The present study focuses on identification and micro-structural characterization of the hydration products formed in high-volume fly ash (HVFA)/portland cement (PC) systems using monochromatic scanning x-ray micro-diffraction (mu-SXRD) and SEM-EDS. Pastes with up to 80% fly ash replacement were studied. Phase maps for HVFA samples using mu-SXRD patterns prove that mu-SXRD is an effective method to identify and visualize the distribution of phases in the matrix. mu-SXRD and SEM-EDS analysis shows that the C-S-H formed in HVFA system containing 50% or more of fly ash has a similar structure as C-S-H(1) with comparatively lower Ca/Si ratio than the one produced in PC system. Moreover, coexistence of C-S-H(1) and stratlingite is observed in the system containing 80% of fly ash, confirming that the amount of alumina and silicate phases provided by the fly ash is a major factor for the formation of C-S-H(I) and stratlingite in HVFA system. (C) 2014 Elsevier Ltd. All rights reserved.