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The companion controlled relative humidity experiments were performed using Cement 116. The paste samples were mixed in a sealed plastic bag for 5 minutes. The specimen holders consisted of two 1.6 mm-thick sheets of hydrophobic polytetrafluoroethylene (PTFE) cut to 127 mm x 152 mm. One of the sheets had been punched with twenty three 25 mm-diameter holes, and the two sheets were adhered together using a silicone sealant. The mixed cement paste was spread over the 25 mm holes, filling them to form discs of uniform thickness. The fresh specimens were then placed in a 25ºC, 100% RH environment.
To assess the combined effects of RH and time on the hydration of cement paste, the specimens were stored at 100% RH for 6 and 12 hours, and 3 and 7 days before being placed into an environment of 25ºC and 90% RH. Here, the specimens were maintained in an atmosphere free of CO2 to avoid carbonation effects, achieved by using an overpressure of pure nitrogen. Prior to entering the 90% RH environment, the two sheets were pried apart, exposing both sides of the hardened specimens (the sheets for the 0.3 w/c pastes were pried apart after 24 hours, or sooner as required). Periodically, the degree of hydration was determined by measurement of the non-evaporable water content, as described above, from a sample consisting of two paste discs.