MICROSTRUCTURE OF HARDENED CEMENT PASTE - A NEW INTERPRETATION

Morphological features of hydrated portland cement paste exhibited on carefully polished surfaces have been critically examined using backscatter mode scanning electron microscopy. A new interpretation and classification of such features is proposed. The microstructure is seen to consist of distinct grains of the order of 10 mum and larger (''phenograins'') set in a cellular groundmass of smaller particles and pores. Most of the phenograins are dense pseudomorphs of the larger cement grains, but some are large calcium hydroxide crystals, and in some cements a few may represent inert filler grains added to the cement. The phenograins that are pseudomorphs of cement particles may be solid, gapped, or hollow; and, at a given age, various stages of hydration are represented in different grains. While many pseudomorph phenograins consist of a simple core of unhydrated material surrounded by a uniform shell of hydration product, more complex and irregular structures are also found. Relationships between these features and earlier classifications of cement hydration product particles are discussed, and implications with respect to difficulties in image analysis of hardened cement paste are examined.