Research on the influence of silica fume on the long-term strength development of lightweight cement

Silica fume (SF) is commonly added to lightweight cement slurry (LWCS) to enhance strength and stability due to its nanoscale properties. However, the long-term strength development of SF-modified cement remains unclear. This study explores the effect of SF on the long-term strength of LWCS cured at 75 °C, 90 °C, and 105 °C, using compressive strength tests and microstructural analyses (XRD, SEM, EDS, TGA). SF initially improves strength through pozzolanic reactions, but over time, the C-S-H gel formed during hydration becomes more porous and looser. The addition of SF promotes the transformation of the C-S-H gel from a high Ca/Si ratio to a lower one, with rapid CH consumption accelerating porosity development. This weakens the bond between cement and hollow glass microspheres (HGMs), causing microcracks from stress differentials. These changes lead to internal bond deterioration and up to 56.76% strength reduction. While SF enhances early strength, it negatively affects long-term strength, raising the risk of cementing failure in oil and gas wells. Future research should focus on addressing long-term strength decline at higher temperatures and identifying alternative materials to mitigate this issue. • The adverse effects of silica fume on the long-term strength development of lightweight cement were found. • The rule and mechanism of strength decline of silica fume lightweight cement are revealed. • The addition of silica fume in lightweight cement should be strictly controlled, and new Recession-resistant materials should be explored.