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The mechanical performance of grade G oil well cement stones declines significantly when subjected to temperatures exceeding 110 °C; the strategy to mitigate the impact of high temperatures is by incorporating siliceous materials. However, it is important to note that the crystalline properties of siliceous materials vary, leading to different effects on the temperature reduction. This study focuses on tricalcium silicate (C 3 S), the primary component of oil well cement. The impact of different types of silica, including amorphous silica (nanosilica, silica fume) and crystalline silica (quartz sand), on the hydration of C 3 S was investigated using 1 H NMR, XRD, TGA, and SEM-EDS analyses. The results show that siliceous materials can significantly prevent the strength decrease of C 3 S hardening products at high temperatures and inhibit the rise of porosity and permeability. Adding excessive amorphous siliceous materials, such as nanosilica, can cause agglomeration, resulting in a porous structure of C 3 S hardening products and hindering their strength. Amorphous silica fume is more reactive than crystalline silica sand and can rapidly initiate a pozzolanic reaction with calcium hydroxide. Siliceous materials also convert high-Ca/Si of C-S-H (hillebrandite, jaffeite, and reinhardbraunsite) into low-Ca/Si of C-S-H (gyrolite, okenite, tobermorite, nekoite). Siliceous materials reduce the porosity and permeability of C3S hardening products and enhance their mechanical properties through the filling and transformation of hydration products., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)