SEM Investigation of Microstructures in Hydration Products of Portland Cement

Portland cement is the one of the main ingredients in the manufacture of many building materials include concrete composites. The phase composition of hydration products is controlled by means of XRD and DTA/TG analysis. Observations of phase changes and microstructure of maturing cement pastes can also be observed using scanning electron microscopy (SEM) techniques combined with chemical analyzes in microareas (EDS analysis). Hydrating in time cement paste is composed mainly of hydrated silicates of calcium so called C-S-H-phase accompanied with a calcium hydroxide (portlandite) and hydration products of calcium aluminate i.e. ettringite. SEM analysis of changes in the morphology and microstructure of cement pastes allow to track the hydration progress observed mainly by changes in the C-S-H phase. The initial stages of hydration of this phase is characterized by radial concentration of fibers or needles, often narrowed at the ends. This fibers grow from the surface of the cement grains. The increase in the degree of C-S-H structure orientation is shown by formation of fibers lattice, sometimes three-dimensional plates so called "honeycomb", which is transformed into the form of a closely-packed, isometric grains. In addition, besides C-S-H cement phase investigation, scanning electron microscopy can also be applied to observation of the crystals formation of tobermorite, ettringite and relicts of portlandite, that often can't be detected by XRD and DTA/TG due to their small amount in mineral composition of concrete.