Influence of calcium nitrate and nitrite on the constitution of AFm and AFt cement hydrates

Abstract

Layered double hydroxides corresponding to Ca4Al2(NO3)2(OH)12·4H2O and Ca4Al2(NO2)2(OH)12·4H2O (nitrate and nitrite AFm phases, respectively) were synthesised and characterised using x-ray diffraction, thermogravimetry, Fourier transform infrared spectroscopy and scanning electron microscopy. Solubility and regions of stability, including thermodynamic data, are presented. Ionic speciations at various temperatures were calculated. The water content of nitrate AFm was found to be sensitive to humidity conditions, unlike nitrite AFm whose water content seems to be independent of the moisture conditions over a wide range of pH2O. Nitrates and nitrites readily displace hydroxide, sulfate and carbonate from the AFm structures so AFm is mainly responsible for storing nitrite and nitrate in cement solids. A comprehensive picture of the phase relations of AFm phases and the binding power for nitrite/nitrate is given for pH∼12 and a temperature of 25°C. The presence of nitrate and nitrite alter the AFm/AFt balance and affect the specific volume of paste solids. Implications for the distribution of nitrate and nitrite in cements undergoing alteration are discussed.