Figure 1: Influence of the mixer on the rheological properties of cement paste. "YS" is the yield stress and "Visc. " is the plastic viscosity.
The goal of testing cement paste instead of concrete is to save materials and labor. But to be useful the cement paste results need to predict concrete performance. Therefore, the cement paste needs to be sheared with about the same intensity as it would have experienced while being mixed in concrete. One way to determine if the mixing method selected is appropriate is to select several cement paste compositions and compare the rheological behavior of the cement paste with the concrete performance. In this paper we selected cement paste with and without mineral admixtures. The key result, which would tell us that we had the correct mixing method, is that the mineral admixture that resulted in the larger reduction in yield stress or viscosity compared to the control, which had no mineral admixture, would be the same in cement paste and in concrete. The two mixers available to us were a Hobart and a blender.
To perform the comparison, we selected three mineral admixtures (MK, UFFA and SF) to test in cement pastes. If the selection of the best mineral admixture was based on the data shown in Figure 1, the result would depend on the mixer used. Consider the yield stress (YS) behavior (bars and left axis). The best admixture should reduce the yield stress compared to the control (no admixtures). If the black bars (paddle mixer) are examined, the best admixture will be the MK, while if the gray bars are examined, the choice will be UFFA. In examining the results obtained with concrete [22], for equivalent slumps the water demand is the lowest for mixtures containing UFFA. In other words, UFFA increases slump in concrete when water content is kept constant. In conclusion, UFFA is the "best" admixture both in concrete and in cement paste if mixed in a blender. Therefore, concrete performance was more accurately predicted by the cement paste mixed in the blender and not in the Hobart mixer. This confirms a study by Helmuth et al. [21] stating that in concrete, during mixing, the cement paste is sheared with an energy and rate more closely reproduced in a blender as opposed to the low shear rate of the Hobart mixer. Therefore, to predict concrete behavior, it is essential to use the correct mixer while preparing cement paste. Thus, the rest of the data reported in this paper were obtained using the blender.
Figure 1: Influence of the mixer on the rheological
properties of cement paste. "YS" is the yield stress and "Visc.
" is the plastic viscosity.
A fluid rheometer for cement paste is not widely available in the construction industry for many reasons. The two main reasons are: 1) the instrument is relatively expensive (on the order of $40,000) and 2) the importance of using such a device for cement paste was not advocated until recently [11, 12]. Therefore, it would be advantageous to be able to use simpler tests such as the mini-slump and the Marsh cone tests. A comparison between the rheometer and the other test results is presented in Figure 2 and Figure 3. These figures are a compilation of all the tests conducted in this research program using a blender to mix the cement paste. Each point represents various mineral admixture additions at various dosages, w/c, and HRWR dosages.
The plot of yield stress versus mini-slump spread diameter (Figure 2) shows a weak correlation: higher yield stress corresponds to a lower spread in the mini-slump. Therefore, an indication of the yield stress could be obtained using the mini-slump. This result was expected because the cement paste in a mini-slump test will only flow if the stress due to the weight of the cement paste contained in the cone is high enough, i.e., higher than the yield stress of the cement paste. It should be kept in mind that the minimum diameter that can be measured is 70 mm, corresponding to the diameter of the bottom of the cone. Therefore, despite some of the scatter of the data shown on Figure 2, an approximation of the yield stress could be obtained by fitting a straight line through the data. This fit will not be done here because it has a limited significance, due to the wide scatter of the data.
In contrast, the plot of time to flow for 300 mL (Marsh cone flow test result) and the viscosity (Figure 3) shows no correlation at all. If outliers are removed, a "shot gun" distribution can be observed. Similar results were obtained when the times to flow for 500 mL or 700 mL were plotted, because the time to flow had a linear relationship with the amount of material measured. Nevertheless, in some limited cases, a lower time to flow does correspond to a lower viscosity. But it would be dangerous to rely on the Marsh cone to select a material for a certain viscosity requirement or even to rank materials based on viscosity due to the overall lack of correlation. This result is somewhat unexpected because it was assumed that the weight of the cement paste was high enough to overcome the yield stress and therefore the speed of the cement paste flow through the flow cone would depend on its viscosity. From the results obtained, it seems that other factors contribute to the flow, such as friction and sedimentation.
