Mechanical properties of geopolymeric materials at elevated temperatures
Creators: name: Pan, Zhu
Publisher: name: Monash University
Year: 2017
Subjects: Uncategorized

description: Geopolymer is an amorphous aluminosilicate binder which is produced by hydrothermal synthesis of aluminosilicates in the presence of concentrated alkaline or alkaline silicate solutions. It is an emerging construction material purported to provide an environmentally-friendly alternative to ordinary Portland cement (OPC) based concrete. Owing to its ceramic-like properties, geopolymer has been touted to be a highly fire resistant material. This claim is further supported by results from investigations of residual strength after thermal exposure. Some studies have found that geopolymers increase in strength after exposure to temperatures of 800°C. In contrast, several recent studies have shown a decrease in residual strength of geopolymers after a similar exposure. These contradictory observations are explored here. The study shows that two opposing processes are occurring simultaneously in geopolymers at elevated temperatures. Process (1) is further geopolymerization, and has the effect of increasing the strength. Process (2) is the damage due to thermal incompatibility which arises because of (i) the temperature gradient and (ii) different movements between matrix and inclusions. Process (2) is also a function of the brittleness level of the material. Whether the strength increases or decreases is dependent on which of the two processes is dominant in the specimen and the test conditions. The detailed examination of process (2) reveals a strong correlation between the degree of strength loss and the brittleness of geopolymers. This correlation suggests that geopolymers are quite brittle. For the first time, the brittleness of geopolymer concrete has been quantitatively determined and compared with OPC concrete. The comparatively high brittleness of geopolymer concrete is important for its fire resistance properties. The high brittleness will also require special structural design measures, similar to the design requirements for high strength concrete. With regard to the fire resistance of geopolymers, most research to date has investigated only residual strength, but the properties of geopolymer while hot have received less attention. Therefore, a number of tests to determine stress-strain curve, ultimate strength, elastic modulus and creep were undertaken in steady and transient heating conditions. In these tests, several critical features of the material’s performance were observed: (1) geopolymers exhibit glass transition behaviour at elevated temperatures; (2) glass transition temperature is improved by the substitution of a sodium-based activator for the potassium-based activator; (3) below glass transition temperature there is a significant increase in hot strength and hot elastic modulus; (4) in the range of 250-550°C thermal transitional creep is absent as compared to OPC concrete. Data obtained from these measurements can serve as input for the development of constitutive models which are essential to predict the response of geopolymer concrete elements in fire. In addition to its deteriorating properties at high temperatures, concrete can also be damaged in fire by a phenomenon called spalling, which is particularly severe in high strength concrete. Since geopolymer is comparatively more brittle than high strength concrete, the issue of spalling in fire is further explored in the last part of this study. It was found that the spalling resistance of geopolymer concrete increases as maximum aggregate size increases. The increasing aggregate size results in an increase in fracture zone length (lp); this in turn reduces the flux of kinetic energy (due to pore pressure) that is released into the fracture front and thereby improves spalling resistance. This theory is further validated by the observation of a good correlation between lp and spalling resistance in this thesis. This study is the first to propose such a hypothesis on the effect of aggregate size on the spalling of concrete, both OPC and geopolymer, and contributes to a better understanding of spalling of concrete in fire.