Confocal imaging of porosity in hardened concrete

We present the results of an exploratory 3D study of fine pore structures in hardened cement paste. A laser scanning confocal microscope was used in reflected (epi) fluorescent imaging mode, allowing for the very high optical resolution of features well below 1 μm in size. Images of pore structures were captured using a ‘reverse imaging contrast’ technique where spaces in the material were impregnated with a fluorescent, dyed epoxy-resin, and scanned in the x–y plane by a laser probe. A vertical ‘stack’ of these optical sections was acquired by imaging sequentially through the z-axis. Using this technique it was possible to observe, partially hydrated cement grains, porous natural quartz aggregate interfaces, micro-cracking in the hydrated cement paste and aggregate particles, as well as fine capillary pores and very small air voids. Operating at the limit of its capability the microscope was able to perform high resolution imaging of the internal areas of partially reacted cement grains, and pore structures approximately 0.17 μm across were measured. 3D models were produced to help visualise the true morphology and distribution of porous features.