Hydration mechanisms and microstructural evolution of oil well cement modified by rice husk ash under low-temperature...

The South China Sea holds abundant oil and gas resources, yet deepwater low-temperature conditions retard cement strength development, increasing cementing costs and threatening early-age zonal isolation. Therefore, this study systematically evaluated rice husk ash (RHA) blended cement at varying dosages (5%, 10%, 15%) and fineness levels (11.4∼56.9 μm). Mechanical testing, calorimetry, and microstructural characterizations were employed to elucidate the underlying enhancement mechanisms. Results indicate that compressive strength exhibits a non-monotonic dependence on RHA parameters. At 1 day of curing, a 5% dosage of the finest RHA maximized early strength, achieving up to a 17.93% increase via rapid pozzolanic activity. However, at 3 and 7 days, the optimal performance shifted; the 7 days strength peaked at 34.42 MPa using a 10% dosage with intermediate fineness (T1-R). Kinetic and microstructural analyses reveal that an optimal low RHA dosage enhances both early (<6 h) and later (>48 h) hydration rates, extends the induction period up to 26.74 h, and densifies the matrix by transforming C-S-H to a ribbon-like morphology (Ca/Si ratio: 1.5∼1.7). Conversely, excessive fineness or high dosages (15%) inhibit later-stage hydration, triggering premature diffusion control (dropping the kinetic parameter to 0.20∼0.30) due to the dilution effect, particle agglomeration, and detrimental CH accumulation. These quantitative insights provide a robust theoretical framework and practical guidance for optimizing green RHA applications in deepwater cementing operations. This study provides a theoretical basis and practical guidance for optimizing RHA application in oil well cement.