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The effect of rheological parameters on static segregation of self-compacting concrete mortar
Author(s)
Jakuja, Mzwandile
Date Issued
2021
Type
Thesis
Publisher
Cape Peninsula University of Technology
Abstract
The stability of mortar is generally understood as the ability of the suspension to remain
homogeneous during and after fresh mortar placement or casting. This is mostly associated
with the segregation of the suspension that can be defined at a static and dynamic level. At a
static level, this phenomenon is simplified by Stokes’ equation, while at a dynamic level the
characterisation becomes more complex due to the horizontal and translation motion of solid
particles that must be considered simultaneously. Static segregation consists primarily of the
downward migration of solid particles from the liquid medium remaining on top of the
suspension (bleeding). Available literature has established a relationship between the rheology
of fresh mortar and its stability, stating that viscosity is the determinant rheological parameter
of the suspension able to dictate mortar stability.
Cement mortar is a suspension with two mediums consisting of sand particles as the solid
phase and cement paste as the liquid phase. It is argued that the overall performance of the
suspension depends on the individual behaviour of the two phases. In accordance with Stokes’
law, solid particles should overcome the physical characteristics of the intermediate medium
to settle effectively. Cement paste has to therefore exhibit microstructural strength to avoid
sand particle settling. This is normally attributed to the yield stress of the cement paste. This
means that the cement yield stress is the strength of the liquid phase that cannot be overlooked
at the expense of overall mortar viscosity, as currently noted in the literature.
High performance cement mortars also require the inclusion of superplasticisers whose
effectiveness depends primarily on their chemical structure (group function) and the dosage at
which they are used. It is thus important to understand the compatibility between the
superplasticisers and cements since their interactions affect the cement paste that can in turn
alter the stability of the cement mortar.
Three different cements and two superplasticisers were used in this study. All cements were
CEMI with distinct contents of aluminate and silicate phases manufactured in three different
plants. The superplasticisers were poly-carboxylates with a specific molecular structure that
defines their impact on the setting time. Mortars with different pastes exhibiting discrete yield
stress values were designed. These yield stresses were achieved at optimum dosage of the
product resulting from the blending of the two superplasticisers. The products consisted of
mixing superplasticisers in different proportions at the set dosage. Rheological measurements
were performed both at mortar and paste scale to estimate their yield stress and viscosity
values. The Total Organic Carbon (TOC) was done at paste scale to determine the adsorption
of superplasticiser on the cement particles within the suspension.
This research confirmed that the stability of mortar depends not only on its overall viscosity,
but also on the yield stress of its cement paste phase that defines the strength that opposes gravity acting on the sand solid particles to cause them to settle. Moreover, the study highlights
the possibility of achieving a high performing superplasticiser by blending two different
superplasticisers at an optimum dosage. In particular, mortar with high yield stress cement
pastes exhibited more stable suspensions with lower segregation indexes. In contrast, mortar
with lower cement yield stress values exhibited higher segregation indexes resulting in a mortar
with poor stability. There is no definitive evidence, according to the results, to indicate that yield
stress and viscosity have an effect on bleeding. Results from TOC measurements were in
agreement with the literature showing that cement pastes with higher adsorption
superplasticisers have lower yield stress values and vice versa.
homogeneous during and after fresh mortar placement or casting. This is mostly associated
with the segregation of the suspension that can be defined at a static and dynamic level. At a
static level, this phenomenon is simplified by Stokes’ equation, while at a dynamic level the
characterisation becomes more complex due to the horizontal and translation motion of solid
particles that must be considered simultaneously. Static segregation consists primarily of the
downward migration of solid particles from the liquid medium remaining on top of the
suspension (bleeding). Available literature has established a relationship between the rheology
of fresh mortar and its stability, stating that viscosity is the determinant rheological parameter
of the suspension able to dictate mortar stability.
Cement mortar is a suspension with two mediums consisting of sand particles as the solid
phase and cement paste as the liquid phase. It is argued that the overall performance of the
suspension depends on the individual behaviour of the two phases. In accordance with Stokes’
law, solid particles should overcome the physical characteristics of the intermediate medium
to settle effectively. Cement paste has to therefore exhibit microstructural strength to avoid
sand particle settling. This is normally attributed to the yield stress of the cement paste. This
means that the cement yield stress is the strength of the liquid phase that cannot be overlooked
at the expense of overall mortar viscosity, as currently noted in the literature.
High performance cement mortars also require the inclusion of superplasticisers whose
effectiveness depends primarily on their chemical structure (group function) and the dosage at
which they are used. It is thus important to understand the compatibility between the
superplasticisers and cements since their interactions affect the cement paste that can in turn
alter the stability of the cement mortar.
Three different cements and two superplasticisers were used in this study. All cements were
CEMI with distinct contents of aluminate and silicate phases manufactured in three different
plants. The superplasticisers were poly-carboxylates with a specific molecular structure that
defines their impact on the setting time. Mortars with different pastes exhibiting discrete yield
stress values were designed. These yield stresses were achieved at optimum dosage of the
product resulting from the blending of the two superplasticisers. The products consisted of
mixing superplasticisers in different proportions at the set dosage. Rheological measurements
were performed both at mortar and paste scale to estimate their yield stress and viscosity
values. The Total Organic Carbon (TOC) was done at paste scale to determine the adsorption
of superplasticiser on the cement particles within the suspension.
This research confirmed that the stability of mortar depends not only on its overall viscosity,
but also on the yield stress of its cement paste phase that defines the strength that opposes gravity acting on the sand solid particles to cause them to settle. Moreover, the study highlights
the possibility of achieving a high performing superplasticiser by blending two different
superplasticisers at an optimum dosage. In particular, mortar with high yield stress cement
pastes exhibited more stable suspensions with lower segregation indexes. In contrast, mortar
with lower cement yield stress values exhibited higher segregation indexes resulting in a mortar
with poor stability. There is no definitive evidence, according to the results, to indicate that yield
stress and viscosity have an effect on bleeding. Results from TOC measurements were in
agreement with the literature showing that cement pastes with higher adsorption
superplasticisers have lower yield stress values and vice versa.
Additional information
Thesis (MTech (Civil Engineering))--Cape Peninsula University of Technology, 2021
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