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.

An assumption usually proposed to explain the water retention capacity of cellulose ethers involves an increase of mortar viscosity [17]. This hypothesis needs further verifications of the mortar consistency effect on water retention. Cellulose ethers can bring about excellent water retention thanks to a possible superposition of two phenomena. Pourchez ventured two hypotheses: (i) a rheological effect similar to those produced by other polysaccharides; (ii) an effect that could be inherent to cellulose ethers, such as a modification of the porous network in the fresh state, osmotic pressure, or the presence of a cellulose ether film playing the role of diffusion barrier [18]. Jenni et al. investigated the role of one type of HEMC on changes in mortar microstructures [19]. They proposed that the air entrapped during mixing process was stabilised by cellulose ethers (due to decrease of the surface tension of the water). Moreover,
they showed that cellulose ethers films were frequently observed between two juxtaposed air voids and also along the pore wall of a single air void.

The rheological properties of fresh concrete are related to cement hydration and chemical interactions in the cement paste system [20]. The concrete’s flow properties such as the relationship between shear rate and viscosity are the subject of ongoing researches. Many simplified methods are used to estimate rheological properties and are well correlated to the rheological parameters (yield stress, plastic viscosity). Ferraris gave an overview of the commonly tests (14 test methods) used to characterize concrete rheology, including slump, penetration, Tattersal two-point tests, etc. [21] Rheological studies were executed on admixed cement-based mortars. Seabra et al. showed that the use of admixtures, such as water-retaining, plasticizer and air entraining, considerably changed the rheological behaviour of admixed lime-based mortars [22,23]. In particular, the introduction of a water-retaining agent promoted thickening fol-lowed, after agitation, by thinning because air was entrained into the mortar. Paiva et al. demonstrated that HPMC thickened the mortar due to an increase of the plastic viscosity. HPMC promoted cohesion among the material particles at fresh state [24]. Nevertheless, the effect of CE structural parameters on the rheological properties of
cement-based mortars is still not well understood. This study aims to identify the main CE parameters which control the water retention of cement-based materials. Secondly, this paper investigates the relationship between rheological behaviour of a
mortar and its capacity to retain water in order to go into details with the hypothesis that both properties are linked together.

Our exploration of water retention mechanisms in CE-admixed mortars proceeds as follows. First, a set of chemical admixtures was selected to study the impact of the average molecular weight and molarsubstitution(MS) onrheologicalproperties andwaterretention of mortars. Samples had identical chemical structure and only differed by their molecular weights or MS. Beforehand, a characterization of all admixtures was performed to quantify their molecular mass. Then, the rheological behaviour was evaluated using rheometry (i.e. yield stress, flow behaviour index and consistency coefficient). Afterwards, the influence of these physico-chemical parameters on mortar water retention capacity and its rheological behaviour were studied. Finally, the effect of some starch derivatives was investigated and compared to the results of CE

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