Alkali-activated slag characterization by scanning electron microscopy, X-ray microanalysis and nuclear magnetic resonance spectroscopy

The alkali activation of blast furnace slag has the potential to reduce the environmental impact of cementitious materials and to be applied in geographic zones where weather is a factor that negatively affects performance of materials based on Ordinary Portland Cement. Blast furnace slag is a highly heterogeneous material. It is well known that its chemical composition affects the physical properties of the alkali activated material, however there is little work on how these inhomogeneities affect the microstructure. In this study we characterize blast furnace slag activated with KOH using several methods: X-ray diffraction (XRD), transmission electron microscopy (TEM), nuclear resonance magnetic spectroscopy (NMR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and quantitative element mapping. Attention is focused on delineating the phases before and after the alkali activation. The unactivated slag shows two distinctive phases. After alkali activation, several chemical changes were detected. One of the phases of the unactivated slag splits into three new phases with distinct chemical composition—one is rich in hydrotalcite and two phases where calcium aluminum silicate hydrate (C-A-S-H) is predominant. A fourth phase was the particles carried over from the unactivated slag, but with significant changes in the chemical composition. NMR results show the formation of tobermorite-like structure in which Al substitutes for Si. •TEM-EDS shows that the unactivated slag is a heterogeneous material with variable chemical composition within the same particle. Phase mapping indicates two phases with different compositions.•The paste obtained after alkali activation of blast furnace slag with a KOH solution is mostly amorphous but some crystalline phases such as hydrotalcite-type, (Ca, Mg)CO3, SiO2, and C-S-H were detected by x-ray diffraction.•Two of the phases observed in the matrix tend to cluster in a region within a narrow ratio of Mg/Ca and Al/Ca while the phase much richer in Mg and Al relative to Ca has greater variability.•NMR spectra show that the Al is approximately equally partitioned between tetrahedral and octahedral coordination. The former is associated with C-A-S-H phase and the latter with hydrotalcite-type phase.