A Re-Evaluation of Hardened Cement Paste Microstructure Based on Backscatter Sem Investigations

Abstract

Backscattered electron imaging of polished cement paste specimens permits a re-evaluation of structural details of hydrated cement paste at the gim level. The primarily microstructural units comprise a highly porous groundmass and large distinct grains (“phenograins”) set in it. The groundmass is composed of several kinds of fine particles, with a significant content of easily detected gross pores. Phenograins are primarily large clinker grains hydrating in-situ, but may be distinct deposits of CH, or may be mineral admixture grains. Detailed EDS analyses indicated that hydrating cement in phenograins has a highly consistent composition, interpreted as C-S-H with a small but regular incorporation of sub-jim CH and calcium monosulfoaluminate. Groundmass particles are highly variable in composition, but appear to consist of C-S-H with variable and occasionally major contents of other hydration products on a sub-μm scale. Incorporation of fly ash does not appear to change the basic microstructure, but silica fume incorporated with superplasticizer drastically modifies the character of the groundmass. Some attempts at quantification of these features by application of image analysis are briefly described.