Page 48 - Riem-Vol6_nº3

400 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 3
Study of the influential factors on the rheological behavior of adhesive mortar available in the market
1. Introduction
Adhesive mortars consist of cement, aggregate and other addi-
tives. The most common industrially produced additives are cel-
lulose ethers (HEC e MHEC) and latex polymers (PVA). Each per-
form specific functions in fresh and hardened mortars [1]. Latex
additives improve the rheological behavior of fresh mortars, but
mainly provides flexibility and tensile strength for hardened mor-
tars [2]. Ethers are water soluble polymers and small amounts of
polymer increases water retention and viscosity in mortars [3, 4].
Quartz sand, with dimensions less than 0.50 mm, is predominantly
used in mortar. Characteristics, such as shape, texture and grain
size, have a significant influence on the workability and adhesion
of mortar [5]. The shape and texture of the grains are primar-
ily responsible for changes in the friction coefficient. Mendes [6]
demonstrated that the more rounded the particles, the lower the
friction coefficient compared to those that are irregularly shaped.
Pereira [7] found that the shape of particles larger than 0.090 mm
significantly influence the flow phenomena of mortar and the more
rough the particles, the greater the shear required for the flow of
the mixture.
Another issue related to the influence of particle size distribution is
the degree of compaction. The ideal condition is to gradually fill the
larger voids with smaller grains [8]. It should be noted, however,
that you should not utilize an excess of fines as they may increase
the surface area, hence increasing the amount of water needed to
coat the particles. Sand with elevated fineness increases the water
consumption in the mixture, which may cause significant shrink-
age in the mortar after water evaporation, and fine sand may fill
the pores of the substrates occupying the spaces of the hydration
products [5, 9].
Moreover, an increase in the proportion of sand, maintaining the
same amount of water in the mixture, decreases the workability of
the mortar, thus influencing the development of adhesive strength
of the coatings. It should also be considered that increased coarse-
ness in the sand could cause inefficient distribution of the grains
in the cement mixture leading to difficulty in mortar flow and com-
promising adhesion [9]. However, it should be emphasized that an
adequate proportion of sand is essential for the formation of the
undeformed skeletal structure of the mixture and to reduce mortar
shrinkage.
The fineness of cement has a significant influence in adhesive
mortar. Different cements exhibit different physical characteristics.
However, fineness is the most significant parameter [10]. The finer
the particles of cement, the more water and additives are required
for the same workability.
From the perspective of rheological behavior, it is assumed that
mortar adhesives are concentrated suspensions of solid particles
immersed in a viscous liquid which has the function of ensuring the
cohesion of the system and providing lubrication and space for the
movement of aggregates [8]. If the volume of the matrix is not suf-
ficient, the rheological characteristics and properties of the mortar
is harmed [11, 6]. With increased matrix content, fluid flows more
easily and fluidity of the system will be governed by the viscosity of
the matrix. Thus, to understand the rheology of the fluid and have
a global view of rheological behavior, it is necessary to analyze the
phenomena that occur in the matrix, such as those derived from
aggregate [12].
To define the behavior of mortar in its fresh state we frequently use
consistency parameter data obtained from tests Flow Table and
the Dropping Ball tests [13]; the so-called monopoint tests. How-
ever, as easy as these tests are to perform, they present concep-
tual limitations in the characterization of mortars. It is not possible
to describe the behavior of material by a single measurement. But,
it should be measured by a rheological profile test that, preferably,
simulates actual conditions since behavior can vary depending on
the characteristics of the actual application [14].
To fill this limitation gap, the Squeeze Flow test was developed. It
provides for the characterization of rheological behavior through
viscosity and yield parameters and is the defined as standard test
NBR 15839 [15]. Despite the aforementioned standard, it is not
specific for the analysis of adhesive mortars. But, this methodology
is being used successfully by various researchers [8,7,16] along
with adaptations and adjustments to the test apparatus.
Within the above-mentioned context, the objective of this work is to
understand the behavior of fresh mortar adhesives based on rheo-
logical characterization of different Brazilian commercial composi-
tions. An understanding of the rheological behavior of adhesive
mortar is the basis for a technological breakthrough, considering
that their application is in a fluid state.
2. Experimental program and materials
The experimental program was designed with the objective of
correlating the physical characteristics of mortar adhesives in the
market, such as particle size, particle morphology and air content,
with their rheological behavior determined from Squeeze Flow test
parameter characteristics.
Table 1 – Detail of the preliminary data of the samples
(Slip and water/dry materials ratio in the pastes and mortars)
Adhesive mortar and paste
( G ) A ( G ) B ( M ) C ( B ) D ( M ) E ( B ) F
Amount of water indicated by the manufacturer
for adhesive mortar (ml/ Kg)
Proportion water/dry mix materials for paste (ml/g)
Slip test results of mortar (mm)
230
1.1
0.5
220
1.1
0.5
200
0.9
0.7
230
0.9
3.4
230
1.2
1.1
200
0.9
5.9
(G) Good quality; (M) Medium quality; (B) Bad Quality