FINAL TECHNICAL REPORT
September 1, 1997, through August 31, 1998
Project Title: EXTRUDED FIBER-REINFORCED CEMENT COMPOSITES CONTAINING FLY ASH
ICCI Project Number: 97-1/3.1D-9
Principal Investigator: Dr. Surendra P. Shah, Notrhwestern University
Other Investigators Dr. Alva Peled, Herman Yost, Notrhwestern University
Project Manager: Dr. Daniel D. Banerjee, ICCI
ABSTRACT
The proposed research is a two years program consisting of two phases: product
development and commercialization feasibility. The major goals are to: 1)
develop extrudable compositions of fiber-reinforced cement composites that
contain substantial quantities of Illinois fly ash, 2) identify the best
compositions for targeted products, and 3) to promote the most promising
products for commercialization. The first year involved development and testing
of different compositions: 1) finding the maximum allowable contents of Illinois
fly ash, 2) evaluating different fiber types such as PVA, cellulose, glass
and acrylic fibers, and 3) developing curing schedules to achieve the desired
mechanical properties. The fly ash was examined as replacement for cement
and silica sand.
The addition of the Illinois Class F fly ash significantly improves the rheology
of the fresh extruded mixtures. This helps the extrusion process and reduced
composite defects. Fly ash used as replacement for cement improves the flexural
performance of the composite for all fiber types. This improvement is different
for each type of fiber. The highest improvement in both flexural strength
and ductility is obtained for composites containing acrylic fibers when as
high as 70% by volume of cement replaced by fly ash. For PVA fibers, only
a slight improvement in the flexural behavior is observed. In this case the
highest flexural strength is obtained when 60% by volume of cement is replaced
by fly ash. It is important to note that even when 80vol% of the cement was
replaced by the Illinois fly ash the flexural strength is not significantly
effected. Fly ash also improves the rheology of the mixture when compared
with sand. Therefore it can be used as a good substitute for sand. Comparable
improvements in flexural response are obtained in the composites cured for
2 days in a steam environment at 90C when compared with the conventional
28 days moist curing. This result is of great practical significance since
shorter curing time is often desirable in a manufacturing plant.
It can be summarized that the Illinois Class F fly ash is particularly suitable
for extruded fiber-reinforced cement composites, since mixtures containing
substantial quantities of this fly ash are extrudable and provide high flexural
properties (flexural strength of about 5000 psi and strain hardening type
of response are observed).
EXECUTIVE SUMMARY
The project is being submitted in the category of Process Research. The
applicable 1997-1998 Research Program Funding Area is Coal Related
Residues/By-Product Management and is within the specific area of Pulverized
Coal Combustion Residues (PCCR). The particular PCCR was Class F fly ash
from the Illinois Basin Coal Combustion Residues Sample Program (CRSP).
The proposed research is a two years program consisting of two phases: product
development and commercializing feasibility. The major goals are to: 1) develop
extrudable compositions of fiber-reinforced cement composites that contain
substantial quantities of Illinois fly ash, 2) identify the best compositions
for targeted products, and 3) to promote the most promising products for
commercialization. The first year involved development and testing of different
compositions: 1) finding the maximum allowable contents of Illinois fly ash,
2) evaluating different fiber types such as PVA, cellulose, glass and acrylic
fibers, 3) optimizing the content of the fibers, and 4) developing curing
schedules to obtain the desired mechanical properties.
Fibers are incorporated in the brittle cement matrix to control the cracking,
to provide high ductility and impact resistance, and to increase the tensile
and flexural strengths. Extrusion is a processing technique that has been
shown to impart high performance characteristics to fiber reinforced cementitious
materials((1)).
Extrusion is a forming process, which forces a highly viscous plastic-like
mixture through a rigid opening (a die) of the desired cross-section. It
has been a widely used process in several industries such as, ceramic, clay,
and polymer industries. There is a growing interest in the use of the extrusion
process for the fiber reinforced cement composites industry. With properly
designed dies and properly controlled material proportions, the fibers can
be aligned in the load-bearing direction. The matrix and fiber packing can
be identified to achieve low porosity and to improve the interface bond between
the fiber and matrix. Extrusion also allows more flexibility in the product
shape than possible with the standard board manufacturing processes. It is
a potential candidate for low cost commercial applications, such as roofing
tiles, flooring tiles, building panels, and pressure pipes. During the extrusion
processing the viscosity (rheology) of the material is a controlling factor.
If the rheology of the material is not close to ideal, defects form in the
material during extrusion leading to reduce performance of the composites.
Besides the economical and ecological reasons for using fly ash in cement-based
products, the round particle morphology of fly ash can improve the rheology
of extrusion systems. Therefore fly ash can be considered as a replacement
for sand, silica fume as well as for cement. Partial replacement of cement
with fly ash not only makes cement-based products inexpensive, but also reduces
the total life cycle energy consumption by lowering the amount of cement
needed.
