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• The hydration products of pure C 3 S and nano-SiO 2 at 80℃ were studied. • The optimum amount of silicon material at 80℃ differs from that at high temperature. • Decalcification will decrease the Ca/Si of hydration products. • Hight Ca/Si products can be converted into low Ca/Si products by nano-SiO 2. The hydration products of G-grade oil well cement can affect its overall performance, which is currently enhanced by nano-silica (nano-SiO 2). However, the hydration process and products of silica nanoparticles and cement have not yet been fully clarified. This study focused on tricalcium silicate (C 3 S), the main component of oil well cement, and analyzed the effects of nano-SiO 2 on the hydration products of C 3 S using 1H low-field NMR, XRD, SEM-EDS, TGA, FTIR, and 29Si MAS NMR at 80 °C. The results showed that after seven days of maintenance and the addition of more than 34% nano-SiO 2, due to the agglomeration and adsorption of nano-SiO 2 , the content of C S H decreases obviously. The added nano-SiO 2 reacted with both the volcanic ash of CH and the high Ca/Si products from C 3 S hydration, resulting in the formation of a large amount of low Ca/Si hydration products and a denser microstructure of the specimen. In addition, the decalcification reaction of nano-SiO 2 facilitated the polymerization of C S H and increased the chain length, but the agglomeration of nano-SiO 2 and decalcification shrinkage increased the porosity of the specimens. [ABSTRACT FROM AUTHOR]