Composite materials are widely used in all types industries. The requirements depend on the type of application. Ceramic composites are popular in applications where materials are expected to encounter high temperatures; such as engine components, exhaust systems and fire barriers. The low density as compared to metals makes them attractive in applications where weight is a critical design parameter. Most of the ceramic composites are fabricated using heat treatment and often the operating temperatures are lower than the temperatures used for the fabrication of the component.

For the research reported in this paper a modified version of an inorganic resin known as Geopolymer was used. Typical Geopolymer can sustain temperatures up to 800°C. This composition was evaluated for a number of applications that require fire-resistance, such as the interior of an aircraft (1). This low-cost, inorganic polymer is derived from naturally occurring geological materials, namely silica and alumina, hence the name Geopolymer. Geopolymer is a two-part system consisting of a silicate liquid and a silica powder and cures at a reasonably low temperature of 150ºC. Hardeners can be added to achieve room temperature curing (22ºC). The matrix has been used to fabricate standard laminate composite plates with carbon, glass, and silicon carbide fibers, sandwich structures using syntactic foam, and strengthening of brick, masonry, and reinforced concrete elements ( 2). The primary objective of the current investigation was to develop a composite that can sustain at least 1350ºC and have a flexural strength of 75 MPa.

Additional objectives were to keep the fabrication temperature to a minimum and use commercially available economical materials. Modifications were made to the basic Geopolymer to attain a higher operating temperature. The modified composition can sustain 1400ºC for long term exposure. Alumina fibers were added to the base matrix for obtaining higher flexural strengths. Three types of alumina fibers were evaluated. The first one was in a paper form and the second fiber type consisted of randomly distributed very short fibers. These fibers were designated as milled fibers.

The third type consisted of discrete short fibers that were much more uniform as compared to the second type.
The third type of fibers was more expensive than the second. Approximately one hundred coupon samples were tested in flexure with varying alumina to silica ratios, fiber content,The results of an experimental investigation on the behavior of milled and short-fiber reinforced composite plates are presented in this paper. The target operating temperature for the plates was 1300°C. The principal variables were the type and volume fraction of fibers and the matrix composition. Three fiber types and five fiber contents ranging from 2.5 to 10 weight percent were evaluated. The density of the samples varied from 1500 to 2800 kg/m3. For the matrix, the ratio between alumina and silica was varied from 1:1 to 5:1. The flexural tensile strength, (modulus of rupture) varied from 10 to 95 MPa. The modulus of elasticity varied from 5 to 60 GPa. There is a strong correlation between the unit weight and the mechanical properties of strength and modulus. Load-deflection response in flexure, strength and stiffness for the various mix formulations and their relation to unit weight are presented.