4.3 Effect of Mineral Admixture Type on Cement Paste
Rheological Properties
In Figure 4, the yield stress and viscosity are shown for mixtures composed of cement paste with the same w/c ratio of 0.35 and varying dosages of HRWR. The amounts of the various mineral admixtures by mass as replacement of cement are indicated on the figure. It is clear that the replacement of cement with UFFA leads to a decrease in the HRWR dosage over the control (no mineral admixtures) at a given yield stress or viscosity. In contrast, the replacement of cement by silica fume significantly increases the HRWR dosage at a given yield stress and viscosity. The addition of MK shows no significant improvement in yield stress and plastic viscosity over the control. Therefore, there are no significant rheological benefits or drawbacks in using MK as a mineral admixture, at least at the dosages tested.
Figure 4: Dosage of HRWR and its effect on the flow properties. The w/c
ratio was 0.35. The error bars represent an estimate percentage error: 17% on
yield stress and 10 % viscosity. This error was estimated from the numerous
tests done.
In Figure 5, the rheological measurements for the four fly ash/cement pastes are plotted against the mean particle size of the fly ashes. All tests were conducted at the same dosage of mineral admixture (12 % replacement of cement by mass), same w/c ratio (0.35) and same dosage of HRWR (0.45 % solid by mass of cement). It is clear that the lowest yield stress and viscosity are obtained at a mean PD of 3 µm. This value corresponds again to UFFA. It also seems that maximum viscosity is reached at a mean PD of about 11 µm. and maximum yield stress at a mean PD of 5.7 µm. This result seems to indicate an optimum and a pessimum PD, with the optimum at 3 µm and the pessimum at 5.7 µm. Unfortunately, a FA with a smaller PD than 3 µm was not available to determine if the correct optimum was reached. Sakai et al. [16] also showed that there is a pessimum at 18 µm, but he used limestone powder and not FA. It is conceivable that the optimum and pessimum value depends on the type of mineral admixture used, and the chemistry and physics of the individual particles.
Figure 5: Influence of mean PD on the flow properties of cement paste. The w/c ratio was 0.35. The error bars represent an estimate percentage error: 17% on yield stress and 10 % viscosity. This error was estimated from the numberous tests done.
Figure 6 shows the results of tests performed on cement pastes with UFFA (at 12 % replacement) at various w/c ratios, plotted vs. HRWR dosage. There are several ways to use or to interpret these results: 1) determine the correct dosage of HRWR needed to obtain the same yield stress and/or viscosity with the UFFA mixes and the control at various w/c; 2) determine the water reduction achieved by using UFFA and maintaining the same yield stress and/or viscosity; 3) determine the reduction in HRWR dosage achieved while maintaining the same yield stress and/or viscosity.
In summary, the addition of UFFA improves the rheological properties. If the goal is to add UFFA and achieve the same yield stress and viscosity as the control, Figure 6 shows that the w/c ratio can be reduced by 10 % and the HRWR dosage can be reduced by 40 %. On the other hand, if the water content is reduced by 20 % (w/c ratio of 0.28) a significant increase of the HRWR dosage (almost double) is needed to maintain the yield stress or viscosity, giving the same rheological behavior as the SF mixes.
Figure 6: Influence of W/C ratio on the rheological properties of cement paste with UFFA at 12% replacement of cement by mass. The numbers in the legend indicate the w/c ratio used. The error bars represent an estimate percentage error: 17% on yield stress and 10 % viscosity. This error was estimated from the numerous tests done.
Figure 7 shows the influence of dosage of UFFA on cement paste rheological properties. The tests were done at a w/c ratio of 0.35 and a fixed HRWR dosage of 0.45 % solid by mass of cement. The plot suggests that a dosage of 12 % by mass is optimal for the best rheological properties. The dosage shows an optimum value corresponding to the lowest value achieved by the yield stress for a 12 % UFFA by mass dosage. The shape of the curve (Figure 7) is an important result because it corresponds to the same type of behavior seen in concrete, as will be shown in Section 6 and in Figure 9.
Figure 7: Influence of the dosage of UFFA on the rheological properties of cement paste with constant dosage of HRWRA (0.44% (13 oz/100lbs of cementitious materials)). The w/c ratio was 0.35. The error bars represent an estimate percentage error: 17% of yield stress and 10% viscosity. This error was estimated from the numerous tests done.
