FINAL TECHNICAL REPORT

November 1, 1998, through October 31, 1999

Project Title: EXTRUDED FIBER-REINFORCED CEMENT COMPOSITES CONTAINING FLY ASH

ICCI Project Number: 98-1/3.1C-1

Principal Investigator: Dr. Surendra P. Shah, Northwestern University

Other Investigators: Dr. Alva Peled, Herman Yost, Northwestern University

Project Manager: Dr. Ronald Carty, ICCI

ABSTRACT

The major goals of the present research 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) promote the most promising products for commercialization. This year involved development and testing of different compositions and specimen cross-sections. Two scales of cellular dies were designed and tested for use in lightweight construction for commercial applications. Compositional development focused on evaluating different fiber types such as glass, polypropylene (PP), PVA, and cellulose fibers, as well as hybrid fibers (i.e., combinations of different fibers). The hybrid system could allow better control of the composite performance. Also, curing schedules were developed to give the desired mechanical properties of the composite for each fiber and hybrid fiber, and the long-term durability of the composites was evaluated.

A significant improvement in flexural behavior of the extruded composites, both in strength and in ductility, was achieved, even though as much as 70% by volume of the cement was replaced with Illinois fly ash. This improvement was especially noticeable when glass, cellulose and acrylic fibers were used. Much lower results were obtained with extruded specimens made with these fiber types when no fly ash was used. The positive effect of the fly ash can be attributed to improvement in fiber durability and fiber-matrix bond, and a more compacted matrix. The combination of improvement in fiber durability and bonding can explain the improved strength and ductility of the acrylic, glass, and cellulose fiber composites. This was more pronounced for steam-cured composites. For polypropylene (PP) fiber composites the improved mechanical performance with the addition of fly ash can be mainly attributed to an improvement in bond strength. For PVA fiber composites the bond as well as durability are relatively high, hence the positive effect of fly ash is not as significant, and the decrease in composite strength for high fly ash content was attributed to the low strength and low toughness of the matrix itself. Combining glass and polypropylene fibers in the same composite made it possible to engineer the composite performance. A composite with cellular cross-sections was successfully extruded in order to produce lightweight products for precast applications. It can be concluded that fly ash can be beneficially used as a replacement for cement in extruded fiber-reinforced cement composites for the building industry. Successful promotion of cellular, extruded products containing Illinois fly ash has already begun.

EXECUTIVE SUMMARY

The major goals of the present research 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) promote the most promising products for commercialization. This year involved development, design and testing of different compositions and specimen cross-sections. Two scales of cellular dies were designed and tested for use in lightweight construction for commercial applications. Extrusion of cellular cross-sections is important where reduction in total weight and increase in structural rigidity are necessary. Compositional development focused on evaluating different fiber types such as glass, polypropylene, PVA, acrylic and cellulose fibers, as well as hybrid fibers (i.e., combinations of different fibers). The hybrid system could allow better control of the composite performance. Moreover, the cost could be reduced if inexpensive fibers can replace part of the expensive fibers. Curing schedules were developed to give the desired mechanical properties of the composite for each fiber and hybrid fiber, and the long-term durability of the composites was evaluated.

Fibers are incorporated in the brittle cement matrix to control the cracking by bridging across the cracks, to provide high ductility and to improve impact resistance as well as increase both the tensile and flexural strength. The modulus of elasticity of the fibers in relation to the modulus of the cement matrix (about 15-30 GPa) influences the mechanical performance of the composite. Hybrid composites containing two or more fiber types can be considered to control the cost and optimize desired properties of the composite by taking advantage of the different properties of different fiber types.

Extrusion is a processing technique that has been shown to impart high performance characteristics in fiber-reinforced cementitious materials.⁽⁽¹⁾) Extrusion is a forming process, which forces a highly viscous plastic-like mixture through a rigid opening (a die) having the geometry of the desired cross-section. There is 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. With the extrusion process, the matrix properties and the packing of fiber can be altered to achieve low porosity and to improve the interface bond between the fiber and the matrix. Extrusion also allows more flexibility in product shape than is 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 ideal, defects form in the material during extrusion leading to reduced performance of the composites. Along with the economical and ecological reasons for using fly ash in cement-based products, the round particle morphology of fly ash improves the rheology of extrusion systems. 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 ram extrusion rheometer was used to extrude the small-scale specimens: cylinder, sheet and small-scale cellular cross-section. An auger extruder was used to extrude the large-scale cellular specimens. Specimens containing Illinois Class F fly ash as a replacement for cement were prepared with different fibers: acrylic, PVA, glass, polypropylene, and hybrid fibers. In most of the cases 70% by volume of cement was replaced by fly ash. The fiber volume fraction was 3% for PVA fibers, 5% for the glass, polypropylene and the hybrid fibers, and 5% or 7% for the acrylic fibers. For studying the different specimen cross-sections, composites with PVA and cellulose fibers with 60% and 50%, respectively, by volume of fly ash as replacement for cement were prepared. After being extruded, the specimens were covered with a plastic sheet for 1-3 days. Then they were either steam cured at 90°C and 100% relative humidity (RH) for 2 or 4 days or cured for 4, 8, 15 and 28 days in 100% RH at room temperature. Thereafter, most of the specimens were dried at 105°C for 24 hours, and then kept at 22^oC and 50% RH for 24 hours before being tested in flexure. In order to study the effect of fly ash on the composite long-term durability, acrylic, glass, PP and hybrid composites as well as plain matrix with and without fly ash were stored in a hot bath at 80°C for 6 to 20 weeks, after being cured in room temperature at 100% RH for 28 days.

