Integrated BSEM-Raman reveals the spatial heterogeneity of mineralogical phases in KOH-GGBS pastes

The large volume utilisation of GGBS from locally available sources remains challenging in China because compositional variability influences the long-term degree of slag reaction, spatial heterogeneity, assemblage, and chemistry of the resulting reaction products. This study integrates optical imaging, Raman spectroscopy, and backscattered electron imaging with energy-dispersive X-ray spectroscopy (BSE-EDX) to elucidate the spatial distribution, chemistry, and long-term evolution of reaction products in the 5-year KOH-activated GGBS pastes prepared from 10 different slag sources. Across all slags, the reaction products exhibit pronounced two-dimensional heterogeneity, with C-(A)-S-H and Mg-Al-LDH intergrown within the matrix, while siliceous hydrogarnet preferentially precipitates in discrete outer reaction product regions. The Mg/Al ratio of Mg-Al-LDH varies with slag source, but is consistently constrained to an effective range of approximately 2–3 when LDH-rich domains are reliably captured. Values below 2 are attributed primarily to BSE-EDX interaction volume effects and phase intermixing, particularly where Al-bearing phases, such as AFm-type phases and siliceous hydrogarnet. In addition, one slag exhibits a distinct Fe-bearing Mg-(Al, Fe)-LDH with aluminosilicate intercalation in an almost fully reacted remanent slag particle. Despite wide differences in slag chemistry, the degree of reaction after 5 years converges to a relatively narrow range of 55–65%, and an empirical relationship is proposed to describe the time-dependent evolution of slag reaction. These findings demonstrate that GGBS can sustain long-term activation and that mix design can be tailored based on slag chemistry to support high slag replacement in both blended slag cements and alkali-activated slag systems.