The cement pastes analyzed in this study were prepared using Cement and Concrete Reference Laboratory (CCRL) portland cement proficiency sample 152,10 issued in January 2004. Its particle size distribution, as measured by laser diffraction, is provided in Figure 1. By volume, the cement contains 69.0 % tricalcium silicate, 8.8 % dicalcium silicate, 12.3 % tricalcium aluminate, and 3.8 % tetracalcium aluminoferrite. Additionally, it contains 6 % calcium sulfates by volume fraction, distributed as approximately 44 % gypsum (calcium sulfate dihydrate), 52 % hemihydrate, and 4 % anhydrite, as determined by x-ray diffraction measurements. Cement pastes with initial w/c = 0.35 and w/c = 0.45 on a mass basis were prepared by mixing for several minutes at either 20 ºC or 40 ºC in a temperature-controlled high speed blender. For the more viscous w/c = 0.25 cement pastes, mixing was performed by kneading the paste by hand in a sealed plastic bag. Cast wafers (≈5 g) of the prepared pastes, approximately 32 mm in diameter and 2 mm to 5 mm in thickness, were placed in small, capped plastic vials to be cured at either 20 ºC or 40 ºC. After about 4 h of curing, any accumulated bleed water was removed from the vials using a pipette, to assess the "true" effective w/c of the pastes. While all of the w/c = 0.25 and w/c = 0.35 cement pastes cured at either 20 ºC or 40 ºC, as well as the w/c = 0.45 cement pastes cured at 40 ºC, contained negligible bleed water, for the w/c = 0.45 pastes cured at 20 ºC, after removing the accumulated bleed water, a paste with an effective w/c = 0.435 remained.
Figure 1. Measured cumulative particle size distribution for CCRL cement 152.
With respect to specimen saturation, two curing conditions were employed. In saturated curing, a small amount of distilled water was placed on top of the paste wafers after removing the bleed water. In sealed curing, the wafers were simply sealed in their plastic vials after removal of the bleed water. The 20 ºC curing was conducted in a walk-in environmental chamber, while the 40 ºC curing was conducted in a temperature-controlled water bath. At various ages between 1 d and 256 d, wafers were removed from their vials and crushed into small pieces, some of which were retained for the LTC analysis as described below. The remaining pieces were further crushed to a fine powder, flushed with methanol in a thistle tube under vacuum to stop their hydration, and divided between two crucibles (for replicates) for determination of degree of hydration via loss-on-ignition (LOI) measurements as described below.