A three-point flexural test was carried out at a rate of 0.0045 inch/sec. The span was 4 inches for the sheet specimens, 8 inches for the cylindrical and small-scale cellular specimens, and 18 inches for the large-scale cellular specimens.

It was found that mixtures containing substantial quantities of Illinois Class F fly ash are extrudable and provide enhanced flexural properties.

The positive effect of the fly ash was observed only when fibers were incorporated to the mixture, and found to be dependent on the type of fiber. Acrylic, glass, and cellulose fibers were affected the most, whereas PP, hybrid, and PVA fiber composites were less influenced by the fly ash.

One of the main advantages of the extrusion technique is that it enables a higher flexibility of product cross-sections than is possible with the standard manufacturing processes. One of the goals of the present work was to produce lightweight products for precast applications. For such applications lightweight products are important in order to reduce the cost of transportation and erection. Cellular cross-sections were extruded successfully for lightweight precast products. It was found that the composite cross-section influenced its mechanical properties. A high content of fly ash resulted in significant improvement in the flexural strength of small-scale cellular composite. The improvement with the large-scale cellular composite was not as great as that for small-scale composite. This was due to fiber clumping at the corner of the cellular cross-section as well as higher porosity of this composite.

Increasing the content of polypropylene fibers while reducing the glass fiber content increased composite toughness and reduced composite strength of the hybrid fiber composite. In order to take advantage of these results and further increase the composite strength and toughness, other fiber types should be considered such as PVA and carbon fibers.

The addition of a high content fly ash improves the long-term durability of acrylic, glass and hybrid fiber composites. An increase in strength and a reduction in toughness over aging were observed for the glass fiber composite with fly ash, due to increase in the interfacial bond. Acrylic and glass fiber composites without fly ash exhibited embrittlement and loss in strength over aging. The accelerated aging improved the flexural performances, mainly the strength, of PP fiber composites with and without fly ash, due to an increase in the fiber-matrix interfacial bond strength. This improvement was greater for composites containing fly ash. The hybrid fibers improve the long-term durability of the composite over glass fiber composites.

During the performance of these research tasks, the investigators have worked closely with the technical and marketing staff of an unnamed major corporation on the commercialization challenges of bringing this process to market through the manufacture of industrial building panels. Tests have been performed to ascertain whether the product would meet the most stringent test specifications of wind load, weight per area, and material handling. The cost of the product is a major factor in determining the marketability. Analysis by the company has determined the product to be cost competitive, this being strongly influenced by the ability to use a significant percentage volume of fly ash.

Summary and Conclusions A significant improvement in the flexural behavior of the extruded composites was obtained when as high as 70% by volume of cement was replaced by fly ash. This was especially true when glass, cellulose and acrylic fibers were used. For these composites, much lower results were obtained with extruded specimens without fly ash. The improved flexural properties of composites containing fly ash can be attributed to improvement in fiber durability, fiber-matrix bond, and a more compact matrix. The combination of improved fiber durability and fiber-matrix bond can explain the increase in strength and ductility of the acrylic, glass, and cellulose fiber composites when fly ash was added. This was more pronounced for steam-cured composites. For PP fiber composites, the improved mechanical performance with the addition of fly ash can be mainly attributed to an improvement in bond strength. For PVA fiber composites, the bond as well as durability is relatively high, hence the positive effect of fly ash is not as significant, and the decrease in composite strength for high fly ash content was attributed to the low strength and toughness of the matrix itself. Combining glass and polypropylene fibers in the same composite made it possible to engineer the composite performance. The extrusion technique enables a higher flexibility of product cross-sections than possible with the standard manufacturing processes. A composite with cellular cross-sections was successfully extruded in order to produce lightweight products for precast applications. It can be concluded that fly ash can be beneficially used to replace cement for extruded fiber-reinforced cement composites.

The collaborating corporation has recently spent considerable time and funds investigating the cost to set up a prototype production facility. Investigations have been made and studies have been performed to determine the viability of equipment suppliers in the US, Japan and Germany. Additional testing and trials will be performed on potential prototype equipment in close collaboration with and guidance of the ACBM investigator team.

¹1 Shao, Y., Marikunte, S., Shah, S. P. 1995. "Extruded Fiber-Reinforced Composites", Concrete International:48-52.