Optimization of SEM-EDS to determine the C-A-S-H composition in matured cement paste samples

•The intermixing of phases is the limiting factor in the analysis of C-A-S-H composition by SEM-EDS•Guidelines to limit beam damage and properly analyze C-A-S-H composition by SEM-EDS are given•SEM-EDS and TEM-EDS give similar results when proper data treatment is made

Microanalysis of characteristic X-rays in the SEM is a powerful method to assess the chemical composition of phases in cement pastes, in particular the calcium silicate hydrate containing aluminium (C-A-S-H). Nevertheless, many variables may influence the results obtained, due mainly to the intimate mixing of C-A-S-H with other hydrate phases and the susceptibility of this phase to damage by the electron beam. In this study the effect of various acquisition parameters was examined, along with methods to determine an “average” C-A-S-H composition. The results acquired in the SEM were compared with the analysis of the same samples in the TEM, where phases can be analyzed without intermixing. A simple method was used to obtain compositions from SEM based analysis that are very close to those which can be obtained in the TEM.