The microstructure of steam-cured precast concrete

This work reports on the microstructure of concrete cubes manufactured at 16°C and steam-cured at 42°C, 60°C and 80°C. Increase in curing temperature leads to the development of distinct hydrate rims around both anhydrous cement grains and Hadley grains. In concrete cured at or above 60°C, 20%-30% of the C-S-H rims had a dark inner zone and light outer shell which were not present at room temperature, and up to 80% of the Hadley grains had distinct hydrate product rims, many being partially or completely in filled with either ettringite or monosulphate-type hydrates. As curing temperature increases the proportion of ettringite-type hydrates decreases, while those associated with monosulphate become more predominant. Sulphate concentrations were about twice as large in the light outer C-S-H as in the dark inner C-S-H for specimens cured at 80°C and, within the hydrate rims of the Hadley grains, the sulphate content at 80°C was about twice that observed at 60°C.