Characterising aggregate surface geometry in thin-sections of mortar and concrete

Measurement of microstructural gradients at the aggregate/cement paste interfacial transition zone (ITZ) in hardened mortar and concrete is commonly performed via quantitative image analysis of multiple micrographs of specimen surfaces, using a scanning electron microscope. However, due to the random orientation of interfaces sectioned by the specimen surface, measurements of the microstructural gradients at the interface have an unknown angular component, and thus have an unknown error. We present a method for the identification of interfaces that are perpendicular to the specimen surface, and therefore, are more suitable for accurate ITZ analysis. This method employs simple optical and electron imaging techniques on petrographic thin-sections. Use of 3D laser scanning confocal microscopy helped to validate the method. Quantitative 2D image analysis of backscattered electron micrographs, captured over three angular classes of interface gives an indication of this error in the determination of interfacial porosity and anhydrous cement content.