On the optimization of BOF slag hydration kinetics

In this study, the early age hydration kinetics of BOF slag is investigated. Isothermal calorimetry and in-situ X-ray diffraction (XRD) are used to examine the effects of K3-citrate concentration and nanosilica addition on slag reactivity. The results reveal that brownmillerite is the most reactive slag phase during the first 24 h of hydration. The kinetics of brownmillerite hydration is controlled by K3-citrate dosage, and the reaction is strongly accelerated with highly concentrated solutions (0.6 M). A similar phenomenon is observed for amorphous C2S at the early hydration stages. Among hydration products, crystalline siliceous hydrogarnet is identified. Precipitation of hydrogarnet (as observed with the XRD technique) coincidences well with the occurrence of a shoulder in the heat flow curves after the heat flow maximum due to brownmillerite dissolution. It is shown that nanosilica slightly delays the hydration of brownmillerite while enhancing the precipitation of C-S-H gel. In consequence, the early age mechanical properties of BOF slag mortars with nanosilica addition are improved.