**1. Introduction**

The use of chemical admixtures in modern mortar technology is a prerequisite for obtaining materials of high quality and durability that meet the standard requirement. The aim is to ensure that the mortars are characterized by good workability, ease of application, adequate adhesion to the masonry, and lowest possible shrinkage deformation.

Cellulose ethers (CE) are a group of polymer admixtures used in dry pre-mix mineral-based binder materials [1–3]. Hydroxyethyl methyl cellulose (HEMC) in addition to hydroxypropyl methyl cellulose (HPMC) is one of the most popular cellulose ethers [3–5]. They are a group of admixtures, with the use of which it is possible to modify the viscosity of pastes and mortars, and, thus, change their rheological or application properties [1,2,5–14], which is important to know how this admixture influences the properties of fresh mortars. The research [2,3,7,8,14,15] showed that the cellulose ethers have an impact on the increase of water retention. Bülichen et al. [3] found that, at low dosages, HEMC achieves water retention by intramolecular sorption of water and concomitant swelling while, at a higher amount of admixture, the molecules agglomerate into large, hydrocolloidal microgel particles. This admixture changes the material rheological properties including plastic viscosity and yield stress [1,3,5,6,12,14]. The high-water retention capacity of the mortar, which can be obtained by using these polymers, prevents rapid water loss from the material and the drainage of some water from the mortar to the substrate. Thanks to this, it is possible to obtain greater adhesion and reduce shrinkage deformations [16,17]. On the other hand, cellulose ethers can cause a delay of hydration processes, as a result of which the setting and hardening time of the binders and mortars become longer [1,18–27]. According to Hua et al. [25], the retardation effect of cellulose ethers favors the increase

of operating time and the decrease of the consistency or slump loss of freshly mortars, but possibly leads to unexpected delay during construction. Study of the influence of this admixture on cement hydration kinetics can facilitate when controlling cement hydration, which is important, especially for application. Izaguirre et al. [27] tested two different additives (cellulose ether and guar gum), which were added to lim-based mortars. Among other things, the setting time was tested. The obtained results showed an increase in setting time from 3 h 15 min for the reference mortar to 10 h 30 min for mortar with an admixture. An adsorption mechanism of this admixture on the Ca(OH)2 crystals was reported to reduce its entanglement between chains and, hence, the viscosity of the pastes as well as its water retention ability. Similar results of the influence on the delay of setting time were observed in ceramic tile adhesive mortars by Petit and Wirquin [1]. Initial setting time increased from 4 h for the reference mortar to 12 h 25 min for mortar modified with an admixture, but final setting time changed from 5 h 25 min to 18 h. Pourchez et al. [19] studied the influence of cellulose ethers on the hydration of C3A and C3A-sulphate systems. It was observed that this admixture leads to a gradual slowing down of the C3A hydration depending on cellulose ether chemistry. Two types of cellulose ethers were used in their research: hydroxypropyl methyl cellulose (HPMC) and hydroxyethyl cellulose (HEC). They observed that HECs include a higher delay than HPMCs. In their opinion, the substitutions group seems to be more important for controlling factors on C3A hydration rather than molecular mass. Betioli et al. [24] observed that cellulose ether HEMC did not change the typical profile of isothermal calorimetry curves. However, it decreased the rate of heat evolution during the acceleration period and decreased the maximum peak. It was expected that the induction period was importantly extended with a hydroxyethyl methyl cellulose (HEMC) admixture. The total heat output after 4 h of hydration was reduced (even 54% for paste with 0.5% HEMC). Oscillatory rheometry and isothermal calorimetry were used for showing the influence of cellulose ether on a hydration process. A steric dispersant barrier effect during the first two hours of hydration associated with a cement retarding nature was displayed, which resulted in reducing the setting speed. Authors of this article observed that the rheology of cement paste changes during the induction period and it was caused by the agglomeration of the particles. Pichniarczyk [22] concluded that methylcellulose has a significant influence on the early hydration of cement. However, that effect practically disappears after a longer period. The XRD (X-Ray Diffraction) pattern of cement paste with cellulose ether admixture peaks of portlandite and ettringite were of significantly lower intensity after 24 h of hydration, but gypsum peaks were of high intensity. These test results indicate a slower hydration process in pastes modified with methylcellulose. After 168 h, hydration differences in the intensity of portlandite and ettringite peaks were lower. X-ray diffraction was complemented with microstructure observations under the SEM. Knapen and Gemert [23] observed that the presence of water-soluble polymers in cement mortars influences the rate and degree of cement hydration as well as the nature and amount of hydration products. The heat evolution was measured by isothermal calorimetry measurements. It was found that the start of the acceleration stage of hydration was postponed by about 30 min in the MC (methyl cellulose) modified pastes (1% MC, HEC cement pastes) and by more than 5 h in the HEC modified paste. The induction period and the dormant period were extended. The rate of the following hydration reactions was slowed down. What was confirmed by lower values of the maximum heat release and broader exothermal peaks in the isothermal calorimetric curves of the cellulose ethers modified pastes.

The correct selection of the admixture, especially its quantity and viscosity, affects the parameters of the finished product. Knowledge of the processes taking place during the setting and hardening of mortars modified with this admixture allows for a correct and comprehensive assessment of its effectiveness and applicability.

Based on the publications available in the literature, it can be concluded that the research results presented so far focus mainly on determining the influence of cellulose ethers on the rheological, physical-mechanical properties of cement [3,5,6,24] or lime [7,8,13,28] pastes and mortars. The cement hydration processes or its impact on the individual phases are also assessed. The subject of setting processes of mortars modified with a cellulose ether admixture focuses on cement-based materials. There is no explanation of the mechanism of action of this admixture and its influence on the hydration of pastes with a cement-lime binder. Therefore, this issue is the subject of these studies. Moreover, in publications for the evaluation of hydration processes, calorimetric measurements, XRD method, or conductometric measurements are usually used [21,22,24,28,29]. In this publication, the authors performed an analysis of the influence of the admixture on the setting processes of mortars, using the ultrasonic technique. Applying the proposed method of examinations, it is possible to describe the rate of setting and hardening of materials based on cement and cement-lime mortars with high accuracy due to the fact that measurements are made on an ongoing basis, which can make it possible to monitor and control the properties of mortars in the plastic state. Using the knowledge gained on the basis of an ultrasonic technique, it is possible to evaluate the influence of individual components on the parameters of the mortar or paste. The results of the ultrasonic technique complement the standard tests. In addition, they allow us to predict the mechanical properties of mortars [30].
