The hydration behavior of Portland cement and C3S with and without stress applied were studied at water to solid (w/s) ratios of 0.35 and 0.50. A greater degree of hydration and denser microstructure were obtained with hydration under stress at an early stage of the hydration process, normally within 48 hours. This observation was confirmed by the determination and comparison of CH content, TGA results, surface area and pore-size distribution of samples hydrated with and without applied stress. Environmental SEMexamination provided the evidence that micro-cracking occurred only in samples hydrated under applied stress. It is suggested that this facilitated the migration of water into the protective layers (around cement grains) that form in the early stages of hydration accelerating the reaction with the unreacted cores of the particles.
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