Tailoring the reaction kinetics of Ca(OH)2-Na2SO4-activated metakaolin system under high-temperature well cementing conditions

Alkali-activated material (AAM) is a promising low-carbon alternative to conventional ordinary Portland cement, offering superior high-temperature and corrosion resistance, making them ideal for high-temperature and high-pressure (HTHP) well cementing. However, a critical challenge for these applications is their rapid, uncontrolled setting under high temperatures, which shortens the thickening time and makes them unpumpable. This study investigates a poly(itaconic acid) (PIA) homopolymer as a high-temperature retarder for a novel, calcium hydroxide and sodium sulfate activated metakaolin system at 120°C. The unretarded system demonstrated a runaway reaction, leading to a thickening time of only 30 min, which was directly correlated with a sharp exothermic peak in calorimetry tests. The poly(itaconic acid) successfully prolonged the thickening time to over 6 h at a 1.0 % dosage, while maintaining a high 1-day compressive strength of approximately 19 MPa. The core finding is that the retardation mechanism is a targeted inhibition of the nucleation and growth of a key intermediate crystalline product, the U-phase. This inhibition delays the associated autocatalytic surge in pH, which is supported by quantitative XRD and TG analyses, aqueous phase analysis, and microstructural evidence from SEM, thereby decoupling the rapid reaction acceleration and providing a wide, controllable workable window. The findings confirm that PIA is a highly effective retarder that governs the reaction kinetics without altering the final stable phase assemblage (katoite and C-(N)-A-S-H gel), offering a practical solution for deploying sustainable AAMs in HTHP well cementing operations. • Poly(itaconic acid) effectively extends slurry thickening time at 120°C. • Retarder targets U-phase formation, delaying pH surge and reaction kinetics. • Compressive strength remains high (∼19 MPa) despite significant setting delay. • A right-angle thickening profile is achieved, ideal for well cementing. • Mechanism revealed via multi-scale analysis: XRD, TG, SEM, and pore solution.