Features of microstructure development in the inner product of reacting slag in blended cement
The morphology of inner product region derived from alite or belite is identical to those present in neat OPC paste. In mature pastes, fully hydrated small grains of both alite and GGBFS often display a coarse morphology.
Generally, inner product in larger grains has a dense homogeneous morphology with very fine porosity, and large slag grains often have a rim of fine textured C-S-H which can exist for many years and merge into outer product C-S-H gel. Mg, Al-rich laths or possibly platelets present within the inner product C-S-H gel, which are oriented either towards the outer boundary of the inner product or randomly even at the outer boundary.
The development of the plates occurs to different extents in different slag grains, possibly depending on composition and also appearing to depend on particle size considering platelets or laths typically appear very early in small grains. When the plates are very well developed they predominate over the C-S-H gel; however, in many inner product regions, even in 3 year old pastes, the plates are less well developed and a high proportion of C-S-H gel is still present. It is not obvious from observations in the early stages that the morphology in the inner product regions is plate-like, thus it is better to be described as laths or needles.
Another microstructural feature of the inner products is the presence of small, round and poorly crystalline particles, which are rich in iron and aluminum and also often contain significant amounts titanium.