3. Materials, Mixing and Testing Details

The materials, mixing, and testing requirements provided here are for the cement paste tests only. The details for the concrete test program are provided in Section 5.

The cement was an ASTM Type I portland cement whose composition is described in Table 1. The high-range water reducer (HRWR) was a naphthalene sulfonate-based product with a mass fraction of 43 % active ingredients. The mineral admixtures used are shown on Table 2 with their mean particle diameters (PD). The mean PD was measured using a laser diffraction particle size analyzer. Four different fly ashes, all from the same plant, were tested. Fly ash (FA) was the standard fly ash available in that plant and used in concrete type applications. Coarse Fly ash (CFA) was a coarse ash that is usually rejected as it does not meet the ASTM C 618 requirements for particle size. Fine fly ash (FFA) is a finer form of ash obtained by separation from a coarser ash using a classifier. Ultra fine fly ash (UFFA) is an ultra-fine ash obtained by still more rigorous separation.

The cement paste composition was varied to explore the influence of mineral admixture dosage and type on the rheological properties. The performance differences in the paste due to the addition of mineral admixture were measured either by the rheological properties at constant water content or by the water reduction at constant mineral and chemical admixture dosage. The compositions of the cement pastes can be summarized as follows:

• water/cement ratio: 0.28 to 0.35
• Dosages of mineral admixtures: 0 % to 16 % of cement, replacing cement by mass
• Dosage of HRWR (based on naphthalene sulfonated condensate): 0.45 % to 0.70 % solid by mass of cementitious materials

Table 2: Mineral Admixtures
No.		Name	Mean PD [µm]
1		Coarse Fly Ash (CFA)	18.0
2		Fly Ash (FA)	10.9
6		Fine Fly Ash (FFA)	5.7
3		Ultra Fine Fly Ash (UFFA)	3.1
4		Metakaolin (MK)	7.4
5		Silica Fume (SF)	» 0.1

The cement paste preparation is very important because the shear history of a mixture will influence its rheological behavior. In this case, we wanted to have the same shear history as in the concrete in order to be able to compare the neat cement paste behavior and the concrete flow. Two types of mixers were considered: a standard paddle mixer [18] and a high-speed blender. The cement paste was mixed using a paddle mixer according to the procedure of ASTM C305, except that no sand was added.

The second mixer used was a large (4 L) blender. This blender was not temperature controlled. The cement paste mixture was prepared according to the following procedure.

• The cement and the mineral admixture (if any) were mixed dry for about 5 s by hand outside the mixer
• Water was poured into the 4 L blender
• The mixer was started at slow speed and the cement-mineral admixture mixture was added over a 50 s period
• The HRWR was added in 5 s
• The mixture was mixed for another 60 s at slow speed

3.3 Cement Paste Testing Details

Fresh cement paste specimens were tested first in a parallel plate fluid rheometer and then using the mini-slump and Marsh cone. The testing details are given below.

3.3.1 Parallel plate rheometer

The parallel plate rheometer was used to determine the yield stress and the plastic viscosity as defined by Bingham. The distance between the two plates of the parallel plate rheometer should be selected based on the cement paste content in the concrete mixture to be characterized. But, as this method was used here as a screening test, and no comparison was sought with a specific concrete mixture design, the gap (distance between the plates) was arbitrarily fixed at 0.4 mm, based on the median value of distance between aggregates in concrete [11]. The shear rate used ranged from 3 s^-1 to 50 s^-1. This range was selected to correspond to the shear rates used in a concrete rheometer [7]. The surfaces of the two plates were serrated to avoid slippage. The sequence of the measurements was:

• One milliliter of cement paste was placed, using a syringe, on the bottom plate
• The two plates were brought together to the desired distance of 0.4 mm
• The computer driven system imposed a slowly increasing shear rate from 0 s^-1 to 70 s^-1 in 160 s. As soon as the highest shear rate was reached, the plate stopped rotating.
• After this first phase (needed to homogenize the specimen), a full cycle of increasing shear rate by 10 steps from 3 s^-1 to 50 s^-1 and back to 0 shear rate with another 10 steps was performed. At each step, the stress was measured and the value of constant stress was recorded. If the constant stress was not achieved in 20 s, the computer took the average of the last 5 values recorded.
• The slope of the down-curve (decreasing shear rate) was used to calculate the plastic viscosity, while the intercept at zero shear rate was used to calculate the yield stress.

3.3.2 Mini Slump Cone Test Details

Kantro developed the mini-slump cone test [19]. The test was conducted as follows:

• A square piece of flat glass on which the diagonals and the median were traced was covered with a plastic sheet. The plastic sheet is needed to avoid water or very fluid paste leakage under the mini-cone.
• The mini-slump cone was placed in the center of the glass plate and filled with the cement paste.
• The cone was gently lifted and after one minute the diameter of the pad formed was measured along the marked line on the glass in 4 directions.
• All four diameters were recorded and the average diameter was calculated.

3.3.3. Marsh Cone Test Details

The Marsh cone is a funnel with a long neck and an opening of 5 mm, currently used to test oil well cement. This test is not a standard test. The test was conducted as follows:

• A Marsh cone was attached to a stand so that the small orifice was pointing down and a glass graduated cylinder was placed under the cone.
• Closing the small orifice with a finger, one liter of cement paste was poured into the cone.
• The orifice was opened and a stop watch started.

The time for a certain amount of cement paste to flow was recorded. The volumes selected were 300 mL, 500 mL and 700 mL. These settings were selected from research conducted by Nehdi et al. [20] that showed a non-linear flow of the cement paste for amounts higher than 700 mL.