**3. Results and Discussion**

#### *3.1. E*ff*ect of Setting Processes by the Ultrasonic Method of Mortars*

The ultrasonic technique used in the research is based on the registration of the speed of the ultrasonic wave, which is related to the change in the viscosity and elasticity of the medium. The publications [14,30,33–36] show the possibilities of using this method. Since the author rightly confirmed [33], the ultrasonic technique is used to evaluate the quality of ready products. A relatively new area of research where this method can be used is the examination of the setting and hardening processes of cement-based materials. According to Zych [33], due to changes in visco-elastic properties caused by hardening, the velocity of wave propagation starts growing. Applying this method, it is possible to describe the rate of setting and hardening of mortars based on cement and cement-lime binders and it is possible to assess the influence of the admixture on the setting processes, which might be useful for assessing the processing time of plastering mortars when choosing a method or establishing the conditions of building, repair, and assembly works [14,33]. The results of measurements obtained with the use of the ultrasonic method, presented in Figure 2 and in Table 4, prove the different properties of mortars. There is a clear influence of the admixture on the reduction of the ultrasonic wave velocity during the entire tested measurement. Thus, it can be concluded that cellulose ether affects the mortar setting processes. In the case of cement mortars, Kulesza et al. [36] obtained similar results. However, the authors focused their research on evaluating the influence of re-dispersible powders on setting and

hardening processes of thin-bed mortars. The influence of cellulose ether was assessed by them as a secondary effect. As for the monitoring of the setting processes of cement-lime mortars, no reference articles were found in the literature.

**Figure 2.** Ultrasonic wave velocity changes in all mortars (according to Table 3).


**Table 4.** Comparison of results read from the graphs (Figure 2).

The setting process varies in intensity depending on the binder used and the amount and viscosity of the cellulose ether. It is clearly visible that the use of an admixture in the amount of 0.3% delays the setting process of both cement and cement-lime mortars (C-0.3-MV and CL-0.3-MV mortars). This is especially true when comparing the induction period of individual materials. This parameter is defined as the time counted from the setting of the measurement to the moment of a sudden change in the velocity of the ultrasonic wave. It should also be noted that, in the case of all modified mortars, the maximum velocity of the ultrasonic wave has been significantly reduced, from 2180.1 m/s, even to 423.3 m/s (more than 5 times), which may be related to the different microstructure (among other porosities) of these materials, resulting from the replacement of cement binder on lime and the use of admixtures. Mortar CL-0.3-MV modified with cellulose ether, in which 50% cement was replaced

with hydrated lime, was characterized not only by the lowest ultrasonic wave velocity after 48 h of maturation, but also the ultrasonic wave velocity recorded during all setting processes was the lowest. The smallest differences in the setting process, in relation to the reference sample (C-0), are visible for the cement mortar with the admixture of 0.05% (sample C-0.05-MV) and viscosity of 25,000 mPa·s. In other cases, it is clearly visible that both the lime and the admixture significantly reduce the velocity of the ultrasonic wave during every period of time for measurements. The induction period of 10 h 54 min and 7 h 18 min indicates that the setting processes of C-0.3-MV and CL-0.3-MV mortars were delayed due to the use of an admixture in the amount of 0.3% and an admixture viscosity of 25,000 mPa·s. The induction time is visible only in the case of mortars modified with a cellulose ether of 0.175% and 0.3%. Despite the fact that the induction time is clearly visible only in the case of four mortars (Table 4), a slow increase in the ultrasonic velocity was recorded in the initial periods for the remaining materials. It is clearly visible that the most important factor in delaying the setting process is the use of admixtures, which is not so noticeable, even in the case of replacing a part of the cement binder with hydrated lime. When testing with the ultrasonic method, the amount of the admixture had a greater influence on the setting process than its viscosity. When analyzing this process for C-0.3-MV and CL-0.05-MV mortar, it is worth noting that, although the ultrasonic wave velocity recorded after 48 h is at a similar level, the induction period and the setting course in the first 12 h are different, which only confirms the conclusions regarding the influence of the amount of the admixture on the processes taking place during the maturation of these materials.

## *3.2. E*ff*ect of Hydration by the Calorimetry Method of Pastes*

The changes in the rate of heat release and the amount of heat released during the hydration of pastes as a function of time are shown in Figures 3 and 4. In the case of C-0 paste, there is a clear peak in the heat flow diagram (Figure 3). In the first 16 h, a rapid hydration reaction was noted, which was confirmed by using the ultrasonic method. In Figure 2, the setting course of the C-0 sample was characterized by a rapid increase in the ultrasonic wave velocity. Based on the micro-calorimetric measurements, it can be concluded that the admixture delays the hydration processes and extends the induction period, which was also observed in literature [10,22,24,37]. The influence of the admixture on the delay of the hydration process is related to the specificity of this admixture–its water retention capacity [2,3,6,10]. Adding a cellulose ether changes the viscosity of the pastes [5,6,9,16] as well as the processes of cement hydration [19,23,24]. The amount of cellulose ether is essential when changing heat flow. Replacing a part of the cement binder with hydrated lime also reduces the heat release energy throughout the measurement and the induction period is also extended. When analyzing Figure 3, it can be noticed that pastes C-0 and C-0.05-MV have three distinct peaks in the curve, pastes C-0.3-MV and C-0.175-HV–two peaks, but pastes CL-0.05-MV, CL-0.3-MV, and CL-0.175-HV have only one clear peak. One peak on curves for this pastes may be caused by replacing part of the cement binder with hydrated lime.

The curve of cumulative heat of hydration (Figure 4) shows that cellulose ether reduces the maximum rate of heat release in 48 h, which was observed in the research [10,24,25]. Cumulative heat after 48 h was reduced by up to 76% in the case of cement pastes and up to 79% in the case of cement-lime pastes (Table 5). The polymer admixture extended the induction time from 2h 50 min to 13 h (in the case of cement pastes) and from 2 h 50 min to 5 h 15 min (in the case of cement-lime pastes). Adsorption between cellulose ethers and hydration products is thought of as the major cause for the retardation, which acts between OH groups of cellulose ethers and metal hydroxides on the surface of the hydration products [25].

**Figure 4.** Cumulative curves of all pastes (according to Table 3).


**Table 5.** Characteristic parameters of heat evolution curves for all pastes (Figures 3 and 4).

In Table 5, some parameters relating to the heat evolution curves were noted. Induction time was read based on Figure 3, but the time of maximum and cumulative heat were read based on Figure 4.

Comparing the results obtained on the basis of measurements using the ultrasonic technique with the results of calorimetric tests, there can be a similarity of the course of the curves showing the change in the velocity of the ultrasonic wave in time (Figure 2, Table 4) and the course of the curves of cumulative heat of hydration (Figure 4, Table 5). Mortar C-0.3-MV and paste C-0.3-MV have the longest induction time, which was measured by an ultrasonic technique and calorimetric measurements. The differences visible in the diagrams of changes in the velocity of the ultrasonic wave of individual mortars (Figure 2) are similar to the diagram of accumulated heat during the hydration processes of pastes (Figure 4).
