Cement grouting of rock fractures will be used to improve mechanical stability and reduce the groundwater inflow of underground construction sites such as high level radioactive waste (HLW) repository. Cementitous grouts will produce a high pH groundwater leading to the dissolution of primary minerals and the precipitation of secondary minerals such as calcium silicate hydrates (C-S-Hs).

A new method using infrared (IR) spectroscopy was proposed here for chacraterizing formation processes of C-S-Hs during the simulated alkaline alteration experiments of granitic rocks. Conventional batch experiments were first conducted at 140°C with granite and SiO2 powders and Ca(OH) 2 solutions. IR spectra on the experimental products showed rapid increases of absorbance around 965 cm−1 due to CSHs within 2 days at 140°C.

An in situ hydrothermal cell with a titanium cell and diamond windows, resistant to high-temperature, high-pressure and high-alkali conditions, was then constructed. This cell was combined with a microscopic Fourier transform infrared (FT-IR) spectrometer and the change in IR spectra with the reaction progress was measured sequentially with a short time interval (5 minutes). In the experiment, quartz, Ca(OH) 2 powders (grains of 5–10 µm and 2–3 µm in diameter), and water were placed in the cell and heated at temperatures of 100, 110, 120, 130 and 140°C and at a pressure of 3 MPa, and IR spectrum was measured at every 5 minutes. A peak around 790 cm−1 due to Si-O-Si of quartz decreased gradually, while another peak around 970 cm−1 due to Si-OH of C-S-H increased gradually within about 1000 minutes at 140°C.

It was found that formed C-S-Hs did not consist of a single phase because the ratio of peak height at 910 cm−1 to that at 970 cm−1 changed with time. Temporal changes in each peak height were analyzed assuming that two types of C-S-Hs were formed, and temporal changes in peak at 970 cm−1 for each phase were obtained. The obtained results can be used to evaluate formation rates of different C-S-H phases possibly with different Ca/Si ratios.

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