Observation of cement paste microstructure evolution

During early hydration in fresh concrete, the cement paste hardens and bonds the aggregates together to make a solid concrete mass. During this process, along with chemical reactions, physical changes in the cement paste at the microlevel and the nanolevel also play major roles in the development of the properties of the concrete. An attempt was made to study the microstructure of cement paste and the evolution of its corresponding lastic modulus during setting. For this purpose, two different in situ testing methods, scanning electron microscopy (SEM) with a Quantomix capsuling system and a nondestructive one-sided ultrasonic technique, ultrasonic wave reflection method, were used. The features in the SEM images were quantified by the use of image analysis techniques, and quantitative analysis was used to examine the relationship between the microstructure of the cement paste and the corresponding dynamic shear modulus during the initial and final setting times. Cement particle growth, motions, and rotations were observed during the evolution of the microstructure of the cement paste. A good correlation between solid-phase and elastic modulus developments was observed after 6 h of mixing for all three pastes tested. Finally, it has been concluded that in situ studies of the microstructures of fresh cement paste can greatly enhance knowledge of the development properties of concrete at an early age.