A small-scale extrusion rheometer was used to extrude the specimens for this
work. Specimens containing different ratio of fly ash as replacement for
cement, silica sand and silica fume were prepared. Different fiber types
were added to the mixtures: PVA, glass, acrylic and cellulose fibers. Four
combinations with different ratios of cement:fly ash were tested: 20:80,
30:70, 40:60, and 50:50. All the sand in the mixtures was replaced with fly
ash (resulting in a 12% replacement by volume). The fiber volume fraction
was 3% for PVA and glass fibers, and 7% - 8% for cellulose and acrylic fibers.
The influence of fiber content on flexural properties was also tested for
PVA fibers. Different volumes of fibers were used: 1%, 2%, 3%, and 4%. The
cement:fly ash ratio was kept constant at 40:60. In most of the cases two
different cross sections of specimens were extruded and tested: 4mm sheet
and 12.7mm diameter cylinders.
After the extrusion the specimens were covered with a plastic sheet. After
24 hours the specimens were placed in 100% RH at 90C for 2 days. Some specimens
were subjected to conventional curing at 100% RH at room temperature for
28 days. Thereafter all the specimens were dried at 105 C for 2 days and
kept for another 24 hours at 50% RH at room temperature, before final testing.
Three-point flexural tests were performed at a rate of 0.0114 mm/sec. The
span was 101.6mm for the sheet specimens and 203.2mm for the cylinder specimens.
It was clear during the extrusion process that the addition of fly ash
significantly improved the rheology of the fresh extruded mixtures. This
lead to the reduction in composite defects and resulted in improved mechanical
performance of the composite. This was observed for all fibers. The improvement
in the flexural behavior on using fly ash varied with the fiber type. For
example, for composites made with acrylic fibers, when 70% of cement was
replaced with fly ash, a four-fold increase in ductility (as measured by
deflection at peak load) was observed. For PVA fibers, only a slight improvement
of the flexural strength was observed with fly ash replacement. In this case
the highest flexural strength was obtained when 60% by volume of cement was
replaced with fly ash. In the cases of cellulose, glass and PVA fibers cement
replacement with fly ash resulted in a two-fold increase in ductility. It
is important to note that even when 80% by volume of the cement was replaced
with the Illinois fly ash the flexural behavior of the composites was not
influenced significantly (only a 20% reduction was observed when compared
with the 60-70% replacement).
Microstructural examination of fractured surfaces using Scanning Electron
Microscope (SEM) indicated a different mechanism of fracture for composites
containing fly ash compared with those without fly ash. Large composite defects
(large pores) were observed in composites that did not contain fly ash. This
can be attributed to the poor rheological properties of mixture used to extrude
these composites, resulting in low flexural properties of composites without
fly ash.
It was also found that the fly ash improved the rheology of the mixture over
sand and achieved similar flexural behavior. Therefore fly ash can be used
as a good substitute for sand.
The flexural strength obtained with fly ash was comparable to that with silica
fume, but the corresponding toughness was lower. This was true even when
half of the silica fume was replaced by fly ash. Therefore, silica fume can
not be replaced when toughness is necessary.
Curing the specimens in a moist environment at 90C for 2 days gave similar
flexural results compared to moist curing at room temperature for 28 days,
for PVA and cellulose fibers. Shorter curing time can be an advantage from
an economical point of view, when considering manufacturing of these composites.
One of the main advantages of extrusion technology is that it allows more
flexibility in product shape than possible with the standard board manufacturing
processes. Complicated solid cross-sections and open or cellular cross-sections
can be extruded, usually with only minor modifications in batch composition.
Where reduction in total weight or increase in structural rigidity is necessary,
cellular cross sections can be extruded. In the second phase of this project
specimen with different cross sections including two scales of cellular dies
will be designed and extruded for use in lightweight construction for commercial
applications.
Summary and Conclusions: It can be summarized that the Illinois Class F fly ash is particularly suitable for extruded fiber-reinforced cement composites, since mixtures containing substantial quantities of this fly ash are extrudable and provide high flexural properties (flexural strength of about 5000 psi and strain hardening type of response). The Illinois Class F fly ash significantly improves the rheology of the fresh extruded mixtures, which leads to a reduction in composite defects and results in improved mechanical performance. Fly ash can also be used to replace cement and sand, but can not replace silica fume when toughness is necessary (since silica fume greatly improved toughness over fly ash). Similar flexural response is obtained for composites cured for 2 days in a steam environment at 90C when compared with the conventional 28 days moist curing. This result is of great practical significance since shorter curing time is often desirable in a manufacturing plant.