Modification of water retention and rheological properties of fresh state
cement-based mortars by guar gum derivatives
Modern factory-made mortars are complex materials, in which several kinds of admixtures are added in order to obtain specific properties, from the fresh state to the hardened material. Indeed, since many years, concretes, mortars or cement grouts with high fluidity have been developed, since their use implies many economical and technical advantages. However, the use of highly flowable mixtures may lead to segregation or excessive bleeding and subsequently, durability issues. In order to overcome this problem by enhancing the sedimentation resistance while maintaining high fluidity, viscosity-enhancing admixtures (VEA) are frequently introduced within the formulation [1–4]. Among these admixtures, natural polysaccharides or their derivatives (such as welan gum, starch derivatives or cellulose ethers) are the most widely used. The incorporation of these VEAs in shotcrete or render mortar is useful to ensure sagging resistance for thick application
on vertical support, and to allow sufficient fluidity for normal pumpability by supplying shear thinning rheological behavior . Indeed, these admixtures provide, generally, high yield stress and apparent viscosity at low shear rate but low resistance to flow at high shear rate . However, their mode of action is not fully understood, since results are sometimes contradictory and strongly dependent of the kind of binder, the polysaccharide nature and the molecular parameters of the admixture (such as molecular weight, nature and content of substitution groups)[7,8–15].
Water retention (WR) is another essential property of monolayer render at fresh state. Indeed, high water retention improves the cement hydration and limits the absorption of the mixing water by a substrate and thus provides good mechanical and adhesive properties to the mortar [16,17]. Among admixtures enhancing water retention capacity of the freshly-mixed mortars, cellulose ethers (CE) are the most widely used . As in the case of the rheological properties, it appears that polymer molecular parameters, such as nature and content of substitution groups,
and molecular weight, have a significant influence on WR [11,18]. The results demonstrated that the WR is strongly improved by the increase in the molecular weight of HydroxyEthylMethylCellulose (HEMC), HydroxyPropylMethylCellulose(HPMC) and HydroxyEthylCellulose (HEC). On the contrary, the molar substitution seems to have a lower impact on the water retention of admixed mortars. Nevertheless, the water retention is improved for low molar substitutions of the CE.
Despite a wide use of CE, HydroxyPropyl Guar (HPG) are now also well-established in the construction industry as water retention agents, as anti-sagging agents and rheology modifiers for mortars[19–23]. Moreover, HPGs are already widely used in various industrial fields, such as textile printing, hydraulic fracturing process, oil production or paper manufacturing, due to their thickening effect [24,25]. Consequently, since HPGs improve the two main properties of mortar, they appear as suitable admixtures to be used in render formulation. However, the study of their impact on mortars is still low, since Plank presented these natural polysaccharides as new promising class of water retaining agents in building materials . Poinot et al. studied the impact of HPGs on WR
and rheological properties of cement-based mortars [21,22]. However, since the main objective of the research was to elucidate the
WR mechanism involved by CEs and HPGs, the formulation did not correspond to an industrially-used standard mixture. Indeed, high Liquid-to-Solid ratio and consequently high admixture dosages were used to be discriminant with respect to WR. Cappellari et al. also studied the effect of HPG and CE on WR and rheological properties of mortars . However, the influence of the molecular parameters could not be studied since the only one commercial HPG was tested on a lime-based mortar.
The aim of this study is to provide an understanding of the effect of chemical composition and structure of HPGs and its dosage on macroscopic properties of mortars. For this purpose, an original guar gum and five HPGs with specific chemical modifications, such as increase in MSHP or substitutions by hydrophobic units, were selected. The impact of admixtures on the water retention capacity and on the rheological behavior of mortars was investigated.