Electron Microscopy Study of Ordinary Portland Cement and Ordinary Portland Cement–Pulverized Fuel Ash Blended Pastes

Many ordinary portland cement (OPC) OPC–pulverized fuel ash (pfa) blended pastes of various ages have been examined using transmission electron microscopy, energy dispersive X‐ray analysis, and scanning electron microscopy. In addition to the phases commonly identified in OPC pastes, such as CH, CSH, AFt, and AFm, we found areas of pooriy crystalline iron‐containing material intermixed with CSH and needles of hydrotalcite‐like composition. The iron‐containing material may be the precursor of hydrogarnet phases identified in older pastes by other authors. In the pfa‐blended cements, some of the pfa particles reacted to give a radially fibrillar gel, and others showed areas of well‐crystallized hydrogarnet within the clearly defined outer boundary of the particle. Unreactive pfa particles had a rim of fibrillar CSH gel around the particle. The general fine‐scale microstructure of the OPC‐pfa‐blended pastes was similar to that in OPC pastes alone, but the CSH gel material had a lower C:S ratio.