect on the fluidity (at W/C=0.35, 305.5 mm), the lowest foaming height (11.000 mm), and the fastest rate of defoaming of the cementitious systems, followed by F-PCE and T-PCE. Complexation, adsorption, surface activity, and polar hydrophobic mechanisms were identified. For unmodified T-PCE, there are more hydrophilic groups (-COOH), stronger surface activity. Therefore, T-PCE shows strong air entrainment; T-PCE has a high density of carboxylate and often forms a complex with 1 Ca2+ in the form of 4 carboxyl groups before reaching the adsorption equilibrium, so the concentration of free Ca2+ with foam stabilizing effect is high, which leads to strong air entrainment of T-PCE. The adsorption and complexation ability of F-PCE with Ca2+ is weaker than that of T-PCE. However, the hydrophobicity of F-PCE molecules is enhanced and the surface tension of F-PCE is reduced due to the ester group and C-F bonds in the structure of HFBMA, so the air entrainment of F-PCE is lower than that of T-PCE through Marangoni effect. P-PCE exhibited the lowest surface activity and the largest surface tension. Defoaming effect is promoted due to increased additional pressure on the bubble surface. Moreover, its adsorption and complexation with Ca2+ surpassed those of T-PCE and F-PCE due to the phosphate group has two negative charges, and the free Ca2+ is reduced, which also reduces the bubble stability. Therefore, P-PCE has minimal air entrainment in the cementitious systems.