**1. Introduction**

The development of new technologies and construction chemicals has contributed to the popularization of dry mortars. The traditionally used cement, cement–lime, and lime plasters are being replaced by materials modified with chemical admixtures and additives. Currently produced mortars consist of several or even a dozen or so raw materials, which are selected so that the final product is characterized by optimal, previously assumed properties. Proper selection of the type and quantity of ingredients in mortars determines the fulfillment of technical and technological requirements. Due to many types and modifications of chemical admixtures, various types of masonry elements, and various substrates, this issue requires a lot of knowledge and practice. A wrong decision during the design of the mortar can lead to failure to meet standard requirements, loss of adhesion, difficulties in plaster application, and the appearance of shrinkage cracks on the material [1–3].

One of the most commonly used polymers in dry-mixed mortars is represented by cellulose ethers. They belong to the group of viscosity-modifying admixtures [4–7], which have been used as ingredients among others of masonry mortars, plasters, and ceramic tile adhesives [2,8–13]. Cellulose ethers are important admixtures in dry-mixed mortars because of their thickening effect and water retention ability [14–16]. Thanks to this admixture, mortars are easy to prepare, comfortable to use, and easy to handle [2,9].

The water retention and rheological properties of cement or lime mortars have been the subject of many studies. Patural et al. [17] found that the molecular weight is crucial to control water retention and consistency in cement mortars. It was noted that, as molecular

**Citation:** Spychał, E.; Dachowski, R. The Influence of Hydrated Lime and Cellulose Ether Admixture on Water Retention, Rheology and Application Properties of Cement Plastering Mortars. *Materials* **2021**, *14*, 5487. https://doi.org/10.3390/ ma14195487

Academic Editor: A. Javier Sanchez-Herencia

Received: 31 August 2021 Accepted: 19 September 2021 Published: 23 September 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

weight increased, the yield stress diminished, the consistency increased, and the water retention improved. Betioli et al. [18] studied the effect of cellulose (HEMC) ether on the rheology of pastes and cement hydration. The correlation between the results from rheology and isothermal calorimetry was investigated. According to this study, the rheology of cement pastes changes during the induction period, when the reactivity of the system is low. The rheologic changes are due to the agglomeration of particles. In the first hours of hydration, cellulose ether increased the critical strain of the cement pastes measured using the strain sweep test when compared to the plain cement paste, probably by acting as a steric barrier, in addition to the retardation effect, according to the authors. A higher admixture content results in a higher and longer steric effect because the HEMC molecules adsorbed onto the cement particles and on C–S–H enhance the suspension stability due to steric repulsion. In [4], the effect of cellulose ether and guar gum on aerial lime-based mortars was explored. Standard and rheological investigations were supplemented with mercury intrusion porosimetry. The amount of mixing water was the greatest when cellulose ether was added, and this additive did not increase the water-retention capacity of the fresh mixtures. As assessed by the authors, high water absorption through capillarity, high permeability, and a long delay in setting time resulted in an undesirable general performance of this admixture in aerial lime-based mortars. Hydraulic lime-based mortars modified with cellulose ether were studied by the authors of [5,19,20]. In [5], the behavior of mortars with four different cellulose ethers was assessed. The results revealed an elevated air content and water retention in mortars with an increasing dose of polymer admixture, resulting in the decreased density of mortars in plastic state. The mechanical properties of modified mortars surpassed the reference ones at the age of 180 days despite the fact that these materials showed higher open porosity and water absorption. The enhanced porosity of mortars resulted in an improvement of their frost resistance and faster carbonation. Summarizing the research conducted, the authors stated that the effect of cellulose ether on hydraulic lime mortars is very similar to that on cement mortars, but it is different to that on air lime-based mortars.

In addition to the consistency, water retention, and rheological properties, the influence of cellulose ethers on cement hydration has also been assessed [6,21–23]. On the basis of the experimental results, it can be concluded that the addition of cellulose ether has a significant influence on the early hydration of cement, and this effect practically disappears after longer period. Cellulose ethers lead to a gradual slowing down of the C3A hydration depending on admixture chemistry. According to observations, substitution groups (nature and content) seem to be more important controlling factors for C3A hydration than molecular mass. The amount of the polymer admixture used is most important in the process of setting pastes and mortars. A much-reduced impact is visible when taking into account the viscosity of the admixture or the type of binder used.

The research on cement hydration was supplemented in [23] with a study of the microstructure observations under SEM (after 24 h of hydration). The microstructure of cement paste with cellulose ether had a high content of gypsum crystals. As shown by the EDS analyses of C–S–H in the paste with cellulose ether, this phase had a very low content of aluminum and sulfur. This confirmed the absence of monosulfate in this sample, as the content of aluminum and sulfur in the classical C–S–H phase increases when it forms a nano-mixture with monosulfate. As the author proved, the cellulose ether admixture inhibited the reaction of the C3A phase with sulfate ions, which would have led to the formation of ettringite and monosulphate. In [24], the morphology of the hydrate crystals and the microstructure of the hardened cement matrix modified with polyvinyl alcohol–acetate and two cellulose ethers were studied. Polymer modification improved the cohesion of the bulk cement paste, while fewer microcracks were observed for the polymer-modified mortars.

Another analyzed issue in the literature is the influence of cellulose ethers on the shrinkage of mortars [25,26]. Messan et al. [25] presented a new testing method for investigating early-age shrinkage of cement-based material modified with glass fiber, cellulose

ether, and redispersible powder. Results of this research showed the importance of the rheology of fresh mortar with regard to early-age shrinkage development. The authors proved that the admixture of cellulose ether decreases the early-age evaporation rate of cement-based materials. This has also been observed in the literature [26]. The use of a viscosity-modifying admixture showed a beneficial effect on mortar shrinkage. According to the authors, the action of cellulose ether reduces the surface tension of the pore solution, thus reducing capillary forces. Long-term water-binding capacity and high-level mortar humidity also contribute to reducing shrinkage deformation.

The nature and the scope of cellulose ether activities are not fully recognized and understood, especially in the case of cement–lime mortars. In addition, there is no information in the literature on how cellulose ether affects their application properties, which is an important issue in the case of plasters. Scientific research in the literature has focused on the assessment of the effect of this admixture on cement [1,9,22,23,27,28], lime [4,5,7,20], and gypsum [29] pastes and mortars. The research carried out by various authors has mainly covered the subject of the influence of this polymer admixture on the standard parameters, consistency, water retention, and rheological properties, among others.

The test results and their analysis presented in this article focus on the assessment of plasters using proprietary methods supplemented with standard methods of pastes and mortars. Importantly, the tests carried out can be applied to both cement-based mortars and cement–lime mortars.
