Structural Regulation, Structure–Activity Relationship, and Dispersion Mechanism of Sodium Alginate-Modified Polycarboxylate Ether Superplasticizers

In this study, a “rigid side chain support-multidentate coordination anchoring-amphiphilic regulation” strategy was adopted to overcome the limitations of the linear conformation of a traditional polycarboxylate superplasticizer (TPCE) while reducing dependence on petroleum-based raw materials. Using the renewable natural polysaccharide sodium alginate (SA) as the functional monomer raw material, the structural regulation, structure–activity relationship, and dispersion mechanism of sodium alginate-modified polycarboxylate water-reducing agent (SA-PCE) were investigated. First, macromolecule SA (M-SA) was degraded into small molecule SA (D-SA) through acid degradation, and the unsaturated carbon–carbon double bond was introduced into D-SA by the Ugi reaction to synthesize Ugi-modified alginate sodium (Ugi-SA). Subsequently, SA-PCE was synthesized by aqueous solution free radical polymerization with Ugi-SA and acrylic acid (AA) as the small monomer, isopreneol polyoxyethylene ether (TPEG) as the large monomer, ammonium persulfate (APS) as the initiator, and sodium methallyl sulfonate (MAS) as the chain transfer agent. The precise regulation of the SA-PCE molecular structure could be achieved by adjusting the dosages of Ugi-SA, MAS, and APS. Finally, the structure–activity relationship and dispersion mechanism of SA-PCE were discussed by combining the fluidity and microstructure of cement paste, the adsorption amount, Zeta potential, the adsorption layer thickness, the hydrodynamic radius, and surface tension. The results showed that as the rigidity of the side chain of SA-PCE increases, the dispersibility and surface activity first increase and then decrease, whereas the hydrodynamic radius, Ca2+ complexation ability, adsorption amount, adsorption layer thickness, and electronegativity increase. Also, as the main chain length increases, the dispersibility, adsorption amount, and electronegativity first increase and then decrease, whereas the hydrodynamic radius and surface tension gradually increase. With increasing polymerization degree, the dispersibility, adsorption amount, hydrodynamic radius, and surface activity first increase and then decrease, whereas the electronegativity shows an overall increasing trend. Compared with TPCE, SA-PCE exhibits rigid side chain support, stronger multidentate coordination, and steric hindrance. The higher carboxylate density and electrostatic repulsion, controllable amphiphilic structure, higher surface activity, and wetting property enhance its dispersion in the cementitious matrix.