The strength of hardened cement paste, mortar and concrete depends on their pore structure and morphology, surface energies and streangth of the hydrate, etc. The pore structure including the pore size distribution, the volume and morphology of pores is the main factor controlling the strength. Strength, morphology and surface energy of hydrate itself is an important strength-controlling factor next to the pore structure. Although a lot of research has been focused on the morphology and surface energy of the hydrate and the influence of pore structure on strength, only a little research on the structural factors controlling the strength of hydrate itself has been carried out. This paper deals with the relationship between the polycondensation degree of silicate anion and the strength of mortar, focusing on the polycondensation degree of silicate anion as a structural factor. Polycondensation degree of silicate anion in C-S-H in a mortar specimen increases with the increase of the curing temperature and age. There are large amounts of silicate anions of heptamer or more and dimer in mortar developing high strength. The higher strengths per unit volume of total pore of autoclaved mortar and silica fume blended white cement mortar cured in water are mainly attributed to the increase in fine pore (20 nm or less) on which the stress caused by load is poorly concentrated. The higher strengths per unit volume of pore of 20 nm or more of white cement mortar and blastfurnace slag blended white cement mortar autocalaved are attributed to the increase in strength of hydrate itself by the increase of polycondensation degree compensating the decrease in cohesive energy caused by the increase in crystallinity and the formation of different kinds of hydrates.
