To improve the latent hydraulicity of metakaolin (MK), sodium hydroxide (NaOH) was added at varying dosages(0-0.5wt%) during the calcination process. Microstructural changes in MK during calcination and subsequent hydration with Portland cement (OPC) were analyzed using thermogravimetric analysis (TG), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results indicate that after calcination, Na is predominantly distributed within approximately 100 nm of the MK surface, leading to an altered amorphous structure of MK, which significantly enhances its latent hydraulicity. The 28-day compressive strength of the paste prepared with 0.05 wt% NaOH-modified MK reached 82.63 MPa, equivalent to 93% of OPC’s compressive strength, whereas unmodified MK yielded only 73% of OPC’s strength. With increasing NaOH content, the compressive strength of the paste slightly decreased but remained above 80% of OPC’s compressive strength. The modified MK exhibits strong compatibility with OPC and stable performance, and fluctuations in NaOH dosage do not cause a sharp decline in properties. The modified MK accelerated the hydration reaction of OPC, generating additional calcium silicate hydrate through synergistic effects, thereby optimizing the microstructure of the cementitious system.
Zhou et al. (Sun,) studied this question.