Effect of Sodium Silicate on Calcium Aluminate Cement Hydration in Highly Alkaline Media: A Microstructural Characterization

The present paper discusses calcium aluminate cement (CAC) hydration in four highly alkaline solutions (8M NaOH and three sodium silicate solutions with SiO2/Na2O moduli of 0.19, 0.69, and 1.17). The reference system was water-hydrated CAC. The hydrated materials were tested for 2- and 28-day compressive and flexural strength and the reaction products were characterized by XRD, fourier transform infrared, and NMR. In highly alkaline media, initial mechanical strength developed much more slowly than in CAC-water systems. Strength in alkali-activated CAC rose gradually with time, however, reaching values of approximately 80 MPa in 28-day specimens, depending on the SiO2/Na2O ratio in the solution used to hydrate the CAC. The chief conclusion drawn from this study was that hydration proceeds differently in alkaline and neutral media, with C3AH6 precipitating in the former. The addition of sodium silicate to the alkaline hydration solutions used to hydrate the CAC induced the precipitation of katoite [Ca2.93Al1.97 (Si0.64O2.56)(OH)9.44] and an alkaline aluminum silicate hydrate.