*2.3. Methods*

All pastes and mortars were prepared in an air-conditional laboratory at a temperature of 20 ± 2 ◦C and a relative humidity of 65% ± 5%. Binders, fine aggregate, and water were weighed with an accuracy of 0.1 g, and the chemical admixture was weighed with an accuracy of 0.0001 g. The mixing method was the same for all samples: 90 s of mechanical stirring at 45 rpm + 30 s pause + 90 s of mechanical stirring at 57 rpm.

Mortar consistency was assessed using two methods: a flow table according to the PN-EN 1015-3:2000 standard [39] and the fall cone method according to the PN-B-04500:1985 standard [40]. The consistency was measured using the method in [39] as the average of two measurements of the mortar flow on the flow table. The consistency measured using the method in [40] was defined as the depth of immersion of the cone in the tested mortar.

The bulk density of fresh mortars was calculated according to PN-EN 1015-6:2000 + A1:2007 standard [41].

The water retention value (WRV) was determined according to the procedure described in [42]. This parameter denotes the percentage water content remaining in the tested mortar after short-term contact with the filter paper. Each mortar was tested three times, determining its value after 10, 30, and 60 min, e.g., WRV10, WRV30, and WRV60. This nonstandard test allowed evaluating the behavior of the mortar in contact with a material (masonry element) with varied adsorption capacity.

Rheological measurements were carried out using the Viskomat NT rheometer (Schleibinger Testing Systems, Buchbach, Germany). The rheological behavior of pastes is described by the Bingham model [43–45], using g (N·mm) and h (N·mm·s) as parameters describing yield stress and plastic viscosity. These properties of the pastes were determined on the basis of the flow curves for decreasing shear rates, after 10 and 60 min. The temperature of pastes between individual measurements was constant (20 ◦C), set by an automatic thermostatic system. Each sample was well protected against water evaporation by being placed in a sealed container for the periods between studies.

The application properties of mortars were estimated on the basis of a subjective rating by an expert plasterer (Edyta Spychał's original method, which was used in her Ph.D. thesis) [46]. The first stage was to build a wall of silicate blocks (the masonry elements were seasoned for a period of 30 days in the research conditions). Then, the wall was primed. An image of the laid brick walls is shown in Figure 1, one with dimensions of 87.5 cm width and 154 cm height, and the other with dimensions of 137.5 cm width and 154 cm height.

**Figure 1.** View of the wall before plastering.

The assessment of mortars was carried out in two stages. On the first day, a specialist plastered the walls and evaluated the fresh materials in terms of consistency, viscosity, the ease of plastering, adhesion to the substrate, and potential difficulties during plastering. On the second day, the ease of surfacing (smoothing, surface correction) was determined.

The analysis of application properties consisted of the following elements:

	- On a 0–10 point scale (10—material with the best application properties);
	- Based on a detailed descriptive assessment;
	- Based on the possible method of application: manual and/or machine-assisted).
	- On a 0–10 point scale (10—material with the best properties);
	- Based on a detailed descriptive assessment.

This article is a part of a wider analysis in which the influence of cellulose ether and hydrated lime on the selected properties of plasters was evaluated [46]. Some research in this direction was previously introduced in [2,6,10].

## **3. Results**

#### *3.1. Consistency and Bulk Density Measurements*

In Table 6, the results of the amount of water in mortars, the consistency, and the bulk density for all samples are presented.


**Table 6.** Selected properties of fresh mortars.

The consistency of the mortars measured using a flow table was selected experimentally, in accordance with the assumptions of the research, i.e., at a constant level of 165 mm. According to the values given in Table 6 and Figure 2, it can be seen that the amount of water needed to obtain the required consistency was highly variable and ranged from 142 g to 250 g. The amount of water needed for the reference mortar (C0) was 195 g. For the three mortars C-3.12MV, C50L-3.12MV, and C25L-3.12 HV, to obtain the required consistency, at least 220 g of water was required. This value was mainly due to the viscosity of cellulose ether used, which, in these cases, was 25,000 mPa·s and 45,000 mPa·s, as well as the maximum added amount of cellulose ether, which, in these cases, was 3.12%. Hence the conclusion, than when selecting the required amount of water, the amount and viscosity of this admixture first of all should be taken into account.

**Figure 2.** Amount of water in mortars.

Obtaining consistency at the same level for cement and cement–lime mortars modified with cellulose ether was associated with an increase in the amount of water when replacing a part of the cement binder with lime. This was due to the smaller grain size of lime compared to the grain size of cement, thus leading to its higher water demand (C-0.52MV and C50L-0.52MV mortars).

The use of cellulose ether in mortars is associated with a greater demand for water in order to obtain a consistency at a similar level compared to unmodified mortar. An increase in the admixture content is usually associated with an increase in the water demand of the mortar [3,12]. This research shows that this relationship was not always true, as observed for the examples of C-0.52MV, C-1.82LV, C50L-1.82LV, C25L-0.52LV, and C25L-0.52HV materials. These plasters required less water to achieve the assumed consistency, compared to the reference mortar C0. The reasons for this phenomenon are due to the properties of cellulose ether. When the viscosity of the liquid phase surrounding the grains of the binder and fine aggregate is too low, the dissolved polymer acts as a "lubricant", facilitating the movements of the mortar grains [29,47]. In this case, the action of the cellulose ether is similar to that of a plasticizing or fluidizing admixture.

