| Summary: | The objective of this study was to develop a fundamental understanding of the microfiller effect
in high-performance concrete. Ultimately, this would help in the development of blended highperformance
cements containing recycled materials and industrial byproducts, offering both
significant economic advantages and environmental relief.
The mechanisms underlying the microfiller effect on the rheology were investigated in cement
paste using a coaxial-cylinders viscometer, a mini slump test, the Marsh cone flow time, and a
pressure bleed test. They were also studied in mortars using the ASTM flow-table test, and in
concrete using a computer-controlled rheometer, a slump-flow test, the conventional slump test,
and an induced bleeding test. It was found that microfillers enhance the superplasticizer
efficiency because they increase the surface layer water and reduce the bulk water through a
reduced void space in the particulate mixture. In the presence of a superplasticizer, microfillers
also decrease the viscosity of concrete mixtures; the finer the particle size of the microfiller the
greater the decrease. This seems to be due to a reduction of the mechanical interlocking between
coarser particles. Ultrafine particles also decrease the bleed water, which reduces the occurrence
of bleed channels and low density microstructural features at interfaces. As a result of the above,
microfillers make the production of fluid and self-leveling concrete much easier. It was also
demonstrated that triple-blended cements containing pozzolanic and non-pozzolanic fillers can
achieve superior rheological properties.
The microfiller effect on mechanical properties was investigated in mortars and in concrete both
at early and later ages. It was discovered that this effect depends on the initial porosity of the
system. At very low w/b ratios, partial replacement of cement with non-cementitious fillers
would not result in lower density hydration products because the initial porosity is already very
low. The hydration reactions in fact yielded denser hydration products. Thus, up to 15%
replacement of cement by a non-cementitious filler caused significant increases in strength. This
was even more significant in triple-blended cements containing combinations of pozzolanic and
non-pozzolanic fillers for which up to 30% partial replacement of cement resulted in significant
strength increases. Ultrafine carbonate fillers increased the very early age strength by about one order of magnitude,
because certain microfillers appear to present energetically preferential substrates for the
germination and growth of calcium hydroxide. Removal of calcium ions from the solution
catalyzes the dissolution of C₃S in an attempt to regain equilibrium. This signals an earlier end of
the induction period and a faster rate of the hydration reactions at early ages.
Quantitative image analysis of backscattered electron micrographs was used to quantify the
microfiller effect on the microstructure of high-performance concrete. Analysis was carried out
on cement paste and concrete both at Id and at 28d. The acceleration of the hydration reactions
at early ages due to carbonate microfillers was confirmed by this technique. Microfillers
generally decreased the porosity and refined the microstructural features. This was accompanied
by increased strength only when the ratio of inner hydration products to outer hydration products
was increased. Densification of the paste-aggregate interface did not seem to necessarily increase
the compressive strength.
The microfiller effect in high-performance concrete was studied from the standpoint of the
theory of particle packing. An insight into particle packing models, the effects of particle
packing on rheology, and the effects of particle size distribution on hydration reactions was
obtained. A new parameter, the microfiller efficiency factor was developed, based on an
estimation of packing density and microfiller effect on hydration rate. A close correlation was
found between the microfiller efficiency factor and compressive strength. In addition, a new
model relating microstructure to strength was proposed. Most available models relate porosity to
strength without accounting for the nature of the solid phase. The model proposed herein
considers for the first time a quantitative value representing the nature of the hydration products
to help estimate strength. This value is the ratio of the dense inner hydration products to the bulk
of the rest of the hydration products. The proposed model achieved good estimations of strength.
Overall, this study proposes a new approach to achieving high-strength materials. Traditionally,
high strength is obtained through increased cement content, reduced w/b ratios, and high rates of
hydration. This work suggests that high strength can be achieved through high initial particle
packing combined with a low rate of hydration, which causes less chemical contraction, less
drying shrinkage and self dessication stresses, and a higher content of inner hydration products. === Applied Science, Faculty of === Civil Engineering, Department of === Graduate
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