On the durability of Portland limestone cement: Effect of pH on the thaumasite formation

Abstract The mechanism of thaumasite formation is studied in a solution of sodium silicate and ettringite stored for 12 months at 7 °C. After 7 months, the mix was carbonated by bubbling CO 2 gas and the pH decreased from 11 to 9.5; at the 9th month the pH was raised again to 12.5 by adding lime water. The phases formed at the different pH ranges were identified by means of X-ray diffraction and infrared spectroscopy. The results indicate that a certain reaction occurred in the mix stored for 7 months at pH 11: A shoulder appeared at frequency of 1030 cm −1 in the infrared spectrogram of the mix and the band in the frequency region ∼500–400 cm −1 broadened; the X-ray patterns show, however, unchanged ettringite patterns with weak calcite phase. The ettringite phase disappears by lowering the pH to 9.5, and an amorphous phase forms instead. Diffraction lines of aragonite and calcite are identified in this sample, and its IR spectra indicate the transformation of the shoulder at 1030 cm −1 to a strong broad band at 1027 cm −1 and the appearance of infrared frequencies characteristic of the carbonate phases. The amorphous phase formed is a carbonated complex of hydrated silicate and of decomposed ettringite. It converts to thaumasite with the supply of lime and the rise of pH to 12.5.