<p>Silica fume is a widely
used pozzolan in the concrete industry that has been shown to have numerous
benefits for concrete including improved mechanical properties, refined pore
structure, and densification of the interfacial transition zone between paste
and aggregates. Traditionally, silica
fume is used as a 5% to 10% replacement of cement; however, newer classes of
higher strength concretes use silica fume contents of 30% or greater. At these high silica fume contents, many
detrimental effects, such as poor workability and inconsistent strength
development, become much more prominent.
</p>
<p> </p>
<p>In order to understand
the fundamental reasons why high silica fume contents can have these
detrimental effects on concrete mixtures, eight commercially available silica
fumes were characterized for their physical and chemical properties. These included traditional properties such as
density, particle size, and surface area.
A non-traditional property, absorption capacity, was also determined. These properties or raw material
characteristics were then related to the hydration and rheological behavior of
pastes and concrete mixtures. Other
tests were performed including isothermal calorimetry, which showed that each
silica fume reacted differently than other silica fumes when exposed to the
same reactive environment. Traditional
hydration models for ordinary portland cement were expanded to include the
effects that silica fumes have on water consumption, volumes of hydration
products, and final degree of hydration.
</p>
<p> </p>
<p>As a result of this
research, it was determined necessary to account for the volume and surface
area of unhydrated cement and unreacted silica fume particles in water-starved
mixture proportions. An adjustment factor
was developed to more accurately apply the results from hydration
modeling. By combining the results from
hydration modeling with the surface area adjustments, an analytical model was
developed to determine the thickness of paste (hydration products and capillary
water) that surrounds all of the inert and unreacted particles in the system. This model, denoted as the “Paste Thickness
Model,” was shown to be a strong predictor of compressive strength
results. The results of this research
suggest that increasing the paste thickness decreases the expected compressive
strength of concretes at ages or states of hydration.</p>
<p> </p>
<p>The rheological behavior
of cement pastes containing silica fume was studied using a rotational
rheometer. The Herschel-Bulkley model
was fit to the rheological data to characterize the rheological behavior. A multilinear model was developed to relate
the specific surface area of the silica fume, water content, and silica fume
content to the Herschel-Bulkley rate index.
The Herschel-Bulkley rate index is practically related to the ease at
which the paste mixes. This multilinear
model was shown to have strong predictive capability when used on randomly
generated paste compositions. </p>
<p> </p>
<p>Additionally, an
analytical model was developed that defines a single parameter, idealized as
the thickness of water surrounding each particle in the cementitious system. This model, denoted as the “Water Thickness
Model,” incorporated the absorption capacity of silica fumes discovered during
the characterization phase of this study and was shown to correlate strongly
with the Herschel-Bulkley rate index.
The Water Thickness Model demonstrates how small changes in water
content can have a drastic effect on the rheology of low w/c or high silica
fume content pastes due to the combined effects of surface area and absorption. The effect of additional water on higher w/c
mixtures is significantly less.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/11303171 |
| Date | 02 December 2019 |
| Creators | Jedadiah Floyd Burroughs (8065844) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Influence_of_Chemical_and_Physical_Properties_of_Poorly-Ordered_Silica_on_Reactivity_and_Rheology_of_Cementitious_Materials/11303171 |
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