The ITZ in concrete – a different view based on image analysis and SEM observations

Abstract The traditional picture of the ITZ in concrete involves an approximately 30 μm zone around each aggregate, within which the porosity increases as the aggregate interface is approached. The results of the writers' extensive image analysis investigations, and examinations of SEM specimens from various concretes provide a very different picture. While the “wall effect” excluded much of the ground cement from the vicinity of the aggregates, the great increases in pore content within a few μm of the aggregate, up to approximately 30% porosity, that have been reported by others, are not found. On average, only modestly higher porosities are observed within the ITZ than in the bulk paste. This is true even in the innermost areas immediately adjacent to the aggregates. In part, the extra space produced by the wall effect is filled in by CH deposits, many of which are anchored directly on the surfaces of aggregates and are essentially non-porous. Strong indications exist that the ITZ contains as high a proportion of C–S–H per unit volume as the bulk paste; thus some of the extra space that was created by the wall effect is filled in by through-solution deposits of C–S–H derived from elsewhere in the system. It is considered that the structure of the ITZ in ordinary concretes is not different enough from that of the bulk cement paste to provide any basis for significant effects on permeance or mechanical properties.