IntroductionPolycarboxylate superplasticizer (PCE) is commonly used in cement-based materials, which has the advantages of low dosage, high water reduction and strong molecular designability, which has been widely concerned by many researchers. PCE adsorbs on the surface of cement particles, and then breaks the cement flocculation structure, playing the effect of dispersing cement particles. However, it is known to all that the capacities of dispersing and dispersing retention of PCE to cement slurry are influenced by many parameters, such as cement components and mixing conditions. The polymer adsorption was shown to be very sensitive to the concentration of divalent ions like sulfate ions, so the sulfate ions of the cement slurry appear to be one of the most important parameters that adversely affect performances. In addition, the molecular structure parameters of PCE have an important influence on the adsorption behavior. Therefore, molecular structure parameters are important factors affecting PCE sensitivity to sulfate.Although previous studies have found that PCE with ionizable side chains (ISC-PCE) has better dispersion retention than PCE with nonionizable side chains (NSC-PCE), the effect of changes in side chain ionizability on its sensitivity to sulfate (sulfate tolerance) is unclear. Therefore, to more effectively realize the performance control of PCE on cement-based materials, the influence of side chain ionizability on sulfate tolerance of PCE is needed. Based on the previously reported ISC-PCE and NSC-PCE, the sulfate concentration was controlled by adding sodium sulfate, under which the new mixed cement slurry flow and its retention and rheological properties were tested to assess the dispersion. The size of PCE in solution, the amount of adsorbed polymer and zeta potential were combined to assess PCE tolerance to SO₄^2-. The salt added species were changed and the effects of SO₄^2- competitive adsorption and ion concentration on PCE performance were analyzed by rheology. The results can help to understand the effect of PCE on cement dispersion in the presence of ion competition and provide a new direction for opening up new structural PCE with good compatibility.MethodsThe fluidities of cement slurries were measured according to the Chinese standard GB/T 8077—2012. A RST-SST rotational rheometer with a paddle rotator of VT40-20 was used (shear rate: 10 s^-1→50 s^-1→10 s^-1). The data was processed with Rheology 3000 software to obtain the yield stress. The hydrodynamic radii (R_h) of the polymers were determined in saline conditions by dynamic light scattering experiments. The adsorbed capacities of the PCEs on cement surfaces, which were calculated from the difference in the concentration of polymers, were measured on a Vario Total Organic Carbon (TOC) analyzer by testing the PCE adsorption after different adsorption times. The zeta potentials of the cement pastes were measured at 25 ℃ on a Malvern Zetasizer Nano ZS90 analyzer. The MD simulations used the Gromacs-4.6.7 software package and a conventional oplsaa force field with RESP charges. The UFF force field was used for the substrate. The equilibrium MD simulation was conducted under the NPT ensemble for a total run time of 20 ns at a relaxed liquid configuration (25 ℃). The solution environment simulations were performed using 3000 water molecules. A relaxation system was selected and the energy was minimized by the steepest descent method with a termination gradient of 100 kJ/(mol·nm) before the relaxation. The system temperature is maintained constant through a Nosé-Hoover thermostat, and periodic boundary conditions were applied to all three dimensions. Long-range electrostatics within a relative tolerance of 1×10^-6 is calculated by the particle mesh Ewald method and a cut-off distance of 1 nm was used as the Ewald interaction and van der Waals interactions. The bond lengths of hydrogen atoms were constrained by the LINCS algorithm. A leap-frog algorithm was used with a time step of 1 fs.Results and discussionCompared with no salt, the hydrodynamic radius of NSC-PCE and ISC-PCE decreased by 36.9% (to 8.40 nm) and 8.1% (to 15.98 nm), the cement slurry mobility decreased by 21.7% and 4.3%, the adsorption volume decreased by 21.7% and 3.9%, and the yield stress increased by 42.7% and 27.8%, respectively.ConclusionsThe main conclusions of this paper are summarized as following. The improved side chain ionizability of PCE can significantly improve the dispersion stability and sulfate tolerance, and reduce its sensitivity to sulfate competitive adsorption and ionic strength. Based on the above results, sulfate tolerance of both PCE was quantitatively assessed (the sulfate resistance index of ISC-PCE was increased by 32% as compared to that of NSC-PCE) and verified by all-atom molecular dynamics simulations. The present results not only deepen the understanding of the impact of sulfate on PCE performance, but also provide a new direction for the development of new PCE with good compatibility.