In this contribution the effect of applied non-hydrostatic stress on the degree of hydration of Portland cement paste with high w/c-ratio is investigated. Small samples (5×5×6 mm) were loaded at a stress level of 0.4 times the compressive strength at an age of 7 or 28 days. Before and after 100 days under load, the degree of hydration (α) of the stressed and reference load-free samples was measured by SEM image analysis and thermogravimetric analysis (TGA). Even though all samples were sealed, sample drying and possible carbonation occurred during the creep tests and this complicated elucidating the effect of applied stress on α. SEM image analysis of unhydrated cement showed that α of samples loaded at 7 days age was 2-3% lower than the one in the corresponding load-free samples after 100 days. This negative effect of applied stress on α was not confirmed by TGA measurements, probably because carbonation or another chemical reaction reduced the calcium hydroxide content. Non-hydrostatic thermodynamics predicts that applied non-hydrostatic stress has a much smaller effect on cement solubility than hydrostatic pressure (by 3 orders of magnitude). Applied non- hydrostatic stress is therefore not expected to significantly increase the driving force of cement dissolution. The observed stress-induced reduction of α is explained by a reduced precipitation rate of calcium hydroxide on the stressed surface of pre-existing calcium hydroxide crystals.
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