ORIGINAL ARTICLE Performance of fly ash based geopolymer concrete made using non-pelletized fly ash aggregates after exposure to high temperatures M. Talha Junaid • Amar Khennane • Obada Kayali Received: 22 May 2014 / Accepted: 19 August 2014 Ó RILEM 2014 Abstract The superior performance of geopolymer paste, mortar and concrete (GPC) when exposed to high temperatures has been well documented. Limi- tations still exist however due to the use of similar aggregates, coarse and fine, in both ordinary portland cement concrete (OPC) and GPC. The behavior of geopolymer concrete at elevated temperatures can be further improved by using a recently developed lightweight non-pelletized aggregates made entirely from fly ash (named Flashag) as replacement for conventional natural aggregates. Tests for the ambient and residual properties of GPC made using these new aggregates exposed to temperatures up to 1,000 °C were carried out. The obtained results in terms of compressive strength, splitting strength, and modulus of elasticity are reported and compared with those obtained for GPC produced using ordinary natural aggregates as well as with those reported in the literature for OPC concretes. The effects of heat cycling, as well as, duration of heating on the strength and deformational behavior of GPC are also investi- gated. At ambient temperatures all the three concretes displayed similar properties. At elevated tempera- tures, however, the performance of GPC made from Flashag was found to be far superior to that of both OPC, and GPC made from ordinary aggregates. Additionally, for up to 4 heating cycles, GPC made with Flashag better retained its strength and stiffness properties as compared to GPC made with ordinary natural aggregates. The study also found that most changes to the strength and microstructure of GPC occur in the first few hours of exposure after which, the duration of heating has no significant effect. Keywords Geopolymer concrete stress–strain curves Á Modulus of elasticity Á Lightweight GPC Á Cyclic heating Á High temperature performance 1 Introduction Alkali liquids (usually a soluble metal hydro-oxide and/or alkali silicate) can be used to react with silica (SiO 2 ) and alumina (Al 2 O 3 ) rich natural source, like metakaolin or with industrial by-products, like fly ash (FA), silica fume (SF), rice husk ash (RHA) or Slag to produce binders [5, 6, 25]. Such binders mixed with typical coarse and fine aggregates can form concrete, usually known as alkali activated concrete or geo- polymer concrete. Currently there is sufficient evi- dence to suggest that geopolymer concrete has a better resistance to fire than OPC concrete [7, 8, 10, 11, 31]. Yet the intrinsic good fire behaviour of geopolymer paste, and to some extent mortars, tends to be reduced by the addition of conventional aggregates. Therefore, M. T. Junaid (&) Á A. Khennane Á O. Kayali School of Engineering and Information Technology, UNSW Canberra, Northcott Drive, Canberra, ACT 2600, Australia e-mail: m.junaid@adfa.edu.au Materials and Structures DOI 10.1617/s11527-014-0404-6