Microstructure of tricalcium silicate and Portland cement systems at middle periods of hydration-development of Hadley grains

The development of the microstructure of C3S paste and a Portland cement paste was studied between 7 and 24 h by means of backscattered electrons in a field-emission SEM. The course of hydration was measured by isothermal calorimetry. While the abundant occurrence of Hadley grains (hollow-shells) in Portland cement systems is well documented from a number of SEM and other microscopy studies, some earlier reports have noted that Hadley grains do not form in C3S or alite paste alone. This report shows evidence of Hadley grains in C3S paste, and follows their development from middle to late hydration stages. At around 10 h the microstructure with respect to Hadley grains were seen to develop in a very similar manner in C3S and cement. In both systems, a narrow gap often developed between the receding anhydrous cores and layer of reaction product enveloping the cores. By 1 day, Hadley grains had continued to develop only in the cement paste, where they became a prominent feature. Only small ‘hollowed-out’ hydration shells were observed in the C3S paste by 1 day. These were presumably reminiscences of the small gapped Hadley grains seen at the earlier hydration stages.