The patch microstructure in concrete: Evidence that it exists and is not a backscatter SEM artifact

Many mortars and concretes have been shown by backscatter SEM to contain dense local patches of cement paste exhibiting few detectable pores adjacent to local patches that are highly porous. It has recently been alleged that the dense patches are artifacts induced in backscatter SEM examination in areas insufficiently impregnated with resin. In the present paper we present evidence that these allegations are incorrect. Fully resin-impregnated specimen surfaces show dense and porous patches, as do completely resin-free areas of specimen surfaces. Secondary electron examination in the resin-free areas shows that the porous patches clearly exhibit the rim brightness effects characteristic of empty pores, but the dense patches do not. The boundaries between dense and porous areas may or may not be sharp, and may vary in sharpness even in the same locality. The alleged “artifact” attributed to backscatter SEM is also readily observed in optical microscope thin section examination. Dense and porous patches are also seen in micro-CT examination, which involves no resin and indeed no specimen preparation at all. The patchy structure in hardened concrete appears to originate in corresponding areas of dense and open assemblages of cement grains in fresh concrete.