Uncovering Novel C6S6H Morphologies: How Nanomaterials Regulate Cement Hydration and Strength at Elevated Temperatures

Summary The effects and mechanisms of three nanomaterials with different reactivities—nano-silica sol (NSS), nano-alumina (NA), and nano-calcium carbonate (NC)—on sand-enhanced oilwell cement cured at 200°C and 20 MPa for 1–28 days were investigated. Incorporation of NSS markedly increased the amorphous fraction, reaching approximately 60% at 5 days and 55% at 12 days of curing. The addition of 1% NC induced the formation of more xonotlite (C6S6H) crystalline phases, with the mass fraction increasing from 18% (5 days) to 28% (12 days). Both additives improved the compactness of the cement after early-stage curing and significantly enhanced the early compressive strength of the sand-added cement. Incorporation of 2.5% NA facilitated the formation of tobermorite (C5S6H), peaking at 23% at 5 days of curing; nevertheless, it produced negligible changes in the mechanical properties of the sand-added cement. Excess NSS (20%) severely inhibited cement hydration. The addition of excessive NSS and NA led to the formation of large numbers of ~500-nm spherical calcium-silicate-hydrate (C–S–H) precipitates (Ca:Si = 1:1) that covered the surfaces of other hydration products. This phenomenon correlates with lower compressive strength.