Mechanics and Phase–Microstructure Evolution of Silica Sand-Modified Oil-Well Cement Cured at 240 °C: From Single-Size Effects to Graded-Packing Design

Oil-well cement sheaths can undergo complex property evolution under ultra-high-temperature curing. This study examines how silica sand particle size and graded packing control the time-dependent performance of Class G oil-well cement cured at 240 °C. Four single-size sands (D50 = 8, 41, 81, and 228 μm; 50% BWOC) and six graded blends (F1–F6) were evaluated using compressive strength and water permeability at 7, 14, and 28 days, supported by XRD and SEM for selected specimens. Contrary to the common assumption that finer silica necessarily yields higher strength, the 240 °C results reveal that coarse silica sand deserves greater attention; while the 8 μm system shows high early strength, it exhibits pronounced late-age retrogression, whereas coarser sands (41–228 μm) maintain continuous strength gain with curing time and display distinct permeability responses. Graded packing further suppresses retrogression; F1 achieves the highest 28-day strength (approaching 50 MPa) and a one-order-of-magnitude reduction in permeability over time. XRD/SEM evidence suggests that the superior performance of optimized designs is associated with reduced residual quartz and enhanced xonotlite development, together with a denser, interpenetrating hydrate framework promoted by graded packing at 240 °C.