III.3. Effect of molecular weight on rheological parameters and water retention
In mortar composition, water-retaining agents are commonly used. Indeed, they retain water inside the mortar during the hydration process caused by absorption through the substrate. The water retention capacity will depend on mortar composition.
According to the NF DTU 26.1, mortars can be divided into three classes [43]. The first category (low retention) is for mortars that have water retention lower than 86%. The second class (intermediate retention) corresponds to values ranging from 86% to 94%. The last one (strong retention) is defined by water retention higher than 94%. Strong retention corresponds to the values sought for good mechanical properties. These limits only refer to the ASTM C1506-09 measurements. Thus, care must be taken when dealing with them.
III.3.1. HEMC
HEMC C mainly differs by their molecular weight (Table 3). The presence of these admixtures had a strong influence on the rheological parameters calculated with Herschel Bulkley model. At first, the yield stress values decreased as the HEMC molecular weight increased (Table 4). They ranged from 5.4 Pa for C1 to 2.0 Pa for C4. These values are lower than those found in the literature obtained for mortars containing superplasticizers and stabilizing agents [44] and [45].
Concerning the flow index, Table 4 shows that this parameter decreases from 0.86 for C1 to 0.59 for C4. This change over molecular weight means that the samples become more shearthinning as the molecular weight of the HEMC increases.
Finally, the effect of CE molecular weight on mortar water retention and on the consistency coefficient is highlighted in Figure 9. The viscosity of the mortar was improved thanks to the presence of cellulose ether in the continuous phase. The viscosity of this phase increased with a rise in admixture molecular weight.
For a constant chemical structure, experimental results of water retention highlighted the impact of HEMC molecular weight (Figure 9). In spite of C1 difference, its influence was in line with the three other HEMC. Hence, for HEMC with molecular mass lower than 400,000 Da, the higher the molecular mass, the better the mortar water retention capacity. These results are in accordance with other data reported in literature [9]. Furthermore, C1 did not provide strong water retention (i.e. lower than 94%). For very low HEMC molecular weights, the mortar water retention was intermediate (class defined by the NF DTU 26.1). On
the contrary, C2, C3 and C4 provided strong water retention capacities to the mortar (respectively 95.7%, 96.4% and 98.8%).
It is important to note that consistency measurements can also be related to the cellulose ether molecular mass (Figure 9). Mortar consistency is also improved when the polymer molecular weight increases. For this HEMC sample group, both mortar consistency and water retention went up when admixture molecular mass increased. Consequently, mortar water retention was plotted versus its consistency. The results are illustrated in Figure 12. For HEMC C, a rise in consistency leads to a similar water retention increase.
References：
[43] NF DTU 26.1. Travaux d'enduits de mortiers (2008).
[44] Z. Toutou, N. Roussel, C. Lanos, Y. Mélinge, P. Monnet, A. Pantet and R. Le Roy, Rhéologie des suspensions à matrice cimentaire. Approche expérimentale multi-échelle, Rev. Eur. Génie Civ. 9 (3) (2005).
[45] H.M. Nguyen, Comportement rhéologique des suspensions concentrées, Master Research Report, INSA Rennes (2003) (63 p.).