*2.1. Materials*

A commercial Portland cement CEM I 42.5 R (Cemex, Chełm, Poland) [30], deeply separated hydrated lime (Alpol, ZSChiM "PIOTROWICE", Sitkówka, Poland) [31], two fractions of quartz sand 0.1–0.5 mm and 0.2–0.8 mm (Grudze ´n Las, Grudze ´n Las, Poland) [32], a polymer admixture of different viscosity and amount (WALOCEL, The Dow Chemical Company, Midland, MI, USA) [33], and tap water were used. Among the wide variety of cellulose ethers, the following were chosen in this research: hydroxypropyl methylcellulose (HPMC) with viscosity 3000 mPa·s and hydroxyethyl methylcellulose (HEMC) with viscosity 25,000 mPa·s and 45,000 mPa·s (for measurements using a Brookfield rheometer at 20 ◦C). These polymer admixtures had the form of a very fine white powder, with a grain size of less than 0.063 mm and a low degree of chemical modification. The chemical composition and physical properties of cement are given in Table 1. The chemical composition of hydrated lime is shown in Table 2.

Silicate blocks of N12 type with dimensions 0.25 m × 0.12 m × 0.22 m (H+H Company, Warsaw, Poland) [34] were used to test the application properties of plasters (for bricklaying the walls). The basic properties of these masonry elements are presented in Table 3.


**Table 1.** Chemical composition and physical properties of cement.


**Table 2.** Chemical composition of hydrated lime.

**Table 3.** Chosen properties of silicate blocks.


<sup>1</sup> For general-purpose mortars and light mortars; <sup>2</sup> for thin pointing mortars.

#### *2.2. Mix Proportions of Pastes and Mortars*

The Box–Behnken experiment plan was used to analyze the plasters [35,36]. Pastes and mortars were tested by taking into account the selected experimental plan. The analysis of the test results was performed using the STATISTICA computer program (StatSoft Company, Cracow, Poland) [37]. A solution was selected, as a result of which it was possible to conduct research for a three-factor, three-level model. A matrix with values of the coded factors is presented in Table 4, while a matrix with selected types of mortars and their composition is presented in Table 5. According to the selected experiment plan, it was necessary to carry out 15 elementary experiments each time—13 different experiments (13 mixtures of different compositions) and two repetitions (for the mixture corresponding to the central point of the Box–Behnken plan). Additionally, a base cement mortar was prepared and tested, which contained neither hydrated lime nor cellulose ether (paste and mortar marked with the symbol C0). The composition of the pastes was similar to the composition of the mortars, excluding fine aggregate.


**Table 4.** Box–Behnken experiment plan.

<sup>1</sup> The percentage of hydrated lime given in relation to the total amount of binder in the mortar; <sup>2</sup> the percentage of cellulose ether given in relation to the amount of binder in the mortar.


**Table 5.** Mortar and paste compositions defined according to the Box–Behnken model.

<sup>1</sup> Base mortar (paste) without hydrated lime or cellulose ether.

Factors that were kept constant throughout this study were as follows:


On the basis of the experiments carried out in accordance with the Box–Behnken model, it was possible to determine the quadratic formula (Equation (1)) for the tested property in each case, which was used to estimate the response area [36,38]. In Equation (1), Yi is the dependent variable or output variable (tested parameter of pastes/mortars), b0–b33 are factors, and X1, X2, and X3 are dependent variables (input factors). For the tested properties of pastes and mortars, the value of the standard deviation was determined for the central point of the Box–Behnken experiment plan, i.e., sample numbers 13, 14, 15 (mortar abbreviated as C25-18.82MV).

$$\mathbf{Y}\_{1} = \mathbf{b}\_{0} + \mathbf{b}\_{1}\mathbf{X}\_{1} + \mathbf{b}\_{2}\mathbf{X}\_{2} + \mathbf{b}\_{3}\mathbf{X}\_{3} + \mathbf{b}\_{11}\mathbf{X}\_{1}^{2} + \mathbf{b}\_{22}\mathbf{X}\_{2}^{2} + \mathbf{b}\_{33}\mathbf{X}\_{3}^{2} + \mathbf{b}\_{12}\mathbf{X}\_{1}\mathbf{X}\_{2} + \mathbf{B}\_{12}\mathbf{X}\_{1}\mathbf{X}\_{3} + \mathbf{b}\_{23}\mathbf{X}\_{2}\mathbf{X}\_{3}.\tag{1}$$
