Water retention additives play an important role in modern building products, particularly in dry-mix mortars [1]. Their function is to prevent uncontrolled water loss into porous substrates such as brick, lime stone, and aerated concrete. In industrial products, cellulose ethers dominate this market because of their favorable cost effectiveness and their good environmental compatibility [2]. The first reports on the preparation of methyl cellulose and its derivates originate from Lilienfeld [3], Leuchs [4] and Dreyfus [5]. Based on their pioneering work in cellulose ether synthesis, production started in Germany in the 1920s and in the United States in 1938. Major current applications of cellulose ethers include wall renders and plasters, joint compounds for gypsum board paneling, cementitious tile adhesives (CTAs), floor screeds, self-leveling underlayments (SLUs) and water-proofing membranes [6]. In dry mortars, cellulose ethers serve to provide water retention and viscosity. Some types retard Portland cement hydration severely [7], [8]. The effect depends on the specific composition (e.g. degree and type of substitution) of the cellulose ether. Application dosages may range between 0.1 and 1.5% by weight of binder, depending on the desired properties. Among the most frequently used cellulose ethers are methyl hydroxyethyl cellulose (MHEC, also referred to as HEMC) and methyl hydroxypropyl cellulose (MHPC) [9], [10], [11]. MHEC is predominantly applied in self-leveling flooring compounds and cementitious tile adhesives (CTAs) while MHPC, because of its air-entraining effect stemming from the hydrophobic hydroxypropyl groups, is the product of choice for wall renders and plasters.

In previous publications, the impact of molecular weight of cellulose ethers on both water retention and rheological properties of mortars has been investigated. The authors conclude that consistency was increased and water retention was improved by higher molecular weight ethers. The mortar rheology is highlighted as one of the key properties relative to water retention [12]. Other studies discussed the influence of cellulose ethers on water transport in the porous structure of cement-based materials and investigated their effect on cement hydration [13], [14]. They found that the type of substituents attached to the anhydro glucose ring of the cellulose ether is critical for water transport and development of the microstructure of fresh and hardened cement. Furthermore, the degree of substitution represents the key parameter relative to cement hydration, as was evidenced by different time periods at which portlandite precipitation occurs.

MHEC HEMC HYDROXYPROPYL METHYL CELLULOSE FOR SELF LEVELING CEMENT LIVELLA CEMENTO SELF LEVELING MIX

In the past, attempts have been made to clarify the working mechanism of methyl cellulose (MC). Early works from Schweizer et al. present the effect of cellulose ethers on water retention and rheology of cementitious mortars and gypsum-based machinery plasters [15], [16]. They demonstrate that adsorption of methyl cellulose is clearly dependent on the degree of substitution (DS). At DS values of > 1.6 which are typical for MC products used in the building industry, only a minor amount (~ 20%) of the MC powder adsorbs. In a very skillful experiment the authors show that during the drying process of a gypsum-based render, MC migrates with the water to the surface of the render and is not retained by adsorption. Thus, they conclude that MC does not adsorb on the binder or its hydrates. Similar observations have been made by Yammamuro et al. [17].

In another study, Jenni et al. stained MHEC with a fluorescent dye and visualized and quantified the migration of MHEC through the capillaries of mortars. They found that MHEC accumulates at system interfaces exhibiting a reduced porosity. From this, they conclude that the cellulose ethers can migrate through the pore system and are therefore not or only partly adsorbed on cement particles under wet conditions [18]. In a later publication from this group they state that the dissolved CE is transported downwards through the capillary pores, but accumulates at the contact layer and substrate surface, which act as micro-filters [19].

Thus, there seems to be agreement that methyl cellulose does not adsorb. None of these works, however, offers an alternative model which clearly can describe the mechanism behind water retention capability of MC. Accordingly, there is still a need for clarification.

Generally, three principle working mechanisms for water retention can come into place whenever a cement paste is subject to filtration on a substrate. These potential mechanisms have been proposed in earlier works by Desbrières and are as follows [20], [21]: (a) Water retention as a result of increased viscosity: increased dynamic filtrate viscosity can decelerate the filtration rate. (b) Water retention as a result of adsorption: anionic polymers may adsorb onto hydrating cement particles and obstruct filter cake pores either by polymer segments which freely protrude into the pore space or even bridge cement particles. Through this mechanism, filter cake permeability is reduced. (c) Water retention as a result of physical pore plugging: polymers may plug pores in the cementitious matrix through formation of polymer films (e.g. latexes), or through large polymer associates which form a 3D network, or through simple swelling and expansion caused by the uptake of an enormous amount of water into the inner sphere and hydrate shells of polymers. This process leads to the formation of large microgel particles. Through this mechanism, a large portion of the mixing water is physically bound and cannot leak-off into a porous substrate. The enormous water-binding capacity of polysaccharides has been described in an earlier study. By using a sorption balance and a microcalorimeter it was found that at the saturation level, neutral polysaccharides (amylose and amylopectin) can bind up to four water molecules per anhydroglucose unit [22].

In recent studies we have investigated the water retention behavior of hydroxyethyl cellulose (HEC) and carboxymethyl hydroxyethyl cellulose (CMHEC) in oil well cement, respectively. We found that the working mechanism of HEC relies on a dual effect, namely its enormous intramolecular water sorption (binding) capacity and a concentration dependent formation of hydrocolloidal associated polymer networks [23]. Contrary to this, the working mechanism of CMHEC changes with dosage. At low dosages (up to ~ 0.3% by weight of cement), adsorption presents the predominant mode of action, whereas above a certain threshold concentration (the “overlapping” concentration), formation of associated polymer networks is responsible for its effectiveness [24].

Here, an attempt was made to establish the working mechanism of MHEC. For this purpose, its water retention effectiveness in cement paste and its dynamic viscosity in cement pore solution were measured. Furthermore, its potential adsorption on cement was probed via a specifically designed static filtration test. Finally, concentration-dependent hydrodynamic diameters of solved MHEC molecules, and their surface tension in cement pore solution were measured to establish the mechanism underlying the effectiveness of MHEC as water retention agent.