Electron energy loss spectroscopy (EELS) is a sophisticated analytical technique used in transmission electron microscopy (TEM) to measure the energy loss of electrons as they interact with a sample. This technique is pivotal in determining the electronic structure, chemical composition, and optical properties of materials at atomic scales. In the context of cement and concrete, EELS is instrumental for analyzing the nanostructure of hydration products, identifying the presence of deleterious phases, and studying the alteration of materials under severe environmental conditions like extreme pH, high salinity, or reactive environments.