*3.3. Rheological Properties of Pastes*

Table 8 and Figure 7 present the results of the rheological properties of pastes after 10 and 60 min. The standard deviation of the yield stress test results was equal to 0.354 N·mm for the measurements after 10 min and 0.514 N·mm for the measurements after 60 min, whereas the standard deviation of the plastic viscosity test results was equal to 0.052 N·mm·s for measurements after 10 min and 0.233 N·mm·s for the measurements after 60 min. When analyzing the above test results, it can be seen that the yield stress and plastic viscosity of the reference paste were the smallest. With an increase in the amount and viscosity of cellulose ether, both parameters increased after 10 and 60 min. Paste C50L-3.12MV

with 50% hydrated lime content, cellulose ether content 3.12%, and viscosity 25,000 mPa·s had the biggest yield stress and plastic viscosity. The test results obtained for C50L-1.82HV, C50L-3.12MV, C25L-3.12HV, and C25L-1.82MV pastes, which translated into large values of rheological parameters, were associated with a higher percentage of cellulose ether and a high viscosity, in combination with the replacement of a part of the cement binder with hydrated lime binder. Information in addition to the measurement results of rheological parameters was also provided by the observations of the paste samples during the research. It is known that hydrated lime and cellulose ether affect the workability and plasticity of the material, as well as its cohesion, which was confirmed during the rheological tests. The appearance of C0 and C-0.52MV pastes after 60 min differed from that of the other samples, as shown in Figure 8a,b. In particular, visible differences were observed for the C0 paste. In the case of this sample (C0 paste), a clear separation of water from the rest of the ingredients could be noticed. The separation of water in the C-0.52MV paste was also visible, but to a smaller extent. Both samples (C0 and C-0.52MV) were characterized by the sedimentation of ingredients and a lack of homogeneity through the mass, confirming the successful modification of the pastes and mortars. Despite the fact that the C-0.52MV paste was modified with an admixture, the amount of cellulose ether seemed to be insufficient to prevent segregation of the ingredients of the paste. Comparing samples with the same amount and viscosity of cellulose ether, but differing in the type of binder (C-0.52MV and C50L-0.52MV), a beneficial effect of hydrated lime was observed. In sample C50L-0.52MV, there was no visible sedimentation of the components, while the leaven was coherent and homogeneous throughout the mass.



**Figure 7.** Correlation between yield stress and plastic viscosity of pastes.

**Figure 8.** Photos showing sedimentation of pastes: (**a**) paste C0; (**b**) paste C-0.52MV.

A correlation between plastic viscosity and yield stress was established, as shown in Figure 7. The trend showed an increase in yield stress with the increase in plastic viscosity, which is consistent with the action of hydrated lime and cellulose ether. According to [50], there is a high correlation and significant interdependence of the examined parameters.

Figure 9 presents the response surface of the influence of X1, X2, and X3 factors on the plastic viscosity after 10 min (Y2). The fit factor of the *R*<sup>2</sup> model in this case was 0.902. As can be seen from Equations (5)–(7), the most important factor influencing the plastic viscosity was the amount of cellulose ether (X2 factor). Statistically, the amount of hydrated lime and cellulose ether viscosity had a lesser effect on the parameter tested. As the amount of admixture in the mortars increased, the plastic viscosity increased (similar to the statistical evaluation of the WRV).

$$\text{Y}\_2 = -7.031 + 0.226 \text{X}\_1 - 0.002 \text{X}\_1^2 + 7.641 \text{X}\_2 - 0.898 \text{X}\_2^2 \tag{5}$$

$$\text{Y}\_2 = -7.031 + 0.226 \text{X}\_1 - 0.002 \text{X}\_1^2 + 0.0003 \text{X}\_3 \tag{6}$$

$$\mathbf{Y}\_2 = -7.031 + 7.641\mathbf{X}\_2 - 0.898\mathbf{X}\_2^\circ + 0.0003\mathbf{X}\_3.\tag{7}$$

**Figure 9.** Utility function for the plastic viscosity after 10 min of pastes: (**a**) correlation between cellulose ether amount and hydrated lime amount; (**b**) correlation between cellulose ether viscosity and hydrated lime amount; (**c**) correlation between cellulose ether viscosity and cellulose ether amount.
