Structure of the hardened paste

Microscopic examination of the hardened cement paste reveals the CSH gel to be an undifferentiated amorphous mass in which unhydrated cement grains, hexagonal crystals of calcium hydroxide, and occasionally small numbers of hexagonal or cubic crystals of aluminate and of sulphoaluminate are embedded. Pores, either filled With water or empty, are also detectable in the mass. As already mentioned, this amorphous mass is a rigid gel, i.e. a solid made up of small particles of colloidal dimensions. The particles are mainly hydrates of calcium silicate with some aluminates and ferrites. The hydrates of the calcium silicate are poorly crystallised and their structure is characterised by a high degree of disorder. Accordingly, these hydrates are sometimes described as ‘crystallites’ or ‘quasi-crystallites’.1,2