Influence of Ca/Si ratio of concrete pore solution on thaumasite formation

To further understand the influence of Ca/Si ratio on thaumasite formation, in this work, reactant solutions with different Ca/Si ratios were prepared, mimicking concrete pore solution environment. Solution samples were then stored at 5 °C for a certain reaction time. Powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope with energy dispersive spectroscopy (SEM/EDS) were used to analyse solids produced in the solutions. It was found that ettringite was produced rather than thaumasite at low Ca/Si ratio (Ca/Si = 3) in reaction time of 6 months. However, the thaumasite was formed at higher Ca/Si ratio (Ca/Si = 6 and Ca/Si = 9), and the formation time of thaumasite became much shorter. Specifically, when Ca/Si ratio reached 9, thaumasite was formed in only 3 days. It could be inferred that in concrete or mortar, if Ca/Si ratio of pore solution was controlled at a certain low level, thaumasite formation could be effectively prevented.