Hydration of mechanically activated granulated blast furnace slag

Ground granulated blast furnace slag (GGBFS) is known to possess latent hydraulic activity, i.e., it shows cementitious properties when in contact with water over a long period of time. Results are presented in this article to show that, in sharp contrast to published literature on the hydration of neat GGBFS, the complete hydration of slag is possible in a short time (days), even without a chemical activator. This is achieved if the slag used for hydration is mechanically activated, using an attrition mill. The nature of the hydration product of the mechanically activated slag depends not only on the initial specific surface area (SSA) of the slag but also on the surface activation, as manifested by the change in the zeta potential (ξ) of the slag during the milling process. Depending upon the SSA and the ξ, the hydration product changed from nonreacted slag with high porosity (slag SSA < 0.3 m²/g, ξ>−29 mV) to hydrated slag with a compact structure (SSA=0.3 to 0.4 m²/g, ξ=−29 to −31 mV), and, finally, to fully hydrated slag with high porosity (SSA>0.4 m²/g, ξ ∼ 26 mV). Unlike the poorly crystalline hydration product formed by the nonactivated slag, even after prolonged hydration for years, the hydration product of mechanically activated slag was crystalline in nature. The crystallinity of the product improved as the duration of the mechanical activation increased. The calcium-silicate-hydrate (C-S-H) phases present in the slag hydration product, characterized by a Ca/Si ratio of 0.7 to 1.5, were similar to those found for the hydraulic cement binder, except for the presence of Mg and Al as impurities. In addition, the presence of a di-calcium-silicate-hydrate phase (α-C₂SH), which normally forms under hydrothermal conditions, and a Ca-deficient and Si-Al-rich phase (average Ca/Si mole ratio < 0.1 and Si/Al ∼ 3) is indicated, especially in the hydration product of slag that was activated for a longer time.