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Performance of geopolymer concrete in fire
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posted on 09.02.2017by Zhao, Ren
Portland cement concrete is a world-wide used construction material. However, when Portland cement concrete is exposed to fire, its mechanical properties are deteriorated. The deterioration of concrete is generally caused by the decomposition of the Portland cement hydrate or the thermal incompatibility between cement paste and aggregate. Spalling, which is a violent or non-violent breaking off of layers or pieces of concrete from the surface of a structural element, may also occur when the concrete is exposed to rapidly rising temperatures. It is generally believed that spalling is influenced by the build-up of pore water pressure and thermal gradient in the concrete when exposed to elevated temperatures.
Geopolymer is an alternative cementitious material which has ceramic-like properties. Geopolymer belongs to the family of inorganic polymers. The chemical composition of geopolymer is similar to natural zeolite, but the microstructure is amorphous. It is suggested that geopolymer processes a potential superior fire resistance due to its amorphous and ceramic-like properties. The objective of this thesis is to study the fire resistance of geopolymer material and to explore the spalling behaviour of geopolymer material when exposed to elevated temperatures.
In this thesis, a method was presented to carry out spalling test in small scale specimen with exposure to rapid temperature rise using a commonly available electric furnace. Hydrocarbon fire and standard fire exposure can be simulated by manipulating the exposure location of the surface of the concrete cylinder. Ordinary Portland cement concrete cylinders with different strengths were tested. The results demonstrated that this method was an effective and convenient technique to predict the spalling risk of a concrete. The spalling behaviour of geopolymer concrete by using the surface exposure test and standard gas furnace fire test was studied. It was shown that 100% fly ash based geopolymer concrete had a better spalling resistance to rapidly rising temperature exposure than that of Portland cement concrete.
The study of sorptivity test of geopolymer concretes results showed that the geopolymer concrete specimen's structure is more porous and more continuous pore structure than Portland cement concrete specimen. The more porous structure of geopolymer than OPC concrete facilitates the release of the internal steam pressure during heating. Hence, less tensile stress is imposed in the geopolymer concrete than Portland cement concrete during heating, reducing the geopolymer's risk of spalling.
When slag was used as a replacement to fly ash in the geopolymer binder, geopolymer paste and concrete specimens developed considerably high strength at room temperature. It was showed that the magnitude of shrinkage of fly ash and slag based geopolymer is significantly higher than that of 100% fly ash based geopolymer and Portland cement concrete. The residual strength of fly ash based geopolymer concrete with slag replacement after exposure to elevated temperature was studied. It was found that the residual strength of 100% fly ash based geopolymer concrete after elevated temperature exposure increased in the temperature range of 200~500°C compared with OPC concretes. Fly ash based geopolymer concrete with slag replacement experienced a strength loss at the temperature range of 200~300°C, then followed by a strength gain at 300~400°C, and another strength loss after 500°C. When slag was used as an additive to fly ash based geopolymer concrete, the overall strength loss of geopolymer concretes with slag replacement after exposure to elevated temperatures ranging from 200~800°C was higher compared with 100% fly ash based geopolymer concrete, however, it was significantly lower than that of the Portland cement concrete specimens.
The investigation of fire resistance property of fly ash and slag based geopolymer material when exposed to hydrocarbon fire was followed. After hydrocarbon fire, no spalling was observed on geopolymer concretes when using varying factors as binder, slag replacement, cation type of alkaline liquid activator, room temperature and elevated temperature curing. Residual strength testing of geopolymer concretes after hydrocarbon fire exposure showed a similar residual strength percentage compared with the result of Portland cement concrete. However, it is noted that high strength Portland cement concrete spalled, while high strength geopolymer concrete still had a considerably high residual strength after fire exposure.