Microstructural features of concrete in relation to initial temperature—SEM and ESEM characterization

Abstract

Both high initial and subsequent curing temperatures of concrete are known to induce decreases in strengths at 28 days of age. This paper deals with observation of the concrete microstructure to point out some differences that would be responsible for these changes in strength. For this purpose, conventional electron microscopy (SEM) and environmental electron microscopy (ESEM) techniques were used to study the microstructural features of normal strength concrete prepared at two initial temperatures (20 °C and 50°C) and cured at either 20°C or 35°C. Using either SEM or ESEM, three major results can be observed on fracture surfaces. When initial temperatures are increased, the bulk paste seems amorphous, with a local poorly textured morphology. Moreover, air voids and capillaries can be seen close to the paste-aggregate interface. High initial temperatures also promote crystallization. Particularly, intermingling of little ettringite rods and large crystals of calcium hydroxide are observed on aggregate imprints, which results in an open microstructure of the paste. This latter observation explains the decrease in concrete strength as a result of a weakening of the interfacial transition zone. There are also outstanding differences between SEM and ESEM observations, which indicate artifacts of drying when using SEM.