LAUSR

Abstract The purpose of this paper was to study the effect of different aluminum phases on the formation of thaumasite in cement paste. In the present study, tricalcium aluminate (C3A) and sodium aluminate (NaAlO2) were chosen as two kinds of aluminum phase sources. Two cement pastes were designed: one was prepared with tricalcium silicate (C3S), calcium carbonate (CaCO3) and C3A, another was prepared with C3S and CaCO3. Two samples were cured for 28 days and then immersed in sodium sulfate (Na2SO4) solution with/without NaAlO2 at 5 °C for a certain time. The cement corrosive products were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Nuclear magnetic resonance (NMR), and Scanning electron microscope with energy dispersive spectroscopy (SEM/EDS). Results showed that thaumasite-ettringite solid solution could be formed in cement paste containing C3A, but the formation time of thaumasite was much longer compared with that of ettringite, in agreement with the woodfordite route. In the case of using NaAlO2 as aluminum phase, ettringite was formed in the sample, while no thaumasite was formed in cement paste after being cured for 14 months. According to the analysis of 27Al NMR, the peak of hexahedrally coordinated aluminum (AlO4) at 58.6 ppm was proved to be Al(OH)3, which would probably restrain the formation of thaumasite. It was indicated that the use of sodium aluminate was unfavorable for the formation of the thaumasite in cement paste.