The chemistry of ground granulated blast furnace slag (GGBFS) is one of the main factors affecting its reactivity. Knowing the composition of GGBFS used in aged concrete may help understand its past performance and estimate remaining service life. It is well known that GGBFS generally react slower than cement clinker. This thesis proposes that partially hydrated GGBFS grains found in aged concrete could be used to determine its chemical composition. The feasibility of using standard-based energy dispersive X-ray spectroscopy (EDS) microanalysis, along with electron imaging, as a tool for quantitative measurement of the chemical composition of GGBFS grains in cement/concrete is explored in this research. X-ray spectra collected from minerals or glasses of known composition are known as standards for quantitative EDS microanalysis. This study initially examined the performance of mineral and slag standards to quantify X-ray spectra obtained from GGBFS-cement paste specimens of known composition. Parameters such as GGBFS properties, experimental conditions, and microanalysis methodologies that affect the EDS quantitative total and accuracy were elaborated. Then, the same standards were used to quantify the unknown composition of GGBFS used in seven concrete specimens collected in the Netherlands representing different service life, which revealed the variation in GGBFS chemistry over the years. Finally, as a supplementary study, the variation in significant hydration products formed due to variation in GGBFS composition in a cement-GGBFS system was predicted using thermodynamic modelling with GEMS. Altogether, it was concluded that standard-based EDS is a solid tool to characterize the chemical composition of GGBFS used in aged concrete.