Percolation due to overlapping ITZs in laboratory mortars? A microstructural evaluation: Fid-Bau Portal

Percolation due to overlapping ITZs in laboratory mortars? A microstructural evaluation

Diamond, S.

In a commonly cited paper (Cem. Concr. Res. 22 (1994) 25), it was found that 28-day-old mortars of sand contents in excess of (similar to)48% by volume showed extra intruded pore space when examined by mercury intrusion porosimetry (MIP). The effect was attributed to 'percolation' of overlapping porous interfacial transition zones (ITZs), as modeled by a hard core/soft shell model. To examine the likelihood of this explanation, duplicate 28-day-old mortars were prepared with the same aggregate and cement using the same unusual mixing procedure and identical curing. The results of examination by backscatter mode SEM were not consistent with the ITZ interpretation. The hardened cement paste (hcp) in all mortars was found to consist of patches of brighter, dense, almost nonporous regions and dark, highly porous patches; the patches indifferently occupying both classical 'ITZ' and classical 'bulk' locations. Many sand grains were surrounded or partly surrounded by dense, almost nonporous hcp. The porous regions appeared to be visibly capable of supporting ready fluid transmission. In the lower sand content mortar, the porous regions were limited in extent and isolated from each other. In the higher sand content mortars, originally considered to percolate by virtue of overlapping ITZs, the proportion of porous hcp was greater and the porous hcp patches were visibly interconnected across the bulk of the mortar. Examination of recovered specimens of the original mortars, now 8 years old, showed the same microstructural features as these duplicate 28-day-old mortars. If percolation does in fact occur in the higher sand content mortars studied, it likely results from the interconnection or overlap of the coarsely porous hcp patches and not from overlap of ITZs.