EARLY AGE MICROSTRUCTURE OF THE PASTE-AGGREGATE INTERFACE AND ITS EVOLUTION

The sequence of microstructural changes occurring at the wet paste-agg regate interface is documented at an age as early as 5 min using the e nvironmental scanning electron microscope (ESEM). Unlike other microsc opic techniques, the ESEM allows pastes of normal water:cement ratio t o be observed at early ages without reducing the paste to a powder. Ev olution of the paste-aggregate microstructure is followed up to an age of 24 h The region adjacent to the aggregate surface contains a phase with a morphology referred to as a ''sheaf of wheat'' morphology. The same interfacial region in a 10-day-old specimen has a microstructure similar to the interfacial transition zone (ITZ) reported in the lite rature. Variations of the ''sheaf of wheat'' morphology due to origina l water-to-cement ratio, mixing energy, incorporation of silica fume, and drying are documented. As revealed by energy dispersive x-ray anal ysis (EDS), the microstructure contains significant amounts of calcium and silica. These results indicate that the observed morphology is li kely to be a calcium silicate hydrate (C-S-H) product that is a precur sor to type I C-S-H. A description of the evolution of the observed mi crostructural features is presented. The ''sheaf of wheat'' morphology appears to be a general precursor to morphologies commonly seen in ma ture pastes.