3. Experimental

Cement and Concrete Reference Laboratory (CCRL) portland cement proficiency sample 152 [16], issued in January of 2004, was used to assess the hydration rates of cement pastes cured under saturated and sealed conditions. Portland cement pastes initially with w/c=0.35 and w/c=0.45 by mass were prepared by mixing the water and cement in a temperature-controlled high-speed blender for several minutes at 20 ºC. For both w/s mass ratios (0.35 and 0.45), cement 152 was also blended with a "fine" limestone powder replacing 20 % of the cement by mass. The limestone powder was obtained by using an air classifier to separate a commercially available material with a cutoff diameter of approximately 30 µm [5], and retaining the finer of the two fractions (which contained approximately 65 % particles finer than 30 µm). Based on its measured loss on ignition, the limestone powder was estimated to be 97 % CaCO₃. Freshly cast wafers (≈5 g) of cement paste were placed in small pre-weighed capped plastic vials to be cured under either saturated (water ponded on top) or sealed conditions at 20 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 or w/s of the pastes. The containers of the sealed paste specimens were simply resealed after removing the bleed water; for the saturated paste specimens, after removing the bleed water and reweighing the vials, a small amount of a fresh supply of distilled water was added to the top of the wafers to maintain saturation, before resealing the vials. While the volume of accumulated bleed water was negligible for the w/s=0.35 pastes, for the w/s=0.45 pastes, the measured effective ratio after removing the accumulated bleed water was found to be about 0.435. At ages of (1, 3, 7, 28, 92, and 182) d, specimens were removed from their vials, crushed to a fine powder using a mortar and pestle, flushed with methanol in a thistle tube connected to a vacuum, and divided between two crucibles. The non-evaporable water content (W_N) of each crucible sample was determined as the mass loss between 105 ºC and 1000 ºC divided by the mass of the ignited sample, corrected for the measured loss-on-ignition of the unhydrated cement (or of the unhydrated cement with 20 % limestone). Previously, the expanded uncertainty in the calculated W_N had been estimated to be 0.001 g/g cement, assuming a coverage factor of 2 [10]. W_N values were converted to estimated degrees of hydration based on the phase composition of the cement and published coefficients for the non-evaporable water contents of the various hydrated cement clinker phases [17]. Based on a propagation of error analysis, the estimated uncertainty in the calculated degree of hydration is 0.004.

Virtual cement pastes were created using CEMHYD3D to match each of the experimental mixtures. Densities of 3200 kg/m³ and 2700 kg/m³ were assumed for the cement and limestone, respectively. The measured particle size distributions, as shown in Figure 1, were utilized for cement 152 and for the limestone filler. The w/c and w/s in the virtual pastes were selected to match those in the real prepared pastes, after accounting for removal of the accumulated bleed water. The chemical composition of cement 152, as measured by scanning electron microscopy (SEM) [18], is provided in Table 1. In addition, the cement contained 6 % calcium sulfates by volume, distributed as approximately 44 % gypsum (dihydrate), 52 % hemihydrate, and 4 % anhydrite, as determined by x-ray diffraction measurements. For all of the simulations conducted using the modified CEMHYD3D software, a conversion factor of 0.00035 h/cycle² was used to convert between model cycles and real time [10, 11]. The same value was used throughout for different w/s (0.35 or 0.435), limestone contents (0 % or 20 %), and curing conditions (saturated or sealed).

Figure 1: Particle size distributions for the materials used in this study.