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Details for the two ASTM Type I portland cements used in this study, Cements 115 and 116 issued by the Cement and Concrete Reference Laboratory, can be found elsewhere . In all cases, the cement and the necessary mixing water, w/c=0.3 or 0.4, were conditioned at 25ºC overnight and then mixed together by hand in a sealed plastic bag for two to three minutes. For hydration under saturated and sealed conditions, samples on the order of ten grams were stored in capped plastic vials at 25ºC until measuring the non-evaporable water content, typically after ages of 1, 3, 7, 14, 28, 56, and 90 days. For the saturated samples, approximately 1 mL of water was added on top of the cement paste to maintain saturation. Non-evaporable water was quantified as the weight loss between 105ºC and 950ºC, corrected for the loss on ignition for the dry cement powder. These values were converted to degrees of hydration based on measurements performed on fully hydrated specimens (w/c=3.0, continuously ground in a jar mill for 28 days), which yielded values of 0.226 and 0.235 g H2O/g cement for Cements 115 and 116, respectively. For Cement 115, two samples, one saturated and one sealed hydration, were removed after 94 days curing and prepared for viewing in the scanning electron microscope (SEM).