Performance of modified lightweight aggregate concrete after exposure to high temperatures

Lightweight expanded clay aggregates (ceramsites) were modified by coating a thin layer of alkali-activated fly ash binder. The modified aggregates were used to prepare concrete specimens, which were exposed to room and high temperatures. Spalling of the concrete specimens during heating and their mechanical properties after exposure were tested. Furthermore, hydration products of the alkali-activated fly ash and microscopic structures of the concrete specimens after exposure were also tested. In addition, in situ images of the concrete specimens were collected to demonstrate the formation and propagation of cracks in the specimens with increasing temperature. All the results were compared with results obtained for concretes prepared with normal lightweight aggregate and with crushed limestone. The results show that, unlike the reference concretes, nearly all the modified concrete specimens were intact, even at a temperature of 1200°C, without spalling and retained good mechanical properties. Nepheline was detected in the alkali-activated fly ash after heating at 800°C. Probably as a result of the strengthening of the alkali-activated fly ash coating, high temperatures induced cracks in the modified concrete that extended along the interfacial transition zone, but did not penetrate the interior of the aggregates.