Effects of carbonation degree on the hydration reactivity of steel slag in cement-based materials

In order to understand the essential relationship between the carbonation treatment and hydration performance of steel slag, the microstructures and reactivities of steel slag supplementary cementitious materials (SCMs) with low and high carbonation degrees were investigated comparatively. Results indicated that carbonation reconstructed the microstructures of steel slags, by forming calcite and SiO₂ gel, which increased the specific surface area and changed the hydration reactivities. Low carbonation degree (15 min of carbonation) increased the hydration reactivity of steel slag and corresponding compressive strengths of cement pastes at all test ages. Instead, highly carbonated steel slags (240 min of carbonation) decreased the compressive strength of cement pastes at early age of 3d but increased the compressive strength at late age of 28d. This is attributed to, on one hand, the excessive consumption of calcium silicates reduced the hydration reactivity. On the other hand, the formation of abundant SiO₂ gel increased pozzolanic reactivity, facilitating the compressive strength development, particularly at late age.