·
Figure 1. Cellulose ether structures used in the studies: HPMC (hydroxypropyl)methyl cellulose (a),
HEMC (hydroxyethyl)methyl cellulose (b), MEHEC methyl ethyl hydroxyethyl cellulose (c).
In cement and cement-lime mortars, cellulose-based admixtures have an impact on both macroscopic
properties: strength, adhesion to substrates as well as consistency and change in water retention [6-9].
In fresh mortar, cellulose particles are adsorbed on the surface of aggregate or cement grains to form
a monolayer. Further adsorption is prevented by the absorbing effect of the solvent [6].
Cellulose, which has not been adsorbed, forms a gel in the intergranular space. As a result of steric interactions of thecellulose chains, the cement grains and aggregates separate. The longer the cellulose chains, the greatertheir interaction, which in turn leads to a more stable suspension.
The influence of cellulose-based admixtures on water retention is caused by their ability to bind water
[10, 11]. The mechanism of water retention by mortars with cellulose admixtures may result from the
difference between the chemical potential of water in the mortar and the chemical potential of clean
water.
As the water evaporates from the wet mortar, the concentration of cellulose admixture increases.
The presence of dissolved substance lowers the potential of water, and thus the hydrostatic pressure,
which helps to retain water in the mortar. In addition, due to the presence of cellulose particles on the
surface of the mortar, the diffusion space may be limited, resulting also in a decrease in water mobility
and stimulating its retention in the mortar [5, 12].
Some of the important physicochemical factors of cellulose admixtures influencing the change of
mortar consistency and water retention include: the type of modification, viscosity, quantity, molecular
weight, granulometry, degree of substitution, etc.[7, 13, 14] .
The available literature is scarce in the investigations of the influence of cellulose admixtures on the
change in consistency, water retention and adhesion to substrates of renovation plasters . This paper
discusses the impact of cellulose ethers, with various modifications and viscosities, on these properties.