Similar to Portland cement, microstructure of slag hydrate could also be classified as inner product, which formed within the boundaries of the original anhydrous grains, and outer product that formed in the originally water-filled spaces.
The major phases present in the outer product region of neat OPC pastes are C-S-H gel, Ca(OH)2, AFm and AFt. The addition of slag affects the morphology of the microstructure of slag blended cement. The morphology of the outer product C-S-H varies with chemical composition, viz. at high Ca/Si ratio it has a fibrillar morphology which gradually changes to foil-like with a reduction in Ca/Si ratio. In pure OPC or with low slag loading, amorphous C-S-H has a strongly linear directional characteristic being fibrillar in its appearance in the TEM. At approximate 75% GGBFS regions of foil-like C-S-H form without these linear characteristics.
As the slag fraction is increased, the foil-like morphology gradually replaces the fibrillar morphology. However, the fibrillar feature of C-S-H gels occurs in all blends except neat GGBFS system. Foil-like morphology may be coarser or finer, which depends on the space constraints upon the development of C-S-H gel. Although the linear and fibrillar morphology has fine porosity, its inefficient filling of space appears to leave some fairly coarse, interconnected pores, while the more evenly distributed pores of the foil-like C-S-H are probably less well interconnected, which may account for the beneficial effects of slag in reducing diffusion rates in blended pastes, thus being largely responsible for the improved durability performance.
The aluminate hydrates AFt and AFm can occur in all the OPC-bearing blends. These phases are found to be identical in morphology to those present in neat OPC pastes, which has a needle-like morphology. Special attention should be paid when preparing the specimen because AFt phase may suffer dehydration.