Adsorption Behavior and Molecular Interaction Mechanism of AMPS-Copolymer/HEDP Complex Retarder on Hydration of Tricalcium Silicate (C 3 S) in Oil Well

Hydroxyethylidene diphosphonic acid (HEDP) is used as an auxiliary material for AMPS (2-acrylamido-2-methylpropanesulfonic acid) copolymer retarders because of its good water solubility and high temperature resistance. This paper investigates how the presence of HEDP significantly enhances the inhibitory effect of AMPS copolymers (PAIND) on cement hydration, focusing on adsorption behavior and interfacial interactions. The adsorption capacity, adsorption thickness and interface interaction were characterized by total organic carbon, ζ-potential, X-ray photoelectron spectroscopy (XPS) and molecular dynamics simulation. Adsorption behavior experiments revealed that the PAIND/HEDP complex retarder exhibited higher adsorption capacity and a thicker adsorption layer on C 3 S surfaces compared to individual PAIND or HEDP. HEDP may enhance the overall adsorption capacity of the system and increase the thickness of the adsorption layer by filling the vacant adsorption sites at the PAIND–C 3 S interface. Interfacial interaction analysis reveals that in PAIND, increasing HEDP content enhances the adsorption of the PAIND/HEDP complex retarder onto C 3 S, resulting in reduced conformational changes and a more stable adsorption layer. This is primarily attributed to a robust network structure formed via multiple chemical interactions─including hydrogen and ionic bonds─between the retarder and C 3 S, which strengthens inhibition of C 3 S hydration under extreme high-temperature conditions.