Taking into account the test results from Table 6 and Figure 3, it can be concluded that the consistency of the mortars varied, ranging from 6.1 cm (C0) to 9.9 cm (C C25L-3.12HV). The standard deviation of the consistency test results [40] was equal to 0.094 cm. All tested mortars achieved a consistency similar to that of typical plasters used in practice [42,48]. The consistency of plastering mortars intended for manual application should be 6–9 cm, whereas the equivalent value for mechanical application should be 8–11 cm. The results obtained in this research fall within the range of consistency given in the literature [48].

**Figure 3.** Consistency results of mortars according to PN-B-04500:1985.

Table 6 and Figure 4 present the range of bulk density values of fresh mortars. The standard deviation of the bulk density test results was equal to 0.000 kg/m3. The lowest bulk density was achieved using the cement mortar modified with cellulose ether in the amount of 3.12% with a viscosity of 25,000 mPa·s (1210 kg/m3—C3.12-MV), while the highest value was achieved by the unmodified mortar (C0)—1950 kg/m3. This parameter can be used to indirectly judge the performance of the plasters (raw material efficiency). The material with the lowest bulk density is characterized by the best performance. Mortar efficiency is important not only from an economic but also from a technological point of view. All modified

mortars had a lower bulk density than the base mortar, which reduced the consumption of materials. Thus, cellulose ether and hydrated lime had a positive effect on increasing the efficiency of the tested mortars, as well as reduced the bulk density. This result is in agreement (for cement mortar) with the literature [3,13].

**Figure 4.** Results of bulk density of fresh mortars.

#### *3.2. Water Retention of Mortars*

Table 7 and Figure 5 present the results of the water retention value (WRV) after 10, 30, and 60 min. The standard deviation of the WRV test results was equal to 0.000% for WRV10 and WRV60 and equal to 0.047% for WRV30.

**Table 7.** Water retention results.


**Figure 5.** Change in water retention of tested mortars over time (WRV parameter).

Comparing the WRV parameter of the mortars, it can be seen that the reference mortar C0 had an initial value approximately 10–15% lower than that of the other mortars. After 15 min, is the water retention was 85.0%; however, after 60 min, it was 76.5%, which is a disadvantage, especially when the plaster is applied on a substrate with high absorbency or when plastering take place in conditions of changing temperature and humidity. In the case of materials C-0.52MV, C50L-0.52MV, C25L-0.52MV, and C25L-0.52MV WRV10, the water retention was greater than 90%, decreasing after 30 and 60 min due to the lowest cellulose ether content of 0.52% in each case. This proves the significant influence of the amount of admixture in terms of water retention in the mortar and its action over time. For the remaining materials, the water retention value remained at a level close to 100% throughout the entire study. Comparing C50L-1.82HV and C50L-1.82LV mortars with the same proportions of binder and amount of admixture, there was no visible influence of the polymer viscosity. The partial change from cement to lime binder was not as noticeable and important as the change in the amount of cellulose ether. According to Brumaud et al. [15], mortars can be classified as follows: C0 mortar would be a material with low water retention (WRV < 86%), C25L-0.52LV mortar would be characterized by medium water retention (86% < WRV < 94%), and the remining samples would be considered as having high water retention (WRV > 94%).

When assessing the test results of the water retention value in mortars after 10 min of testing, similar relationships were obtained in comparison with the literature data. The presence of cellulose ether in cement or lime mortars increases water retention [7,16,49]. Information about the effective action of the admixture in cement–lime mortars, which is not available in the literature, was confirmed in this article. In the case of mortars with a cement–lime binder, a strong influence of the polymer in terms of water retention was visible throughout the entire measurement. Figure 6 presents the response surface of the influence of X1, X2, and X3 factors on WRV10 (Y1). The fit factor of the *R*<sup>2</sup> model in this case was 0.962, which denotes that this model explained 96.2% of changes in the response value. As can be seen from Equations (2)–(4), the most important factor influencing WRV10 was the amount of cellulose ether (X2 factor). For this factor, the significance level was *p* < 0.05, suggesting that the X2 factor had a statistically significant influence on the examined feature. 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 WRV10 increased. Thus, modifying plasters with cellulose ether is a favorable solution from the point of view of water retention in the mortar. The utility level shown in the range from 0 to 1 in Figure 6 determines the influence of the defined factors on the tested parameter, suggesting the optimal arrangement of the factors X1, X2, and X3. Adding as little as 1.2% cellulose ether enabled obtaining a high WRV10 (Figure 6).

$$\text{Y}\_1 = 91.519 - 0.06 \text{X}\_1 + 0.0016 \text{X}\_1^2 + 6.002 \text{X}\_2 - 1.213 \text{X}\_2^2,\tag{2}$$

$$\mathbf{Y}\_1 = \mathbf{9}\mathbf{1}.5\mathbf{1}\mathbf{9} - \mathbf{0}.06\mathbf{X}\_1 + \mathbf{0}.001\mathbf{X}\_1\mathbf{1}^2 + \mathbf{0}.0001\mathbf{X}\_{\mathbf{3}\prime} \tag{3}$$

$$\text{Y}\_1 = 91.519 + 6.002\text{X}\_2 - 1.213\text{X}\_2^2 + 0.0001\text{X}\_3. \tag{4}$$

**Figure 6.** Utility function for the WRV10 test of mortars: (**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.
