The influence of both early and late heat treatments on the microstructure and on the hydration products of Portland cement mortars has been investigated. The mortars were given either a 4-h or 28-day precure at 20 °C before heating at 90 °C for 12 h and were subsequently stored in distilled water at 20 °C. The microstructure, studied by backscattered electron (BSE) imaging, shows the formation of distinct rims of inner C-S-H with different grey levels during the different stages of the curing cycles. The grey levels and corresponding BSE coefficients of these different rims were determined by image analysis and their chemical compositions by EDS microanalysis. It was found that the compositions depend on the temperature and time at which the rims had developed. The lighter C-S-H formed at 90 °C was denser and contained much more sulfate than the darker C-S-H formed at 20 °C, especially when the heat cure took place at early ages. The sulfate incorporated within the lighter C-S-H was released gradually over time. © 2002 Elsevier Science Ltd. All rights reserved.