Cellulose ethers influence on water retention and consistency in
cement-based mortars

1. Introduction
Factory-made mortars have been mostly implemented as masonry renderings, fixing tiles, self-levelling floors and so on. When mortar is applied on substrate, water may be absorbed by the substrate. This phenomenon can induce insufficient hydration of cement, and thus decrease mechanical properties of the mortar. Water retention capacity of a mortar is thus a key element when choosing an appropriate formulation as a function of the substrate, climatic conditions, and industrial applications of the mortar, etc.

A wide variety of chemical admixtures are present in industrial mortars currently used in construction. They are classified according to the function they perform, e.g. air entrainment, water retention, set retardation or acceleration, etc. [1] Among the organic admixtures widely used in mortar and concrete, polysaccharides are polymers that can be classified as water reducer, set retarder, anti-washout and water retention agent [1,2]. Many authors demonstrated that mortar and concrete properties can significantly be modified at both the fresh and hardened states by the addition of polysaccharides [3–6]. Among all polysaccharides, cellulose ethers are commonly introduced into industrial mortar formulations in order to provide some required properties to the mortar, from the fresh paste to the hardened material [7]. These cellulose derivatives are suitable molecules to improve water retention and workability of the fresh material, together with adhesion to the substrate [3]. However, the major drawback of these macromolecules in mortar formulation is the cement hydration delay [2,6,8]. Pourchez et al. highlighted various delays on cement hydration induced by cellulose ethers (from 10 min up to several hours) [9,10]. This delay seemed to mainly depend on the chemical structure of the molecule and, in particular, on the degree of substitution.

When the support material absorbs water, this can induce insufficient hydration of the cement and therefore provoke a loss in mechanical performances. Water retention is a mortar property that prevents the rapid loss of water to the substrate by suction. This
property avoids bleeding or “water loss” whenthe mortar is in contact with relatively permeable surfaces. Water retention is a fundamental property, which affects workability and bonds between mortar and masonry. Water-retaining agents, also known as thickening or viscosity enhancing additives, are essential components in mortar formulation because they also reduce segregation and improve workability. However, they can slightly reduce compressive strength of the hardened concrete depending on the W/C [2,11–13]. The most widespread cellulose ethers used in practice as admixtures are hydroxypropyl methyl cellulose (HPMC) and hydroxyethyl methyl cellulose (HEMC) [2,11]. Some publications deal with HEMC and with other organic additives such as latexes mixed with a silicone emulsion, and starches [14–16]. They showed that mortar water retention capacity is improved thanks to these admixtures. This property is also increased with a rise in polymer to cement ratio. Moreover, Pourchez et al. studied the influence of a few HEMC and HPMC on water retention capacity and their results revealed the significant influence of the admixture molecular weight [9]. However, a better understanding of the admixture–cement interactions is required to explain this water retention enhancement.

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