EXPERIMENTAL HYDRATION OF 2 SYNTHETIC GLASSY BLAST-FURNACE SLAGS IN WATER AND ALKALINE-SOLUTIONS (NAOH AND KOH0.1N) AT 40-DEGREES-C - STRUCTURE, COMPOSITION AND ORIGIN OF THE HYDRATED LAYER

The hydration of blast furnace slags has been modelled using two synthetic (CaO, SiO2, Al2O3, MgO) glasses with different Al2O3/MgO values. Experiments (duration: 16 h to 150 d) were performed at 40° C in deionized water (pH 6.5) and in NaOH and KOH (0.1 N) solutions (pH=12.9). The hydrated layer was characterized from a combination of several techniques at different scales: surface analysis by XPS and SEM; TEM of ultrathin diamond-cut sections including electron microdiffraction and EDS analysis; X-ray diffraction of scratched powders. In water, the hydrated zone is only about 0.5 μm thick after 150 d with a leached layer covered by a thin siliceous film on which are scattered rare amorphous lamellae. In alkali media, the hydrated zone is composed of three parts: an inner layer made of modified residual glass, calcium-depleted and richer in magnesium and aluminium than the initial glass; an intermediate lamellar of constant thickness (O.3 μm) after 15 d with magnesium and aluminium as major components ("hydrotalcite type" composition) and labelling the initial solid-solution interface, an outer layer with initially abundant C-S-H more or less carbonated after reaction with atmospheric CO2. The "hydrotalcite-type" layer separates the inner domain dominated by the formation and evolution of a leached glass layer from an outer one, where the precipitation of C-S-H and other amorphous or crystalline compounds, followed by carbonation, are the major processes. © 1990 Chapman and Hall Ltd.